JP2001211166A - Chaos block ciphering - Google Patents

Abstract

PROBLEM TO BE SOLVED: To adopt EXOR operation between an input code and cipher key (permutation) as preprocessing and postoprocessing for chaos block ciphering to obtain a result which is a combination of the final purpose of the ciphering to make cipher codes to be so difficult that they cannot be deciphered and avoiding the cipher system to have a large load and become less reliable by making the complicated combination more complicated. SOLUTION: A common key system requires a common cipher key to generate a cipher code as well as to generate a deciphering code. The cipher key is attached to the cipher code and stored, or stored separately. For chaos block ciphering, the cipher key is given as a permutation of digital blocks and EXOR operation between an input sentence and the cipher key is added as preprocessing and then effectively usable to cancel a featured distribution of block cipher codes. The permutation which should be stored and held is used for preprocessing and postprocessing for the input sentence to improve the robustness of the cipher codes without making the constitution of the system complicated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[Industrial application] In many cases, information on characters, data, graphs, tables, and the like is stored in an ASCII code file. This is the result of the spread of word processing input from the keyboard and the creation of diagrams using a mouse. The basic unit of the filed information is a 1-byte (8-bit) block.

[0002] Cryptography is used to guarantee correct access to information and to prevent unauthorized access. The complexity of the system due to the use of cryptography must be avoided. Chaos block cipher (quantization resolution n =
8, step τ = 4) performs encryption by dividing a unit of information (1 byte = 8 bits) into two 4-bit blocks while leaving the feature that a large number of encryption keys can be generated.
This is an extremely easy encryption restoration method.

[0003] The present invention does not cover the case where the amount of information of the original text to be encrypted is extremely small, such as the prevention of unauthorized access to an electronic key or a magnetic card. PC (personal us
It is to be noted that information (2 kB / A4.1 page) whose ecomputer is typified by a document is encrypted with a unique encryption key and stored in a memory.

[0004]

2. Description of the Related Art A chaos block cipher is a permutation-type cipher that makes use of the characteristics of chaos (initial value sensitivity, branching, divergence, convergence). The components are an encryption key, an encryption table, and a restoration table. The flexibility of chaos makes it possible to issue almost unlimited encryption keys, making the creation of encryption and recovery tables flexible.

[0005] In communications between computers or between devices equipped with a CPU, transmission codes are often subject to various restrictions. It is a feature of chaos block cipher that it can flexibly cope with such restrictions.

[0006]

The distribution of character codes has an inherent bias. When typing English text from the keyboard, the space bar is repeatedly pressed. When entering Japanese from the keyboard, the Kana-Kanji conversion key is repeatedly struck. If you look at the distribution of character codes, you can easily tell whether the original text is English or Japanese. If the encryption code is generated using the chaos block cipher as it is, the distribution of the encryption code naturally reflects the bias. This creates the need for pre-processing.

[0007] The ideal distribution of the cryptographic code is a well-diffused distribution, similar to the distribution of a chaotic time series (which is equivalent to a cryptographic table) quantized such that the distribution of the cryptographic code is linear. This is the case. The distribution of the internal state of chaos is never uniform, no matter how long it is observed. Not uniform,
The bias inherent in chaos is left fair, making it difficult to determine the original text. This is a fundamental difference from encryption using random numbers.

[0008]

The preprocessing condition is that the processing can be executed at a high speed, and the information amount after the preprocessing does not increase more than the information amount of the original sentence. For that purpose, the operation must be a simple basic operation that today's digital computers are good at. The first candidate is an EX-OR (exclusive OR) operation. To perform an EX-OR operation on the original text, a PN (pseudo noise) signal is required, and no spreading is performed to prevent an increase in the amount of information. That is, only one-to-one EX-OR operation between the original text and the PN signal period is performed.

The encryption key, which is a component of the chaotic block cipher, is a permutation of 0 to 15 when a 4-bit block cipher with n = 8 and τ = 4. In order to facilitate system management and enable centralized management with small data, the in-phase transform quantization n of the logistic map is used.
In most cases, it is sampled from pseudo random numbers generated when = 1. When used as an encryption key, the auto-correlation and cross-correlation are checked in advance. The permutation of 0 to 15 assured through such selection is set as a condition that can be said to be optimal even as a one-to-one PN signal.

Normally, in document processing of chaotic block cipher belonging to the common key cryptosystem, an encryption key is stored attached to an encryption code. Of course, basic operations have been added, and the encryption key is embedded in the encryption code. Utilizing the encryption key as a PN signal for preprocessing means using an existing key for another purpose of preprocessing. The system is not complicated by the pre-processing.

When the pre-processing is performed, the post-processing must of course also be performed. If EX-OR is adopted as the operation, the same processing can be repeated, which is simple and convenient.

