# Patent

## Title
Device and method for calculating a result of a sum with a calculating unit with limited word length

## Abstract

For calculating the result of a sum of a first operand and a second operand, a modified second operand is calculated, which is negative and less than the modulus. Based on this modified second operand, a sum is calculated which is less than a maximally processable number of a calculating unit executing the calculation. Finally, the sum calculated using the modified second operand is reduced, namely with respect to the modulus, to obtain the result of the sum of the first and second operands.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority from German Patent Applications No. 10 2006 025 677.8, filed on Jun. 1, 2006, and No. 10 2005 051 772.2, filed on Oct. 28, 2005, which are incorporated herein by reference in their entirety.
BACKGROUND
The present invention relates to the calculation of the modular multiplication, such as it is required, in particular, in cryptographic applications, and particularly to a calculation of the modular multiplication with operands having a larger number of digits than bit slices are present in a long number calculating unit.
The modular multiplication is a central operation used in modular exponentiation, such as it is usually used in cryptography. For example, as shown in FIG. 2a, a key pair is generated in public key cryptography, i.e. in asymmetric cryptography, such as in the RSA method. The key pair consists of a public key e and a private key d. The private key is only known to one entity. The public key serves this entity, but is provided to another entity which wants to send, for example, encrypted data to the one entity to which the private key belongs. As shown in FIG. 2a, an encryption of an unencrypted message M to an encrypted message C is done by calculating a so-called modular exponentiation, in which the message is raised to a higher power with the public key, to then perform a modular reduction with respect to the modulus N, which is also known publicly. For the decryption, the same operation is performed, but now with the private key as exponent, so that the one entity to which the private key belongs and by which the public key was originally distributed to the other entity, again obtains the plain text message M.
These public key methods may also be used as signature/verification methods. An entity generates a digital signature by encrypting the message M to be signed with the private key of this entity to generate the signature S, such as it is also illustrated in FIG. 2a. The verification is then done by the verifying entity subjecting the signature to modular exponentiation with the public key e of the signing entity to then obtain a plain text message M that may be compared to the plain text message M to which the signature is assigned. If the plain text message obtained in the verification matches the plain text message to which the signature is assigned, it may be assumed that the signed document is authentic.
As mentioned above, a cryptographic calculation including modular exponentiation, such as illustrated in FIG. 2b, is split into several modular multiplications. For example, it is usually preferred to calculate a modular exponentiation by applying modular multiplications consecutively. In particular, due to the increased security requirements for the RSA algorithm, there is an interest to execute a modular multiplication with a width of 2048 bits, i.e. with key lengths and/or modulus lengths of 2048 bits.
Generally in modular multiplication as part of a cryptographic calculation, both the multiplier A and the multiplicand B and the modulus N represent parameters of the cryptographic calculation, because the final results, such as plain text message, encrypted message, signature, etc. depend on these parameters.
As already mentioned, there is an interest to steadily increase the key lengths of public key cryptography, because this allows to still prevent so-called brute force attacks with increasingly fast processors. For example, the effort of a brute force attack is correlated with the key length, so that increasingly long keys also require increasingly more complex brute force attacks which, with currently available computers, take so much time that a cryptographic algorithm may be considered to be safe. However, what is problematic with increasingly larger key lengths is that the key length that a crypto co-processor in a chip card or a computer (for example in a TPM module) has is limited by the long number calculating unit included in this crypto co-processor. Such a long number calculating unit is shown, for example, in FIG. 4c, where a so-called bit-slice structure of a long number calculating unit is illustrated.
In the embodiment shown in FIG. 4c, each bit slice includes an arithmetic unit, which may, for example, be a one-bit full adder, which may receive a carry from a lower bit slice and which may output a carry to a higher bit slice. Furthermore, at least one register is associated with such a bit slice. However, it is preferred to associate a certain number of registers, for example two or, even better, for example five registers. In a currently existing crypto co-processor with a bit slice number of 1408 slices, a bit slice includes five registers, i.e. register Z, register C, register N, register CR0 and register CR4, as indicated in the left subimage in FIG. 4a. In that case, this processor operates in long mode. With this number of bit slices, the processor is well-suited to perform RSA calculations with key lengths of 1024 bits, because, for a calculation with 1024 bits key length, a calculating unit that would also have only 1024 bit slices would not be quite sufficient. In the calculating unit with 1408 bit slices, slightly longer key lengths may also be calculated, but there should always be slightly more bit slices than key bits to be able to compensate certain overflow or underflow situations.
The calculating unit 40 shown in FIG. 4b may be provided with data and/or flow sequences and/or controlled by a controller 41. Furthermore, there is a register configuration means 42 which may configure the registers of the calculating unit, i.e. the five registers in long mode in this embodiment, to ten registers in short mode. Each long mode register of a certain length thus results in two short registers of half the length, respectively, in this embodiment, so that two N registers, two C registers, two Z registers and one CR0 register, one CR2 register, one CR4 register and one CR6 register are created. Still each bit slice has an arithmetic unit, i.e. for example a one-bit full adder, which now, however, has twice the number of registers in short mode in contrast to the situation in FIG. 4c representing the long mode.
If the crypto co-processor with 1408 bits now is to calculate RSA key lengths of, for example, 2048 bits, this is no longer easily possible, because there are not enough bit slices.
It is apparent that, although an increase in key lengths is very desirable from the security point of view, each increase in key lengths causes already existing coprocessors to be no longer readily usable. Thus, always new longer calculating units would have to be developed, which requires development time and costs.
In order to avoid this, methods have been developed with which larger numbers may be processed on smaller calculating units. For example, there are generally methods for doubling a calculating unit in software. Such a method is, for example, the calculation of the modular multiplication using the Chinese Remainder Theorem (CRT), as it is described in section 14.5 on pages 610-613 of “Handbook of Applied Cryptography”, A. Menezes, P. van Oorschot, S. Vanstone, 1996. Generally, a modular exponentiation with a long modulus is split into two modular exponentiations with a short modulus using the Chinese remainder theorem, wherein these results are then combined. In that way, a calculating unit may, so to speak, be doubled “software-wise”.
However, this concept only allows doubling, which is inconvenient for situations in which doubling of the key lengths is not necessarily required, but in which key lengths are to be used that are maybe only 50% larger than the architectural calculating unit length, i.e. the number of bit slices. If such 100% doubling algorithms are used, when perhaps only key lengths larger by 50% are to be processed, the calculating unit is used only with (100+50) %/2=75%. In principle, hardware resources are thus wasted.
In addition to the CRT doubling method, there are also further calculating unit doubling algorithms, such as the Montgomery multiplication, a multiplication with Karatsuba-Offman and subsequent reduction by means of, for example, the Barrett reduction, or the doubling method using the MultModDiv operation, such as it is, for example, discussed in German patent DE 10219158 B4.
Considering, for example, FIG. 4d, a calculating unit for a 1024 bit key length is indicated at 43. Software doubling using, for example, the Chinese remainder theorem or using one of the above further methods, is useful when 2048 bits are required, such as illustrated in block 44 in FIG. 4d. In this way, the whole calculating unit is used, i.e. no unused bit slices remain. However, if a key length with, for example, 1536 bits is to be enough, software doubling using, for example, the Chinese remainder theorem (CRT) will result in 2×768 bits being required. The remaining 2×256 bits would remain unused in this case.
Not only regarding the utilization of the calculating unit, but also when register lengths of a calculating unit cannot be divided, the calculating unit will always have a maximum processing capacity also including intermediate results of the calculation. In other words, the word length that may maximally be processed by the calculating unit will be limited. This word length is, for example, the total length of the register memories, i.e. the register cells associated with the individual bit slices. Alternatively, the maximally processable word length is also an effective word length larger than the register length which is achieved by various software/hardware concepts. In general, however, the situation will be such that this maximally processable word length does not only apply to input operands or the result of the calculation, but also to all intermediate results within the calculation, which are stored in a memory and which will then be required again for a later calculation on the path from the input operand to the result.
If, within a calculation, there is a situation in which an intermediate result becomes larger than the maximally processable word length, this results in a significant slowing down of the processor, because in that case auxiliary routines have to be started, which require special measures for this intermediate result, such as more sophisticated measures of further enlargement of the calculating unit by further increase of the number of operand fragments. Alternatively, such a situation may even result in an error and/or in an output to the user that there has been an overflow and thus the calculation desired by the user cannot be performed and/or smaller input operands have to be supplied by the user.
Even if such an “overflow” does not result in a computing abort or in a calculating error, it still results in a significant slowing down when corresponding service routines have to be started to manage this exceptional situation.
Conventionally, there is thus a lack of a more flexible and faster calculating unit concept.
BRIEF SUMMARY
Among other aspects, the present invention concerns a method and a device for calculating a result of a sum of a first operand and a second operand smaller than a modulus by means of a calculating unit designed to maximally process numbers whose amount is less than or equal to a product of the modulus and an integer larger than 1 with a means for storing the first operand in the calculating unit, wherein a magnitude of the first operand is less than or equal to the maximally processable number; a means for calculating a modified second operand, wherein the modified second operand is equal to the second operand minus the modulus, so that the modified second operand is negative; a means for calculating a sum of the first operand and the modified second operand or the first operand and a product of the modified second operand times a factor less than or equal to the integer, and for storing the sum in the calculating unit; and a means for reducing the sum with respect to the modulus to obtain the result.



BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)
Next, there will be given a detailed description of the preferred embodiments of the present invention with respect to the accompanying drawings, in which:
FIG. 1a shows a schematic representation of the device for calculating a result of a modular multiplication according to a preferred embodiment of the present invention;
FIG. 1b shows a register representation of the operands A, B, N of FIG. 1a and the division of the operands into portions;
FIG. 1c shows a schematic representation of the functionality of the inventive device for combining the intermediate results;
FIG. 1d shows a representation of the modular multiplication operation;
FIG. 1e shows a representation of the modular multiplication with addition;
FIG. 1f shows a schematic representation of the multiplication with addition;
FIG. 1g shows a schematic representation of the reduction operation;
FIG. 1h shows a schematic representation of the sequential calculation of the intermediate results;
FIG. 2a shows a general representation of the field of application of the modular exponentiation;
FIG. 2b shows a schematic representation of the decomposition of a modular exponentiation into modular multiplications;
FIG. 2c shows a schematic representation of the multiplication operation;
FIG. 2d shows a special representation of the inventive MultAdd operation;
FIG. 2e shows a special representation of the MultModAdd operation;
FIG. 2f shows a special representation of the modular multiplication operation;
FIG. 2g shows a schematic representation of the MultModDiv operation sequentially used in the present invention;
FIG. 3a shows a general representation of the inventive MultAdd operation;
FIG. 3b shows a base version of the reduction operation;
FIG. 3c shows a first preferred version of the reduction operation;
FIG. 3d shows a second preferred version of the reduction operation;
FIG. 3e shows a third preferred version of the reduction operation;
FIG. 3f shows a fourth preferred version of the reduction operation;
FIG. 4a shows a representation of the register situation of a calculating unit in long mode and in short mode;
FIG. 4b shows a schematic representation of a configurable calculating unit;
FIG. 4c shows a schematic representation of a bit slice structure of a calculating unit;
FIG. 4d shows a schematic representation of the various possibilities of software doubling as compared to the inventive software extension by three or more splittings,
FIG. 4e shows a tabular comparison of various algorithms;
FIG. 5 shows a flow diagram of the inventive calculation;
FIG. 6a shows a preferred implementation of the modular multiplication algorithm;
FIG. 6b shows a preferred register implementation of the algorithm of FIG. 6a; 
FIG. 7a shows a preferred implementation of the MMA operation;
FIG. 7b shows an alternative implementation of the MMA operation;
FIG. 7c shows a register implementation of the MMA operation;
FIG. 8a shows a preferred implementation of the MMA operation;
FIG. 8b shows a preferred register implementation of the MMA operation of FIG. 8a; 
FIG. 8c shows a schematic representation of the input and output operands in the present invention;
FIG. 8d shows a schematic block circuit diagram of the method and the device of the present invention;
FIG. 8e shows a calculating example for the present invention illustrating the register loading of the short auxiliary and result registers for each intermediate step;
FIG. 9a shows a preferred implementation of the TC operation (TC=treat carry);
FIG. 9b shows a preferred implementation of the TB operation (TB=treat borrow);
FIG. 9c shows a preferred implementation of the reduction operation;
FIG. 9d shows a preferred implementation on the register level of the reduction operation of FIG. 9c; 
FIG. 10a shows a representation of the MMD operation;
FIG. 10b shows a preferred register implementation of the MMD operation;
FIG. 11a shows an implementation of the MMD operation;
FIG. 11b shows a register implementation of the MMD operation of FIG. 11a; 
FIG. 12 shows a register implementation of the calculation (estimation) of ε and/or e;
FIG. 13a shows an implementation of a transform rule for the modulus;
FIG. 13b shows an implementation of the DIV operation;
FIG. 13c shows a schematic representation of a reduction algorithm for the final reduction;
FIG. 14 shows an inventive calculating unit with a maximally processable number and an associated sign bit;
FIG. 15 shows a block circuit diagram and/or flow diagram of the inventive device and/or the inventive method for calculating a result of a sum;
FIG. 16 shows an association of the inventive procedure with the modular multiplication described in FIGS. 8 and 9 for representing the embedding of the present invention in the preferred processor for the calculation of the modular multiplication;
FIG. 17a shows an inventive calculating example; and
FIG. 17b shows a comparative example that may be calculated without the invention on the processor of FIG. 14 only or with significant time/complexity disadvantage.



