JP4294500B2 - Stream cipher evaluation apparatus and stream cipher evaluation method - Google Patents

Description

  The present invention relates to a stream cipher evaluation device and a stream cipher evaluation method.

  In recent years, various services using computers have been provided. In this service providing system, encryption is usually used for the purpose of concealing communication. As one of the encryption methods, a common key encryption method that uses one key for both encryption and decryption is known. Common key cryptosystems are broadly divided into the most common block ciphers and stream ciphers. Recently, stream ciphers have received attention because they have the advantage of faster processing speed than block ciphers. I'm starting.

There are various methods for evaluating the cipher, and for example, a method for evaluating the cost required to solve the cipher is known (for example, see Patent Document 1). As another evaluation method, one that evaluates the security of a cipher by verifying the periodicity of the cipher is known. Conventionally, many stream ciphers have the characteristic that they have a very long period and include a non-linear part. Therefore, when verifying the periodicity of a stream cipher, it was obtained by operating the stream cipher algorithm to be evaluated. A random number sequence (data sequence of a key for stream encryption) is sampled, and the periodicity of the sampled data is examined.
Japanese Patent Laid-Open No. 11-231773

  However, in the conventional security evaluation technology based on the periodicity verification of the stream cipher described above, sampling data is used in the periodicity verification. Therefore, an error due to sampling may occur in the verification result. There is a risk that the reliability of the sex evaluation is lowered.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a stream cipher evaluation apparatus and a stream cipher evaluation method capable of improving the performance of periodic verification of stream ciphers. .

  In order to solve the above-described problem, a stream cipher evaluation apparatus according to the present invention includes an algorithm input unit that inputs a stream cipher algorithm, and a reduced algorithm that reduces the scale while maintaining the algorithm structure of the input stream cipher algorithm. A reduced algorithm generating means for generating, a reduced algorithm executing means for generating a random number as a key for stream encryption by operating the reduced algorithm, and outputting the random number sequence; and a periodicity for verifying the periodicity of the random number sequence And a verification means.

  In the stream cipher evaluation apparatus according to the present invention, the reduced algorithm creating means divides the input stream cipher algorithm into a non-linear transformation part and a part for inputting data to the non-linear transformation part. It is characterized by performing reduction.

  The stream cipher evaluation method according to the present invention includes a step of inputting a stream cipher algorithm, a step of creating a reduced algorithm with a reduced scale while preserving the algorithm structure of the input stream cipher algorithm, and operating the reduced algorithm And a process of generating a random number as a key for stream encryption and a process of verifying the periodicity of the random number sequence.

  According to the present invention, the periodicity is verified by reducing the scale without changing the algorithm structure of the original stream encryption algorithm, so the verification result is equal to the verification of the periodicity of the original stream encryption algorithm. It will be a thing. Thereby, the performance improvement of the periodicity verification of a stream cipher can be aimed at. As a result, it is possible to obtain an excellent effect that contributes to improving the reliability of the safety evaluation of the stream cipher.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a block diagram showing a configuration of a stream cipher evaluation apparatus according to an embodiment of the present invention. In FIG. 1, an algorithm input unit 1 inputs a stream cipher algorithm via a GUI (Graphical User Interface) unit 5. The reduction algorithm creation unit 2 creates a reduced version of the input stream encryption algorithm. The reduction algorithm execution unit 3 operates the reduction algorithm to generate a random number that is a key for stream encryption, and outputs this random number sequence. The periodicity verification unit 4 verifies the periodicity of the random number sequence and outputs the verification result via the GUI unit 5.

FIG. 2 is a block diagram illustrating an example of a stream encryption algorithm. This stream cipher algorithm includes a non-linear conversion unit 11 called S-Box and a linear feedback shift register (LFSR) 12 that inputs data to the S-Box 11.
The LFSR 12 includes a shift register including six registers R1 to R6 and one exclusive OR calculator (XOR) 13 that outputs feedback data. The register length of each of the registers R1 to R6 is 16 bits, for example. The linear feedback shift register (LFSR) is known to generate pseudo random numbers, and pseudo random numbers are output from the LFSR 12 to the S-Box 11.
The S-Box 11 has a template (replacement table) for realizing nonlinear transformation. Then, the input data from the LFSR 12 is nonlinearly converted using the template, and the converted data is output as a random number for the key.