[0012]

The generation of chaos using the one-dimensional mapping may be performed by an electronic circuit (integrated circuit) or by calculation of a logistic map using a digital computer. In the present case, a case of calculation using a logistic map will be specifically described. Logistic map x (t +
1) = 4x (t) {1-x (t)} is calculated by binary decimal 5
This is performed with 2 bits (double precision). In-phase transform quantization Y (t) = [2 / π · arcsin (x (t) ^1/2 ) with the same precision]
²ⁿ ], n = 8, and calculate the time series Y (t)
Find −t. When n = 8, t _max = 2 ¹⁶ = 65
536.

Although the chaos block cipher can be designed using the time series Y (t) -t as the cipher table as it is, if the step to the past or the future is limited to τ = 4, the encryption using degeneracy is performed. Can organize tables. That is, the time series Y (t) -t is replaced with the past time series Y (t-4)-regarding the 16 degenerated quantum boxes.
Re-arranging to t enables faster search.

16 degenerated quantum boxes and character code 4
The correspondence with the bit block is the permutation of the encryption keys 0 to 15. Not only one encryption key but also two or more encryption keys may be used in combination. The structured design of the encryption key means that system management can be structured and performed.

Even when one encryption key is used for one document, when issuing the next encryption key to the next document, it is better that the two encryption keys are sufficiently separated from each other. The cross-correlation must be examined to ensure that. 2
Even when a pair of encryption keys are used, it is preferable that the mutual distance between the encryption keys is long.

A 64-bit encryption key having such features is a PN signal in which "0" and "1" are appropriately mixed. Although n = 8 and τ = 4 seem to limit the flexibility of chaos too much, it can be said to be a reasonable choice for managing documents in units of characters.

[0017]

When n = 8 and τ = 4, 16 types of 8-bit codes are distributed to 16 degenerate quantum boxes as standard. This distribution is not absolute. It is free to add operations such as exchanges and shifts. Once fixed as an encryption table, when the same document is repeatedly encrypted with the same encryption key, the encryption code will be different each time, but the types of 8-bit codes that appear will be limited.

FIG. 1 shows the distribution of encryption codes when 2000 English characters are encrypted with a certain encryption key. The 16 peaks come from the space key code. Similar results are obtained for Japanese. It is difficult to judge whether the original sentence is English or Japanese by looking at the distribution alone, but this is not a favorable tendency as an encryption code.

For example, if n = 10 and τ = 5, that is, if a chaotic block cipher of a 5-bit block is adopted, character codes are divided in the middle, so that the distribution of the cipher codes does not have the features shown in FIG. Instead, a program for processing the fraction is required, and as a result, extra processing such as counting the number of characters and checking the number of blocks is required.

When installation on a general-purpose CPU such as Z-80 is considered, nothing is more convenient than selecting n = 8 and τ = 4. The encryption key, which is a permutation of 0 to 15, is P
FIG. 2 shows the distribution of encryption codes that have been preprocessed as N signals and then encrypted with a 4-bit block cipher. The peaks dependent on the type of code distributed to the degenerate quantum box disappear. It is equivalent to the state distribution of the chaotic time series quantized by the in-phase transform.

[0021]

The 4-bit block chaos block cipher can be used as an industrial general-purpose CPU, such as a Z-block, even if the encryption key / encryption table / reconstruction table is given as a database or if it is sequentially calculated by inverse calculation of a logistic map. 80 and the like. That is, a PC card can be used. If a personal PC holds such a PC card, data in the PC can be encrypted and managed.

In the 4-bit block process, since one character code is divided into two, the characteristics of the characters are likely to be different from the encryption code, but if the encryption key given in the permutation of 0 to 15 is preprocessed as a PN signal, It is diffused to the original distribution of chaos. The effect of incorporating preprocessing into chaotic block ciphers is significant.

[Brief description of the drawings]

FIG. 1 is a frequency distribution of an encryption code y (t) (n = 8, τ =
4).

FIG. 2 is a frequency distribution (n = 8, τ = 4) of a pre-processed encryption code y (t).

Claims (2)

[Claims] 1. A cryptographic code is generated using a branch to the past of a chaotic time series, which is quantized and observed to be linear as a result of a nonlinear one-dimensional mapping circuit, and proceeds to the same step in the future. In a chaotic block cipher that generates a restoration code, a pre-process of performing an EX-OR operation using an encryption key as a PN signal is added to the original text to be encrypted to generate an encryption code, and the restoration code is subjected to the same processing as the pre-processing. Block cipher characterized by returning to the original text. 2. A bifurcation to the past of a chaos time series (τ = 4), wherein a non-linear one-dimensional map is a logistic map, and a quantization observation is in-phase transformation quantization (n = 8) so as to be linear. ) Is used to generate an encryption code, and in the chaos block cipher in which a restoration code is obtained by proceeding to the same step in the future, an EX-OR in which an encryption key (permutation of 0 to 15) is used as a PN signal in the original text to be encrypted.
A chaotic block cipher characterized by generating a cryptographic code by adding preprocessing to perform an operation, applying the same processing to the restored code as in preprocessing, and returning to the original text.