DETAILED DESCRIPTION
In preferred embodiments, in a calculating unit in which there may maximally be processed numbers whose magnitude is less than or equal to a product of a modulus and an integer larger than 1, when a sum of two operands is to be calculated with respect to a modulus, and when one operand of the two operands is less than the modulus, this operand is made negative in a way that the modified operand is equal to the non-modified operand minus the modulus. Then, a sum of the one operand with the modified operand will still be less than the maximally processable number, although the sum of the two operands without the modulus subtraction to make an operand negative would have yielded a number that would have been larger than the maximally processable number of the calculating unit.
The result of the sum calculation according to the invention is thus still within the allowed number range. This result, however, carries an error as compared to the number preceding the number to be processed.
This error, however, is eliminated according to the invention when this result is modularly reduced, namely with respect to the modulus that has previously been subtracted from the operand to achieve the modified operand.
The inventive device is particularly advantageous in that now, in a calculating unit designed to process negative and positive numbers, i.e. which processes a sign bit, still no overflow prevention measures are required, even if the sum of the two operands to be calculated resulted in an intermediate result larger than the maximally processable number of the calculating unit.
Thus, the inventive calculating unit is faster and more flexible and also less error prone in that no overflow is generated and thus no overflow managing measures are required that significantly slow down the calculating unit per se. According to the invention, there may thus also be calculated more complex calculations in which a sum and a subsequent reduction of the sum are required, wherein there will only be intermediate results smaller than the maximally processable number, because the sum is always smaller than the maximally processable number prior to its final reduction, because an addend is made negative, while the other addend is positive. According to the invention, the sum is thus always transformed into a difference, wherein the difference partner is obtained by modulus subtraction, and the thus introduced error is again eliminated in the final modular reduction with respect to this modulus.
The only additional step required is that the second operand is made negative, namely by subtracting the modulus from its actual value without the inventive measure. This additional step, however, only requires a simple subtraction of the modulus, thus takes place only on the arithmetic side and does require any exceptional managing or overflow managing routine. Thus, there is no slowing down, which is not desirable.
It is particularly preferred if the inventive reduction by one N “too much” is embedded or integrated in a larger reduction in which there is a reduction by subtracting a multiple of N. In this case, the multiple simply needs to be incremented by one, wherein the subsequent subtraction is then performed considering the incremented multiple.
The inventive concept is further advantageous in that the inventive intermediate step of the “negative-making” of the second operand may be executed irrespective of whether the result of the sum of the two operands is really above the maximally processable number or not. Instead, it is preferred to operate, according to the invention, independent of the values of the first and second operands, so that no value-dependent current profile of the circuit is externally visible. Thus, even by a side channel attack, for example by means of differential power analysis, it is not possible to determine whether an intermediate result is above the maximally processable number or not, because, irrespective of whether the intermediate result is actually larger than or less than the maximally processable number, always the same step sequence and thus essentially the same power consumption and thus also essentially the same time consumption and/or the same performance/time profile is output by the circuit.
In connection therewith, it is to be noted that the present invention is particularly suitable for long number calculating units, because these calculating units are designed for operands with lengths (significantly) above 300 bits anyway. On the other hand, increasing requirements of cryptographic securities and particularly also drastically increasing computing power that allows more and more brute force attacks to become increasingly feasible, result in increasingly longer keys and thus increasingly longer operands and/or numbers to be processed. Each measure with which calculations may be performed, although the intermediate results already become “too large” for the calculating unit, is thus highly welcome, because these measures as a whole result in a less expensive product, particularly due to the reduced hardware requirements for the same security, i.e. for the same key length. Especially in the area of chip cards, which are a mass product where price differences in the order of cents are important if a product is to survive on the market, the requirements of chip area (hardware requirements), security features and price are particularly tight. In addition, time aspects also play a role, because customers do not want to wait a long time when authenticating themselves somewhere with their chip cards. This expectation, however, does not prevent customers from expecting a maximum security standard of their chip cards.
It is exactly in this field of conflict that the present invention has its advantages, providing maximum security due to the reliability of actually too large intermediate results with, at the same time, fast execution of a cryptographic calculation, because no overflow measures are taken which would otherwise significantly slow down the operation of the chip card.
In order to further increase the efficiency, in preferred embodiments of the present invention, at least the multiplicand of a modular multiplication is divided into at least three portions, wherein each portion has a number of digits less than half the number of digits, wherein the at least three portions of the multiplicand include all digits of the multiplicand. Furthermore, means for sequentially calculating is provided to sequentially calculate an intermediate result for each portion of the multiplicand, and to then obtain a result of the modular multiplication using these intermediate results.
By splitting the multiplicand into at least three portions, a dividable calculating unit may preferably be used, in which the multiplicand and preferably also the multiplier and the modulus is divided into three or more parts, so that each third of the number may be accommodated in one half of the co-processor. Thus, the calculating unit itself may also be utilized in full length, and no hardware resources are wasted.
In preferred embodiments, a division of all registers of the calculating unit is done, namely into registers of the same length, and furthermore all operands, i.e. both the multiplier and the multiplicand and the modulus, are divided into three or more parts as well, so that in the end, for the calculation of the (long) modular multiplication, only logic and arithmetic operations are required that are done with numbers whose length, i.e. whose number of digits, is maximally equal to the number of digits of one portion of the numbers. Preferably, in order to obtain optimum utilization of a calculating unit, the portion into which a number is divided, i.e. the number of bit slices of the calculating unit that has to perform these operations with a smaller number of digits, is selected such that they correspond to one half of the dividable calculating unit.
According to the invention, a calculating method using the MultModDiv operation is used, in which there are preferably never used more than ten short registers of the calculating unit, wherein two short registers of the calculating unit in short mode correspond to one register of the calculating unit in long mode.
In particular, in order to calculate the modular multiplication, the whole task is divided into three multiplication modulus addition operations to be performed sequentially in the present invention, wherein another portion of the multiplicand B is used for each one of these individual operations. Each such multiplication modulus addition operation is, in turn, divided into a multiplication addition operation and a subsequent reduction operation, wherein, in the multiplication addition operation, there is always used the currently considered portion of the multiplicand B, and in individual iteration steps, corresponding portions of the intermediate result C obtained from the previous step and the multiplier A are used.
According to the invention, this multiplication addition operation is now divided into several MultModDiv operations, i.e. into modular multiplications, which respectively provide the integer quotient, i.e. the DIV result, and the remainder, i.e. the MOD result. Both the DIV result and the MOD result are short numbers that may be stored in short registers. The short registers in which results of the MMD operation are stored are also referred to as auxiliary registers, because they are written to several times in the process of the iterative processing of the multiplication addition operation. In other words, the results of an MMD operation are only required for the subsequent updating operation, in which a piece of the result number, namely a portion of the result number fitting into a short register, is successively calculated. Specifically, during updating, the result of the previous MMD operation is updated using an addition of portions of the third operand C, i.e. the intermediate result of a previous step.
According to the invention, each updating step provides two entries into a result register, wherein the more significant entry into the result register already represents a final result that is not changed anymore, while the less significant entry of the two obtained results will still be changed by a result of an updating step depending on the current number situation.
The present invention thus only needs a calculating unit for a multiplication addition operation that has a word width equal to the length of only one portion and not the whole length of an operand, i.e. which has a short word width. In other words, such a calculating unit only requires internal registers of short length and not of long length. In addition, only in a preferred embodiment of the present invention only two auxiliary registers and—for a division into three portions—four short result registers are required. The multiplication addition operation may thus be calculated with only six short registers. In this case, the calculating unit is a bit slice stack, wherein each bit slice has a full adder function, i.e. receives a carry from a lower bit slice and passes a carry on to a higher bit slice, wherein “higher” and “lower” refers to the significance of the processed binary digits. If there is only a calculating unit with six internal registers, the calculating unit must be able to receive the additional operands from an external memory, i.e. the portions of the intermediate result from a previous iteration step and the required portions of the multiplier A and, if necessary, the current portion of the multiplicand B.
In the preferred embodiment of the present invention, a number of 10 or 12 short registers is sufficient for calculating the multiplication addition operation, which may be obtained by halving five or six long registers, wherein there is further available a working memory, which is typically called “XDATA”, in which further portions are stored. This working memory, however, only has to be accessed with respect to a single portion of a single operand in each cycle, so that an operation may be efficient and with a small number of working memory accesses, but with maximum utilization of the internal registers. It is to be noted that the inventive concept of calculating the result of a multiplication addition operation may be used not only as part of a modular multiplication, but in all cases where, using a processor allowing only short word lengths due to its construction, a multiplication addition operation is to be calculated that includes long operands, i.e. operands having a word length that may not be processed by the calculating unit in one go.
Before discussing the preferred use of the inventive concept as part of the efficient calculation of a modular multiplication with an actually too small calculating unit in detail with respect to FIGS. 1 to 13c, first the present invention is illustrated with respect to FIGS. 14 to 17b, as it may be also used in a calculating unit in which there is no division of the operands. Also in such a calculating unit having, for example, registers and/or a number of bit slices equal to the digits of the maximally processable number, in which there is thus no operand division, the inventive concept will also result in a performance advantage. The preferred application, however, is in a device and/or a method and/or a computer program usable for calculating the modular multiplication, wherein there is register division such that long operands and thus cryptographic key lengths may be calculated with a too short calculating unit.
FIG. 14 shows such a calculating unit 1400, for example a binary calculating unit, such as it is illustrated with respect to FIG. 4b. The calculating unit is defined by a maximally processable number which, in the binary case, is typically +/−2(m+1)−1. This maximally processable number may also be represented by a product of the modulus N and a number Z, wherein the number Z is an integer larger than 1 and, in preferred embodiments of the present invention to be described later, is used to achieve a division of a long number into several short numbers.
The property of the calculating unit to process only numbers smaller than or equal to a maximally processable number is illustrated in FIG. 14 by an exemplary long register 1402 which exemplarily comprises a sign bit as MSB which, in a set state, indicates for example a negative number, while it indicates a positive number in the unset state. The digits of the number are symbolized by digits 1, 2, 3, 4, . . . , n in FIG. 14. Alternatively, however, the calculating unit could also be, for example, a decimal calculating unit, in which the digits would correspond exactly to the digits of the decimal system, wherein in that case both calculations of the maximally processable number have to take no longer the number “2”, but the number “10” as the basis of the exponentiation.
Since the MSB and/or the highest digit of the maximally processable number is a sign bit, the calculating unit also includes means 1404 for interpreting the most significant bit (MSB) and/or the most significant digit of the maximally processable number as sign bit and not as “value bit”.
As will be discussed later, the calculating unit may include such a long register. The long register, however, may also be implemented by various short registers, wherein again the most significant bit of the short register storing the most significant portion and/or the most significant “subnumber” is again interpreted as sign bit. With respect to the division of the numbers into operands, which will be discussed further below, this most significant portion would correspond, for example, to the portion A2, B2 and N2, respectively, of an operand A, an operand B and a modulus N.
As a long number has only a single sign, no individual sign bit is required for the two remaining portions, i.e. for example A1, A0 and/or B1 or B0.
FIG. 15 shows a schematic block circuit diagram of a device and/or a method for calculating a result obtained at 1500 using a sum of a first operand X stored in means 1502 and a second operand Y that may be provided by means 1504. In the embodiment shown in FIG. 5, the second operand Y is smaller than the modulus N. Furthermore, the whole calculation of the sum is performed in a modular way, i.e. the result of the sum with respect to the remainder class defined by N is wanted, as it is illustrated at 1506 in FIG. 15.
As already discussed with respect to FIG. 14, the calculating unit 1400 including the elements shown in FIG. 15 is designed to process maximum numbers whose magnitude is less than or equal to a product of the modulus N and the integer Z larger than 1.
In particular, the calculating unit 1400 of FIG. 14 and/or the calculating unit illustrated in FIG. 15 includes the means 1502 for storing the first operand X in the calculating unit, wherein a magnitude of the first operand is less than or equal to the maximally storable number. It is to be noted that, if the first operand X is larger than the maximally storable number, storing would not be possible without special further measures anyway. Specifically, the preferred embodiment of the present invention further assumes that X is positive and less than the maximally storable number, as illustrated on the right-hand side with respect to block 1502 in FIG. 15. However, the first operand X may also become negative for other alternatives, as long as it has a magnitude smaller than N times Z, i.e. the maximally storable number.
The inventive device further includes means 1508 for calculating a modified second operand Y′, wherein the modified second operand is equal to the second operand minus the modulus N, so that the modified second operand is negative. A special integrated use within the modular multiplication of this “modulus subtraction” from an already reduced value is particularly illustrated in FIG. 1d or in FIG. 9c at “Red′” and will be discussed in more detail below.
The means 1508 and the means 1502 feed means 1510 for calculating a sum of the first operand and the modified second operand or the first operand and a product of the second operand times a factor less than or equal to the integer Z. The result of the means 1510 for calculating is thus larger than or equal to the product of the modulus and the integer, and is always smaller than the product of the modulus and the integer. The result of the sum calculation means 1510 is thus still within the allowed range, i.e. less than the maximally processable number, in spite of the fact that the sum of the first operand X and the non-modified second operand Y would actually have become larger than the maximally processable number, i.e., in the present case, larger than or equal to N, Z.
The calculated sum is finally supplied to means 1512 which performs a modular reduction of the sum with respect to the modulus to obtain the result 1500 which is, in turn, larger than or equal to zero and in any case less than the modulus N, and which is now again positive particularly in comparison with the output value of block 1508.
In other words, the error introduced by means 1508 is thus again eliminated due to the further subtraction of a modulus from the actually already reduced second operand by the modular reduction in step 1512. This small detour for calculating the modified operand now, however, allows to also execute the calculation, as illustrated in FIG. 15, although intermediate results are reached that are larger than the maximally processable number, as will be illustrated based on a comparison of FIG. 17a and FIG. 17b. 
Only as an example, FIGS. 17a and 17b show a calculation in which the two operands X equal to 95 and Y equal to 7 are to be modularly reduced with respect to the modulus N equal to 10. The maximally processable number of the calculating unit is N times Z. In other words, the calculating unit may thus process numbers between −99 and +99.
The simple sum of X+Y, however, already yields 102, wherein the number 102 is no longer processable and would result in an overflow managing routine and/or even in an error.
According to the invention, the modulus is subtracted from the second operand Y, so that a negative number −3 results from the number 7. The sum of −3 and 95 now yields the number 92, which is an easily processable number. The modular reduction of the number 92, however, yields the same result as the modular reduction of the number 102 which is no longer processable, so that the error introduced by the modulus subtraction step is again eliminated in the end due to the modular reduction.
At this point, it is further to be noted that the second operand Y=7 has already been the result of a previous modular reduction, that this value is thus already in the remainder class and thus actually should not be reduced and/or subjected to a subtraction of the modulus again. However, not executing the subtraction of the modulus from the second operand Y would result in, as in the comparative example, the calculating unit no longer being able to calculate the result of the sum of the first operand and the second operand with respect to the modulus.
As already mentioned, the inventive calculating unit-efficient sum calculation as part of a modular multiplication is executed as illustrated in FIG. 1d. Specifically, the inventive calculation for calculating the result of the sum of the first and the second operands with respect to a modulus represents the MMAZ operation for the second portion of operand B1 and for the first portion B0 of this operand, as also illustrated in the “correspondence” in FIG. 16. The result of the means 1512 for reducing the sum with respect to the modulus N obtained at the output 1500 of FIG. 15 thus corresponds to the value E+N in FIG. 7b, because the further subtraction of the modulus has already been executed in step 1508 by the reduction Red′Z (D; N).
The means 1508 of FIG. 15 thus corresponds to the functionality for calculating the value C of the second MMAZ step in FIG. 1d and/or the functionality of step MMA'Z in FIG. 7b, wherein Red′Z is executed to obtain the modified second operand E in FIG. 7b as the result of the second MMAZ step. Then, the sum of D is calculated in the last step MMAZ according to FIG. 7a to then perform the functionality RedZ D−N of FIG. 7a by the reduction in block 1512 of FIG. 15 to finally obtain the result C of the modular multiplication of FIG. 1d. 
In the following, a preferred embedding and/or implementation of the inventive concept to keep intermediate results below the number to be maximally processed is discussed in connection with FIGS. 1a to 13c, which shows a preferred implementation of the modular multiplication with a calculating unit having registers shorter than the operand length.
FIG. 1a shows a schematic representation of an inventive device for calculating a result of a modular multiplication with a multiplier A, a multiplicand B and a modulus N. Originally, the multiplier A, the multiplicand B and the modulus are each numbers extending from a least significant digit (the LSB in the binary case) to a most significant digit (the MSB in the binary case). The operands A, B, N have a length less than or equal to a certain number of bits, such as 1536 bits in the scenario in block 46 described in FIG. 4d. 
Each portion of the multiplicand B provided by means 10 for providing the portions has a length of 512 bits in the embodiment shown in FIG. 1b, i.e. a length equal to a third of the original length of the multiplicand B. Thus, all portions are equal in length. The number B may then be written as illustrated in FIG. 1b. The number Z represents the “register shift number” or the corresponding multiplier to be multiplied to the second and/or, in squared form, to the third portion to recombine the number B from the portions B0, B1, B2. i directly means the number of digits and/or the number of bits that a portion has. It is to be noted that the embodiment shown in FIG. 1b is exemplary for a uniform division of a number into three portions. According to the invention, however, there may also be generated more than three portions and preferably an odd number of portions, and there may also be generated portions having unequal lengths, i.e. that, for example, the first portion is somewhat shorter than a third and the second portion is somewhat longer than a third, etc. However, with respect to an optimum adaptation of the division into portions by the means 10 of FIG. 1a to the calculating unit, portions of equal length are preferred.
The portions of a number may thus represent the number directly, so that the portions directly have the digits of the number and yield the number when they are cut out, so to speak, and put together. Alternatively, and sometimes even preferably, the number is calculated from the portions using the division number Z, so that the portions represent the number here as well, but the representation is not done via directly putting them together, but via a calculation with the division number Z, as indicted in FIG. 1b at “generally”.
The means 10 for providing the multiplicand in at least two portions thus receives the number B on the input side and provides the three or more portions B0, B1, B2 on the output side, wherein each portion has a number of digits less than half the number of digits, and wherein the means 10 for providing is further selected to perform the portion division so that the generated portions together include all digits of the multiplicand.
The means 10 provides the portions to means 12 for sequentially calculating a sequence of steps. In particular, the means 12 for sequentially calculating a sequence of steps is designed to calculate a first intermediate result for the use of a more significant portion of the multiplicand, as illustrated at 14 in FIG. 1h. This first intermediate result is then used to calculate a second intermediate result also using a less significant portion B1. This second intermediate result is then used to calculate a third intermediate result using again a less significant portion B0 of the multiplicand. The third intermediate result may already be the result of the modular multiplication, if only three portions have been used for the multiplicand. The third intermediate result may then be further processed according to the procedure shown in FIG. 1h, if further portion divisions have been performed, to then finally obtain the final result of the modular multiplication.
Although, in preferred embodiments, the means 10 for providing is designed to provide not only the multiplicand, but also the multiplier and the modulus into individual portions, the embodiment shown in FIG. 1a, in which only one operand of the multiplication is divided, already results in an advantage in that no long register is required for the multiplicand itself, but that a short register is sufficient there, because the whole multiplicand is never required due to the sequential calculating nature of means 12, but always only a portion of the multiplicand.
For calculating units in bit slice architecture, however, a division into portions of all operands and the modulus is preferred, as discussed below, to only have to use registers that have the same (short) length. In this context, there is also preferred a division of all parameters of the modular multiplication into portions of the same length, because the best calculating unit utilization is achieved when (short) registers of the same length are used.
According to the invention, it is preferred that a calculating unit is used for performing the modular multiplication that has at least one register having a length less than a length of the multiplicand, but larger than or equal to a portion of the multiplicand, wherein the means for calculating is designed to sequentially load a portion of the multiplicand into the register or read it from the register.
In a further preferred embodiment, a division of the numbers into exactly three portions is performed, and a calculating unit is used that is operated in a short mode, i.e. which is divided into two calculating unit halves in which the three portions of the respective numbers are processed.
Subsequently, a preferred embodiment of the present invention is given in which a 2048 bit multiplication is implemented. First, however, an overview of certain used notations and operations is given. Essentially the following is about the calculation of the modular multiplication as illustrated in FIG. 2f. 