Next, the operation of the reduced algorithm creating unit 2 will be described by taking the stream encryption algorithm of FIG. 2 as an example.
First, the reduction algorithm creation unit 2 divides the input stream encryption algorithm of FIG. 2 into a non-linear transformation part (S-Box 11) and a part for inputting data to the non-linear transformation part (LFSR 12). Next, each part is reduced.

  For the data input part (LFSR12), the number of bits of data input to the nonlinear conversion part (S-Box 11) is reduced while the entire structure is preserved. For example, when the register length of each of the registers R1 to R6 of the LFSR 12 is 16 bits, the register length is changed to 8 bits.

  The non-linear conversion portion (S-Box 11) is replaced with one that performs a small-scale non-linear conversion having a similar non-linearity related to the non-linear conversion. For example, when the original template is for performing non-linear conversion of 16-bit data, the original template is changed to a non-linear conversion of 8-bit data. For this purpose, various templates are prepared in advance in the reduction algorithm creation unit 2.

  Next, the reduction algorithm creation unit 2 outputs the reduced algorithm to the reduction algorithm execution unit 3.

Next, the operation of the reduction algorithm execution unit 3 will be described.
The reduction algorithm execution unit 3 operates the reduction algorithm input from the reduction algorithm creation unit 2. For example, an operation clock is supplied to the reduction algorithm of the stream cipher algorithm of FIG. 2, thereby operating the reduced LFSR to input data to the reduced S-Box every clock, and reducing the reduced S-Box template. The input data is nonlinearly transformed to output a random number. Next, the reduction algorithm execution unit 3 sequentially outputs the random numbers output by the operation of the reduction algorithm to the periodicity verification unit 4 as a random number sequence.

Next, the operation of the periodicity verification unit 4 will be described with reference to FIG.
The periodicity verification unit 4 verifies the presence or absence of periodicity with respect to a finite number of random number sequences input from the reduction algorithm execution unit 3.
In FIG. 3, a random number sequence a1, a2, a3, a4,..., AN is input from the reduction algorithm execution unit 3. The periodicity verification unit 4 uses four variables: a pattern start offset (pos), a pattern length (len), a comparison destination start offset (tgp), and a repetition number (rep).

  First, the periodicity verification unit 4 determines whether or not the first random number a1 matches the next random number a2 (step S1). The values of the variables at the first time point are pos = 0, len = 1, tgp = 1, and rep = 0. Next, when the random numbers a1 and a2 match, in step S2, it is determined whether or not the random number a1 matches the next random number a3, and 1 is added to each of tgp and rep, and tgp = 2 and rep = 1. And Next, if the random numbers a1 and a3 match, in step S3 it is determined whether or not the random number a1 matches the next random number a4, and 1 is added to each of tgp and rep, and tgp = 3 and rep = 2. And Thereafter, the same process is repeated until a random number different from the random number a1 appears. If a different random number appears, the same processing as step S4 shown below is performed.

  If the random numbers a1 and a3 do not match in step S2, the same random numbers a1 and a2 and the different random number a3 are connected in the order of appearance in step S4, and the connected random numbers a1 | a2 | a3 after this connection (| Represents a concatenation) and the next equal number of concatenated random numbers a4 | a5 | a6 are determined to match. At this time, len is added to 3, and tgp is added by 1, resulting in 2. If they match, the same processing is repeated until the same number of connected random numbers appear, as in steps S2 and S3. On the other hand, if they do not match, the connected random numbers are connected, and the same processing as in step S4 is performed.

