Cryptography – Particular algorithmic function encoding
Reexamination Certificate
1999-08-27
2004-05-04
Barrón, Gilberto (Department: 2132)
Cryptography
Particular algorithmic function encoding
C380S059000, C380S037000
Reexamination Certificate
active
06731754
ABSTRACT:
BACKGROUND OF THE INVENTION
This continuation-in-part application claims the benefit of U.S. Application Ser. No. 08/879,041 filed on Jun. 19, 1998, for the subject matter that both contained in this application and 09/879,041.
1. Field of Invention
This invention relates to a coding method, including a decoding method, for transmitting and storing the content of plain-text in the form of digital information while securing the secrecy thereof. More particularly, the invention relates to a chain coding method, wherein a Latin square, which is a conversion table, is used as a cipher key for coding and decoding, and a plain-text signal, which is used as a data to be input, consisting of a predetermined number of bits (even 1 bit will do), is used as a unit block for feedback and feedforward decoding.
2. Description of Related Art
There have been various proposed coding methods used as the means for securing secrecy of data. The typical examples of such methods are DES (Data Encryption Standard) of the United States and FEAL-8 developed by NTT (Nippon Telegraph and Telephone Corporation).
These methods use a kind of block cipher that codes or decodes input data by block unit under the control of a secret coding key and obtains secrecy-secured code (cipher) by repeating conversion.
However, such methods have a shortcoming that, as long as the same key is used, the same coding block (hereinafter referred to simply as a coded signal) is obtained from the same plain-text block (hereinafter referred to as a plain-text signal).
Such a shortcoming does not immediately allow easy analysis of code; however, when many of the same coded signals are used for coding a large amount of input data, this shortcoming may suggest some hint how to analyze such code.
For overcoming such a shortcoming, a feedback coding method as shown in
FIG. 10
has been adopted. In this feedback coding method, in the coding stage A, in generating a coded signal C
i
from a plain-text signal P
i
(block No. i=1, 2, . . . ) using a key, a preceding coded signal C
i−1
is used as a feedback signal C
b
. This feedback signal C
b
and the plain-text signal P
i
are combined to form the coded signal C
i
. In the decoding stage B, a plain-text signal P
i
is produced from the coded signal C
i
using the same key, a preceding coded signal C
i−1
is used as a feedforward signal C
1
and this feedforward signal C
f
and coded signal C
i
are combined to produce the plain-text signal P
i
by decoding.
IV is an initial value signal used for coding and decoding initial plain-text signal P
i
and decoding the coded signal C
i
. For IV, a predetermined value is used (the value of the initial value signal IV should be assigned in advance for both the coding stage A side and decoding stage B side or assigned together with plain-text signal P
i
).
Given below are the numerical expressions of the above processes.
Coding Stage A Side:
1st block (i = 1)
V
i
= g(IV)
C
i
= xor (P
i
, f
k
(V
i
))
2nd and subsequent blocks (i = 2,..)
V
i
= g (C
i-1
)
C
i
= xor (P
i
, f
k
(V
i
))
Decoding Stage B Side:
1st block (i = 1)
V
i
= g(IV)
P
i
= xor (C
i
, f
k
(V
i
))
2nd and subsequent blocks (i = 2,..)
V
i
= g(C
i-1
)
P
i
= xor (C
i
, f
k
(V
i
))
In the above numerical expressions, a coding function f represents a predetermined coding algorithm such as DES and FEAL-8, and function g represents a specific discrete function defined for all the coded signals C
i
. For example, when the simplest identity function is chosen, V
i
=C
i−1
, the operation “xor”, performed by arithmetic unit
5
, combines a plain-text signal P
i
and a feedback signal C
b
for the purpose of coding or combines coding signal C
i
and feedforward signal C
f
for the purpose of decoding. This “xor” operation is the operation for “exclusive or” (addition by modulo
2
) and is also a mathematically reversible operation.
Further simplifying the above numerical expressions, we obtain
Coding Stage A Side:
1st block (i = 1)
C
i
= xor (P
i
, IV)
2nd and subsequent blocks (i = 2,...)
C
i
= xor (P
i
, C
i-1
)
Decoding Stage B Side:
1st block (i = 1)
P
i
= xor (C
i
, IV)
2nd and subsequent blocks (i = 2,...)
P
i
= xor (C
i
, C
i-1
)
To code the 1st data block, the “xor” operation is performed on the plain-text signal P
i
(binary number of 1 digit) and the initial value signal IV (binary number of 1 digit) to obtain coded signal C
i
. To code the 2nd and subsequent data blocks, the “xor” operation is performed on the plain-text signal P
i
and the preceding coded signal C
i−1
(binary number of 1 digit), which is the feedback signal C
b
, to obtain coded signal C
i
.
Further, to decode the 1st data block, the “xor” operation is performed on the coded signal C
1
with the initial value signal IV given as a feedforward value, to obtain decoded plain-text signal P
i
.
For decoding the 2nd and subsequent data blocks, the “xor” operation is performed on the coded signal C
1
with the preceding signal C
i−1
given as a feedforward signal C
f
to obtain decoded plain-text signal P
1
.
As described above, according to the prior art, the “xor” operation always ends in a fixed value, that is, when the “exclusive or” (addition by modulo
2
) operation is applied to the plain-text signal P
i
and the input value of preceding coded signal C
1-1
or applied to the input values of the coded signal C
i
and preceding signal C
i−1
.
However, according to the prior art described above, when the “xor” operation, that is, the “exclusive or” operation (addition by modulo
2
) for coding/decoding by feedback coding method is performed, the result of the operation for the input value and the feedback value always ends in fixed values due to the addition by modulo
2
. In other words, for the input value data blocks (binary number of 1 digit), feedback value data blocks and output value data blocks, the output value is 0 when both the input value and the feedback value is 0 or 1, while the output value is 1 when the input value and the feedback value are 0 and 1, or 1 and 0. Therefore, the result of the output is fixed for each bit, thereby causing a problem that such a system may provide some hint for data decoding.
Thus, this invention is devised to solve such a problem of the prior art by accomplishing a technical task, that is, to freely vary the values resulting from the “xor” operation for coding/decoding by the feedback coding method or to freely vary the values resulting from the “exclusive or” operation (addition by modulo
2
), thereby making it extremely difficult to decode the coded input data.
SUMMARY OF THE INVENTION
In the following explanation, natural numbers are used as the elements of the matrix of a Latin square, but the Latin alphabet may also be used.
To accomplish the aforesaid technical task, an illustrative embodiment of the invention includes a coding method and a feedback coding method. The coding method provides a secrecy security system designed for transmitting and storing the content of plain-text converted into digital information while securing the secrecy of the content thereof. The feedback coding method codes using a plain-text signal having a predetermined number of bits as a unit block. An illustrative embodiment of the invention also uses a coding Latin square as a conversion table for a converter in conversion stage of the feedback coding method, uses the plain-text signal and the initial value signal for the initial conversion, and uses a plain-text signal and coded signal converted from a preceding plain-text signal as a feedback signal to specify positions of the rows and columns of the coding Latin square and to use the value of the specified position as coded signal. A feedforward decoding method for decoding by unit block of coded signal may be used. Additionally, a decoding Latin square is used as a conversion table, when the decoding Latin square corresponds to t
Barrón Gilberto
Lanier Benjamin E.
Netcomsec Co., Ltd.
Oliff & Berridg,e PLC
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