Abstract:
A method and apparatus are disclosed for improving public key encryption and decryption schemes that employ a composite number formed from three or more distinct primes. The encryption or decryption tasks may be broken down into sub-tasks to obtain encrypted or decrypted sub-parts that are then combined using a form of the Chinese Remainder Theorem to obtain the encrypted or decrypted value. A parallel encryption/decryption architecture is disclosed to take advantage of the inventive method. 
     REEXAMINATION RESULTS 
     The questions raised in reexamination request No.  90 / 005 , 733 , filed May  18 ,  2000  and reexamination request No.  90 / 005 , 776 , filed on Jul.  28 ,  2000 , have been considered and the results thereof are reflected in this reissue patent which constitutes the reexamination certificate required by  35  U.S.C.  307  as provided in  37  CFR  1 . 570 ( e ) .

Description:
This application claims the benefit of U.S. Provisional Application No. 60/033,271 for PUBLIC KEY CRYTOGRAPHIC APPARATUS AND METHOD, filed Dec. 9, 1996, naming as inventors, Thomas Colins  Collins, Dale Hopkins, Susan Langford and Michale  Michael Sabin, the discolsure  disclosure of which is incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to communicating data in a secure fashion, and more particularly to a cryptographic system and methods using public key cryptography. 
     Computer systems are found today in virtually every walk of life for storing, maintaining, and transferring various types of data. The integrity of large portions of this data, especially that portion relating to financial transactions, is vital to the health and survival of numerous commercial enterprises. Indeed, as open and unsecured data communications channels for sales transactions gain popularity, such as credit card transactions over the Internet, individual consumers have an increasing stake in data security. 
     Thus, for obvious reasons, it is important that financial transaction communications pass from a sender to an intended receiver without intermediate parties being able to interpret the transferred message. 
     Cryptography, especially public key cryptography, has proven to be an effective and convenient technique of enhancing data privacy and authentication. Data to be secured, called plaintext, is transformed into encrypted data, or ciphertext, by a predetermined encryption process of one type or another. The reverse process, transforming ciphertext into plaintext, is termed decryption. Of particular importance to this invention is that the processes of encryption and decryption are controlled by a pair of related cryptographic keys. A “public” key is used for the encryption process, and a “private” key is used to decrypt ciphertext. The public key transforms plaintext to ciphertext, but cannot be used to decrypt the ciphertext to retrieve the plaintext therefrom. 
     As an example, suppose a Sender A wishes to send message M to a recipient B. The idea is to use public key E and related private key D for encryption and decryption of M. The public key E is public information while D is kept secret by the intended receiver. Further, and importantly, although E is determined by D, it is extremely difficult to compute D from E. Thus the receiver, by publishing the public key E, but keeping the private key D secret, can assure senders of data encrypted using E that anyone who intercepts the data will not be able to decipher it. Examples of the public key/private key concept can be found in U.S. Pat. Nos. 4,200,770, 4,218,582, and 4,424,414. 
     The prior art includes a number of public key schemes, in addition to those described in the above-identified patents. Over the past decade, however, one system of public key cryptography has gained popularity. Known generally as the “RSA” scheme, it is now thought by many to be a worldwide defacto standard for public key cryptography. The RSA scheme is described in U.S. Pat. No. 4,405,829 which is fully incorporated herein by this reference. 
     The RSA scheme capitalizes on the relative ease of creating a composite number from the product of two prime numbers whereas the attempt to factor the composite number into its constituent primes is difficult. The RSA scheme uses a public key E comprising a pair of positive integers n and e, where n is a composite number of the form
 
n=p·q   (1) 
 
where p and q are different prime numbers, and e is a number relatively prime to (p−1) and (q−1); that is, e is relatively prime to (p−1) or (q−1) if e has no factors in common with either of them. Importantly, the sender has access to n and e, but not to p and q. The message M is a number representative of a message to be transmitted wherein
 
0≦M&lt;n−1.   (2) 
 