    
    
        #N is defined to be the bit length of N, i.e., if n=#N, then N∈[2n−1, 2n[.
        A mod N denotes the usual remainder of A modulo N, i.e. A mod N∈[0, N[.
        A mod′ N denotes the negative remainder of A modulo N, i.e. A mod′N∈]−N,0], i.e. A mod′ N=A mod N−N, if A mod N>0.
        There will be used several notations for integers: Let Z≧2 be some integer, then there will be written for the integer N≧0

N=(N2|N1|N0)Z 

wherein

N0:=N mod Z, 

N1:=(N div Z)mod Z, 

N2:=N div Z2.

    
    


Although there may be used the notation

N=N2·Z2+N1·Z+N0 

=(N2,N1,N0)Z 

this latter notation does not imply that N1 and N0 are reduced modulo Z, while the first notation (N2|N1|N0)Z does imply this: In this notation N1 and N0 are in [0,Z[. However, N2 may be larger than Z, this is in the case of N≧Z3. Equivalently, N2 may be negative in the case of N<0.

    
    
        Furthermore the generalizations (Nm−1| . . . |N0)Z as well as (Nm−1, . . . , N0)Z are analogously used in the obvious way.
        Think of Z as a power of two, e.g. Z=21024. But it is not necessary that Z is a power of two, neither any non-trivial power of an integer!
    
    


The following basic algorithms are fundamental and always used. Their implementation will be discussed later. Let K∈N.
The usual multiplication: A·B
The modular multiplication of bit length K: A·B mod N
The MultModDiv operation (FIG. 2g) of bit length K:

(A·B div N,A·B mod N)

Furthermore there is needed the MultAdd algorithm (FIG. 2d):

A·B+C·2K 

and the MultModAdd (FIG. 2e):

A·B+C·2K mod N 

There is often written:

MK(A,B)=A·B, 

MMK(A,B;N)=A·B mod N, 

MMDK(A,B;N)=(A·B div N,A·B mod N).

The performance and/or speed of these algorithms, which is not classified any further at this point, depends on their implementation. So in the following it will be denoted by mK, mmK, mmdK, etc.
Note that the index is used in a very free manner, sometimes, if the exact K is not important, the index is left out, sometimes K is replaced by the actual number base 2K. Even other bases are used. More about this in the following sections.
Remark 1: Note the very important fact that for (Q,R):=MMDK (A,B;N) there is the identity

A·B=Q·N+R. 

This is a fundamental fact for many implementations, which are following.
In this section there are given some hints as to how the basic algorithms are implemented in the Crypto@1408—if they can be implemented in a direct way. Furthermore there are discussed some very basic and general methods to break down long integer arithmetic into smaller pieces.
Multiplication
On the Crypto@1408, multiplications of a length up to 1400 bits (including sign bits) are possible, i.e. A·B for #A+#B≦1400. The average performance on the Crypto@1408 for this operation is given by




  
    
      
        #
        ⁢
        
          
        
        ⁢
        B
      
      +
      1
    
    
      1
      ,
      75
    
  
  ⁢
  
    au
    .
  




For more about this multiplication algorithm, see [5, 6, 8]. Usually, in order to break down a long multiplication into smaller pieces, the well-known high-school method is used: Set, for example, Z:=2k for some suitable k and write A=(Am−1, . . . , A0)Z as well as B=(Bm−1, . . . , B0)Z, then the method can be roughly described as it is shown in FIG. 2c. 
The line in the loop will be read in the following way: The old value of the partial integer (Ci+j+1,Ci+j)Z=Ci+j+1·Z+Ci+j is added to the partial product Mk(Ai,Bj) yielding the result X. Then set Ci+j+1:=X div Z and Ci+j:=X mod Z. Of course, hidden in these instructions are treatments of carries, which will not be discussed any further here.
There are faster ways to implement a multiplication, e.g. with KARATSUBA-OFFMANN, cf. [9]. But, although these algorithms are very good in theoretical performance, they often have the disadvantage that they are not optimal for implementation, e.g. they need very much resources like memory.
Modular Multiplication
On the Crypto@1408, modular multiplications of a length up to 1400 bits are possible, i.e. A·B mod N for LR:=#N+1≦1400. The realization is done via the so-called ZDN algorithm—[11]. The average performance on the Crypto@1408 for this operation is given by




  
    
      L
      ⁢
      
        
      
      ⁢
      R
    
    α
  
  ⁢
  au




The factor α=αLR is a parameter which depends on the statistical properties of the ZDN algorithm. For αLR, values are usually between 2.5 and 2.7.
One way to implement the multiplication for longer bit lengths is to break it down into smaller pieces. Looking at the equation for m=3




  
    
      
        
          
            A
            ·
            B
          
          ⁢
          
            
          
          ⁢
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                    B
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                      ⁢
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                      ⁢
                      
                        
                      
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                  )
                
                ⁢
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              +
              
                A
                ·
                
                  B
                  0
                
              
            
            )
          
          ⁢
          mod
          ⁢
          
            
          
          ⁢
          N
        
      
    
  




it can be see that a modular multiplication

A·B mod N 

can be realized as in FIG. 1d, wherein operation MMA is shown in FIG. 1e. 

Of course, this is only one way to do this. Some derived versions of this are presented in this paper.
The MultModDiv Operation
The MultModDiv operation is a recently introduced operation, cf. [7], which does a little bit more than a modular multiplication: It not only computes the modular product (A·B mod N), but also the quotient (A·B div N). Implemented in HW, the additional implementation overhead is small, since this last integer is just a protocol of what the modular reduction was doing during the modular multiplication. In SW the overhead is significant, but surprisingly only 100%! The algorithm can be implemented as shown in FIG. 10a. 
Note that this algorithm only works for positive and reduced A and B. It will be needed later on also for a (reduced) negative multiplicand, but in this case only the negative integer is inverted, the latter algorithm is applied and finally the output is inverted. Also, it is possible to run the two modular multiplications in parallel (mode) if the modulus is small enough. This means again a doubling of the performance. More about this with respect to FIGS. 11a and 11b. As one can see there, the performance for MMDk is given by:




  
    m
    ⁢
    
      
    
    ⁢
    m
    ⁢
    
      
    
    ⁢
    
      d
      k
    
  
  :=
  
    {
    
      
        
          
            m
            ⁢
            
              
            
            ⁢
            
              m
              k
            
          
        
        
          
            
              if
              ⁢
              
                
              
              ⁢
              L
              ⁢
              
                
              
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            ≤
            695
          
        
      
      
        
          
            
              2
              ·
              m
            
            ⁢
            
              
            
            ⁢
            
              m
              k
            
          
        
        
          
            
              if
              ⁢
              
                
              
              ⁢
              L
              ⁢
              
                
              
              ⁢
              R
            
            >
            695
          
        
      
    
  




In the algorithm representations, according to usual pseudo code notation, the term “input” stands for the algorithm input parameters. The term “output” stands for the algorithm output. The term “return” stands for jumping back and/or giving back the corresponding value to a hierarchically higher program that invoked the algorithm. The argument of “return” is thus the actual result of the algorithm that has been calculated. Furthermore, “for” stands for a repetition loop that is to execute something given by the term “do” starting from a start parameter “to” to an end parameter. “end” stands for the end of a loop. Furthermore, “if” stands for a conditional loop, wherein “when” indicates what to do if the condition of the if loop is fulfilled. Correspondingly, “else if” indicates a further condition that has to be fulfilled instead of a first condition to perform a certain calculation introduced by “then”. The term “treat carry” stands for treating a carry, wherein borrow stands for a negative carry, i.e. so to speak a “carry forward”.
In the following, there are further given some register implementations, such as they are to be seen, for example, in FIG. 6b. Under “Crypto@1408”, the registers of the preferably used crypto co-processor with 1408 bit slices can be found, which are, however, operated in the short mode due to the division in the middle. The register notation in all register implementations is as illustrated in FIG. 4a in the right subimage. For example, the second field in the right column stands for the register CR0 of the processor. Furthermore, the numbers in the fields stand for the corresponding values stored into the corresponding register. If there are “asterisks” in a register, this means that the register is unused, i.e. the register may be occupied with undetermined numbers, which, however, do not play any further role. Furthermore, the vertical column of fields described with “XDATA” stands for an external memory, i.e. refers to a RAM working memory of the processor, while the twelve registers are the internal registers of the memory. Thus, if data are to be loaded from the external RAM memory into the registers of the co-processor, data movement commands (move commands) are necessary.
Register Architecture of Crypto@1408
The following will illustrate the implementations of the algorithms with the assignments of the Crypto registers with the intermediate results. There is shown the Crypto@1408 in the two modes, namely the long mode and the parallel mode (FIG. 4a).

    
    
        In long mode, there are 5 registers of a length of 1.408 bits: Z, C, N, CR0 and CR4.
        In parallel mode, there are 10 registers of a length of 704 bits: CR0, CR2, CR4 and CR6, as well as three registers Z, C, N for each side.
    
