Data encrypting/decrypting conversion methods and...

Cryptography – Particular algorithmic function encoding

Reexamination Certificate

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Reexamination Certificate

active

06606385

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to encryption/decryption techniques for encrypting/decrypting digital data transferred among computers, home-use-destined electric/electronic equipment and the like.
In the digital home-use-destined electric/electronic equipment promising further development in the future, the encryption/decryption technology is indispensably required for preventing or disenabling unauthorized or illegal copying of digital data.
As the encryption technology known heretofore there has already been proposed what is known as the RC
5
encryption algorithm in which data-dependent cyclic shift operation (also called end-around, circular or ring shift operation) is adopted, as is disclosed in R. L. Rivest: “The RC
5
Encryption Algorithm”, FAST SOFTWARE ENCRYPTION, 2nd International Workshop, Springer-Verlag, (1995). The RC
5
encryption algorithm is designed such that processed data length (i.e., the length of data to be processed) of w bits, secret key length of b bytes and processing round number r are variable. For having better understanding of the concept underlying the present invention, the RC
5
encryption algorithm will be explained below in some detail.
For the text data which has not undergone any encrypting conversion processing (hereinafter referred to simply as the plain-text data) and which is given by “L[
0
] and R[
0
]”, where L[
0
] represents more significant w/2 bits of the processed data length of w bits, and R[
0
] represents least significant w/2 bits thereof, there can be obtained through the RC
5
encryption algorithm an encrypted text “L[2r+1], R[2r+1]” which can be derived through the procedure defined by the following expressions:
L
[
1
]=
L
[
0
]+
S
[
0
],
R
[
1
]=
R
[
0
]+
S
[
0
],
L[N
+1
]=R[N],
where 1≦N≦2r, and
R[N
+1]=((
L[N] EOR R[N
])<<<
R[N
])+
S[N
+1],
where 1≦N≦2r.
In the above expressions, the repetition represented by “1≦N≦2r” is illustrated for “N” in
FIG. 23
of the accompanying drawings. In conjunction with the above definition, arithmetic expression “A +B” in general represents a remainder resulting from division of a sum of “A” and “B” by the x-th power of “2”, and operation symbol “EOR” represents an exclusive-OR on a bit-by-bit basis. Further, expression “x<<<y” in general represents arithmetic operation of shifting repetitionally “x” to the left (leftward shift) by least significant log(w) bits of “y”. According to the RC
5
encryption algorithm, twice repetition of the arithmetic operation shown in
FIG. 23
is referred to as one stage operation. The encrypted text can be generated by repeating the one-stage operation r times.
Major features of the RC
5
encrypting algorithm can be seen in that the length of the secret key is variable on a user-by-user basis and the cyclic shift can be varied or changed dynamically. However, because such algorithm structure is adopted that the dynamic change of the cyclic shift depends on the data for encryption the RC
5
encryption algorithm suffers a drawback of not being sufficiently hard against the selective plain-text attack, one of the cryptanalysis methods. For more particulars in this respect, reference should be made to Lar R. Knudsen, Willi Meier: “IMPROVED DIFFERENTIAL CRYPTANALYSIS ON RC
5
”, Advances in Cryptology-CRYPTO'96, Springer-Verlag, 1996.
SUMMARY OF THE INVENTION
In the light of the state of the art described above, it is an object of the present invention to provide encrypting conversion method and apparatus which are capable of controlling dynamically the cyclic shift independent of data for conversion and additionally capable of realizing the encrypting conversion with highly enhanced randomness with a simplified system configuration.
Another object of the present invention is to provide method and system for decrypting the encrypted text.
Yet another object of the present invention is to provide a data communication system in which the encrypting/decrypting conversion techniques taught by the invention are adopted.
In view of the above and other objects which will become apparent as the description proceeds, there is provided an encryption system or apparatus for generating a encrypted text data of a predetermined length as an encrypted block from a plain-text data and key or keys as inputted, which apparatus includes:
(1) at least two fixed cyclic shift processing modules for cyclically shifting data leftward or rightward,
(2) a cyclic shift processing selecting module for selecting the fixed cyclic shift processing means, and
(3) a cyclic shift processing sequence determining module for determining an order or sequence for the selection of the cyclic shift processing selecting module on the basis of data for determining the shift number selecting sequence.
Thus, there is provided according to an aspect of the present invention an encrypting conversion apparatus which receives as inputs thereto at least one key and plain-text data to thereby output encrypted text data, which apparatus can be implemented in hardware fashion or software fashion and includes a cyclic shift processing module for determining a shift number on the basis of data for determining a shift number selecting sequence, a module for dividing inputted plain-text data into first data and second data and setting the first data as data L[
1
] while setting the second data as data R[
1
], at least one stage of an encrypting conversion processing module for receiving as inputs thereto data L[N] and R[N] to thereby output data L[N+1] and data R[N+1], wherein the encrypting conversion processing module is so arranged as to perform at least once for the data L[N] a conversion processing by using the key and a cyclic shift processing by means of the cyclic shift processing module, respectively, to thereby generate data X and wherein a value derived from arithmetic operation of the data R[N] and the data X is set as the data L[N+1] while the data L[N] being set as the data R[N+1], and a module for outputting a combination of two output data from a final stage of the encrypting conversion processing module as an encrypted text.
In a mode for carrying out the invention, the cyclic shift processing module may be so arranged as to include at least two different fixed cyclic shift processing modules each for performing cyclic shift by a fixed number of bits leftward or alternatively rightward, a cyclic shift processing selecting module for selecting the fixed cyclic shift processing module, and a cyclic shift processing sequence determining module for determining a selecting sequence for the cyclic shift processing selecting modules on the basis of data for determining the shift number selecting sequence.
In another mode for carrying out the invention, the data for determining the shift number selecting sequence may be generated on the basis of the aforementioned key.
Further, according to another aspect of the present invention, there is provided a decrypting conversion apparatus which receives as inputs thereto at least one key and encrypted text data to thereby output plain-text data, which apparatus can be implemented hardware-wise or softwarewise and includes a cyclic shift processing module for determining a shift number on the basis of data for determining a shift number selecting sequence, a module for dividing inputted encrypted text data into first data and second data and setting the first data as data L[
1
] while setting the second data as data R[
1
], at least one stage of a decrypting conversion module for receiving as inputs thereto data L[N] and R[N] to thereby output data L[N+1]

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