The sender enciphers M to create ciphertext C by computing the exponential
 
C=M e (mod n)  C≡M e ( mod n ).   (3) 
 
     The recipient of the ciphertext C retrieves the message M using a (private) decoding key D, comprising a pair of positive integers d and n, employing the relation
 
M=C d (mod n)  M≡C d ( mod n )   (4) 
 
     As used in (4), above, d is a multiplicative inverse of
 
e(mod(lcm((p−1), (q−1))))   (5) 
 
so that
 
e·d=1(mod(lcm((p−1), (q−1))))  e·d≡ 1 ( mod ( lcm (( p− 1   ), ( q− 1   ))))   (6) 
 
where lcm((p−1), (q−1)) is the least common multiple of numbers p−1 and q−1. Most commercial implementations of RSA employ a different, although equivalent, relationship for obtaining d:
 
d=e −1 mod(p−1) (q−1)  d≡e −1   mod (( p− 1   )·( q− 1   )).   (7) 
 
This alternate relationship simplifies computer processing.
 
     Note: Mathematically (6) defines a set of numbers and (7) defines a subset of that set. For implementation, (7) or (6) usually is interpreted to mean d is the smallest positive element in the set.) 
     The net effect is that the plaintext message M is encoded knowing only the public key E (i.e., e and n). The resultant ciphertext C can only decoded using decoding key D. The composite number n, which is part of the public key E, is computationally difficult to factor into its components, prime numbers p and q, a knowledge of which is required to decrypt C. 
     From the time a security scheme, such as RSA, becomes publicly known and used, it is subjected to unrelenting attempts to break it. One defense is to increase the length (i.e., size) of both p and q. Not long ago it was commonly recommended that p and q should be large prime numbers 75 digits long (i.e., on the order of 10 75 ). Today, it is not uncommon to find RSA schemes being proposed wherein the prime numbers p and q are on the order of 150 digits long. This makes the product of p and q a 300 digit number. (There are even a handful of schemes that employ prime numbers (p and q) that are larger, for example 300 digits long to form a 600 digit product.) Numbers of this size, however, tend to require enormous computer resources to perform the encryption and decryption operations. Consider that while computer instruction cycles are typically measured in nanoseconds (billionths of seconds), computer computations of RSA steps are typically measured in milliseconds (thousandths of seconds). Thus millions of computer cycles are required to compute individual RSA steps resulting in noticeable delays to users. 
     This problem is exacerbated if the volume of ciphertext messages requiring decryption is large—such as can be expected by commercial transactions employing a mass communication medium such as the Internet. A financial institution may maintain as Internet site that could conceivably receive thousands of enciphered messages every hour that must be decrypted, and perhaps even responded to. Using larger numbers to form the keys used for an RSA scheme can impose severe limitations and restraints upon the institution&#39;s ability to timely respond. 
     Many prior art techniques, while enabling the RSA scheme to utilize computers more efficiently, nonetheless have failed to keep pace with the increasing length of n, p, and q. 
     Accordingly, it is an object of this invention to provide a system and method for rapid encryption and decryption of data without compromising data security. 
     It is another object of this invention to provide a system and method that increases the computational speed of RSA encryption and decryption techniques. 
     It is still another object of this invention to provide a system and method for implementing an RSA scheme in which the components  factors of n do not increase in length as n increases in length. 
     It is still another object to provide a system and method for utilizing multiple (more than two), distinct prime number components  factors to create n. 
     It is a further object to provide a system and method for providing a technique for reducing the computational effort for calculating exponentiations in an RSA scheme for a given length of n. 
     SUMMARY OF THE INVENTION 