    


The basic configurations are illustrated as shown in FIG. 4a: 
Moving Data
Depending on the fact that the data may lie in the cache or in the XRAM (external memory), it can take more or less time to move an integer into or out of the Crypto@xxxx. The following assumes an average value for the performance movk to move a k-bit integer into or out of the Crypto. Some examples show that the moves take a significant time comparable to multiplications.
I.
II. The Modular Multiplication Algorithms
There are several algorithms for implementing a modular multiplication on the basis of simpler elements like (small) multiplications or a smaller modular multiplication. By virtue of these algorithms, it is possible to implement a modular exponentiation by “square and multiply” or Lucas-chain methods (Montgomery ladder). We are not going the way of finding optimal performance algorithms for square and multiply respectively, since this eliminates the possibility of a secure implementation of RSA if needed.
Although what is of interest is actually the algorithm MM2048, it can be seen that A·B mod N=A(B2Z2+B1Z+B0)mod N, or equivalently this expression can be written as ((A·B2 mod N)Z+A·B1 mod N)Z+A·B0 mod N. Therefore, the implementation of MMK may sometimes be broken down to some “smaller” algorithms like MMAk, for some k<K.
Although sometimes these algorithms need additional data (hence some precomputations may be necessary), this is not taken into account and they are not counted. Usually, they have no impact on the performance of the full RSA computation. They may, however, have an impact on the performance of a “short” RSA, like a verification with a small exponent F4=216+1.
Montgomery Multiplication
Without doubt, the most famous algorithm for implementing a modular multiplication is the Montgomery Multiplication [10]. This multiplications algorithm actually does not implement the algorithm MMK(A,B;N)=AB mod N, but rather

A·B·2−K mod N 

Without going into detail, with this strange kind of modified multiplication, it is possible to implement a K-bit-RSA calculation with the same number of multiplications as in the usual implementations which use MMK.
Multiplication with Barrett Reduction
At the moment, a discussion about this method is left out, since usually Barrett reduction has the same performance as the last method. It is not expected that in this context there will be a much better implementation than the one in the last section.

    
    
        A. Fischer-Sedlak-Seifert's Algorithm with MMD
    
    


This algorithm was designed in order to make a 1 k-bit RSA co-processor fit for 2 k-bit operations needing only a minor hardware add-on. The algorithm, described in [7,2], is specifically used for doubling the bit length. It uses the MultModDiv algorithm, which has to be built into the hardware, cf. [4], or may be emulated in software, cf. [3], with two modular multiplications.
Preferred Inventive Algorithm with MMD
This algorithm implements the modular multiplication in the classical way, by computing




  
    
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      ·
      B
    
    ⁢
    
      
    
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    ⁢
    
      
    
    ⁢
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  -
  
    
      ⌊
      
        
          A
          ·
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        N
      
      ⌋
    
    ·
    N
  




However, because of practical reasons and architectural restrictions, this will be done in three steps—as described above—by implementing for K=m·k with m=3 and k=┌K/3┐. Thus, MMK will be implemented as in FIG. 1d. 
Now, MMAZ is the algorithm given (FIG. 1e).
N2 will be very close to Z. However, at the moment, there are no restrictions with respect to the integer Z, except that Z has to have about the correct size of k bits. But more about that later.
Again, this last algorithm will be implemented in two steps, namely: First the multiplication shown in FIG. 1f is performed.
Note the following estimation.
Remark 5: For the output of MAZ




  
    
      
        
          
            
              D
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        (
        1
        )
      
    
  




and in particular for D=(D3| . . . |D0)Z 

D3=D div Z3 

∈[−N2−1,N2]

⊂[−Z,Z[

After the multiplication step, there is the reduction of FIG. 1g. 
In the following, there is first of all presented the mathematical description of MAZ and RedZ with a little theory, which will be important for the implementation.
Description of the Algorithm
From now on, any algorithms will be given for the case of m=3. Because this is the case needed. However, k will not be fixed yet.
The multiplication operation MAZ (FIG. 3a), i.e.,

(A2|A1|A0)Z·Bi+(C2|C1|C0)Z·Z 

or equivalently

(A2|A1|A0)Z·Bi+(C2|C1|C0|0)Z 

will be implemented in the straightforward way:





  
    
      A
      ⁢
      
        
      
      ⁢
      
        B
        i
      
    
    +
    
      C
      ⁢
      
        
      
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        A
        0
      
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    +
    
      
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              A
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          +
          
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        )
      
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    +
    
      
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              A
              2
            
            ⁢
            
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          +
          
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        )
      
      ⁢
      
        Z
        3
      
    
    +
    
      
        C
        3
      
      ⁢
      
        Z
        3
      
    
  




Since Aj·Bi is a 2 k integer, this product is written as

Aj·Bi=(BAij)1·Z+(BAij)0 

and therefore the result is





  
    
      (
      
        BA
        
          i
          ⁢
          
            
          
          ⁢
          0
        
      
      )
    
    0
  
  +
  
    
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            (
            
              BA
              
                i
                ⁢
                
                  
                
                ⁢
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          1
        
        +
        
          
            (
            
              BA
              
                i
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                ⁢
                1
              
            
            )
          
          0
        
        +
        
          C
          0
        
      
      )
    
    ⁢
    Z
  
  +
  
    
      (
      
        
          
            (
            
              BA
              
                i
                ⁢
                
                  
                
                ⁢
                1
              
            
            )
          
          1
        
        +
        
          
            (
            
              BA
              
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                ⁢
                
                  
                
                ⁢
                2
              
            
            )
          
          0
        
        +
        
          C
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      )
    
    ⁢
    
      Z
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  +
  
    
      (
      
        
          
            (
            
              BA
              
                i
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            )
          
          1
        
        +
        
          C
          2
        
      
      )
    
    ⁢
    
      Z
      3
    
  




Note that the large brackets still may be ≧Z!
The reduction operation RedZ (FIG. 3b), i.e. E:=D mod N will be implemented as




  
    E
    :=
    
      D
      -
      
        
          [
          
            D
            ⁢
            
              
            
            ⁢
            div
            ⁢
            
              
            
            ⁢
            N
          
          ]
        
        ·
        
          N
          .
          
            

          
          ⁢
          Here
        
      
    
  
  ,
  
    
      [
      
        D
        ⁢
        
          
        
        ⁢
        div
        ⁢
        
          
        
        ⁢
        N
      
      ]
    
    :=
    
      
        ⌊
        
          D
          N
        
        ⌋
      
      .
    
  




However, since Q0:=D div N cannot be computed directly, the strategy is to first approximate Q0 by {tilde over (Q)}0, wherein

{tilde over (Q)}0:=D3·Z div N2.

Hence, Q0={tilde over (Q)}0+ε can be written. A computation shows that ε∈{−2, −1, . . . , 4}, and in this context it will even be

ε∈{−2,−1,0,1,2,3}.

Remark 6: In fact, ε=−2, 3 will almost never happen and ε=2 only very seldom.
Thus, the first version of the plain (base) algorithm of FIG. 3b will be as in FIG. 3c. 
Remark 7: The range of Q0 is given by:




  
    
      
        
          Q
          0
        
        =
        
        ⁢
        
          ⌊
          
            D
            N
          
          ⌋
        
      
    
  
  
    
      
        ∈
        
        ⁢
        
          [
          
            
              ⌊
              
                
                  
                    -
                    N
                  
                  ⁢
                  
                    
                  
                  ⁢
                  Z
                
                N
              
              ⌋
            
            ,
            
              ⌊
              
                
                  
                    N
                    ⁢
                    
                      
                    
                    ⁢
                    Z
                  
                  -
                  1
                
                N
              
              ⌋
            
          
          ]
        
      
    
  
  
    
      
        =
        
        ⁢
        
          [
          
            
              -
              Z
            
            ,
            
              Z
              [
            
          
        
      
    
  




Unfortunately, the problem of the exact computation of the division




  D
  N




was only postponed. But since this worked very well, it will be done a second time: ε is approximated by {tilde over (ε)} such that

δ:=ε−{tilde over (ε)}∈{−1,0,1}.  (2)

Then the reduction looks like in FIG. 3d. 
Now, how is ε approximated? Look at the following equation:




  
    
      
        
          
            (
            
              D
              -
              
                
                  
                    Q
                    ~
                  
                  0
                
                ⁢
                N
              
            
            )
          
          ⁢
          
            
          
          ⁢
          mod
          ⁢
          
            
          
          ⁢
          N
        
        =
        
          D
          ⁢
          
            
          
          ⁢
          mod
          ⁢
          
            
          
          ⁢
          N
        
      
    
  
  
    
      
        =
        
          
            (
            
              D
              -
              
                
                  
                    Q
                    ~
                  
                  0
                
                ⁢
                N
              
            
            )
          
          -
          
            ɛ
            ⁢
            
              
            
            ⁢
            N
          
        
      
    
  




This yields

ε=(D−{tilde over (Q)}0N)div N, 

and therefore D−{tilde over (Q)}0N is computed: Set

{tilde over (Q)}0:=D3Z div N2 and {tilde over (R)}0:=D3Z mod N2,

so that D3Z={tilde over (Q)}0N2+{tilde over (R)}0. Now





  
    D
    -
    
      
        
          Q
          ~
        
        0
      
      ⁢
      N
    
  
  =
  
    
      
        (
        
          
            D
            2
          
          +
          
            
              R
              ~
            
            0
          
          -
          
            
              (
              
                
                  
                    Q
                    ~
                  
                  0
                
                ⁢
                
                  N
                  1
                
              
              )
            
            1
          
        
        )
      
      ⁢
      
        Z
        2
      
    
    +
    
      
        (
        
          
            D
            1
          
          -
          
            
              (
              
                
                  
                    Q
                    ~
                  
                  0
                
                ⁢
                
                  N
                  1
                
              
              )
            
            0
          
          -
          
            
              (
              
                
                  
                    Q
                    ~
                  
                  0
                
                ⁢
                
                  N
                  0
                
              
              )
            
            1
          
        
        )
      
      ⁢
      Z
    
    +
    
      (
      
        
          D
          0
        
        -
        
          
            (
            
              
                
                  Q
                  ~
                
                0
              
              ⁢
              
                N
                0
              
            
            )
          
          0
        
      
      )
    
  




Here, the notation ({tilde over (Q)}0Ni)1:={tilde over (Q)}0Ni div Z and ({tilde over (Q)}0Ni)0:={tilde over (Q)}0Ni mod Z was used, so that Q0Ni=({tilde over (Q)}0Ni)1Z+({tilde over (Q)}0Ni)0.
From all this, there may now be given an approximation for ε by computing

{tilde over (ε)}:=(D2+{tilde over (R)}0−({tilde over (Q)}0N1)1)div N2.

In fact, the operands are approximated by their uppermost (say, for example) 16 bits. There still remains the work of proving that

δ:=ε−{tilde over (ε)}∈{−1,0,1}.

This will be done later. Now, it is possible to give the following version of RedZ shown in FIG. 3e. 
Observe the following computation:




  
    
      
        
          D
          -
          
            
              
                Q
                ~
              
              0
            
            ⁢
            N
          
          -
          
            
              ɛ
              ~
            
            ⁢
            N
          
        
        =
        
        ⁢
        
          
            
              (
              
                
                  D
                  2
                
                +
                
                  
                    R
                    ~
                  
                  0
                
                -
                
                  
                    (
                    
                      
                        
                          Q
                          ~
                        
                        0
                      
                      ⁢
                      
                        N
                        1
                      
                    
                    )
                  
                  1
                
                -
                
                  
                    ɛ
                    ~
                  
                  ⁢
                  
                    N
                    2
                  
                
              
              )
            
            ⁢
            
              Z
              2
            
          
          +
        
      
    
  
  
    
      
        
        ⁢
        
          
            
              (
              
                
                  D
                  1
                
                -
                
                  
                    (
                    
                      
                        
                          Q
                          ~
                        
                        0
                      
                      ⁢
                      
                        N
                        1
                      
                    
                    )
                  
                  0
                
                -
                
                  
                    (
                    
                      
                        
                          Q
                          ~
                        
                        0
                      
                      ⁢
                      
                        N
                        0
                      
                    
                    )
                  
                  1
                
                -
                
                  
                    ɛ
                    ~
                  
                  ⁢
                  
                    N
                    1
                  
                
              
              )
            
            ⁢
            Z
          
          +
        
      
    
  
  
    
      
        
        ⁢
        
          (
          
            
              D
              0
            
            -
            
              
                (
                
                  
                    
                      Q
                      ~
                    
                    0
                  
                  ⁢
                  
                    N
                    0
                  
                
                )
              
              0
            
            -
            
              
                ɛ
                ~
              
              ⁢
              
                N
                0
              
            
          
          )
        
      
    
  
  
    
      
        =
        
        ⁢
        
          
            
              (
              
                
                  D
                  2
                
                +
                
                  
                    R
                    ~
                  
                  0
                
                -
                
                  
                    (
                    
                      
                        Q
                        0
                        ′
                      
                      ⁢
                      
                        N
                        1
                      
                    
                    )
                  
                  1
                
                -
                
                  
                    ɛ
                    ~
                  
                  ⁢
                  
                    N
                    2
                  
                
              
              )
            
            ⁢
            
              Z
              2
            
          
          +
        
      
    
  
  
    
      
        
        ⁢
        
          
            
              (
              
                
                  D
                  1
                
                -
                
                  
                    (
                    
                      
                        Q
                        0
                        ′
                      
                      ⁢
                      
                        N
                        1
                      
                    
                    )
                  
                  0
                
                -
                
                  
                    (
                    
                      
                        Q
                        0
                        ′
                      
                      ⁢
                      
                        N
                        0
                      
                    
                    )
                  
                  1
                
              
              )
            
            ⁢
            Z
          
          +
          
            (
            
              
                D
                0
              
              -
              
                
                  (
                  
                    
                      Q
                      0
                      ′
                    
                    ⁢
                    
                      N
                      0
                    
                  
                  )
                
                0
              
            
            )
          
        
      
    
  




By virtue of this computation, the final version of the algorithm may be given, as shown in FIG. 3f. 
Remark 8: Note the slight difference in the first lines: ({tilde over (Q)}0, {tilde over (R)}0):=MMD(D3,Z;N2) was replaced by

({tilde over (Q)}0,{tilde over (R)}0):=MMD(D3,Z−N2;N2)

Q0:=Q0+D3 

First of all, it is easy to check that this new equation still holds! This change was done because it is not desired that operands are larger than the modulus, and in this case Z>N. However, since




  N
  ∈
  
    [
    
      
        
          2
          3
        
        ⁢
        
          Z
          3
        
      
      ,
      
        
          Z
          3
        
        [
        
          
        
        ⁢
        
          
            
              
                
              
            
          
          
            
              
                
              
            
          
        
      
    
  




or more precisely





  
    N
    2
  
  ∈
  
    [
    
      
        
          2
          3
        
        ⁢
        Z
      
      ,
      
        Z
        [
        
          
        
        ⁢
        
          ,
          
            
          
          ⁢
          
            
              
                
                  
                
              
            
            
              
                
                  
                
              
            
          
        
      
    
  




it is certain that





  
    Z
    -
    
      N
      2
    
  
  ∈
  
    [
    
      0
      ,
      
        
          1
          3
        
        ⁢
        
          
            N
            2
          
          [
          
            
          
          ⁢
          
            .
            
