Abstract:
An object of the present invention is to prevent secret information that is being internally processed from being inferred through operational information of a secured device, including the current consumption information. One solution is provided by an information processing device having at least a key generation apparatus that generates key data automatically, an encryption unit that encrypts data with the corresponding key data, a register that stores a plurality of encrypted data items with the corresponding encryption key data items, and an arithmetic unit that performs operations using data expressed with the corresponding encryption key data and new key data as the input, encrypts the operation result with new input key data, and outputs the result, thereby being capable of performing internal processing on an encrypted data expression. Accordingly, only encrypted data is transferred on the internal or external data bus line.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an information processing device, and more particularly to a tamper proof device such as an integrated circuit card (IC card) having a high degree of security. 
     2. Description of the Related Art 
     The IC cards are intended for holding information that must not be tampered by encrypting data with secret encryption keys and decrypting the encrypted text. An IC card has no internal power source and becomes operable only when inserted into a card reader/writer by which it is powered. When it becomes operable, the IC card receives commands from the card reader/writer and transfers data as commanded. The general descriptions of IC cards can be found in books such as IC Card, Junichi Mizusawa, The Institute of Electronics, Information and Communication Engineers, published by Ohm. 
     As shown in FIG. 1, an IC card  101  includes an IC card chip  102 . An IC card generally has a set of contacts, through which power is supplied and data communication is performed. 
     The structure of an IC card chip is basically the same as that of a microprocessor. As shown in FIG. 2, the IC card chip is organized into a central processing unit (CPU)  201 , a memory unit  204 , an I/O port  207 , and a coprocessor  202 . The central processing unit (CPU)  201  performs logic and arithmetic operations, and the memory unit  204  stores programs and data. The I/O port  207  communicates with external card reader/writers. The coprocessor  202  is specifically used for performing modulo arithmetic, such as operations required in the RSA public key cipher. There are also many IC card processors without coprocessors. A data bus  203  provides links among these components. 
     The memory unit includes a read-only memory (ROM), a random access memory (RAM), and an electrically erasable programmable read-only memory (EEPROM). ROM is not modifiable, and mainly stores program code. RAM is rewritable, but its contents are lost when power is off. RAM therefore cannot be used to retain data after the IC card is withdrawn from the reader/writer such that its power supply is stopped. EEPROM is rewritable, and it retains its contents even without power. EEPROM is used to store data that must sometimes be rewritten and must be retained even when the IC card is removed from the reader/writer. EEPROM is used, for example, in a prepaid card that retains data indicating the amount of use, which has to be rewritten at every use and must be retained after the card is withdrawn from the card reader/writer. 
     IC cards store programs and data inside an enclosed IC card chip so as to store important information and perform cryptographic processing. The degree of difficulty in deciphering cryptographic processing in IC cards has been considered to be similar to the difficulty of deciphering cryptographic algorithms. It is suggested, however, that there is a risk that information being cryptographically processed in IC cards and the cryptographic keys used for such processing may be inferred through observation and analysis of current consumption during the cryptographic processing, which is easier than deciphering cryptographic algorithms. The current consumption can be observed by measuring current that is supplied from the card reader/writer. Such risks are described in ‘8.5.1.1 Passive protective mechanisms’ p.263 of Smart Card Handbook written by W. Rankl &amp; W. Effing, John Wiley &amp; sons Co. 
     The CMOS circuits in an IC card chip consume current when their output changes from ‘1’ to ‘0’, and vice versa. The data bus  203  has a particularly large electrical capacitance such that it draws a large current when the value placed on it changes from ‘1’ to ‘0’, or vice versa. This suggests the possibility that observation of the current consumption can reveal the operations inside the IC card chip. 
     FIG. 3 is a graph showing current consumption waveforms over one processing cycle in an IC card chip. The waveforms vary as indicated with lines  301  and  302  depending on the data being processed. The variations are caused by differences in data carried on the data bus  203  and data being processed in the CPU  201 . 
     Therefore, it is possible to infer which component is operating or what kind of data is being processing from the current consumption. 
     As countermeasures against such risks, the prior art provides two general methods: one method keeps the values of current consumption constant; the other method changes the current consumption while performing the same processing. An example of the former method provides a positive data bus, a negative data bus and a plurality of arithmetic units, which perform dummy and real operations concurrently to keep the current consumption constant regardless of the input data and operational results (PCT WO 99/67766). This method, however, raises problems of increased hardware scale, such as a doubling of the bus width and a quadrupling of the number of arithmetic units. As an example of the latter method, a method for encrypting data transferred on the bus or stored in memory has been suggested (JP-A-5731/2001). This method imposes a programming restriction, however, because the difference in life time of a plurality of data sharing the same key information places a limitation on the timing of updating of the encryption key. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to reduce the correlation between data being processed and current consumption in an IC card microprocessor chip without adding substantial hardware scale or programming restrictions. 