     The present invention discloses a method and apparatus for increasing the computational speed of RSA and related public key schemes by focusing on a neglected area of computation inefficiency. Instead of n=p·q, as is universal in the prior art, the present invention discloses a method and apparatus wherein n is developed from three or more distinct random prime numbers; i.e., n=p 1 ·p 2 ·. . . ·p k , where k is an integer greater than 2 and p 1 , p 2 , . . . p k  are sufficiently large distinct random primes. Preferably, “sufficiently large primes” are prime numbers that are numbers approximately 150 digits long or larger. The advantages of the invention over the prior art should be immediately apparent to those skilled in this art. If, as in the prior art, p and q are each on the order of, say, 150 digits long, then n will be on the order of 300 digits long. However, three primes p 2 , p 1 , and p 3  employed in accordance with the present invention can each be on the order of 100 digits long and still result in n being 300 digits long. Finding and verifying 3 distinct primes, each 100 digits long, requires significantly fewer computational cycles than finding and verifying 2 primes each 150 digits long. 
     The commercial need for longer and longer primes shows no evidence of slowing; already there are projected requirements for n of about 600 digits long to forestall incremental improvements in factoring techniques and the ever faster computers available to break ciphertext. The invention, allowing 4 primes each about 150 digits long to obtain a 600 digit n, instead of two primes about 350  300 digits long, results in a marked improvement in computer performance. For, not only are primes that are 150 digits in size easier to find and verify than ones on the order of 350  300 digits, but by applying techniques the inventors derive from the Chinese Remainder Theorem (CRT), public key cryptography calculations for encryption and decryption are completed much faster—even if performed serially on a single processor system. However, the inventors&#39; techniques are particularly adapted to be  advantageously apply enable  RSA public key cryptographic operations to parallel computer processing. 
     The present invention is capable of using  extending the RSA scheme to perform encryption and decryption operation using a large (many digit) n much faster than heretofore possible. Other advantages of the invention include its employment for decryption without the need to revise the RSA public key encryption transformation scheme currently in use on thousands of large and small computers. 
     A key assumption of the present invention is that n, composed of 3 or more sufficiently large distinct prime numbers, is no easier (or not very much easier) to factor than the prior art, two prime number n. The assumption is based on the observation that there is no indication in the prior art literature that it is “easy” to factor a product consisting of more than two sufficiently large, distinct prime numbers. This assumption may be justified given the continued effort (and failure) among experts to find a way “easily” to break large component  composite numbers into their large prime factors. This assumption is similar, in the inventors&#39; view, to the assumption underlying the entire field of public key cryptography that factoring composite numbers made up of two distinct primes is not “easy.” That is, the entire field of public key cryptography is based not on mathematical proof, but on the assumption that the empirical evidence of failed sustained efforts to find a way systematically to solve NP problems in polynomial time indicates that these problems truly are “difficult.” 
     The invention is preferably implemented in a system that employs parallel operations to perform the encryption, decryption operations required by the RSA scheme. Thus, there is also disclosed a cryptosystem that includes a central processor unit (CPU) coupled to a number of exponentiator elements. The exponentiator elements are special purpose arithmetic units designed and structured to be provided message data M, an encryption key e, and a number n (where n=p 1 *p 2 * . . . p k   n=p 1   ·p   2   · . . . ·p   k , k being greater than 2) and return ciphertext C according to the relationship,
 
C=M e (mod(n))  C≡M e ( mod n ). 
 
     Alternatively, the exponentiator elements may be provided the ciphertext C, a decryption (private) key d and n to return M according to the relationship,
 
M=C d (mod(n))  M≡C d ( mod n ). 
 