              
                
                  
                    
                  
                
              
              
                
                  
                    
                  
                
              
            
          
        
      
    
  




However, because of equation 1, the first operand is within [−Z, Z[, but it will be seen that this is no problem, since





  
    
      
        
          
            D
            3
          
          ·
          
            (
            
              Z
              -
              
                N
                2
              
            
            )
          
        
        ∈
        
        ⁢
        
          [
          
            
              -
              Z
            
            ,
            
              Z
              [
              
                ·
                
                  [
                  
                    0
                    ,
                    
                      
                        1
                        3
                      
                      ⁢
                      
                        
                          N
                          2
                        
                        [
                        
                          
                            
                              
                                
                              
                            
                          
                          
                            
                              
                                
                              
                            
                          
                        
                      
                    
                  
                
              
            
          
        
      
    
  
  
    
      
        
          
            =
            
            ]
          
          -
          
            
              1
              3
            
            ⁢
            
              ZN
              2
            
          
        
        ,
        
          
            1
            3
          
          ⁢
          
            
              ZN
              2
            
            [
          
        
      
    
  
  
    
      
        
          
            ⊆
            
            ]
          
          -
          
            N
            2
            2
          
        
        ,
        
          
            N
            2
            2
          
          [
        
      
    
  




Furthermore, note the following:
Remark 9: Because of remark 7 and equation (2),

Q′0∈[−Z−1,Z].  (3)

Mathematical Performance

For the first part MAk, there are needed 3 mmdk, for the second part Redk, there are also needed 3 mmdk. Since this computation has to be done 3 times, the result is

mmK=18·mmdk.

Implementation for (m, k)=(3, k)

The implementation of the algorithm is shown starting from FIG. 5.
System Performance for (m, k)=(3, k)
It can be seen that the implementation of algorithm MAk needs 3 mmdk+movk and the implementation of Redk needs 3 mmdk. This will be used three times and thereafter the result has to be moved outside of the Crypto, so that the performance will be: 3(6 mmdk+movk)+movK, i.e.




  
    m
    ⁢
    
      
    
    ⁢
    
      m
      K
    
  
  =
  
    
      
        18
        ·
        m
      
      ⁢
      
        
      
      ⁢
      m
      ⁢
      
        
      
      ⁢
      
        d
        k
      
    
    +
    
      
        6
        ·
        m
      
      ⁢
      
        
      
      ⁢
      o
      ⁢
      
        
      
      ⁢
      
        v
        k
      
    
  




The Range of ε

The parameter ε was defined to be




  
    ɛ
    =
    
      
        
          Q
          0
        
        -
        
          
            Q
            ~
          
          0
        
      
      =
      
        
          ⌊
          
            D
            N
          
          ⌋
        
        -
        
          ⌊
          
            
              
                D
                3
              
              ⁢
              Z
            
            
              N
              2
            
          
          ⌋
        
      
    
  
  ,




wherein D∈[−NZ, NZ[, in particular D3∈[−Z, Z[. In order to give an estimation of ε, first the real number





  ɛ
  :=
  
    
      D
      N
    
    -
    
      
        
          D
          3
        
        ⁢
        Z
      
      
        N
        2
      
    
  




is computed and then the following lemma is used.

Lemma 1 For r, s∈R, there is always




  
    
      ⌊
      r
      ⌋
    
    -
    
      ⌊
      s
      ⌋
    
  
  =
  
    {
    
      
        
          
            
              
                ⌊
                
                  r
                  -
                  s
                
                ⌋
              
            
          
          
            
              
                
                  ⌊
                  
                    r
                    -
                    s
                  
                  ⌋
                
                +
                1
              
            
          
        
        ⁢
        
          

        
        ⁢
        Now
        ⁢
        
          
        
        ⁢
        set
        ⁢
        
          
        
        ⁢
        ɛ
      
      :=
      
        
          
            D
            N
          
          -
          
            
              
                
                  D
                  3
                
                ⁢
                Z
              
              
                N
                2
              
            
            ⁢
            
              
            
            ⁢
            and
            ⁢
            
              
            
            ⁢
            get
            ⁢
            
              
            
            ⁢
            ɛ
          
        
        =
        
          
            
              D
              N
            
            -
            
              
                
                  D
                  3
                
                ⁢
                Z
              
              
                N
                2
              
            
          
          ⁢
          
            

          
          =
          
            
              
                1
                
                  N
                  ⁢
                  
                    
                  
                  ⁢
                  
                    N
                    2
                  
                
              
              ⁡
              
                [
                
                  
                    D
                    ⁢
                    
                      
                    
                    ⁢
                    
                      N
                      2
                    
                  
                  -
                  
                    
                      D
                      3
                    
                    ⁢
                    Z
                    ⁢
                    
                      
                    
                    ⁢
                    N
                  
                
                ]
              
            
            ⁢
            
              

            
            =
            
              
                
                  1
                  
                    N
                    ⁢
                    
                      
                    
                    ⁢
                    
                      N
                      2
                    
                  
                
                ⁡
                
                  [
                  
                    
                      
                        
                          (
                          
                            
                              D
                              3
                            
                            ⁢
                            
                              |
                            
                            ⁢
                            
                              D
                              2
                            
                            ⁢
                            
                              |
                            
                            ⁢
                            
                              D
                              1
                            
                            ⁢
                            
                              |
                            
                            ⁢
                            
                              D
                              0
                            
                          
                          )
                        
                        Z
                      
                      ⁢
                      
                        N
                        2
                      
                    
                    -
                    
                      
                        D
                        3
                      
                      ⁢
                      
                        
                          Z
                          ⁡
                          
                            (
                            
                              
                                N
                                2
                              
                              ⁢
                              
                                |
                              
                              ⁢
                              
                                N
                                1
                              
                              ⁢
                              
                                |
                              
                              ⁢
                              
                                N
                                0
                              
                            
                            )
                          
                        
                        Z
                      
                    
                  
                  ]
                
              
              ⁢
              
                

              
              =
              
                
                  
                    
                      1
                      
                        N
                        ⁢
                        
                          
                        
                        ⁢
                        
                          N
                          2
                        
                      
                    
                    ⁡
                    
                      [
                      
                        
                          
                            
                              (
                              
                                
                                  D
                                  2
                                
                                ⁢
                                
                                  |
                                
                                ⁢
                                
                                  D
                                  1
                                
                                ⁢
                                
                                  |
                                
                                ⁢
                                
                                  D
                                  0
                                
                              
                              )
                            
                            Z
                          
                          ⁢
                          
                            N
                            2
                          
                        
                        -
                        
                          
                            D
                            3
                          
                          ⁢
                          
                            
                              Z
                              ⁡
                              
                                (
                                
                                  
                                    N
                                    1
                                  
                                  ⁢
                                  
                                    |
                                  
                                  ⁢
                                  
                                    N
                                    0
                                  
                                
                                )
                              
                            
                            Z
                          
                        
                      
                      ]
                    
                  
                  ⁢
                  
                    

                  
                  ⁢
                  hence
                  ⁢
                  
                    

                  
                  ⁢
                  ɛ
                
                ∈
                
                  
                    1
                    
                      N
                      ⁢
                      
                        
                      
                      ⁢
                      
                        N
                        2
                      
                    
                  
                  ⁢
                  
                    (
                    
                      [
                      
                        0
                        ,
                        
                          
                            Z
                            3
                          
                          [
                          
                            
                              
                                ·
                                
                                  N
                                  2
                                
                              
                              -
                              
                                
                                  [
                                  
                                    
                                      -
                                      
                                        (
                                        
                                          
                                            N
                                            2
                                          
                                          +
                                          1
                                        
                                        )
                                      
                                    
                                    ,
                                    
                                      N
                                      2
                                    
                                  
                                  ]
                                
                                ⁢
                                
                                  Z
                                  [
                                  
                                    0
                                    ,
                                    
                                      
                                        
                                          Z
                                          2
                                        
                                        [
                                        )
                                      
                                      ⁢
                                      
                                        

                                      
                                      =
                                      
                                        1
                                        
                                          N
                                          ⁢
                                          
                                            
                                          
                                          ⁢
                                          
                                            N
                                            2
                                          
                                        
                                      
                                    
                                  
                                  ]
                                
                              
                              -
                              
                                
                                  Z
                                  3
                                
                                ⁢
                                
                                  N
                                  2
                                
                              
                            
                            ,
                            
                              
                                
                                  2
                                  ·
                                  
                                    Z
                                    3
                                  
                                
                                ⁢
                                
                                  N
                                  2
                                
                              
                              +
                              
                                
                                  
                                    Z
                                    2
                                  
                                  [
                                  
                                    

                                  
                                  =
                                  ]
                                
                                ⁢
                                
                                  
                                    -
                                    
                                      Z
                                      3
                                    
                                  
                                  N
                                
                              
                            
                            ,
                            
                              
                                
                                  2
                                  ·
                                  
                                    Z
                                    3
                                  
                                
                                N
                              
                              +
                              
                                
                                  
                                    
                                      Z
                                      2
                                    
                                    
                                      N
                                      ⁢
                                      
                                        
                                      
                                      ⁢
                                      
                                        N
                                        2
                                      
                                    
                                  
                                  [
                                  
                                    

                                  
                                  ⊆
                                  ]
                                
                                ⁢
                                
                                  
                                    -
                                    3
                                  
                                  2
                                
                              
                            
                            ,
                            
                              
                                6
                                2
                              
                              +
                              
                                
                                  
                                    2
                                    
                                      
                                        -
                                        2
                                      
                                      ⁢
                                      
                                        
                                      
                                      ⁢
                                      k
                                    
                                  
                                  ⁡
                                  
                                    [
                                    =
                                    ]
                                  
                                
                                ⁢
                                
                                  
                                    -
                                    3
                                  
                                  2
                                
                              
                            
                            ,
                            
                              3
                              +
                              
                                
                                  2
                                  
                                    
                                      -
                                      2
                                    
                                    ⁢
                                    
                                      
                                    
                                    ⁢
                                    k
                                  
                                
                                [
                              
                            
                          
                        
                      
                    
                  
                
              
            
          
        
      
    
  




Therefore, there is obtained └ε┘∈{−2, . . . , 3} and by virtue of the lemma ε∈{−2, . . . , 4}. Nevertheless, assuming




  N
  ∈
  
    [
    
      
        
          3
          4
        
        ⁢
        
          Z
          3
        
      
      ,
      
        
          Z
          3
        
        [
        ,
      
    
  




then





  
    
      
        ɛ
        ∈
      
      ]
    
    ⁢
    
      
        -
        4
      
      3
    
  
  ,
  
    
      8
      3
    
    +
    
      
        
          2
          
            
              -
              2
            
            ⁢
            
              
            
            ⁢
            k
          
        
        ⁡
        
          [
          =
          ]
        
      
      ⁢
      
        
          -
          4
        
        3
      
    
  
  ,
  
    3
    [
  




So it can be seen that in this case └ε┘∈{−2, −1, 0, 1, 2} and ε∈{−2, −1, 0, 1, 2, 3}.
How to Estimate ε
It could be seen that ε=aZ2+bZ+c, wherein a=(D2+{tilde over (R)}0−({tilde over (Q)}0N1)1), b=(D1−({tilde over (Q)}0N1)−({tilde over (Q)}0N0)1), c=(D0−({tilde over (Q)}N0)0), and {tilde over (ε)}:=a div N2 was defined. Now set:



  
    r
    =
    
      
        
          
            
              a
              ⁢
              
                
              
              ⁢
              
                Z
                2
              
            
            +
            
              b
              ⁢
              
                
              
              ⁢
              Z
            
            +
            c
          
          N
        
        ⁢
        
          
        
        ⁢
        s
      
      =
      
        
          a
          
            N
            2
          
        
        .
        
          

        
        ⁢
        Then
      
    
  




  
    x
    :=
    
      
        r
        -
        s
      
      =
      
        
          1
          
            N
            ⁢
            
              
            
            ⁢
            
              N
              2
            
          
        
        ⁢
        
          (
          
            
              
                N
                2
              
              ⁡
              
                (
                
                  
                    b
                    ⁢
                    
                      
                    
                    ⁢
                    Z
                  
                  +
                  c
                
                )
              
            
            -
            
              
                (
                
                  
                    N
                    1
                  
                  ⁢
                  
                    |
                  
                  ⁢
                  
                    N
                    0
                  
                
                )
              
              ⁢
              a
            
          
          )
        
      
    
  




  Obviously




  
    
      
        
          
            
              -
              4
            
            ⁢
            
              
            
            ⁢
            Z
          
          <
          a
          <
          
            3
            ⁢
            
              
            
            ⁢
            Z
          
        
      
    
    
      
        
          
            
              -
              5
            
            ⁢
            
              
            
            ⁢
            Z
          
          <
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It can be seen that x=r−s is indeed very small, since Z will be in the range of 2700! So, practically, there will never be the case that ε≠{tilde over (ε)} (for general integers).
The actual approximation will be done by computing s using only the uppermost (e.g.) 16 bits of the involved integers, therefore there will be made an error of about the size 2−16. This is still very small, and only in a few cases the estimation of {tilde over (ε)} will be incorrect by 1. And this is the reason why a final reduction step is needed at the end of Red.
III. Analysis of the Algorithm
In this section, the three algorithms described in the previous sections are compared. These multiplication methods are denoted as Algorithm I, Algorithm II and Algorithm III, respectively.
Comparison of the Algorithms
Performance values—only for the time-consuming parts of the algorithms—are given in FIG. 4e. Of course, a real implementation will need about 10%-20% longer for all the software overhead, which is not described here.
Advantages/Disadvantages


    
    
        Below 2.064 bits, the fastest multiplication is Algorithm III.
        Above 2.065 bits, the only working algorithm is Algorithm II.
        Algorithm III needs the fewest external memory.

