KR101583285B1

KR101583285B1 - Block cipher method using expansion key and apparatus thereof

Info

Publication number: KR101583285B1
Application number: KR1020140083716A
Authority: KR
Inventors: 최진영; 송유현
Original assignee: 고려대학교 산학협력단
Priority date: 2014-07-04
Filing date: 2014-07-04
Publication date: 2016-01-07

Abstract

The present invention relates to a method of encrypting a block using an extended key and an apparatus according to the method, in which a plurality of random keys are generated, subkeys are generated from each random key by the number of rounds of block encryption, Generates the round keys by generating XOR operations on the selected sub keys by selecting one of the generated sub keys different from each other, and generating the ciphertext using the round keys for the plain key The block encryption method extends the short key without changing the block encryption structure, thereby enhancing security and ensuring high-speed encryption.

Description

[0001] The present invention relates to a block ciphering method using an extended key,

The present invention relates to a block encryption method and apparatus, and more particularly, to a block encryption method using a round key generated by XORing a combination of subkeys generated from a plurality of random keys, .

Block cipher refers to an encryption method in which a cipher key and an algorithm are applied in units of data blocks to form a cipher text. A typical block cipher method is DES (Data Encryption Standard). The National Institute of Standards and Technology (NIST) adopted the DES as the national standard, and the encryption algorithm was public and licensed for royalty-free use worldwide.

DES is a symmetric cryptosystem in which the sender and receiver use the same secret key, divides the plaintext to be encrypted into blocks of a certain size, and applies encryption using a random key and a sub key for each block . Since DES has a short key bit length of 56 bits, it is vulnerable to poor security. To overcome this problem, Triple-DES has been proposed. Triple-DES is a method of repeating DES three times, which is very safe, but has a drawback that it is slow.

AES-128 bit / 192 bit / 256 bit, which is a high-speed encryption scheme, has been developed and used as a standard in the current block cipher. However, since AES DES was adopted as standardization in the US, And Triple-DES, which is a variant of DES, are still in use, technical means are needed to supplement the existing DES to contribute to the improvement of the weakened cryptographic strength.

H. X.MEL, "CRY PTO GRAPHYDECRY PTED ", ADDISON WESLEY, 2001.

A first problem to be solved by the present invention is to provide a block encryption method capable of enhancing security by extending a short key and at the same time guaranteeing the processing speed of the existing encryption method.

A second problem to be solved by the present invention is to provide a block encryption method capable of ensuring an additional security improvement without changing the block encryption structure.

A third problem to be solved by the present invention is to provide a block encrypting apparatus which can be commonly applied to a plurality of encrypting schemes and which extends a short key to enhance security.

It is another object of the present invention to provide a computer-readable recording medium storing a program for causing a computer to execute the above-described method.

In order to achieve the first object of the present invention, Generating a plurality of random keys; Generating a sub key by the number of rounds of block encryption for each random key; Selecting one sub-key among the sub-keys generated from the random keys, and XORing the selected sub-keys to generate round keys; And generating a ciphertext using the round keys for the plaintext, wherein the selected subkeys are generated from different random keys, and the order of generation is different. do.

According to an embodiment of the present invention, in the step of generating the round keys, the generation order of the selected sub keys may differ by a predetermined number.

According to another embodiment of the present invention, the step of generating the ciphertext may XOR the plaintext using the round key for each round.

According to another embodiment of the present invention, the block encryption is DES encryption, and the number of random keys may be four or more.

In order to achieve the second object, the present invention provides a block encryption method further comprising an OTP (OTP) conversion step of applying a hash function having a part of the cipher text and the round keys as a variable to the generated ciphertext to provide.

According to an embodiment of the present invention, the hash function may use an XOR operation and an MD5 operation.

In order to achieve the third object of the present invention, A key generation unit for generating a plurality of random keys and generating a sub key for a number of rounds of block encryption for each generated random key; An XOR operator for selecting subkeys one by one from the subkeys generated from the respective random keys and generating XORs for the selected subkeys to generate round keys; And a ciphertext generation unit for generating a cipher text using the round keys for the plain text, wherein the XOR operation unit is configured to generate the random intermediate key in which the selected intermediate subkeys are generated from different random keys, And a block encryption device.