     According to one aspect of the invention, the information processing device includes a memory unit; an arithmetic unit; first encryption means for encrypting data written into/read from the memory unit or data input into/output from the arithmetic unit with a first cryptographic algorithm into first data including first key data and first encrypted data; and transfer means for transferring the first data into/from the memory unit or the arithmetic unit such that only encrypted data is transferred thereby. 
     According to another aspect of the invention, the information processing device includes an arithmetic unit; first encryption means for encrypting data to be input into the arithmetic unit into first data including first key data and first encrypted data; at least one decoder for decrypting the first data; at least one encoder for encrypting output of the arithmetic unit into second data including second key data and second encrypted data; transfer means for transferring data into/from the arithmetic unit such that only encrypted data is transferred thereby. The decoder and the encoder are disposed close to the arithmetic unit so as to reduce current consumption therebetween. 
     According to a third aspect of the invention, the information processing device includes a memory unit; an arithmetic unit; first encryption means for encrypting data written into/read from the memory unit or data input into/output from the arithmetic unit with a first cryptographic algorithm into first data including first key data and first encrypted data; and second encryption means for encrypting the first data with a second cryptographic algorithm into second data including second key data and second encrypted data to be stored in at least one of ROM/EEPROM and RAM of the memory unit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and additional features and characteristics of the present invention will become more apparent from the following detailed description considered with reference to the accompanying drawings in which like reference numerals designate like elements and wherein: 
     FIG. 1 shows a semiconductor integrated circuit device in an IC card; 
     FIG. 2 shows the basic structure of a microprocessor; 
     FIG. 3 is a drawing showing current consumption waveforms in one processing cycle in an ordinary IC card chip; 
     FIG. 4 is a block diagram showing an embodiment of the present invention; 
     FIG. 5 shows an embodiment of an adder according to the present invention; 
     FIG. 6 is a truth table of an embodiment of an encryption full adder according to the present invention; 
     FIG. 7 is a truth table of another embodiment of an encryption full adder according to the present invention; 
     FIG. 8 shows an embodiment of a logic circuit synthesized from the truth table shown in FIG. 6; 
     FIG. 9 shows an embodiment of a logic circuit synthesized from the truth table shown in FIG. 7; 
     FIG. 10 is a block diagram showing an embodiment of the present invention; 
     FIG. 11 shows an embodiment of an encryption unit and a decryption unit used in the present invention; 
     FIG. 12 shows an embodiment of an encryption unit and a decryption unit used in the present invention; 
     FIG. 13 shows an embodiment of the present invention; 
     FIG. 14 shows an embodiment of the present invention; 
     FIG. 15 shows an embodiment of the present invention; 
     FIG. 16 shows an embodiment of the present invention; 
     FIG. 17 is an example of a correspondence table between instruction codes and instructions; and 
     FIG. 18 is another example of the correspondence table between instruction codes and instructions of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     There are two methods of reducing the correlation between the data being processed and the current consumption: (1) changing the current consumption in an unpredictable way even if the value of data being processed is the same, and (2) keeping the current consumption constant even if the value of the data to be processed varies. 
     The present invention reduces the correlation between current consumption and data by the first method (1). Basically, this method makes the correlation between the true value of the data and its electrical expression unpredictable by encrypting the data and changing the encryption key at short time intervals. In this case, if the encryption key is shared among a plurality of data items, changing the encryption key entails the necessity of rewriting all of the data items sharing it. Therefore, the amount of data that shares an encryption key should be minimized, and more preferably should be the same as the size of the access unit. For example, keys may be provided per byte. The number of bits of a key may be 1 bit at the minimum. A 1-bit key can also be considered to be a selection bit for selecting two types of keys. A simple exemplary method is to invert the data when the key bit is a ‘1’, and not to invert the data when the key bit is a ‘0’. The cryptography used in this method can be considered to be a kind of Vernam cipher, equivalent to the use of 255 as a key for a key bit of ‘1’, and the use of 0 as a key for a key bit of ‘0’. The Vernum cipher performs encryption by an exclusive-or (xor) operation using a secret key K, and performs decryption by xoring the encrypted data again with the secret key K that was used for encryption. 
     