IV. Implementation Aspects

    
    


In this section, the 2.048-bit RSA implementation on the Crypto@1408 will be described in detail. Of course, the main focus is on the implementation of the modular multiplication. It is clear how to set the multiplication in the framework of an exponentiation. So this will be described only very briefly. The modular multiplication A·B mod N presented here has a certain restriction: The integers A, B and N have to be transformed into a special form, namely it has brought from the binary form into the Z-ary form, e.g. (A2, A1, A0)z with three “digits”. A and B, of course, have to be reduced. The precomputation will first decide the length k of the base parameter Z, transform the input values A, B and N into the right form, such that it is usable for the modular multiplication algorithm. Here, A and B will be just brought from the binary form into the Z-ary. The modulus N will be—as it is known for the usual implementation of RSA on the Crypto@xxxx—multiplied by a certain integer, and the exponentiation will be performed with this multiple of N. After the exponentiation, it is necessary to make the final reduction modulo the original N. And the result in Z-ary form is computed back into the old binary form.
It is superfluous to say that, with the pre- and post-computation, this algorithm is not well-suited for a single modular multiplication, although it is possible to do it. On the other hand, all other multiplication algorithms, e.g. the ones presented here, usually need some kind of pre- and post-computation, and in fact there is no really better way to do a simple modular multiplication.
Structure of the RSA Implementation
The framework of the RAS implementation is equal to any other implementation. First, there is the pre-computation, transforming the input parameters baseB* and modulus N* into the right form B and N. Then, the actual RSA implementation starts: It will be decided whether a squaring or the multiplication with the base will be done. Due to this decision, either the operation A←MM(A, A, N) or A←MM(A, B, N) will be carried out. There will be no description how to make this decision—this is standard for an RSA implementation. At the end, in post-computation, the result A will be reduced modulo the input modulus N* and transformed back into the binary form necessary for the output.
The following will describe the implementation of A←MM(A, B, N). For the squaring, i.e. A←MM(A, A, N), one can use A for the parameter B. It can even have the same allocated memory, since the result will be copied into the container of A at the very end.
Note that the exponentiation/modular multiplication only needs external memory for A2, A1, A0, B2, B1, B0, N1 and N0, i.e. maximally




  
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An overview of the algorithm is given in FIG. 5.
FIG. 5 thus shows, so to speak, a flow diagram of the inventive modular multiplication algorithm for three portions. The processing direction and/or time direction is illustrated by an arrow 50 in FIG. 5. In order to perform modular multiplication, there are thus shown, as illustrated for example with respect to FIG. 1d, three MMA operations to be performed consecutively, which are designated 51, 52 and 53 in FIG. 5. The most significant portion B2 of the multiplicand is used for the MMA operation 51. For the second MMA operation 52, the result of the first MMA operation and the next less significant portion B1 of the multiplicand are used. The result of the second MMA operation is finally used together with the least significant portion B0 of the multiplicand to obtain the final result of the modular multiplication. Next, the result is read out from the internal registers, i.e. E2, E1 and E0, by means of a move command 54, to free the internal registers for a new modular multiplication.
Each MMA operation, for example the MMA operation 51, is divided into an MA operation 55a and a reduction operation 55b, wherein the MA operation, in turn, is again divided into various operations illustrated in FIG. 5, while the reduction operation is also divided correspondingly.
The Modular Multiplication Algorithm
The input for this modular multiplication is the modulus N, the multiplicand A∈[0, N[ and the multiplier B∈[0, N[. Formally, there is an input parameter k defining the length of the computation. The output will be A·B mod N stored in the place of A residing in the external memory.
The entry conditions for this algorithm already discussed above are
N is encoded in three integers N2, N1 and N0, such that Ni∈[0, Z[ and N=N2·Z2+N1·Z+N0, shortly written as N=(N2, N1, N0)Z. 
Furthermore, N2∈[0, Z[, such that N2 is transformed according to the Crypto@xxxx architecture.
A is encoded as three integers A2, A1 and A0, such that Ai∈[0, Z[ and A=A2·Z2+A1·Z+A0, shortly written as A=(A2, A1, A0)Z.
B is encoded in three integers B2, B1 and B0, such that Bi∈[0, Z[ and B=B2·Z2+B1·Z+B0, shortly written as B=(B2, B1, B0)Z.
The modular multiplication algorithm is shown in FIG. 6a. 
It is illustrated in FIG. 6b. 
In the external memory XDATA, there are respectively the portions of the multiplier A and the multiplicand B and the least significant and the next more significant portion N1 and N0, while the most significant portion N2 of the modulus is already in the CR6 register of the crypto co-processor operated in the short mode. The other three registers CR4, CR2 and CR0 are set to zero. The intermediate result of the first MMA′ operation, i.e. E1′, E2′, E0′, then replaces the zeros in the corresponding registers prior to the first MMA′ step. The second MMA′ step results in replacing the values E0′, E1′ and E2′ by E0′′, E1′′ and E2′′. By means of the next MMA operation, there is again replacing, so that after the third MMA operation the final result of the modular multiplication is present in the form of the lowest portion E0, the next higher portion E1 and the highest portion E2. This result E is thus obtained by the algorithm in FIG. 6a, specifically also portion-wise.
The result portions E2, E1 and E0 replace A2, A1 and A0 in the working memory, so that the result of a previous modular multiplication step now provides the new multiplier A for the next modular multiplication step, which will again be performed in the same way, wherein now, however, the original operand A is replaced by the newly calculated operand E.
In this algorithm, besides the already known MMA algorithm, a variation of it, namely MMA′ is used. Roughly, the difference between the two algorithms is given in the formula MMA′=MMA−N. They are defined as shown in FIGS. 7a and 7b. 
The register implementation is illustrated in FIG. 7c. 
Both variations use the algorithms MAZ and RedZ, wherein the last one again has two variants, namely RedZ itself and Red′Z. Roughly, the difference between the two algorithms is given in the formula Red′=Red−N.
First the algorithm MAZ will be discussed. The algorithm is illustrated in FIG. 8a. 
The register implementation of the algorithm of FIG. 8a is illustrated in FIG. 8b. A preferred implementation of the inventive concept illustrated algorithmically in FIG. 8a is shown in FIG. 8d, wherein the register movements referred to in FIG. 8d are summarized in FIG. 8c, and wherein FIG. 8e gives an example of the inventive multiplication addition algorithm and the use of the two auxiliary registers and the four result registers. Before discussing the algorithm in detail, the meaning of the term “short register length” and “long number length” is first illustrated with respect to FIG. 8c. For this, a register block 800 is illustrated, which includes nine registers only for reasons of clarity. Each register of these nine registers has a certain number length and/or a number of binary digits and may thus store maximally one portion Ai, Bi, Ci of the operand A, the operand B and the operand C. In the example shown here, each operand is divided into three portions. The index i thus has the values 0, 1, 2.
Considering each register for itself, each register has a number between zero and 2k−1. If, however, the least significant bit of a register is given a certain initial significance (and/or valency) per convention, which is common in calculating unit technology, a large register may, so to speak, be imitated by correspondingly interpreting the numbers in registers from these small registers. Equally, a row of the register block at 800 in FIG. 8c could include a single large register having a length equal to three times a short register length. In this case, the middle short register, in which there is stored A1 or B1 or C1, would have to be given, for example, an initial significance of 2k and/or generally speaking an initial significance of a number Z (the fourth operand), while the initial significance of the corresponding least significant register, in which there is stored A0, B0, C0, would be 20. Correspondingly, the initial significance of a register in which there is stored A2, B2 or C2 would be 22k or Z2.
Corresponding conventions also apply to the individual output or result registers 802. They are again four registers with short register length in which there is respectively stored a portion D0, D1, D2 or D3 of the result value, wherein, depending on the position and/or identification of a short register, there is an initial significance of 20, Z, Z2 or Z3, which has to be given to register content when the total (absolute) number and not only a number within a register is important.
At 804, there is shown an example of a multiplication addition operation, i.e. an operation between a first operand A, a second operand Bi, a third operand C and a fourth operand Z, wherein the first operand A and the third operand C are longer than the second operand Bi or the fourth operand Z, and wherein portions of the first operand A or the third operand C are shorter than the first operand or the third operand per se. In the individual result registers 802a, 802b, 802c, 802d, there are stored iteratively calculated results, wherein updated MOD and/or DIV results of 3 MMD operations are stored in the registers 802a to 802c, and wherein the MOD result of the last (third) MMD operation is stored in the least significant short register 802d. 
It is to be noted that any number of iterations may be used, i.e. that the long operands do not necessarily have to be divided into three portions, but may also be divided into two portions or into more than three portions, such as four or five portions. The number of iterations would then increase correspondingly. The number of auxiliary registers, however, will not increase. The number of required result registers would, however, increase according to the number of portions (+1). Still, an embodiment will be discussed below in which the long operands are divided into three portions of the same length, although the division of equal length is not necessarily required either. Although it results in a regular and well-manageable value situation of the initial significances of the individual registers, it is not necessarily a precondition. If portions of unequal length are selected, the initial significances of the individual short registers are set correspondingly, so that the “combining” of the result number from the individual portions is done correctly.
FIG. 8d illustrates the inventive concept with respect to a device and/or a method for calculating the result 802 by a multiplication addition operation 804 between a first operand A, a second operand Bi, a third operand C and a fourth operand Z, wherein the first and the third operands are longer than the second or the fourth operands, and wherein portions of the first or third operands are shorter than the fourth operand, i.e. they are, so to speak, the number indicating the initial significance.
The inventive device includes means 810 for calculating results of an MMD operation using the second operand, a more significant portion A2 of the first operand and the fourth operand as modulus. These results include a DIV result providing the integer quotient of the operation, and an MOD result yielding the remainder of the integer division. These two results are provided to means 811 for storing the results, as shown in FIG. 8d. The means 811 is designed to store the results in the form of U1 and U0 in two short auxiliary registers 812a, 812b. The values stored in the short auxiliary registers, i.e. the results of the first MMD operation, are then provided to means 813 for updating the DIV result and the MOD result, wherein the update is performed using an addition of portions of the third operand. This update thus takes into account the addition term of the multiplication addition operation, as shown at 804 in FIG. 8c. The means 813 for updating is further designed to store updated results in a fourth result register 814a and a third results register 814b. The memory contents of the result register 814a are designated D3′, while the portion of the result in the third result register is designated D2′.
Depending on the appearance of the portions of the third operand C, the update in means 813 for updating results in a change of the supplied DIV result or the supplied MOD result or not. If the whole situation of the third operand is, for example, so that the DIV result or the MOD result of the first MMD operation is not changed, the corresponding value U1 or U0 in the auxiliary register 812a, 812b may be entered directly into a result register 814a, 814b. In this case, “updating” thus means that there has been no change of the result of the MMD operation. If, however, the third operand is such that the results of the MMD operation executed at 810 are changed, this results in a change of the auxiliary register values and in the changed auxiliary registers values being fed into corresponding result registers, such as 814a, 814b. 
The present invention further includes means 815 for executing the MMD operation and the update again using another portion of the first operand, until all portions of the first operand are processed. The registers in which updated results are stored, and a register in which an MOD result of a last MMD operation is stored, then together provide the result of the multiplication addition operation according to the initial significance associated with the registers, as shown at 802.
The means 815 for executing again may be designed as iteration means that again activates the means 810, 811, 813 in a cyclic processing, but provides them with the corresponding other portions of the operands. Alternatively, when no iterative processing is desired, the means 815 for executing again may also be designed as simple doubling and/or tripling of the elements 810, 811, 813, which, however, are fed with correspondingly different values. For reasons of efficiency, however, the embodiment is preferred in which the means 815 for executing again drives the existing means 810, 811, 813 again, but with other input operands, until all portions of the first operand A are processed.
In the first step, Aj, Bi and Z as well as Cj, Cj−1 are required as input operands.
In the second step, Aj−1, Z and Cj−2 are required as input operands.
In the third step, Aj−1, Bi and Z are required as input operands.
If there is a division into only two portions, the calculation is completed after as few as two steps.
If, however, there is a division into more than three portions, Cj−3 will be used in addition to Cj−2 in the second step, and Aj−3 and Cj−4 will be used in the third step, and there would be a fourth and last step, in which Aj−4 would be used.
In this case, the result register would also have five individual short result registers, instead of the four individual result registers used in the case of three portions, in which the result values D3′, D2′′ and D0 are stored, wherein W0 represents the MOD result of the last MMD operation, while the other three inputs into the whole result register 802 will be updated MMD results.
For the purpose of illustration, FIG. 8e illustrates an example and the three iteration steps for calculating the result of the multiplication addition operation with respect to an arbitrarily selected example.
For each step, the loading of the two auxiliary registers 812a, 812b and the contents of the result registers 802a to 802d obtained in these steps are illustrated. In the embodiment shown in FIG. 8e, only registers are required that may store a single decimal digit, and there are never required registers that have to store two decimal digits.
The register implementation is illustrated in FIG. 8b. 
The main part of the algorithm thus consists of the MMD operations. Their implementation will be discussed in a following section. In addition to these, there are an elementary operation, namely adding components of integers and treating a possible carry.