According to the embodiment of the present invention, the generation order of the selected sub keys may be different by a predetermined number.

According to the embodiment of the present invention, the block encryption is DES encryption, and the number of the random keys may be four or more.

According to an embodiment of the present invention, there is provided an OTP (One Time Pad) converter for applying a hash function having a part of the ciphertext and the round keys as a variable to the generated ciphertext, Operation can be used.

According to another aspect of the present invention, there is provided a computer-readable recording medium storing a program for causing a computer to execute the above-described embodiments.

According to the present invention, it is possible to provide a block encryption method capable of enhancing security by extending a short key and at the same time ensuring a processing speed of an existing encryption method. Furthermore, security can be improved through additional OTP steps without changing the block encryption structure.

1 is a flowchart of a block encryption method using an extended key according to an embodiment of the present invention.
2A is a flowchart of a block encryption method that further includes an OTP conversion step according to another embodiment of the present invention.
2B is a conceptual diagram of a block encryption method that further includes an OTP conversion step according to another embodiment of the present invention.
3 is a table showing subkeys generated from a plurality of random keys.
FIG. 4A is an example in which a sub key is selected so that the generation order is different from the sub key generated from each random key by one.
FIG. 4B is an example of selecting a subkey among the subkeys generated from each random key.
5 is a table showing a round key generated by XORing selected sub keys.
6 is a conceptual diagram illustrating generation of a 64-bit cipher text by applying a block encryption method using an extension key according to an embodiment of the present invention to a 64-bit plain text.
7A is an OTP conversion code when encrypting a block encryption method including an OTP conversion step according to another embodiment of the present invention.
FIG. 7B is an OTP conversion code when decrypting a block encryption method including an OTP conversion step according to another embodiment of the present invention.
8 is a block diagram of a block encrypting apparatus using an extended key according to another embodiment of the present invention.
9 is a round key generation example for implementing a block encryption method further including an OTP conversion step according to another embodiment of the present invention.
FIG. 10A is a table illustrating a step of encrypting a plaintext of a text format by a block encryption method further including an OTP conversion step according to another embodiment of the present invention.
FIG. 10B is a table showing a step of decrypting the cipher text in FIG. 10A by a block encryption method that further includes an OTP conversion step according to another embodiment of the present invention.
11A is a diagram illustrating an encryption screen in which an original screen is encrypted by a block encryption method that further includes an OTP conversion step according to another embodiment of the present invention and a picture format original screen.
FIG. 11B is a diagram illustrating a decrypted screen in which the original screen and the encrypted screen of FIG. 11A are decrypted again by a block encryption method including an OTP conversion step according to another embodiment of the present invention.

Prior to the description of the concrete contents of the present invention, for the sake of understanding, the outline of the solution of the problem to be solved by the present invention or the core of the technical idea is first given.

One of the block encryption methods used as a standard is the DES encryption method. DES has been adopted as a standard for financial services in the United States and its algorithms are open and royalty free, allowing it to be widely used around the world. The symmetric cipher uses the same secret key for both the sender and receiver, and the encryption is fast because the key to be encrypted is small. A smart card is one that uses DES around us. It was developed by IBM and certified by the US government to be used for general data transfers below Level 1 secrets. DES divides the message into 64-bit blocks for encryption and mixes it with the key through a complex round of 16 steps each to generate a cipher text.

However, since DES has a short key length of 56 bits, it is vulnerable to security. In order to solve this problem, AES-128 bit / 192 bit / 256 bit encryption with a sufficiently increased key length has been devised. AES encryption is strong enough for high-speed encryption because the length of the key is long enough. However, there is a cost problem to encrypt using the new structure, AES, where DES is utilized. For this reason, Triple-DES in which DES is used as it is repeated three times has been devised and used. However, Triple-DES has a disadvantage that its stability is improved but its speed is slow.