       
         Encrypted data=(key bit×255) xor  plaintext data  (Eq. 1)  
       
     
     Suppose the decimal number 63 is to be encrypted and the key bit is ‘1’. The encryption equation can be expressed in binary notation as follows. 
     
       
         (1×11111111( b )) xor  00111111( b )=11000000( b )  (Eq. 2)  
       
     
     If the key bit is added as the most significant bit, then 63 is encrypted to 448 as follows. 
     
       
         Encrypted data=111000000( b )=448  (Eq. 3)  
       
     
     The encrypted data can then be decrypted by taking the most significant bit as the key bit and the lower 8 bits as the encrypted data as follows. 
     
       
         Plaintext data=(key bit×255) xor  encrypted data  (Eq. 4)  
       
     
     This can be expressed in binary notation as follows. 
     
       
         (1×11111111( b )) xor  11000000( b )=00111111( b )=63  (Eq. 5)  
       
     
     Other key values can be selected by the bits in (Eq. 1) and (Eq. 4). In this case, the two key data items selected by the one key bit should become all ‘1’s when xored. This is because two key data items having bits of the same value constantly produce the same encrypted value of the bit to be processed. The values 0 and 255 satisfy this condition. Therefore, if the data to be processed is 8-bit data, the value of the encryption key is K, and the key bit is 1 bit, the encryption and decryption equations are the following. 
     
       
         encrypted data=(not(key bit×255)  xor  key value)  xor  plaintext data  (Eq. 6)  
       
     
     and 
     
       
         plaintext data=(not(key bit×255)  xor  key value)  xor  encrypted data  (Eq. 7)  
       
     
     The key bit may be stored by adding it at the highest or any other bit position. Suppose the key value is 170 (=10101010(b)) and the plaintext data value is 123. If the key bit is ‘1’, then 
     
       
         (not(1×255)  xor  170)  xor  123=(not(1×11111111( b ))  xor  10101010( b ))  xor  1111011( b )=(not(11111111( b ))  xor  10101010( b ))  xor  1111011( b )=10101010( b )  xor  111011( b )=11010001( b )=209  (Eq.  8 )  
       
     
     If the key bit is added to the highest position, then the encrypted value becomes 209+256=465. Similarly, if the key bit is ‘0’, then 
     
       
         (not(0×256)  xor  170)  xor  123(not(0×11111111( b ))  xor  10101010( b )  xor  1111011( b )=(not(0)  xor  10101010( b ))  xor  1111011( b )=01010101( b )  xor  1111011( b )=00101110( b )=46  (Eq.  9 )  
       
     
     The value of the key bit is ‘0’, so the value obtained by adding the key bit is also 46. The applicable register is able to store encrypted data with the encryption bit. The encrypted data is stored as is and is decrypted only when the true value is necessary, such as at the time of arithmetic operations. 
     Next, in decryption, since 465 includes the key bit of ‘1’ and the encrypted data value of 209, 
     
       
         (not(1×255)  xor  170)  xor  209=170)  xor  209=123  (Eq.  10 )  
       