(D′3|D′2)Z:=(C2+U1,C1+U0)Z 

means

D′2:=C1+U0 

D′3:=C2+U1 

(D′3,D′2):=TC(D′3,D′2)

and

(D′′2|D′′1)Z:=(D′′2+V1,C0+V0)Z 

means

D′1:=C0+V0 

D′′2:=D′2+V1 

(D′′2,D′1):=TC(D′′2,D′1)

[(D′3,D′′2):=TC(D′3,D′′2)]

Here, the last action was set in brackets, since this is not really necessary: If this was necessary, the carry would be resolved in the last step. Namely, as D′′2≦2(Z−1), it will still be possible to hold the carry from the next step, and it can be resolved without additional problems. Finally

(D′′1|D0)Z:=(D′1+W1,W0)Z 

can be implemented as

D0:=W0 

D′′1:=D′1+W1 

(D′′2,D′′1):=TC(D′′2,D′′1)

The algorithm TC is nothing else than simply treating a carry in the Z-ary integer representation, as shown in FIG. 9a. 
In the preferred embodiment of the present invention, the algorithm TC is performed as part of the updating step, i.e. after the performed addition and prior to the loading of the result registers 814a, 814b in FIG. 8d. The algorithm for TC is shown in FIG. 9a. The algorithm TC only steps in when the second input X, i.e. the value of (C1+U0) in the algorithm shown in FIG. 8a, is larger than Z. If this value is less than Z, there is no carry, and the function TC of FIG. 9a is transparent. If, however, it is the case that X is larger than Z or equal to Z, Z is subtracted from X, and Y is incremented by “+1” to take the carry into account. The value Y then represents the contents of the fourth register D3′, while the value X represents the contents D2′ of the third register 814b in FIG. 8d. 
It is to be noted that, when such numbers are calculated in which there is never a carry, the TC function is not required. For universal applicability, however, this function is preferred and is used within the means for updating after the addition of portions of the third operand C.
Next, a process of the inventive method is illustrated in more detail with respect to a preferred register implementation. A register situation of a calculating unit with 5 long registers divided into ten short registers is assumed. The register loading at the start of the algorithm is illustrated at 840 in FIG. 8b. It can be seen that only the top four registers are loaded with N2, C1, C2 and C0. N2 is the top portion of the transformed modulus, which itself is not required for the multiplication addition calculation, but which is already in the register due to the previous and/or subsequent calculations. In principle, however, it is not required for the execution of the multiplication addition operation.
Furthermore, the state of the external memory and/or working memory XDATA 850 is shown in FIG. 8b. The external memory 850 includes three portions of the first operand A, three portions of the second operand B, and the middle and lowest portions of the transformed modulus N, which, however, are not required for the multiplication addition operation either.
In a memory loading step Movk, the internal register memory is now loaded, namely with the fourth operand Z in the fourth row and the left column, shown at 840′. The numbers Z+1 and the new loading of Bi into a further short memory are not required for the multiplication addition operation in its basic execution. Then the first MMD operation is performed by the means 810. The results U0, U1 are fed into the two still free register memories, as illustrated at 841. Now there is an update, which is illustrated by an addition function and a TC function in FIG. 8b. Here, the registers C2 and C1, are overwritten with the values D3′ and D2′. This is possible because the values C2 and C1, are no longer needed, as can be seen in FIG. 8a. Furthermore, the memory loading is illustrated in FIG. 8b after the first updating step (Add, TC) such that the two auxiliary registers in which U0, U1 were stored are again cleared, as illustrated at 841′.
Then, the second MMD operation is performed and the results V0, V1, are again stored into the two auxiliary registers, as can be seen at 842. Then there is an update, i.e. an addition operation and a TC operation are executed to achieve a memory loading 842′. It can be seen that the register memory in which C0 was written has been overwritten by D1′, because C0 is no longer needed after the second update (Add, TC).
Furthermore, the third and last MMD operation is performed to obtain a loading of the register memory as shown at 843. Again, a loading of the two auxiliary registers by W0 and W1, i.e. the results of the MMD operation, is achieved, wherein then there is a last update to obtain a memory loading as illustrated at 843′. In the embodiment shown in FIG. 8b, the value N2 was shifted, and W0 was entered as least significant result register value.
A final treat carry step for the use of the contents of the memory, as shown at 843′, results in the ultimate output state, illustrated at 844.
It is to be noted that the internal memory loading has been selected so that the processor, i.e. the means 810 for calculating the MMD operation or the means 813 for updating, which may be one and the same calculating unit or may be separate calculating units, always has to access only a single value in the external memory. In the first MMD step, this is the value A2. In the second MMD step, this is the value A1, and in the third MMD step, this is the value A0.
In the case of a processor that does not execute any external access, but is to operate only with its internal registers, the value for A2, A1 and/or A0 would have to be stored into an available short register prior to each execution of the MMD operation, or all three values could be loaded as part of Movk at the beginning.
Later the algorithm for treating a negative carry (borrow), which is analogous to TC, will be used after a subtraction. It is shown in FIG. 9b. 
The final reduction step only has to take care of the upper two parts of D, since the lower two parts have been treated in the last step, so

(D3|D2|D1|D0)Z:=(D′3,D′′2,D′′1,D0)Z 

(also denoted as TC) is in fact implemented as

D1:=D′′1 

(D3,D2):=TC(D′′3,D′′2)

Remember that D3 may become positive or negative, so that it is “free floating”.
Finally, a further part of the whole algorithm is the modular reduction step. It has to versions, one that computes the usual remainder ∈[0, N[ and one that computes the remainder decremented by N, i.e. ∈[−N, 0]. The two algorithms are shown in one step, since the differences only lie in the computation of {tilde over (ε)} and in the final reduction (FIG. 9c).
The register implementation is illustrated in FIG. 9d. 
Again, the main part of the algorithm consists of the three MMD operations. Their implementation will be discussed in a following section. The remaining part consists of the estimation of ε, which will be discussed in a later section, and some elementary operations of addition or subtraction of components and treating possible carries or borrows.
The first two additions Q′0:=Q0+D3 and D′2:=D2+R0 are not subjected to any carry treatment. In any case, remark 7 has shown that Q0 will not become much larger than Z. On the other hand, D′0 may become as large as 2Z, but it is non-negative and therefore this integer has to be interpreted as unsigned integer!
With reference to FIG. 12, it is shown how to implement the next three lines together:

ε:=estimate(D′2−(Q′0N1)1 div N2)[+1]

D′′2:=D′2−εN2 

Q′′0:=Q′0+ε

The subtraction

(D′1|D)Z:=(D1−U1,D0−U0)Z 

will be done similarly to the MAZ algorithm, with the difference that borrows have to be treated instead of carries. But note that, if Q0′′<0, then U1 and U0 are also negative, hence the subtraction is actually an addition, and carries have to be treated again:

D′0:=D0−U0;D′1:=D1−U1 

if Q′′0≧0 then

(D′1,D′0):=TB(D′1,D′0)

(D′2,D′1):=TB(D′2,D′1)

else

(D′1,D′0):=TC(D′1,D′0)

(D′2,D′1):=TC(D′2,D′1)

end

Of course, there is a possibility to change the order of the operations. In the same way, the other subtraction

(D′′′2,D′′1)Z:=(D′′2−V1,D′1−V0)Z 

is treated as:

D′1:=D′1−V0;D′′′2:=D′′2−V1 

if Q′′0≧0 then

(D′′2,D′′1):=TB(D′′2,D′′1)

else

(D′′2,D′′1):=TC(D′′2,D′′1)

end

Remember that D′′2 will not be resolved yet. This is done in the final reduction step following now:
In RedZ, it is desired to get a result E∈[0, N[. But sometimes, the result is slightly larger than N or less than 0. In this case, N has to be subtracted or added once. In order to check whether E>N, E has to be subtracted from N. But unfortunately, N does not fully lie in the Crypto@1408. So E2−N2 is computed and there is a check whether this difference is ≧0. If E>N, this is certainly the case. Note that also E2−N2=0 may be a hint that E>N, since it is always possible that E1>N1. However, this cannot be checked immediately, since the full N first has to be loaded into the Crypto@1408. It happens only in very few cases that E2=N2, while E≦N. Since all this happens very rarely, one takes the time and loads the complete N into the Crypto@1408 and performs the final reduction: N is subtracted from E. If then the new integer E2≧0, this is the end. If E2 becomes negative, N has to be added again. If E2 was negative from the beginning—which also happens very rarely—then N is also loaded into the Crypto@1408 and added to E. This algorithm is formally given by

if E2−N2≧0 then

(E2|E1|E0)Z:=(E2−N2,E1−N1,E0−N0)Z 

end

if E2<0 then

(E2|E1|E0)Z:=(E2+N2,E1+N1,E0+N0)Z 

end

Note that normally, in 99.9% of all cases, both if-conditions will not be fulfilled. So the implementation must consider this fact. It should be avoided to load the full N into the Crypto@1408, unless one of the conditions is true, because this needs a lot of time! For Red′Z, the final reduction is quite similar.

if −N2−E2<0 then

(E2|E1|E0)Z:=(E2+N2,E1+N1,E0+N0)Z 

end

if E2≧0 then

(E2|E1|E0)Z:=(E2−N2,E1−N1,E0−N0)Z 

end

In both cases, one always has to be aware of carries and borrows and resolve them like in the additions and subtractions described earlier in this section. Finally, note the following important remark.
Remark 11: Because of equation (3), Q′′0∈[−Z −1, Z]. Since in the second and third MMD operations the second factor is definitely reduced mod Z, the product Q′′0·Ni will always have an absolute value ≦(Z+1)(Z−1)=Z2−1.
Implementation of MMDk 
Input for the MMD algorithm is a transformed modulus N (in this case this will be N2 or Z), the multiplicand X (in this case Bi, D3, Q′′0) and the multiplier Y (Ai, Z, Z−N2 and Ni), which in most but not all cases will lie outside the Crypto@1408.
1. If #N=:k>704−8−1=695 (sign bit, not counted), the MMD operation has to be computed in long mode of the Crypto@1408. Otherwise, the parallel mode discussed below may be used. In long mode, the algorithm is given in FIG. 10a. 

    
    
        For the Crypto@1408, the algorithm is illustrated in the following FIG. 10b. 
    
    


2. At least in one case, the algorithm will be needed for a negative multiplicand. But this is not a problem, as long as X∈]−N, 0]: In this case, (Q, R):=MMD(−X, Y; N) is computed and (−Q+1, N−R) is returned, if R>0, and (−Q, −R), if R=0. This is legitimate by the following observation: If

−X·Y=Q·N+R, with R∈[0,N[, 

then

X·Y=−Q·N−R=(−Q+1)·N+(N−R).

In fact, in this case, it will be sufficient to just return (−Q, −R), since this algorithm works in this part with negative remainder R.
3. It is even not really necessary that X and Y∈[0, N[ (or more generally in ]−N, N[). It is sufficient that the product X·Y lies in [0, N2[. Therefore, it may be allowed that X or Y is a little bit larger than N, as long as

    
    
        Crypto@1408 does not interpret the integer in a wrong way (sign bit).
        the product X·Y is not too big, i.e. lies in [0, N2[.
    
    


In FIG. 11a, the algorithm is given in the way it is preferably used.
The algorithm implemented on Crypto@1408 is illustrated in FIG. 11b. 
Making the Side Computation
In this section, there is given an implementation of the three lines

ε:=estimate(D′2−(Q′0N1)1 div N2)

D′′2D′2−εN2 

Q′′0:=Q′0+ε

of Red.

The main point in this implementation is the estimation of D′2−(Q′0N1)1 div N2. By the same technique already used several times an approximation of a division may be obtained by using only the top bits of a dividend and divisor. In this case, the 16 top bits (including sign bit) will be used, i.e.:

D′2top:=D′2 div 2k−16 

Q′0top:=Q′0 div 2k−16 

N1top:=N′1 div 2k−16 

Ztop:=Z div 2k−16 

N2top:=ZtopN2 div 2k−16 

Since ( . . . )1 in (Q′0N1)1 means division by Z, the fraction is multiplied by Z and there is hence an approximation in the following way:

ε:=(ZtopD′2top)div N2top 

D′2top and Q′0 are obtained by reading the most significant word of D2 and Q′0. The top two bytes—sign extended—are loaded into some CPU register. Also N1top is prepared in the same way, but in this case this has to be done only once during the whole exponentiation. (Pre-computation!) Then the product Q′0topN1top will be computed. It is a 32 bit word and it is subtracted from ZtopD′2top. Call the result X. This result will be compared with N2top, which is prepared in the same way as N1top, but with an additional factor of Ztop. The rest is obvious and shown in FIG. 11.
Remark 12: Note that for Red′ one has to add one additional block

D′2:=D′2−N2 

Q′0:=Q′0+1

in any case.

Pre-Computation

The pre-computation gets the base B* and the modulus N* for the exponentiation. N* has a bit length K, i.e. N* ∈[2K−1, 2K[.

(N′2|N′1|N′0)Z:=N* 

B|B2|B0)Z:=B* 

Since the pre-computation is not relevant for the performance, the implementation aspects will not be discussed in too much detail. Only some remarks and comments regarding these points:
Transforming N* into (N2|N1|N0):
Now set W:=2k−1 the largest power of two to less than Z and write N*=(N2*|N1*|N0*)W, i.e. divide N* into three (k−1) bit blocks. The transformation into Z-ary representation (N2N1|N0):=N* is given in FIG. 13a: 
Note that Z is transformed, so that the MMD implementation of FIGS. 11a and 11b may really be used. The two addition parts are done exactly like in the main implementation presented starting from FIG. 6: Make the addition component-wise and treat the carry!
Transforming B* into (B2|B1|B0)Z:
This works exactly in the same way as in the last point.
Making the Final Reduction (Post-Computation)
The final reduction takes the output (A2|A1|A0)Z of the last modular multiplication of the pure exponentiation, reduces this number modulo N*=(N′2|N′1|N′0)Z:=N* and transforms it back into the binary representation A*. The result is

(A′2|A′1|A′0)Z:=(A2|A1|A0)Z mod(N′2|N′1|N′0)Z  1.

A*:=(A′2|A′1|A′0)Z  2.

wherein (N′2|N′1|N′0)Z is known from section 5.5.

For 1. The Reduction

will be done in the already known way: A:=A−[A div N]·N, so the algorithm can be given as shown in FIG. 13c. 

The division is the one from the last section, and the entire already known technique is used.
For 2. Transforming into Binary Form
This is actually just the computation

A:=A′2·Z2+A′1·Z+A′0 

Three different methods to implement a 2048-bit RSA computation have been described. Furthermore, the performance of such an implementation of the algorithm was evaluated, taking into account some system aspects which are performance-dominating, such as moving integers into and out of Crypto@1408. It was found that in terms of speed and usage of external memory, the inventive preferred algorithm is the best one. Although for m=3 it is only suitable to implement RSA up to 2048 bits (+16 for randomization). If there is a need for longer bit lengths, then the Algorithm II (Fischer-Sedlak-Seifert with MMD) seems to be the best reasonable method. Alternatively, m=4, 5, . . . may also be chosen.
Depending on the circumstances, the inventive method for calculating a result may be implemented in hardware or in software. The implementation may be done on a digital storage medium, particularly a floppy disk or CD with control signals that may be read out electronically, which may cooperate with a programmable computer system so that the method is executed. Generally, the invention thus also consists in a computer program product with a program code stored on a machine-readable carrier for performing an inventive method when the computer program products runs on a computer. In other words, the invention may thus be realized as a computer program with a program code for performing the method when the computer program runs on a computer.
While this invention has been described in terms of several preferred embodiments, there are alterations, permutations, and equivalents which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and compositions of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and equivalents as fall within the true spirit and scope of the present invention.