     According to this decryption aspect of the invention, the CPU receives a task, such as the requirement to decrypt cyphertext  ciphertext data C. The CPU will also be provided, or have available, a public  private key e  d and n, and the factors of n (p 1 , p 2 , . . . p k ). The CPU breaks the encryption  decryption task down into a number of sub-tasks, and delivers the sub-tasks to the exponentiator elements. When the  The results of the sub-tasks are returned by the exponentiator elements to the CPU which will , using a form of the CRT, combines the results to obtain the message data M. An encryption task may be performed essentially in the same manner by the CPU and its use of the exponentiator elements. However, usually the factors of n are not available to the sender (encryptor), only the public key, e and n, so that no sub-tasks are created. 
     In a preferred embodiment of this latter aspect of the invention, the bus structure used to couple the CPU and exponentiator elements to one another is made secure by encrypting all important information communicated thereon. Thus, data sent to the exponentiator elements is passed through a data encryption unit that employs, preferably, the ANSI Data Encryption Standard (DES). The exponentiator elements decrypt the DES-encrypted sub-task information they receive, perform the desired task, and encrypt the result, again using DES, for return to the CPU. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified block diagram of a cryptosystem architecture configured for use in the present invention. 
         FIG. 2  is a memory map of the address space of the cryptosystem of  FIG. 1 ; and 
         FIG. 3  is an exemplary illustration of one use of the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     As indicated above, the present invention is employed in the context of the RSA public key encryption/decryption scheme. As also indicated, the RSA scheme obtains its security from the difficulty of factoring large numbers, and the fact that the public and private keys are functions of a pair of large (100-200 digits or even larger) prime numbers. Recovering the plaintext from the public key and the ciphertext is conjectured to be equivalent to factoring the product of two primes. 
     According to the present invention, the public key portion e is picked. Then, three or more random large, distinct prime numbers, p 1 , p 2 , . . . , p k  are developed and checked to ensure that each ( p   i   − 1   ) is relatively prime to e. Preferably, the prime numbers are of equal length. Then, the product n=p 1 , p 2 , . . . , p k   n=p 1   ·p   2   · . . . p   k , is computed. 
     Finally, the decryption key  exponent, d, is established by the relationship:
 
d=e −1 mod ((p 1 −1) (p 2 −1)) . . . (p k −1))  d≡e −1   mod (( p   1   − 1   )·( p   2   − 1   )· . . . ·( p   k   − 1   )) , or equivalently  
 
 d≡e   −1   mod ( lcm (( p   1   − 1   ) ,  ( p   2   − 1   ) , . . .  ( p   k   − 1   ))) 
 
     The message data, M is encrypted to ciphertext C using the relationship of (3), above, i.e.,
 
C=M e mod n  C≡M e ( mod n ). 
 
     To decrypt the ciphertext, C, the relationship of (3)  (   4   ), above, is used:
 
M=C d mod n  M≡C d ( mod n ) 
 
where n and d are those values identified above.
 
     Alternatively, a message data M can be encoded with the private key to a signed message data M s    using a relationship of the form      M   s   ≡M   d ( mod n ) .    
       The message data M can be reproduce from the signed message data M   s    by decoding the signed data with the public key, using a relationship of the form    M≡M s   e ( mod n ) .    
     Using the present invention involving three primes to develop the product n, RSA encryption and decryption time can be substantially less than an RSA scheme using two primes by dividing the encryption or decryption task into sub-tasks, one sub-task for each distinct prime. (However, breaking the encryption or decryption into subtasks requires knowledge of the factors of n. This knowledge is not usually available to anyone except the owner of the key, so the encryption process can be accelerated only in special cases, such as encryption for local storage. A system encrypting data for another user performs the encryption process according to (3), independent of the number of factors of n. Decryption, on the other hand, is performed by the owner of a key, so the factors of n are generally known and can be used to accelerate the process.) For example, assume that three distinct primes, p 1 , p 2 , and p 3 , are used to develop the product n. Thus, decryption of the ciphertext, C, using the relationship
 
M=C d (mod n)  M≡C d ( mod n ) 
 
is used to develop the decryption sub-tasks:
 
M 1 =C 1   d     1   mod p 1  M   1   ≡C   1   d     1   ( mod p   1 ) 
 
M 2 =C 2   d     2   mod p 2   M 2   ≡C   2   d     2   ( mod p   2 ) 
 
M 3 =C 3   d     3   mod p 3  M   3   ≡C   3   d     3   ( mod p   3 ) 
 
where
 
C 1 =Cmod p 1 ;  C 1   ≡C ( mod p   1 ) ;  
 
C 2 =Cmod p 2 ;  C 2   ≡C ( mod p   2 ) ;  
 
C 3 =Cmod p 3 ;  C 3   ≡C ( mod p   3 ) ;  
 
d 1 =dmod (p 1 −1)  d 1   ≡d ( mod ( p   1   − 1   )); 
 
d 2 =dmod (p 2 −1)  d 2   ≡d ( mod ( p   2   − 1   )); and 
 
d 3 =dmod (p 3 −1)  d 3   ≡d ( mod ( p   3   − 1   )). 
 
