RU2140710C1 - Process of block ciphering of discrete data - Google Patents

Abstract

FIELD: electric communication, computer engineering, ciphering of data by cryptographic processes and devices. SUBSTANCE: proposed process includes formation of ciphering key in the form of collection of subkeys, breaking of data block into N≥2 subblocks and conversion of subblocks in turn by implementation of two-position operation with subblock and subkey. Process differs from traditional methods by realization of substitution operation with subkey depending on j-th subblock, where j≠i, before implementation of two-position operation with i-th subblock and subkey. EFFECT: increased speed of ciphering. 3 dwg

Description

The invention relates to the field of telecommunications and computer technology, and more particularly to the field of cryptographic methods and devices for encrypting messages (information). In the aggregate of the features of the proposed method, the following terms are used:
- a secret key is a combination of bits known only to a legitimate user;
- the encryption key is a combination of bits used to encrypt information data signals; the encryption key is a removable cipher element and is used to convert a given message or a given set of messages; the encryption key is generated by deterministic procedures for the secret key; in a number of ciphers, the secret key is used directly as the encryption key;
- a cipher is a set of elementary steps for converting input data using a cipher key; the cipher can be implemented as a computer program or as a separate electronic device;
- a subkey is a part of the encryption key used in individual elementary steps of encryption;
- encryption is a process that implements some way of converting data using a cryptographic key, converting data into a cryptogram, which is a pseudo-random sequence of characters from which obtaining information without knowing the encryption key is practically impossible;
- decryption is the opposite of the encryption process; decryption provides information recovery from the cryptogram with knowledge of the encryption key;
- cryptographic strength is a measure of the reliability of information protection and represents the complexity, measured in the number of elementary operations that must be performed to recover information from the cryptogram with knowledge of the conversion algorithm, but without knowledge of the encryption key.

 Known methods for block data encryption, see, for example, the US standard DES [W. Diffie, M.E. Hellman. Security and Immitability: Introduction to Cryptography // TIIER. 1979. T. 67. N 3. S. 87-89], the encryption method according to US patent N 5222139 of June 22, 1993, the code FEAL-1 and the cryptographic algorithm B-Crypt [C. Maftik. Protection mechanisms in computer networks. - M., Mir, 1993. S. 49-52]. In the known methods, the encryption of data blocks is performed by forming an encryption key in the form of a set of subkeys, splitting the converted data block into subblocks, and changing the latter one by one using substitution, permutation, and arithmetic operations performed on the current subblock and the current subkey.

However, the known analogue methods do not possess sufficient resistance to differential cryptanalysis [Berson TA Differential Cryptanalysis Mod 2 ³² with application to MD5 // EUROCRYPT'92. Hungary, May 24-28, 1992, Proceedings. P 67-68], because for all input data blocks for a given conversion step, the same subkey is used unchanged.

в соответствии с формулой B₂: = B₂

Q_i, где 1 ≤ i ≤ 8, после чего над полученным новым значением подблока B₂ выполняют операцию подстановки, затем операцию циклического сдвига влево на одиннадцать бит, т.е. на одиннадцать двоичных разрядов в сторону старших разрядов, а затем на полученное значение B₂ накладывают подблок B₁ с помощью операции поразрядного суммирования по модулю два (⊕) в соответствии с формулой B₂: = B₂ ⊕ B₁. Операция подстановки выполняется следующим образом. Подблок разбивается на 8 двоичных вектора длиной по 4 бит. Каждый двоичный вектор заменяется двоичным вектором из таблицы подстановок. Выбранные из таблицы подстановок 8 4-битовых вектора объединяются в 32-битовый двоичный вектор, который и является выходным состоянием подблока после выполнения операции подстановки. Всего выполняется 32 аналогичных шага изменения подблоков, причем для всех преобразуемых входных блоков данных на фиксированном шаге преобразования подблоков используется один и тот же подключ с неизменным значением.The closest in technical essence to the claimed method of block encryption is the method described in the Russian standard of cryptographic data protection [USSR Standard GOST 28147-89. Information processing systems. Cryptographic protection. Cryptographic Transformation Algorithm]. The prototype method includes generating an encryption key in the form of a sequence of 8 subkeys 32 bits long, splitting the input 64-bit data block into two 32-bit subblocks B ₁ and B _2, and alternately converting the sub-blocks. One step of converting a subblock, such as subblock B ₂ , is to overlay the current subkey Q _i , which is fixed for this step, using the addition operation modulo 2 ³²