  If the random numbers a1 and a2 do not match in step S1, the different random numbers a1 and a2 are concatenated in order of appearance in step S5, and the concatenated random number a1 | a2 after the concatenation and the next same number of concatenated random numbers a3. Judge coincidence with | a4. At this time, len is set to 2 and tgp is set to 1 by adding 1. If they match, the same process is repeated as in steps S6 and S7 until the same number of connected random numbers appear, as in steps S2 and S3. On the other hand, if they do not match, the connected random numbers are connected, and the same processing as in step S4 is performed.

  When the end of the random number sequence is reached, the connected random number with repeatability measured so far and its repetition cycle are output as the verification result. Here, if there is a connected random number that has been repeated just before the end, it is determined that there is periodicity. If a fraction occurs at the end of the random number sequence, the fractional part is not processed.

  Next, the process returns to step S1, and pos = 1, len = 1, tgp = 2, and rep = 0, and the same processing is repeated from the random number a2. Thereafter, every time the end of the random number sequence is reached, the process returns to step S1 and pos and tgp are shifted one by one, and the same process is repeated from the corresponding process start random number.

  According to the above-described embodiment, the periodicity is verified by reducing the scale without changing the algorithm structure itself of the original stream cipher algorithm, so that the verification result (repeated concatenated random number and its repetition period, period) The determination result of the presence / absence of the property is equal to the verification of the periodicity of the original stream encryption algorithm. Thereby, the performance improvement of the periodicity verification of a stream cipher can be aimed at. As a result, it is possible to obtain an excellent effect that contributes to improving the reliability of the safety evaluation of the stream cipher.

  Note that the stream cipher evaluation apparatus according to the present embodiment may be realized by dedicated hardware, or is configured by a computer system such as a personal computer, and implements each function of the apparatus shown in FIG. The function may be realized by executing a program to do so.

In addition, an input device, a display device, and the like (none of which are shown) are connected to the stream cipher evaluation device as peripheral devices. Here, the input device refers to an input device such as a keyboard and a mouse. The display device refers to a CRT (Cathode Ray Tube), a liquid crystal display device or the like.
The peripheral device may be connected directly to the stream cipher evaluation device or may be connected via a communication line.

  The embodiment of the present invention has been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and includes design changes and the like within a scope not departing from the gist of the present invention.

It is a block diagram which shows the structure of the stream encryption evaluation apparatus which concerns on one Embodiment of this invention. It is a block diagram which shows an example of a stream encryption algorithm. It is a figure for demonstrating operation | movement of the periodicity verification part 4 shown in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Algorithm input part, 2 ... Reduction algorithm preparation part, 3 ... Reduction algorithm execution part, 4 ... Periodicity verification part, 5 ... GUI part.

Claims (3)

An algorithm input means for inputting a stream cipher algorithm;
A reduced algorithm creating means for creating a reduced algorithm that reduces the scale while preserving the algorithm structure of the input stream cipher algorithm;
To operate the reduction algorithm key become random number for the stream cipher generated by the reduction algorithm execution means for outputting the generated random number sequence, as a random number sequence,
Periodicity verification means for verifying the periodicity of the random number sequence;
A stream cipher evaluation apparatus comprising:   The reduction algorithm creating means divides the input stream encryption algorithm into a non-linear conversion part and a part for inputting data to the non-linear conversion part, and performs reduction for each part. 2. The stream cipher evaluation apparatus according to 1. A stream cipher evaluation method in a stream cipher evaluation device comprising an algorithm input means, a reduction algorithm creation means, a reduction algorithm execution means, and a periodicity verification means,
The algorithm input means inputs a stream cipher algorithm;
The reduction algorithm creating means creating a reduced algorithm whose scale is reduced while preserving the algorithm structure of the input stream encryption algorithm;
The reduction algorithm executing means operates the reduction algorithm to generate a random number as a key for stream encryption, and sequentially outputs the generated random number as a random number sequence ;
The periodicity verification means verifies the periodicity of the random number sequence;
A stream cipher evaluation method comprising:

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