     
     Similarly, since 46 includes the key bit of ‘0’ and the encrypted data value of 46, 
     
       
         (not(0×255)  xor  170)  xor  46=85  xor  46=123  (Eq.  11 )  
       
     
     The correct value is obtained by decryption in both cases. 
     FIG. 11 shows an exemplary logic circuit implementing the above process (Eq. 6 and Eq. 7). 
     If there is only one key bit, current consumption differs depending on whether the value of the key bit is ‘1’ or ‘0’. In a bus of the pre-charged type, the voltage of which is first raised, then decreased according to the value of the bit, current consumption varies depending on the value of the key bit and on whether discharging is performed after charging. Therefore, it is preferable to use a plurality of bits to express the key bit, to prevent variations in current consumption depending on the key bit value. It is assumed that two bits are used to express the key bit. If two bits (key bit  1  and key bit  2 ) are used to express key bits ‘0’ and ‘1’, the ‘1’ state of the key bit is expressed by (key bit  1 =1, key bit  2 =0), and the ‘0’ state of the key bit is expressed by (key bit  1 =0, key bit  2 =1). FIG. 12 shows an embodiment of the encoder/decoder circuit in which one key bit is expressed by a pair of signal-flow paths. 
     An information processing device according to the present invention provides encryption and decryption units at positions such that a link to conventional memory units and buses is possible. More specifically, the information processing device disposes the decryption unit at the point from which data is output to the outside, and the decryption unit decrypts the data and then outputs the decrypted data to a conventional bus or memory unit. The encryption unit is disposed at the point from which data is input from the outside, and the encryption unit encrypts data from a conventional bus or memory unit and then passes the encrypted data into the information processing device. 
     There are two methods of performing arithmetic operations on encrypted data. One is to place a decryption unit at the input interface of the arithmetic unit and an encryption unit at the output interface from which operational results are obtained. The other is to construct an arithmetic circuit capable of operation on the encrypted data as is. A method of implementing an encrypted full adder that receives encrypted data as input and outputs encrypted arithmetic results is described by taking a 1-bit full adder as an example. 
     First, with an ordinary full adder that is not encrypted, suppose the two inputs are A′ and B′, the carry bit from the lower-order position is C′in, the carry bit to the higher-order position is C′out, and the addition result is R′. The relationships of A′, B′, C′in, C′out, and R′ can be expressed as follows. 
     
       
         R′=A′ xor B′ xor C′in  (Eq.  12 )  
       
     
     
       
         C′out=A′ and B′ or (A′ xor B′) and C′in  (Eq. 13)  
       
     
     Next, an encrypted full adder is assumed. It is assumed that the first encrypted input data is A, the key bit of the first input data is Ak, the second encrypted input data is B, the key bit of the second input data is Bk, the carry bit produced to the higher-order position is Cout, the carry bit from the lower-order position is Cin, the addition result is R, and the encryption bit carrying Cout, Cin, and R is Rk. 
     In this case, in an encrypted full adder that inverts bits when the encryption bit is ‘1’, the relationship between A′, B′, C′in, C′out, and R′ of the input and output of the unencrypted full adder, and A, k, B, Bk, Cin, Cout, R, and Rk of the input and output of the encrypted full adder can be expressed as follows. 
     
       
         A′=A xor Ak  
       
     
     
       
         B′=B xor Bk  
       
     
     
       
         C′ in=Cin xor Rk  
       
     
     
       
         C′ out=Cout xor Rk  
       
     
     
       
         R′=R xor Rk  (Eq. 14)  
       
     
     Substituting (Eq. 14) with equations (Eq. 12, Eq. 13) yields the logic expressions of the encrypted full adder as the following. 
     
       
         R=(A xor Ak xor B xor Bk xor Cin xor Rk) xor Rk=A xor B xor Cin xor Ak xor Bk  (Eq. 15)  
       
     
     
       
         Cout=((A xor Ak) and (B xor Bk) or ((A xor Ak) xor (B xor Bk)) and (Cin xor Rk) xor Rk=((A xor Ak) and (B xor Bk) or (A xor B xor Ak xor Bk)) and (Cin xor Rk)) xor Rk  (Eq 16)  
       
     
     Using De Morgan&#39;s theorem, 
     
       
         X or Y=not ((not X) and (not Y))  (Eq. 17)  
       
     
     (Eq. 16) may be written as the following. 
     