according to the formula B ₂ : = B ₂

Q _i , where 1 ≤ i ≤ 8, after which the substitution operation is performed on the new value of the sub-block B ₂ , then the operation of cyclic left shift by eleven bits, i.e. eleven binary digits in the direction of the higher digits, and then the sub-block B _{1 is} superimposed on the obtained value of B ₂ using the bitwise summing operation modulo two (⊕) in accordance with the formula B ₂ : = B ₂ ⊕ B ₁ . The substitution operation is performed as follows. The subblock is divided into 8 binary vectors with a length of 4 bits. Each binary vector is replaced by a binary vector from the lookup table. The 8 4-bit vectors selected from the lookup table are combined into a 32-bit binary vector, which is the output state of the subblock after performing the lookup operation. A total of 32 similar steps are taken to change the sub-blocks, and for all the converted input data blocks, at the fixed step of converting the sub-blocks, the same subkey is used with the same value.

 However, the prototype method has drawbacks, namely, microelectronic encryption devices based on it are expensive and do not provide a high encryption speed [Andreev N.N. On some areas of research in the field of information security // Collection of materials of the international conference "Information Security". Moscow, April 14-18, 1997. M. 1997. S. 96], necessary for the construction of real-time information protection tools. Based on the prototype method, it is very difficult to create devices on the modern element base that provide an encryption speed of more than 10 Mbps. This disadvantage is due to the fact that to ensure resistance to differential cryptanalysis in the prototype method, a large number of operations of four types are used, including substitution operations.

 The basis of the invention is to develop an encryption method in which the input data is converted in such a way as to reduce the number of conversion operations per one bit of input data, while ensuring high resistance to differential cryptanalysis, thereby increasing the encryption speed.

 The problem is achieved in that in the method of block encryption of discrete data, which includes generating an encryption key in the form of a set of subkeys, dividing a data block into N ≥ 2 subunits and alternately converting the subunits by performing a two-step operation on the subunit and the subconnect, new according to the invention is that before performing a double operation on the i-th subunit and subkey on the subkey, a permutation operation is performed depending on the j-th subunit, where j ≠ i.

 Thanks to this solution, the structure of the subkeys used at a given encryption step depends on the data being converted, and thus various modified subkey values are used for various input blocks at this conversion step, which ensures high resistance to differential cryptanalysis while reducing the number of conversion operations performed, which and provides an increase in the speed of cryptographic conversion.

 Below the essence of the claimed invention is explained in more detail by examples of its implementation with reference to the accompanying drawings.

 In FIG. 1 presents a generalized encryption scheme according to the claimed method.

 In FIG. 2 schematically shows the structure of a block of controlled permutations, consisting of a set of the same elementary blocks that implement a permutation of two adjacent binary bits (bits) depending on the control signal u.

 In FIG. 3 shows a block diagram of an elementary controlled switch, which is the basic element of a controlled permutation block. For u = 1, the input bits are not rearranged, i.e. output signals coincide with input signals. For u = 0, the input bits are rearranged.

 In FIG. 4 is a table of input and output signals of an elementary controlled switch with a high potential of the control signal.

 In FIG. 5 is a table of input and output signals of an elementary controlled switch with a low potential of the control signal.