     The results of each sub-task, M 1 , M 2 , and M 3  can be combined to produce the plaintext, M, by a number of techniques. However, it is found that they can most expeditiously be combined by a form of the Chinese Remainder Theorem (CRT) using, preferably, a recursive scheme. Generally, the plaintext M is obtained from the combination of the individual sub-tasks by the following relationship:
 
Y i =Y i−1 +[(M i −Y i−1 ) (w i   −1 mod p i )mod p i ]·w i mod n  Y i   ≡Y   i−1 +(( M   1   −Y   i−1 )( w   i   −1 ( mod p   i ))) mod p   i )) ·w   i ( mod n )
 
where
 
i≧ 2  2≦i≦k  
 
where k is the number of prime factors of n, and 
         M   =     Y   k       ,       Y   1     =         C   1     ⁢           ⁢   and   ⁢           ⁢     w   i       =       ∏     j   &lt;   1               ⁢           ⁢     p   j               
 
Encryption is performed in much the same manner as that used to obtain the plaintext M, provided (as noted above) the factors of n are available. Thus, the relationship
 
C=M e (mod n)  C≡M e ( mod n ), 
 
can be broken down into the three sub-tasks,
 
C 1 =M 1   e1 mod p 1   C 1   ≡M   1   e     1   ( mod p   1 ) ,  
 
C 2 =M 2   e2 mod p 2   C 2   ≡M   2   e     2   ( mod p   2 )  and  
 
C 3 =M 3   e3 mod p 3   C 3   ≡M   3   e     3   ( mod p   3 ) ,  
 
where
 
M 1 =M(mod p 1 )  M 1   ≡M  ( mod p   1 ), 
 
M 2 =M(mod p 2 )  M 2   ≡M  ( mod p   2 ), 
 
M 3 =M(mod p 3 )  M 3   ≡M  ( mod p   3 ), 
 
e 1 =emod (p 1 −1)  e 1   ≡e  ( mod ( p   1   − 1   )), 
 
e 2 =emod (p 2 −1)  e 2   ≡e ( mod ( p   2   − 1   ), and 
 
e 3 =emod (p 3 −1)  e 3   ≡e ( mod ( p   3   − 1   )) .  
 
     In generalized form, the decrypted  ciphertext C ( i.e., encrypted  message M) can be obtained by the same summation  a recursive scheme as identified above to obtain the ciphertext C from its contiguous constituent sub-tasks C i . 
     Preferably, the recursive CRT method described above is used to obtain either the ciphertext,  C,  or the deciphered plaintext (message) M due to its speed. However, there may be occasions  implementations when it is beneficial to use a non-recursive technique in which case the following relationships are used: 
         M   ≡       ∑     i   =   1               ⁢           ⁢           M   i     ⁡     (       w   i     -   1       ⁡     (     mod   ⁢           ⁢     p   i       )       )       ·         w   i     ⁡     (     mod   ⁢           ⁢   n     )       ⁢     
     [     [                 ⁢   M         =       ∑     i   =   1               ⁢           ⁢         M   i     ⁡     (       w   i     -   1       ⁢   mod   ⁢           ⁢     p   1       )       ⁢     w   i     ⁢   mod   ⁢           ⁢   n   ⁢                       ]     ]             
 where 
           [     [               ⁢     w   i       =       ∏     j   ≠   1               ⁢           ⁢       p   j     ⁢         ]     ⁢         ]             
           w   i     =       ∏     j   ≠   1               ⁢           ⁢     p   j         ,           ⁢   and       
 
k is the number (3 or more) of distinct primes chosen to develop the product n.
 