Accordingly, the present invention provides a method for encrypting data, comprising: receiving a plaintext to improve security while utilizing a DES encryption structure; Generating a plurality of random keys; Generating a sub key by the number of rounds of block encryption for each random key; Selecting one sub-key among the sub-keys generated from each random key, and XORing the selected sub-keys to generate round keys; And generating a cipher text using the round keys with respect to the plain text, wherein the selected sub keys are generated from different random keys, and the generation order is different.

1 is a flowchart of a block encryption method using an extended key according to an embodiment of the present invention.

In step S110, a plain text is input. A plain text is a target to be encrypted, and can be various forms such as a text file, a picture file, and a multimedia file. The plaintext is encrypted by the predetermined block size according to the block encryption method. In an embodiment of the present invention, it is exemplified that the plaintext is divided into 64-bit blocks.

In step S120, a plurality of random keys are generated. In the case of DES encryption, one random key is used, but according to the present invention, a plurality of random keys are required to generate the extended key. When the block encryption method is DES encryption, the number of random keys is determined according to the length of the key, and may be four or more when the length of the key is 64 bits. The present invention will be described by generating four random keys in the embodiment of Figs. 9A, 9B, 10A, and 10B, which will be described below. It is clear that the length of the random key is 64 bits and may vary depending on the block encryption method.

In step S130, a sub key is generated by the number of rounds of block encryption for each random key. In the case of DES, a random key of 64 bits is reduced to 56 bits in order to generate a sub key, and can be reduced to a general DES algorithm. The method of generating the subkey may be by a DES standard key-generator. Specifically, sub keys are generated by the number of rounds of block encryption for one random key, and sub keys are generated for each of the plurality of random keys. Therefore, if the number of random keys is n and the number of rounds of block encryption is m, the sub key is generated by n X m. In the case of DES, the number of rounds of block encryption is 16, and in the embodiment to be described below, 4 random keys are generated, so that a total of 4 X 16 = 64 subkeys are generated. This is shown in FIG. 3 as a table. It can be seen that the random keys K1, K2, K3, and K4 are generated, and the subkeys are generated by 16 rounds for each random key in the vertical direction of the table.

In step S140, subkeys are selected one by one from the subkeys generated from the respective random keys, and XOR operations are performed on the selected subkeys to generate the round keys. The selected subkeys are generated from different random keys and may be characterized in that the order of generation is different. That is, one of the sub keys generated from one random key and identified in the generation order is selected, and sub-keys generated from the other random keys are selected in a different generation order from those already selected. For example, if 16 subkeys are generated for block encryption with 16 rounds from 4 random keys, 16 subkeys are generated from the same random key, and one subkey is selected for each of 4 randomkeys. have.

According to an embodiment of the present invention, the order of generation of the selected sub-keys may differ by a predetermined number. FIG. 4A shows one of the sub-keys K1_1, K2_2, K3_3, and K4_4 that are different from each other so that the generation order increases by one. The predetermined number will be an integer smaller than the number of rounds, and can be selected to increase or decrease, and it is natural that a person skilled in the art can appropriately determine and carry out the present invention. At this time, the generation order of the selected sub keys should not be the same.

Also, different sub keys may be selected one by one as shown in FIG. 4B. Specifically, among the subkeys generated from K1, the generation order is 2 (K1_2), the sub-keys generated from K2 are 10 (K2_10), and the sub-keys generated from K3 are 12 K3_12) indicates that the generation order is 6 (K4_6) among the sub keys generated from K4.

According to an embodiment of the present invention, the order of generation of the selected sub-keys may differ by a predetermined number. FIG. 4A shows one of the sub-keys K1_1, K2_2, K3_3, and K4_4 that are different from each other so that the generation order increases by one. The predetermined number will be an integer smaller than the number of rounds and can be selected to increase or decrease. At this time, the generation order of the selected sub keys should not be the same. This is because, when XOR operation is performed in the same order with respect to the XOR operation, only the effect of generating a subkey from one random key is generated. Therefore, if XOR operation is performed by selecting another one in the order of generation, a combination effect of random keys is generated and the round key is expanded.