       
         Cout=(((A xor Ak) nand (B xor Bk)) nand ((A xor B xor Ak xor Bk) nand (Cin xor Rk))) xor Rk  (Eq. 18)  
       
     
     Equations (Eq. 15) and (Eq. 18) can be expressed in the logic circuit shown in FIG.  8 . 
     Next, it is assumed that an encrypted full adder that reverses bits when the encryption bit is ‘0’. The bits Cin and Cout are assumed to be inverted when the encryption bit is ‘1’. Then the relationships between A′, B′, C′in, C′out, and R′and A, Ak, B, Bk, Cin, Cout, R, and Rk can be expressed by equation (Eq. 19). 
     
       
         A′=A xor not Ak  
       
     
     
       
         B′=B xor not Bk  
       
     
     
       
         C′in=Cin xor Rk  
       
     
     
       
         C′out=Cout xor Rk  
       
     
     
       
         R′=R xor not Rk  (Eq.  19 )  
       
     
     By substituting (Eq. 19) with equations (Eq. 12) and (Eq. 13) and applying De Morgan&#39;s theorem, the logic expression of the encrypted full adder that inverts bits when the encryption bit is ‘0’ can be determined in the following way. 
     
       
         R=(A xor (not Ak) xor B xor (not Bk) xor Cin xor Rk) xor (not Rk)=not (A xor B xor Cin xor Ak xor Bk)  (Eq. 20)  
       
     
     
       
         Cout=(((A xor not Ak) nand (B xor not Bk)) nand ((A xor B xor not Ak xor not Bk) nand (Cin xor Rk))) xor Rk=(((A xor Ak) or (B xor Bk)) nand ((A xor B xor Ak xor Bk) nand (Cin xor Rk))) xor Rk  (Eq.  21 )  
       