     Thus, for example above (k=3), M is constructed from the returned sub-task values M 1 , M 2 , M 3  by the relationship
 
M=M 1 (w 1   −1 mod p 1 ) w 1 mod/n+M 2 (w 2   −1 mod p 2 ) w 2 mod n+M 3 (w 3   −1 mod p 3 ) w 3 mod n  
 
M=M 1   −1 ( w   1   −1 ( mod p   1 )) ·w   1 ( mod n ) +M   2 ( w   2   −1 ( mod p   2 )) ·w   2 ( mod n ) +M   3 ( w   3   −1 ( mod p   3 ))· w   3 ( mod n ) 
 
where
 
w 1 =p 2 p 3 , w 2 =p 1 p 3 , and w 3 =p 1 p 2 . 
 
     Employing the multiple distinct prime number technique of the present invention in the RSA scheme can realize accelerated processing over that using only two primes for the same size n. The invention can be implemented on a single processor unit or even the architecture disclosed in the above-referenced U.S. Pat. No. 4,405,829. The capability of developing sub-tasks for each prime number is particularly adapted to employing a parallel architecture such as that illustrated in FIG.  1 . 
     Turning to  FIG. 1 , there is illustrated a cryptosystem architecture apparatus capable of taking particular advantage of the present invention. The cryptosystem, designated with the reference numeral  10 , is structured to form a part of a larger processing system (not shown) that would deliver to the cryptosystem  10  encryption and/or decryption requests, receiving in return the object of the request—an encrypted or decrypted value. The host would include a bus structure  12 , such as a peripheral component interface (PCI) bus for communicating with the cryptosystem  10 . 
     As  FIG. 1  shows, The cryptoprocessor  10  includes a central processor unit (CPU)  14  that connects to the bus structure  12  by a bus interface  16 . The CPU  14  comprises a processor element  20 , a memory unit  22 , and a data encryption standard (DES) unit  24  interconnected by a data/address bus  26 . The DES unit  24 , in turn, connects to an input/output (I/O) bus  30  (through appropriate driver/receiver circuits—not shown). 
     The I/O bus  30  communicatively connects the CPU to a number of exponentiator elements  32   a ,  32   b , and  32   c    32 a,  32 b and  32 c. Shown here are three exponentiator elements, although as illustrated by the “other” exponentiators  32   n    32 n, additional exponentiator elements can be added. Each exponentiator element is a state machine controlled arithmetic circuit structured specifically to implement the relationship described above. Thus, for example, the exponentiator  32 a would be provided the values M 1 , e 1 , and p 1  n  to develop C 1 . Similarly, the exponentiator circuits  32 b and  32 c develop C 2  and C 3  from corresponding subtask values M 2 , e 2 , P 2   p 2 , M 3 , e 3 , and P 3   p 3 . 
     Preferably, the CPU  14  is formed on a single integrated circuit for security reasons. However, should there be a need for more storage space than can be provided by the “on-board” memory  22 , the bus  30  may also connect the CPU  14  to an external memory unit  34 . 
     In order to ensure a secure environment, it is preferable that the cryptosystem  10  meet the Federal Information Protection System  Processing Standard (FIPS)  140 -   1   level  3 . Accordingly, the elements that make up the CPU  14  would be implemented in a design that will be secure from external probing of the circuit. However, information communicated on the I/O bus  30  between the CPU  14  and the exponentiator circuits  32  (and external memory  34 —if present) is exposed. Consequently, to maintain the security of that information, it is first encrypted by the DES unit  24  before it is placed on the I/O bus  30  by the CPU  14 . The exponentiator circuits  32 , as well as the external memory  34 , will also include similar DES units to decrypt information received from the CPU, and later to encrypt information returned to the CPU  14 . 
     It may be that not all information communicated on the I/O bus  30  need be secure by DES encryption. For that reason, the DES unit  24  of the CPU  14  is structured to encrypt outgoing information, and decrypt incoming information, on the basis of where in the address space used by the cryptosystem the information belongs; that is, since information communicated on the I/O bus  30  is either a write operation by the CPU  14  to the memory  34 , or a read operation of those elements, the addresses assigned to the secure addresses and non-secure addresses. Read or write operations conducted by the CPU  14  using secure addresses will pass through the DES unit  24  and that of the memory  34 . Read or write operations involving non-secure addresses will by-pass these DES units. 