The selected subkeys generate round keys by XOR operation. FIG. 5 is a table showing that a total of 16 round keys are generated by sequentially selecting the subkeys generated from the four random keys and sequentially performing the XOR operation. For example, the first round key R1 becomes a value calculated by K1_1 XOR K2_2 XOR K3_3 XOR K4_4.

In this step, the block encryption method has an effect of expanding a key, and the term 'extended key' means a round key. Compared to encrypting a subkey in correspondence with each round by generating a subkey for each round number from one random key in the DES encryption, which is a general block encryption method, the round key according to the present invention is further provided with a plurality of random keys Keys are generated by the number of rounds, and XOR operations are performed by combining the generated subkeys, so that the Round Key in the original scheme is expanded. Using such an extended round key means increasing the number of combinations of keys to further increase the difficulty of finding the key.

In step S150, a cipher text is generated using the round keys for the plain text. It is possible to finally generate a cipher text by performing an XOR operation using the round key corresponding to each round. In the case of DES encryption, the operations in each round are defined by the published algorithm and the initial permutation before the first round and the final permutation after the last round can be added. Of course, it is also possible to perform an operation by an algorithm other than the disclosed algorithm, and in an embodiment of the present invention, it is disclosed that each step is operated using an XOR operation. 6 is a conceptual diagram illustrating generation of a 64-bit cipher text by applying a block encryption method using an extension key according to an embodiment of the present invention to a 64-bit plain text.

The final ciphertext according to FIG. 1 is encrypted on a block-by-block basis, and it is theoretically impossible to attempt self-decryption without a key because security is improved by an extended key. In more detail, when the sub keys are XORed in step S140, it is impossible to divide the original value by Shannon's theorem, so the extended key can not be theoretically known. In terms of encryption speed, the round key is substituted for the block encryption process without changing the length of the bits and the number of keys, and the value generated through the encryption does not have a problem of slowing down because there is no bit change. Therefore, the short key length is maintained, and the fact that the advantage of DES is fast is maintained. In addition, when four random keys of 64 bits are used in combination, they are reduced to 56 bits and eventually a key of 224 bits is used. All 224-bit keys must match to generate the same round keys, and the effort to find all 56-bit four keys is a bit length that can not be practically solved. On the other hand, since standard round keys are generated depending on one 56-bit random key, it is vulnerable to security because it can find all the round keys if only one key is found.

Among the block encryption methods, most of the Feistel ciphers involve the final permutation process. The final replacement may further include an OTP conversion step to increase the security effect of the block encryption method using the extended key when the security is weak or there is no final replacement process. FIG. 2A is a flowchart of a block encryption method that further includes an OTP conversion step according to another embodiment of the present invention, and FIG. 2B is a conceptual diagram of a block encryption method further including an OTP conversion step according to another embodiment of the present invention .

Steps S210 through S250 of FIG. 2A correspond to steps S110 through S150 of FIG. 1, respectively, and the duplicated description is replaced with the detailed description of FIG.

In step S260, OTP (One Time Pad) conversion is performed to apply a hash function to the ciphertext. The ciphertext is already encrypted, but you can add an OTP translation step to improve security. It is possible to apply any block encryption method because it has the advantage of not changing the structure of the existing block encryption because it is performed by receiving the already generated ciphertext. In the OTP conversion of the present invention, a part of a ciphertext, a random key, and a counter variable are concatenated to perform a XOR operation on a result of a HASH operation and a part of a cipher text. HASH can be a general hash function such as MD5. In another embodiment of the present invention, the ciphertext is OTP-converted by the code of Fig. 7A, and the ciphertext is decrypted by the code of Fig. 7B through the OTP conversion step.

