JP4658150B2 - Encryption method and decryption method - Google Patents

Description

  The present invention relates to an encryption method and a decryption method.

  When a secret document is encrypted using a block cipher such as AES (Advanced Encryption Standard), the number of bytes of each plaintext block constituting the plaintext (where n ≧ 2) is a fixed number N The CBC (Cipher Block Chaining) mode can be used for encryption of each plaintext block. When the number of bytes of the nth plaintext block is less than N, padding is performed by adding padding data to the nth plaintext block so that the number of bytes of the plaintext block is N. For example, padding ends when an unsigned integer is sequentially added to the plaintext block, such as “1”, “2”, and “3”, and the number of bytes becomes N. Each plaintext block obtained is encrypted by the encryption device. The decryption device for decrypting each ciphertext block obtained in this way is configured based on a padding mechanism, and thus, plaintext can be obtained by distinguishing padding data.

  On the other hand, when the number of bytes of each of the n-1 or less plaintext blocks is a fixed number N and the number of bytes of the nth plaintext block is an arbitrary number of N or less, the CFB is used for encryption and decryption of each plaintext block (Cipher Feed Back) mode and OFB (Output Feed Back) mode can be used.

  An initial vector is required for encryption and decryption in each CBC mode, CFB mode, and OFB mode. According to the Appendix C standard of Non-Patent Document 2, the initial vector must be different each time. Furthermore, in CBC mode and CFB mode, the initial vector must be unpredictable. For this reason, for example, in IPSec, which is an encryption standard for layer 3 frames in TCP / IP communication, a field for an initial vector is provided in addition to plaintext, and the data used for generating the initial vector in encryption is used as a field. Store and decode with initial vector generated using field data.

  However, in this method, since the field is provided, the total number of bytes increases, and encryption of each plaintext block in the Ethernet (registered trademark) frame (one of the layer 2 frames) in which the upper limit of the number of bytes is defined. Cannot be used for decryption.

  In Patent Document 1, for example, one plaintext block is encrypted in ECB (Electric Code Book) mode, and the remaining plaintext block is encrypted in OFB mode using the obtained ciphertext block as an initial vector. Is decrypted in ECB mode, and the remaining ciphertext block is decrypted in OFB mode using the ciphertext block as an initial vector.

However, in this method, since encryption is performed in the ECB mode, the obtained ciphertext block is easily deciphered. In the ECB mode, since encryption / decryption is performed without using an initial vector, it is weak to decryption using the statistical properties of plain text. Therefore, it is not recommended for encryption and decryption for transmitting each plaintext block as described above (for example, refer to 6.1 of Non-Patent Document 2).
JP 2001-285281 A S. Kent, R. Atkinson, IPSec, 'IP Encapsulating Security Payload (ESP)', RFC 2406, November 1998 NIST (National Institute of Standards and Technology) Special Publication 800-38A, 'Recommendation for Block Cipher Modes of Operation Methods and Techniques', December 2001

  The present invention has been made in view of the above-described problems, and an object of the present invention is to provide a technique related to encryption capable of obtaining a ciphertext that is short and difficult to decipher and decryption suitable for the ciphertext. It is in.

In order to solve the above problems, the present invention of claim 1 is an encryption method performed by an encryption device that encrypts n (where n ≧ 2) plaintext blocks, wherein the plaintext blocks are: Each of the 1st to (n−1) th plaintext blocks has a certain number of bytes N (where N is a positive integer), and the nth plaintext block has a number of bytes M (where M is M ≦ N). And an encryption unit configured in the encryption device encrypts the n-th plaintext block, and a CBC (Cipher Block Chaining) mode encryption configured in the encryption device. The encrypting unit encrypts each n−1 or less plaintext block in the CBC mode using the ciphertext obtained by the encrypting unit as an initial vector, and the OFB (Output Feed Back) mode configured in the encrypting device The encryption unit is the n-1th plaintext. With an encryption method characterized by encrypting said n-th plaintext block OFB mode as an initial vector encryption result of the lock and solutions.