       FIG. 2  diagrammatically illustrates a memory map  40  of the address space of the cryptosystem  10  that is addressable by the processor  20 . As the memory map  30  shows, an address range  30  provides addresses for the memory  22 , and such other support circuitry (e.g., registers—not shown) that may form a part of the CPU  14 . The addresses used to write information to, or read information from, the exponentiator elements  32  are in the address range  44  of the memory map  40 . The addresses for the external memory  34  are in the address ranges  46 , and  48 . The address ranges  44  and  46  are for secure read and write operations. Information that must be kept secure, such as instructions for implementing algorithms, encryption/decryption keys, and the like, if maintained in external memory  34 , will be stored at locations having addresses in the address range  46 . Information that need not be secure such as miscellaneous algorithms data, general purpose instructions, etc. are kept in memory locations of the external memory  34  having addresses within the address range  48 . 
     The DES unit  24  is structured to recognize addresses in the memory spaces  44 ,  46 , and to automatically encrypt the information before it is applied to the I/O bus  30 . The DES unit  24  is bypassed when the processor  20  accesses addresses in the address range  48 . Thus, when the processor  20  initiates write operations to addresses within the memory space within the address range  46  (to the external memory  34 ), the DES unit  24  will automatically encrypt the information (not the addresses) and place the encrypted information on the I/O bus  30 . Conversely, when the processor  20  reads information from the external memory  34  at addresses within the address range  46  of the external memory  34 , the DES unit will decrypt information received from the I/O bus  30  and place the decrypted information on the data/address bus  26  for the processor  20 . 
     In similar fashion, information conveyed to or retrieved from the exponentiators  32  by the processor  20  by write or read operations at addresses within the address range  44 . Consequently, writes to the exponentiators  32  will use the DES unit  24  to encrypt the information. When that (encrypted) information is received by the exponentiators  32 , it is decrypted by on-board DES units (of each exponentiator  32 ). The results  of the task performed by the exponentiator  32  is then encrypted by the exponentiator&#39;s on-board DES unit, retrieved by the processor  20  in encrypted form and then decrypted by the DES unit  24 . 
     Information that need not be maintained in secure fashion to be stored in the external memory  34 , however, need only be written to addresses in the address range  48 . The DES unit  24  recognizes writes to the address range  48 , and bypasses the encryption circuitry, passing the information, in unencrypted form, onto the I/O bus  30  for storing in the external memory  34 . Similarly, reads of the external memory  34  using addresses within the address range  48  are passed directly from the I/O bus  30  to the data/address bus  26  by the DES unit  24 . 
     In operation, the CPU  14  will receive from the host it serves (not shown), via the bus  12 , an encryption request. The encryption request will include the message data M to be encrypted and, perhaps, the encryption keys e and n (in the form of the primes p 1 , p 2 , . . . p k ). Alternatively, the keys may be kept by the CPU  14  in the memory  22 . In any event, the processor  20  will construct the encryption sub-tasks C 1 , C 2 , . . . , C k  for execution by the exponentiators  32 . 
     Assume, for the purpose of the remainder of this discussion, that the encryption/decryption tasks performed by the cryptosystem  10 , using the present invention, employs only three distinct primes, p 1 , p 2 , p 3 . The processor  20  will develop the sub tasks identified above, using M, e, p 1  p 2 , p 3  Thus, for example, if the exponentiator  32 a were assigned the sub-task of developing C 1 , the processor would develop the values M 1 ,  and e 1 , and (p 1 −1)  and deliver units  (write) these values, with n,  p 1  to the exponentiator  32 a. Similar values will be developed by the processor  20  for the sub-tasks that will be delivered to the exponentiators  32 b and  32 c. 