     
     FIG. 9 shows a logic circuit implementing these equations (Eq. 20 and Eq. 21). An array of the 1-bit full adders shown in FIGS. 8 and 9 can implement an adder for a plurality of bits. 
     For example, an encrypted full adder for performing encryption with 0x55 when the encryption bit is ′‘0’ and encryption with 0xAA when the encryption bit is ‘1’ can be implemented by an array of eight encrypted full adders expressed alternately by the logic circuit in FIG.  9  and the logic circuit in FIG. 8, with the logic circuit in FIG. 9 is in the lowest-order position. 
     There are two ways to implement an instruction decoder that interprets and executes encrypted data: one is to connect a decryption unit at the point at which instructions from the instruction decoder are received into the information processing device; the other is to provide a decoder with a many-to-one correspondence between instruction codes and instructions such that the encrypted data can be directly interpreted and executed. Encrypting instruction codes yields a number of encrypted values equal to the number of different encrypted bit values. All of the values obtained through encryption are made to correspond to the instruction that corresponds to the original instruction code. A correspondence between all the instruction codes and instructions is established in this way, and a table showing the many-to-one correspondence between the encrypted instruction codes and instructions is generated. Constructing a decoder according to the table makes it possible to implement a decoder that can interpret encrypted instruction codes without decrypting them. 
     FIG. 4 is a block diagram showing the basic structure of an information processing device for presenting a brief description of a first embodiment of the present invention. FIG. 4 shows only the main components of the parts associated with the present invention in the information processing device. Some conventional structures may suffice for the other parts of the information processing device. The CPU  401  comprises a key generator  1009  that generates keys used for encryption of operation results, an arithmetic unit  406  that receives encrypted values as inputs and outputs the results in encrypted form, an encoder  409  that encrypts data received from an internal bus  402 , a register  403 , an instruction decoder  404 , and external ports, a key generator  410  that generates keys for encryption in the encoder  409 , and a decoder  408  that decrypts data when the data is output to the external ports and elsewhere. The CPU  401  has the structure shown in the drawing. The instruction decoder  404  uses both encrypted data and key bits sent from the internal bus  402  to determine the instruction to be executed. This type of instruction decoder can be implemented easily, for example, by configuring a decoder circuit under the assumption that (n+m) bit data obtained by combining n data bits and m key bits is used as an instruction code. In the present embodiment, n=8 bits and m=1 bit. First, there is a processor for which 8-bit instruction codes correspond to instructions as shown in FIG. 17 in the unencrypted state. The encryption method adapted adds 1-bit key bit to the highest-order position, which is xored with 0x55 (i.e., 0x55=5x16+5) when the key bit is ‘0’ and xored with 0xAA (i.e., 0xAA=10x16+10) when the key bit is ‘1’. The unencrypted instruction code of BSR is 0x5C. If the instruction code is encrypted, the instruction code for BSR can take two values as follows depending on the value of the key bit. 
     0x5C xor 0x55+0=0x009 (when key bit=0) 
     0x5C xor 0xAA+0x100=0x1F6 (when key bit=1) 
     Providing a many-to-one correspondence table of instruction codes and instructions by which the two values can be made to correspond to BSR can implement a decoder capable of interpreting and executing instructions without the need for decryption thereof. FIG. 18 shows a many-to-one correspondence table of instruction codes and instructions which is obtained by converting the correspondence table of instruction codes and instructions shown in FIG. 17 such that the encrypted instruction codes can be interpreted directly without decryption. 
     FIG. 5 shows an embodiment of an adder according to the present invention, which comprises full adders  501 ,  502 , and  503  that are capable of operating on encrypted data directly. The encrypted full adder  501  uses one bit of first input data AO ( 511 ), the key bit Ak ( 512 ) of the first input data, one bit of second input data B0 ( 513 ), the key bit Bk ( 514 ) of the second input data, and a third encryption bit Rk ( 510 ) which is used to encrypt the result of the add operation and the carry input, and to generate an encrypted result R0 ( 515 ) of the add operation and an encrypted carry Cout ( 516 ) to the next bit. An embodiment of the encrypted full adder  501  for encryption that inverts bits when the encryption bit is ‘1’ is shown in FIG.  8 . This type of encrypted full adder can be implemented by a logic circuit other than that shown in FIG. 8, and it can also be embodied with a circuit synthesized according to the truth table shown in FIG.  6 . 
     An embodiment of the encrypted full adder  501  for encryption that inverts bits when the encryption bit is ‘0’ is shown in FIG.  9 . This type of encrypted full adder can be implemented by a logic circuit other than that shown in FIG. 9, and it can also be embodied with a circuit synthesized according to the truth table shown in FIG.  7 . 