7A and 7B are as follows. K1, K2, K3, and K4 are 64-bit random keys, and '+' means connecting variables. Therefore, K5 is a length of 256 bits by connecting K1 to K4. RC is the right 32 bits of one block of ciphertext, and LC is the left 32 bits of one block of ciphertext. I is a counter variable, and xor means an XOR operation. OTP conversion is performed for all ciphertexts by iterative operation (For-Next syntax). When encrypting plaintext with block encryption, block ciphers mode is additionally required, since the size of plaintext exceeds the encryption unit (for example, 64 bits, 128 bits, 256 bits, etc.). OTP conversion is a kind of block cipher mode, and others include CBC, OFB, PCBC, and CTR.

Through the OPT conversion step according to the present invention, there is no need to include an additional block cipher mode, and it is possible to overcome the problem of security vulnerability such as the appearance of image contour which is a disadvantage of an electronic codebook (ECB).

FIG. 8 is a block diagram of a block encrypting apparatus 40 using an extended key according to another embodiment of the present invention.

The block encryption apparatus 40 includes an input unit 41, a key generation unit 42, an XOR operation unit 43, and a cipher text generation unit 44, and may further include an OTP conversion unit 45. The respective components of the block encryption apparatus 40 correspond to the detailed steps of FIG. 1 and FIG. 2 as follows, and a detailed description thereof will not be repeated.

The input unit 41 receives a plain text. It will be obvious that the input unit can be variously implemented according to the environment in which the block encryption device is implemented and can be changed according to the kind of the plain text. Corresponds to step S110 in FIG.

The key generation unit 42 generates a plurality of random keys. In the case of DES, the 64-bit random key is reduced to 56 bits, and the sub keys are generated by a round number of 16 steps from a plurality of reduced random keys. The key generation unit may be changed by an operator and may be a DES-standard key generator. And S120 and S130 in Fig.

The XOR operation unit 43 receives the subkeys from the key generation unit 42, selects the different subkeys, performs an XOR operation on the selected subkeys, and generates a round key. DES generates 16 round keys. The generation order of the selected sub keys may differ by a predetermined number. And corresponds to step S140 in FIG.

The ciphertext generation unit 44 receives the plain text and the round keys, and generates ciphertexts through each round. In the case of DES encryption, each round step may be based on an open algorithm and may be based on an XOR operation according to an embodiment of the present invention. And corresponds to step S150 of FIG.

The OTP conversion unit 45 is configured to additionally include a plurality of random keys and ciphertexts, and applies a hash function. The hash function can use XOR operation and MD5 operation. And corresponds to step S260 of FIG.

The block cipher apparatus 40 of the above-described embodiment is used to generate a cipher text at a high speed of a plain text, and the generated cipher text theoretically assures the degree of security that the attack is impossible. It is possible to encrypt and extend the key by a safe length without changing the structure of the existing block encryption method. In particular, since the structure of DES encryption, which is the representative and most widely used block encryption, can be applied without modification, it is possible to use DES in the future because it is highly utilized.

9 is a round key generation example for implementing a block encryption method further including an OTP conversion step according to another embodiment of the present invention. Specifically, the first row is an XOR in order to generate a round key, the second row is a random key of 64 bits K1, K2, K3, K4, and the second row to the 18th row is the second row It is the result of calculating the formula of the first row for each 16 sub keys generated from the random key. So, finally, the rightmost column becomes the round key.

An application example in the case where the plain text is text by applying the round key in Fig. 9 will be described with reference to Figs. 10A and 10B, and an application example in the case where the plain text is a picture file will be described below with reference to Figs. 11A and 11B.

FIG. 10A is a table showing a step of encrypting plain text in a text format by a block encryption method further including an OTP conversion step according to another embodiment of the present invention. FIG. And an OTP conversion step according to a block encryption method according to an embodiment of the present invention.

The plaintext is divided into 64 bits, which is the block size, each row of the table represents each round, and the column represents a concrete value encrypted by applying a round key corresponding to each round. 10A includes an initial permutation and a final permutation process and includes an OTP conversion step by applying a HF function which is a hash function after the final replacement. FIG. 10B shows a HF function, which is an OTP conversion step, applied to a cipher text as a result of FIG. 10A, decodes it so as to correspond to each round after initial substitution. At the end of the round, the left 32 bits and the right 32 bits of the cipher text are SWAP, and after the final replacement step, the text is converted to plain text. Since the decryption step of FIG. 10B is to perform encryption inversely, it corresponds to each step of FIG. 10A upside down.