     In turn, the exponentiators  32  develop the values C 1 , C 2 , and C 3  which are returned to (retrieved by) the CPU  14 . The processor  20  will then combine the values C 1 , C 2 , and C 3  to form C, the ciphertext encryption of M, which is then returned to the host via the bus  12 . 
     The encryption, decryption techniques described hereinabove, and the use of cryptosystem  10  ( FIG. 1 ) can find use in a number of diverse environments. Illustrated in  FIG. 3  is one such environment.  FIG. 3  shows a host system  50 , including the bus  12  connected to a plurality of cryptosystems  10  ( 10 a,  10 b, . . . ,  10 m) structured as illustrated in  FIG. 1 , and described above. In turn, the host system  50  connects to a communication medium  60  which could be, for example, an internet connection that is also used by a number of communicating stations  64 . For example, the host system  50  may be employed by a financial institution running a web site accessible, through the communication medium, by the stations  64 . Alternatively, the communication medium may be implemented by a local area network (LAN) or other type network. Use of the invention described herein is not limited to the particular environment in which it is used, and the illustration in  FIG. 3  is not meant to limit in any way how the invention can be used. 
     As an example, the host system, as indicated, may receive encrypted communication from the stations  64 , via the communication medium  60 . Typically, the data of the communication will be encrypted using DES, and the DES key will be encrypted using a public key by the RSA scheme, preferably one that employs three or more distinct prime numbers for developing the public and private keys. 
     Continuing, the DES encrypted communication, including the DES key encrypted with the RSA scheme, would be received by the host system. Before decrypting the DES communication, it must obtain the DES key and, accordingly, the host system  50  will issue, to one of the cryptosystems  10  a decryption request instruction, containing the encrypted DES key as the cyphertext C. If the (private) decryption keys, d, n (and its component primes, p 1 , p 2 , . . . p k ) are not held by the cryptosystem  10 , they also will be delivered with the encryption request instruction. 
     In turn, the cryptosystem  10  would decrypt the received cyphertext in the manner described above (developing the sub-tasks, issuing the sub-tasks to the exponentiator  32  of the cryptosystem  10 , and reassembling the results of the sub-task to develop the message data: the DES key), and return to the host system the desired, decrypted information. 
     Alternatively, the post  host-system  50  may desire to deliver, via the communication medium  60 , an encrypted communication to one of the stations  64 . If the communication is to be encrypted by the DES scheme, with the DES key encrypted by the RSA scheme, the host system would encrypt the communication, forward the DES key to one of the cryptosystems  10  for encryption via the RSA scheme. When the encrypted DES key is received back from the cryptosystem  10 , the host system can then deliver to one or more of the stations  64  the encrypted message. 
     Of course, the host system  50  and the stations  64  will be using the RSA scheme of public key encryption/decryption. Encrypted communications from the stations  64  to the host system  50  require that the stations  64  have access to the public key E (E, N)  E=( e, n ) while the host system maintains the private key D (D, N,  D=( d, n ) and the constituent primes, p 1 , p 2 , . . . , p k ). Conversely, for secure communication from the host system  50  to one or more of the stations  64 , the host system would retain a public key E′ for each station  64 , while the stations retain the corresponding private keys E′  D′. 
     Other techniques for encrypting the communication could used. For example, the communication could be entirely encrypted by the RSA scheme. If, however, the message to be communication ed is represented by a numerical value greater than n−1, it will need to be broken up into blocks size M where
 
0≦M≦N− 1  0≦M≦n−1 . 
 
     Each block M would be separately encrypted/decrypted, using the public key/private key RSA scheme according to that described above.