    [1] W. Fischer, “Vorrichtung und Verfahren zum Berechnen eines Ergebnisses aus einer Division,” DE patent #102,05,713, Aug. 7, 2003.
    [2] W. Fischer, H. Sedlak, J. P. Seifert, “Vorrichtung und Verfahren zum Berechnen eines Ergebnisses einer modularen Multiplikation,” DE patent #102,19,158, Dec. 9, 2004.
    [3] W. Fischer, J. P. Seifert, “Vorrichtung und Verfahren zum Umrechnen eines Termes,” DE patent application #102,19,161,A1, Nov. 20, 2003.
    [4] W. Fischer, H. Sedlak, J.-P. Seifert, “Vorrichtung und Verfahren zum Berechnen eines ganzzahligen Quotienten,” DE patent #102,19,164, Dec. 2, 2004.
    [5] W. Fischer, H. Sedlak, J. P. Seifert, “Vorrichtung und Verfahren zum Berechnen einer Multiplikation mit der Verschiebung des Multiplikanden, insbesondere bei der kryptographischen Berechnung,” DE patent #102,60,655, Jun. 24, 2004.
    [6] W. Fischer, H. Sedlak, J. P. Seifert, “Modulare Multiplikation mit paralleler Berechnung der Look-Ahead-Parameter u.s. bei der kryptographischen Berechnung,” DE patent #102,60,660, Jun. 9, 2004.
    [7] W. Fischer, J. P. Seifert, “Increasing the bitlength of a crypto-coprocessor,” Proc. of CHES '02, Springer LNCS, Vol. 2523, pp. 71-81, 2002.
    [8] W. Fischer, J. P. Seifert, “Unfolded modular multiplication,” Proc. of ISAAC '03, Springer LNCS, 2003.
    [9] A. Menezes, P. van Oorschot, S. Vanstone, “Handbook of Applied Cryptography,” CRC Press, 1997.
    [10] P. L. Montgomery, “Modular multiplication without trial division,” Math. of Computation, 44:519-521, 1985.
    [11] H. Sedlak, “The RSA cryptographic Processor: The first High Speed One-Chip Solution,” Proc. of EUROCRYPT '87, Springer LNCS, Vol. 293, pp. 95-105, 198.

## Claims

1. A device for calculating a result of a sum of a first operand and a second operand less than a modulus by means of a calculating unit formed to maximally process numbers whose magnitude is less than or equal to a product of the modulus and an integer larger than 1, comprising:
a storage unit formed for storing the first operand in the calculating unit, wherein a magnitude of the first operand is less than or equal to the maximally processable number;
a modified second operand calculator formed for calculating a modified second operand so that the modified second operand is equal to the second operand minus the modulus, and that the modified second operand is negative;
a sum calculator formed for calculating a sum by adding the first operand and the modified second operand or the first operand and a product of the modified second operand times a factor less than or equal to the integer, and for storing the sum in the calculating unit; and
a reducing unit formed for reducing the sum with respect to the modulus to get the result.

2. The device of claim 1, wherein the calculating unit is formed to further store a sign bit for a number, indicating whether the number is positive or negative, and
wherein the calculating unit is formed to store and/or interpret the first operand as positive or negative number.

3. The device of claim 1, wherein the calculating unit is formed to calculate the first operand by a modular reduction with the modulus, wherein, prior to the modular reduction, the first operand had a value less than or equal to the maximally processable number and was stored in the calculating unit.

4. The device of claim 1, wherein the modified second operand calculator is formed to calculate the modified second operand by subtracting a multiple of the modulus, wherein the multiple is larger than or equal to 1 and is selected so that the modified second operand is negative and is less than the modulus in magnitude.

5. The device of claim 1,
wherein the first operand is equal to a product of a first operand of a multiplication addition operation and a portion of a second operand of the multiplication addition operation, wherein the second operand is the result of a product of a modified second operand of a previous step and the integer, and
wherein the first operand is a multiplier of the multiplication addition operation, wherein the portion of a second operand of the multiplication addition operation is a portion of the multiplicand of the multiplication addition operation.

6. The device of claim 5, wherein the sum calculator comprises:
a result calculator of an MMD operation using the second operand, a more significant portion of the first operand and a fourth operand as modulus, and for storing a DIV result in a first auxiliary register and an MOD result in a second auxiliary register;
an updating unit formed for updating the DIV result and the MOD result using an addition of at least one portion of a third operand and for storing updated results in a fourth result register and a third result register; and
an executing unit formed for executing again the MMD operation and the updating using another portion of the first operand, until all portions of the first operand are processed, wherein result registers in which updated results of an MMD operation are stored and a register in which an MOD result of a last MMD operation is stored together represent the result.

7. A device for calculating a result of a modular multiplication with a multiplier, a multiplicand and a modulus, comprising:
a multiplicand providing unit formed for providing the multiplicand in at least three portions, wherein each portion comprises a number of digits less than half the number of digits of the multiplicand, and wherein the at least three portions include all digits of the multiplicand; and
a sequential calculator, wherein the sequential calculator is formed
to calculate a first intermediate result using a more significant portion of the multiplicand,
to calculate a second intermediate result using a less significant portion of the multiplicand and the first intermediate result, and
to calculate and store a third intermediate result using a still less significant portion of the multiplicand and the second intermediate result, wherein the third intermediate result represents the result of the modular multiplication, if the multiplicand is divided into exactly three portions, or wherein the result of the modular multiplication is derivable from the third intermediate result by a further sequential calculation, if the multiplicand is divided into more than three portions,
wherein the sequential calculator comprises a device for calculating a result of a sum of a first operand and a second operand less than a modulus by means of a calculating unit formed to maximally process numbers whose magnitude is less than or equal to a product of the modulus and an integer larger than 1, the device for calculating comprising:
a storage unit formed for storing the first operand in the calculating unit, wherein a magnitude of the first operand is less than or equal to the maximally processable number;
a modified second operand calculator formed for calculating a modified second operand so that the modified second operand is equal to the second operand minus the modulus, and that the modified second operand is negative;

a sum calculator formed for calculating a sum by adding the first operand and the modified second operand or the first operand and a product of the modified second operand times a factor less than or equal to the integer, and formed for storing the sum in the calculating unit; and
a reducing unit formed for reducing the sum with respect to the modulus to get the result, and

wherein the first operand is a product of the multiplier and a portion of the multiplicand, and wherein the second operand is a product of an intermediate result and the number.

8. The device of claim 7, wherein the calculator is formed to execute the following calculation:


C1:=MMAZ(A,B2,0;N)−N 



C2:=MMAZ(A,B1,C1;N)−N 



C3:=MMAZ(A,B0,C2;N),

wherein N is the modulus, A is the multiplier, B2 is the most significant portion of the multiplicand, B1 is a less significant portion of the multiplicand, B0 is a least significant portion of the multiplicand, C1 is the first intermediate result, C2 is the second intermediate result, and C3 is the third intermediate result, and MMAZ represents a MultModAdd operation with a respective portion of the multiplicand.

9. The device of claim 8,
wherein the MMA operation comprises a reduction operation in addition to the multiplication addition operation.

10. The device of claim 7,
wherein the multiplicand providing unit is formed to comprise at least one register comprising a length less than a length of the whole multiplicand, but larger than or equal to a portion of the multiplicand,
and wherein the calculator is formed to load a portion of the multiplicand into the register during a calculation.

11. The device of claim 7,
wherein the multiplicand providing unit is formed to also divide the multiplier and the modulus into at least three portions each, and
wherein the calculator is formed to use both the portions of the multiplier and the modulus for one or more calculations.

12. The device of claim
wherein the sequential calculator is formed to comprise ten or less registers with a length at least as large as a length of a portion and less than a whole length of the modulus.

13. The device of claim 7,
wherein the sequential calculator is formed to execute the following equation:


C=[(A·B2 mod N)·Z+A·B1 mod N]·Z+A·B0 mod N, 


wherein C is the third intermediate result, A is the multiplier, Z=2i, wherein i is a number of digits of the portions, B2 is the most significant portion of the multiplicand, B1 is a less significant portion of the multiplicand, B0 is the least significant portion of the multiplicand, N is the modulus, and mod indicates a modular reduction operation.

14. The device of claim 7, formed as a configurable calculating unit, wherein the configurable calculating unit comprises:
a bit slice structure, wherein each bit slice comprises a calculating unit part and a register part; and
a register configuration unit formed to configure the configurable calculating unit into a long mode or a short mode,
wherein the configurable calculating unit in the long mode comprises a certain first number of long registers, the configurable calculating unit in the short mode comprises a second number of short registers, the second number is larger than the first number, and a length of a short register is such that a portion of the multiplicand is storable in the short register.

15. The device of claim 7,
formed to perform the modular multiplication within a cryptographic calculation, wherein the multiplier, the multiplicand and the modulus are parameters of the cryptographic calculation.

16. The device of claim 15,
wherein the cryptographic calculation is an encryption, a decryption, a signature generation or a signature verification.

17. The device of claim 7,
wherein all portions of the multiplicand comprise the same number of digits.

18. The device of claim 7,
wherein the calculator is formed to only perform a multiplication of numbers of a length less than or equal to the number of digits in a portion.

19. A method for calculating a result of a sum of a first operand and a second operand less than a modulus by means of a calculating unit formed to maximally process numbers whose magnitude is less than or equal to a product of the modulus and an integer larger than 1, comprising:
storing, by a storage unit, the first operand in the calculating unit, wherein a magnitude of the first operand is less than or equal to the maximally processable number;
calculating, by a modified second operand calculator, a modified second operand, wherein the modified second operand is equal to the second operand minus the modulus, so that the modified second operand is negative;
calculating by a sum calculator, a sum of the first operand and the modified second operand or the first operand and a product of the modified second operand times a factor less than or equal to the integer, and storing the sum in the calculating unit; and
reducing, by a reducing unit, the sum with respect to the modulus to get the result,
wherein the storage unit, the modified second operand calculator, the sum calculator or the reducing unit comprises a hardware implementation.

20. A method for calculating a result of a modular multiplication with a multiplier, a multiplicand and a modulus, comprising:
providing, by a multiplicand provider, the multiplicand in at least three portions, wherein each portion comprises a number of digits less than half the number of digits of the multiplicand, and wherein the at least three portions include all digits of the multiplicand; and
sequentially calculating, by a sequential calculator, by using a more significant portion of the multiplicand to calculate a first intermediate result, by using a less significant portion of the multiplicand and the first intermediate result to calculate a second intermediate result, and by using a still less significant portion of the multiplicand and the second intermediate result to calculate and store a third intermediate result, wherein the third intermediate result represents the result of the modular multiplication, if the multiplicand is divided into exactly three portions, or wherein the result of the modular multiplication is derivable from the third intermediate result by a further sequential calculation, if the multiplicand is divided into more than three portions,
wherein, in the step of sequentially calculating, a method for calculating a result of a sum of a first operand and a second operand less than a modulus by means of a calculating unit formed to maximally process numbers whose magnitude is less than or equal to a product of the modulus and an integer larger than 1, is executed, the method comprising storing the first operand in the calculating unit, wherein a magnitude of the first operand is less than or equal to the maximally processable number; calculating a modified second operand, wherein the modified second operand is equal to the second operand minus the modulus, so that the modified second operand is negative; calculating a sum of the first operand and the modified second operand or the first operand and a product of the modified second operand times a factor less than or equal to the integer, and storing the sum in the calculating unit; and reducing the sum with respect to the modulus to get the result,
wherein the first operand is a product of the multiplier and a portion of the multiplicand, and wherein the second operand is a product of an intermediate result and the number, and
wherein the multiplicand provider or the sequential calculator comprises a hardware implementation.

21. A non-transient storage medium having stored thereon a computer program with a program code for performing a method for calculating a result of a sum of a first operand and a second operand less than a modulus by means of a calculating unit formed to maximally process numbers whose magnitude is less than or equal to a product of the modulus and an integer larger than 1, when the computer program runs on a computer, the method comprising storing the first operand in the calculating unit, wherein a magnitude of the first operand is less than or equal to the maximally processable number; calculating a modified second operand, wherein the modified second operand is equal to the second operand minus the modulus, so that the modified second operand is negative; calculating a sum of the first operand and the modified second operand or the first operand and a product of the modified second operand times a factor less than or equal to the integer, and storing the sum in the calculating unit; and reducing the sum with respect to the modulus to get the result.

22. A non-transient storage medium having stored thereon a computer program with a program code for performing a method for calculating a result of a modular multiplication with a multiplier, a multiplicand and a modulus, when the computer program runs on a computer, the method comprising providing the multiplicand in at least three portions, wherein each portion comprises a number of digits less than half the number of digits of the multiplicand, and wherein the at least three portions include all digits of the multiplicand; and sequentially calculating by using a more significant portion of the multiplicand to calculate a first intermediate result, by using a less significant portion of the multiplicand and the first intermediate result to calculate a second intermediate result, and by using a still less significant portion of the multiplicand and the second intermediate result to calculate and store a third intermediate result, wherein the third intermediate result represents the result of the modular multiplication, if the multiplicand is divided into exactly three portions, or wherein the result of the modular multiplication is derivable from the third intermediate result by a further sequential calculation, if the multiplicand is divided into more than three portions, wherein, in the step of sequentially calculating, a method for calculating a result of a sum of a first operand and a second operand less than a modulus by means of a calculating unit formed to maximally process numbers whose magnitude is less than or equal to a product of the modulus and an integer larger than 1, is executed, the method comprising storing the first operand in the calculating unit, wherein a magnitude of the first operand is less than or equal to the maximally processable number; calculating a modified second operand, wherein the modified second operand is equal to the second operand minus the modulus, so that the modified second operand is negative; calculating a sum of the first operand and the modified second operand or the first operand and a product of the modified second operand times a factor less than or equal to the integer, and storing the sum in the calculating unit; and reducing the sum with respect to the modulus to get the result, wherein the first operand is a product of the multiplier and a portion of the multiplicand, and wherein the second operand is a product of an intermediate result and the number.

23. A device for calculating a result of a sum of a first operand and a second operand less than a modulus by means of a calculating unit formed to maximally process numbers whose magnitude is less than or equal to a product of the modulus and an integer larger than 1, comprising:
means for storing the first operand in the calculating unit, wherein a magnitude of the first operand is less than or equal to the maximally processable number;
means for calculating a modified second operand, wherein the modified second operand is equal to the second operand minus the modulus, so that the modified second operand is negative;
means for calculating a sum of the first operand and the modified second operand or the first operand and a product of the modified second operand times a factor less than or equal to the integer, and for storing the sum in the calculating unit; and
means for reducing the sum with respect to the modulus to get the result.