     FIG. 10 shows the second embodiment of the present invention. This embodiment performs arithmetic operations and instruction interpretation using an arithmetic unit  406  and an instruction decoder  404  that operate on conventional unencrypted data and instruction codes. A decoder  1006  is placed at the input point of the instruction decoder  404  such that the decrypted values are input. Similarly, this embodiment also links decoders  1007  and  1008  to each operation input point of the arithmetic unit  406 , decrypts data therein, then performs operations on the decrypted data in the arithmetic unit  406 , encrypts the results with encryption keys generated in the key generator  1009  in an encoder  1005 , and outputs the encrypted results to the internal bus  402 . The register  403  can store encrypted n-bit data and m-bit key information that was used for the encryption. FIG. 11 shows an embodiment of an encryption unit and a decryption unit for encryption algorithms used in the present invention, which uses 1-bit key bit  1101  and data bits  1104  as input to perform encryption and decryption. In this embodiment, the encryption and decryption units are implemented by the same circuit. FIG. 12 shows an embodiment in which a pair of key bits is used for encryption. 
     FIG. 13 is the third embodiment of the present invention. In this embodiment, a RAM  1305  and a ROM/EEPROM  1304  are linked to the CPU  401  through an external bus  1301 . The RAM  1305  can store both data encrypted in an encoder  1005  in the CPU  401  and an encryption key. The ROM/EEPROM  1304  can also store both the encryption key and data, and ROM data is encrypted in advance such that it can be decrypted by decoders  1006 ,  1007 , and  1008  in the CPU  401 . This structure has encrypted data everywhere except inside the arithmetic unit  406  and the instruction decoder  404  thereby discouraging external attacks. 
     FIG. 14 shows the fourth embodiment of the present invention. The RAM  1305  is linked to the CPU  401  through the external bus  1301 . The RAM  1305  can store data that was encrypted by the encoder  1005  and encryption keys. The ROM/EEPROM  1304  is linked to the external bus  1301  through an encoder  1402  that performs encryption on the true data stored in the ROM/EEPROM  1304  according to keys generated in a key generator  1403  that generates encryption keys automatically. When the CPU  401  reads data, the data is encrypted in the encoder  1402 . This embodiment provides an advantage in that ROM data sent to the external bus is encrypted. Therefore, if the same data is sent repeatedly, the data acquires different key information and its expression does not become a fixed value such that analysis by an external attacker is impeded. This embodiment can avoid placing key information in ROM data thereby resulting in reduced ROM area. Of course, it is also possible to encrypt ROM data in advance and allow the encoder  1402  to perform an operation similar to converting the key used for encryption. 
     FIG. 15 shows the fifth embodiment of the present invention. The RAM  1305  and ROM  1304  are linked to the CPU  401  through the external bus  1301 . RAM data is encrypted by a predetermined method. In writing RAM data, first, data encrypted using a type I cryptographic algorithm in the CPU is decrypted in a decoder  1503 , then the decrypted data is re-encrypted in a RAM data encoder  1506  into a type II encrypted data, then the type II encrypted data is output to the external bus  1301  and written into the RAM  1305 . In reading RAM data, the type II encrypted data is read from the RAM  1305 , then decrypted by a decoder  1504 . Thereafter, the data is encrypted with the type I cryptographic algorithm in the encoder  1502 , then sent to the internal data bus. ROM data is encrypted by a predetermined method, decrypted by a ROM data decoder  1505 , then encrypted by an encoder  1502  with a key generated by a key generator as in the case of ROM data. This method can encrypt key information without storing the type I keys in RAM or ROM by appropriately selecting methods of encrypting and decrypting RAM and ROM data. 
     FIG. 16 shows the sixth embodiment of the present invention, in which the disposition of the RAM decoder  1504  and the ROM data decoder  1505  and the disposition of the encoder  1502  using the cryptographic algorithm in the CPU is interchanged. The embodiment in FIG. 15 first decrypts data that was encrypted by a RAM data encryption method to restore it to plaintext data (i.e., true data), then encrypts the plaintext data according to the CPU internal encryption method (i.e., twice encryption). The embodiment in FIG. 16 appropriately selects an encryption method, further encrypts data that was encrypted by a RAM data encryption method by using the CPU&#39;s internal encryption method (i.e., double encryption), and decrypts the resultant data by a decoder adopting a decryption method corresponding to the RAM data encryption method thereby making it possible to obtain the data encrypted by the CPU&#39;s internal encryption method. One possible cryptographic method that can be applied in this cryptographic algorithm is the Vernum cipher (A primer of cryptography written by Eiji Okamoto, published by KYORITSU SHUPPAN). Accordingly, no plain text data is in existence during the transition between the type I encrypted data and the type II encrypted data in the six embodiment. 
     According to the embodiments of the present invention, it is possible to provide information processing devices with higher security. It is also possible to provide IC card components and information processing systems with higher security. 
     The principles, preferred embodiments and modes of operation of the present invention have been described in the foregoing specification. However, the invention which is intended to be protected is not limited to the particular embodiments disclosed. The embodiments described herein are illustrative rather than restrictive. Variations and changes may be made by others, and equivalents employed, without departing from the spirit of the present invention. Accordingly, it is expressly intended that all such variations, changes and equivalents which fall within the spirit and scope of the present invention as defined in the claims, be embraced thereby.