операцию суммирования по модулю 2ⁿ. Жирные сплошные линии обозначают шину передачи n-битовых сигналов, тонкие пунктирные линии - передачу одного управляющего бита, жирные пунктирные линии - шину передачи n управляющих сигналов, в качестве которых используются биты преобразуемых подблоков.The invention is illustrated by a generalized scheme of cryptographic conversion of data blocks based on the proposed method, which is presented in FIG. 1, where: P is a block of controlled permutations; A and B are convertible n-bit subunits; K _2r , K _2r-1 - elements of the encryption key (plug in); the sign ⊕ denotes the operation of bitwise summation modulo two, the sign

summation operation modulo 2 ⁿ . Bold solid lines indicate the transmission bus of n-bit signals, thin dashed lines indicate the transmission of one control bit, bold dotted lines indicate the transmission bus of n control signals, which are used as bits of the converted subunits.

 FIG. 1 shows one (rth) round of encryption. Depending on the specific implementation of the controlled permutation block and the required transformation speed, from 6 to 20 or more rounds can be set.

 Consider a specific example of the implementation of the proposed method of cryptographic transformations of binary data blocks.

Example. This example explains the encryption of 64-bit data blocks. The encryption key is formed in the form of 16 connections K ₁ , K ₂ , K ₃ , ... K ₁₆ , each of which has a length of 32 bits. The input data block is divided into two 32-bit subunits A and B. The encryption of the input block is described by the following algorithm:
1. Set the counter for the number of rounds r = 1.

2. Convert subblock B to the expression
B: = B ⊕ P _A (K _2r ),
where P _A (K _2r ) denotes the operation of the permutation of the bits of the subkey K _2r , performed depending on the value of subunit A.

3. Converter subunit A in accordance with the expression
A: = A + B.

4. Converter subunit A in accordance with the expression
A: = A ⊕ P _B (K _2r-1 ),
where P _B (K _2r-1 ) denotes the operation of the permutation of bits of the subkey K _2r-1 , performed depending on the value of subunit B.

5. Converter subunit B in accordance with the expression
B: = B + A.

 6. If r ≠ 8, then increment the counter r: = r + 1 and go to step 2, otherwise STOP.

In FIG. 2, where thin solid lines indicate the transmission of one sub-bit, a possible implementation of a controlled permutation block using a set of elementary switches S is shown. This example of a controlled permutation block corresponds to an ⁸ P block with an 8-bit input for sub-signals and an 8-bit input for control signals (bits of the data sub-block) indicated by dashed lines in the same way as in FIG. 1. The structure of the ³² P controlled permutation block with a 32-bit input for sub-signals and a 32-bit input for data sub-block control signals is similar to the ⁸ P block shown in FIG. 2. Such a structure of the controlled permutation block sets the number of different variants of the permutation operation equal to the number of possible code combinations at the control input. For ³² P, the number of different permutations is 2 ³² . This means that when encrypting two different data blocks, the probability of repeating some permutation at a given step is 2 ^-32 , and the repetition of permutations at z given steps is 2 ^-32z . Thus, the set of modified connections used to convert each input message is unique. FIG. 3, 4 and 5 explain the operation of the elementary switch, where u is the control signal, a and b are the transmission lines of single-bit input signals, c and d are the output signal lines. The tables in FIG. 4 and 5 show the dependence of the output signals on the input and control signals. It can be seen from these tables that, for u = 1, line a commutes with line c, and line b commutes with line d. For u = 0, line a commutes with line d, and line b connects with line c. For modern planar integrated circuit manufacturing technology, the described structure of controlled permutation blocks is simple, which makes it easy to manufacture cryptographic microprocessors containing controlled permutation blocks with input sizes of 32, 64, and 128 bits.

 The above examples show that the proposed method of block encryption of discrete data is technically feasible and allows us to solve the problem.

 The inventive method can be implemented, for example, in specialized cryptographic microprocessors that provide an encryption speed of about 300 Mbit / s, sufficient for real-time encryption of data transmitted via high-speed fiber-optic communication channels.

Claims (1)

 A method of block encryption of discrete data, including the formation of an encryption key in the form of a set of subkeys, dividing a data block into N≥2 subunits and alternately converting subunits by performing a two-place operation on the subunit and subkey, characterized in that before performing the two-place operation on the ith subunit and subkey over the subkey perform a permutation operation depending on the j-th subunit, where j ≠ i.

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