11A is a diagram illustrating an encrypted screen in which an original screen is encrypted by a block encryption method that further includes an OTP conversion step according to another embodiment of the present invention and a picture format original screen. In the original screen, the shape of the red circle with three circles overlapped on the encryption screen.

11B is a diagram illustrating a decrypted screen in which the original screen and the encrypted screen of FIG. 11A are decrypted again by a block encryption method including an OTP conversion step according to another embodiment of the present invention. The encrypted screen shown in FIG. 11A can be decrypted again to obtain an original screen as shown in FIG. 11B.

Furthermore, the present invention can be embodied as computer-readable codes on a computer-readable recording medium. A computer-readable recording medium includes all kinds of recording apparatuses in which data that can be read by a computer system is stored.

Examples of the computer-readable recording medium include a ROM, a RAM, a CD-ROM, a magnetic tape, a floppy disk, an optical data storage device and the like, and also a carrier wave (for example, transmission via the Internet) . In addition, the computer-readable recording medium may be distributed over network-connected computer systems so that computer readable codes can be stored and executed in a distributed manner. In addition, functional programs, codes, and code segments for implementing the present invention can be easily deduced by programmers skilled in the art to which the present invention belongs.

While the present invention has been described in connection with what are presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, Various modifications and variations may be made thereto by those skilled in the art to which the present invention pertains.

Accordingly, the spirit of the present invention should not be construed as being limited to the embodiments described, and all of the equivalents or equivalents of the claims, as well as the following claims, belong to the scope of the present invention .

40: Block encryption device
41: input unit 42: key generation unit
43: XOR operation unit 44: cipher text generation unit
45: OTP converter

Claims

Receiving a plain text;
Generating a plurality of random keys;
Generating a sub key by the number of rounds of block encryption for each random key;
Selecting one sub-key among the sub-keys generated from the random keys, and XORing the selected sub-keys to generate round keys; And
And generating a cipher text using the round keys for the plaintext,
Wherein the selected subkeys are generated from different random keys, and the order of generation is different, and the order of generation of the selected subkeys is different by a predetermined number.

The method according to claim 1,
Wherein the predetermined number is an integer smaller than the number of rounds.

The method according to claim 1,
The step of generating the ciphertext includes:
Wherein the XOR operation is performed using the round key for each round of the plaintext.

The method according to claim 1,
The step of generating the ciphertext includes:
And an OTP (One Time Pad) conversion step of applying a hash function having a part of the ciphertext and the random keys as variables to the generated ciphertext.

5. The method of claim 4,
Wherein the hash function uses an XOR operation and an MD5 operation.

The method according to claim 1,
Wherein the block encryption is DES encryption.

The method according to claim 6,
Wherein the number of random keys is four or more.

A computer-readable recording medium storing a program for causing a computer to execute the method according to any one of claims 1 to 7.

An input unit for receiving a plain text;
A key generation unit for generating a plurality of random keys and generating a sub key for a number of rounds of block encryption for each generated random key;
An XOR operator for selecting subkeys one by one from the subkeys generated from the respective random keys and generating XORs for the selected subkeys to generate round keys; And
And a ciphertext generation unit for generating a ciphertext using the round keys for the plaintext,
Wherein the XOR operation unit is generated from random keys different from each other in the selected subkeys, and the order of generation of the selected subkeys is different by a predetermined number.

10. The method of claim 9,
Wherein the predetermined number is an integer smaller than the number of rounds.

10. The method of claim 9,
Wherein the block encryption is DES encryption, and the number of the random keys is four or more.

10. The method of claim 9,
And an OTP (One Time Pad) converter for applying a hash function having a part of the cipher text and the random keys as a variable to the generated ciphertext,
Wherein the hash function uses an XOR operation and an MD5 operation.