The present invention of claim 2 is a decryption method performed by a decryption device that decrypts n (where n ≧ 2) ciphertext blocks, and the ciphertext blocks are numbered from 1 to the (n−1) th ciphertext. Each sentence block has a fixed number of bytes N (where N is a positive integer), and the nth ciphertext block has a number of bytes M (where M is a positive integer M ≦ N) , and the said OFB (Output Feed Back) mode decoding unit configured to decode device, the n-1 th ciphertext block decoding the n-th ciphertext block in OFB mode as the initial vector, the decoding An encryption unit configured in the device encrypts a plaintext block decrypted from the n-th ciphertext block, and a CBC (Cipher Block Chaining) mode decryption unit configured in the decryption device is operated by the encryption unit. The obtained ciphertext is Decoding the n-1 th or less of each ciphertext block in CBC mode and solutions with a decoding method characterized as torr.

The present invention of claim 3 is an encryption method performed by an encryption device for encrypting n (where n ≧ 2) plaintext blocks, wherein the plaintext blocks are numbered from 1 to (n−1) th. Each plaintext block has a certain number of bytes N (where N is a positive integer), and the nth plaintext block has a number of bytes M (where M is a positive integer M ≦ N). And an encryption unit configured in the encryption device encrypts one plaintext block from the second to nth, and a CFB (Cipher Feed Back) mode encryption unit configured in the encryption device However, the solving means includes an encryption method characterized in that the first to nth plaintext blocks are encrypted in the CFB mode using the ciphertext obtained by the encryption unit as an initial vector.

The present invention of claim 4 is a decryption method performed by a decryption device that decrypts n (where n ≧ 2) ciphertext blocks, wherein the ciphertext block is the 1st to (n−1) th ciphertext. Each sentence block has a fixed number of bytes N (where N is a positive integer), and the nth ciphertext block has a number of bytes M (where M is a positive integer M ≦ N) And a first CFB (Cipher Feed Back) mode decryption unit configured in the decryption apparatus uses the first ciphertext block as an initial vector and the second to nth ciphertext blocks in the CFB mode. The encryption unit configured in the decryption device decrypts the plaintext block decrypted from one of the second to nth ciphertext blocks, and the second CFB configured in the decryption device The mode decryption unit is operated by the encryption unit. The resulting the first ciphertext block ciphertext as an initial vector with a decoding method characterized by decoding in CFB mode and solutions.

  The fifth aspect of the present invention provides a solving means having a computer program for causing an encryption apparatus constituted by a computer to execute the encryption method according to the first or third aspect.

  According to a sixth aspect of the present invention, there is provided a computer program for causing a decoding apparatus constituted by a computer to execute the decoding method according to the second or fourth aspect.

  The seventh aspect of the present invention uses a computer-readable recording medium on which the computer program according to the fifth or sixth aspect is recorded as a solving means.

  ADVANTAGE OF THE INVENTION According to this invention, the technique regarding the encryption from which the short and difficult-to-decipher ciphertext is obtained, and the decoding suitable for the said ciphertext can be provided.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[First Embodiment]
FIG. 1 is a configuration diagram of an encryption device 1 according to the first embodiment.

  The encryption device 1 is, for example, an encryption device that encrypts plaintext that is document data including secret information and the like, and n (where n ≧ 2) plaintext blocks 101,. For example, encryption is performed using AES (Advanced Encryption Standard).

  The encryption device 1 encrypts the n-th plaintext block 10n, and the ciphertext obtained by the encryption unit 11 is used as an initial vector in the CBC (Cipher Block Chaining) mode for n−1 or less. The CBC mode encryption unit 12 for encrypting each plaintext block 101,..., 10n-1, and the nth in OFB (Output Feed Back) mode using the encryption result of the n-1st plaintext block 10n-1 as an initial vector. And an OFB mode encryption unit 13 for encrypting the plaintext block 10n. Although not shown in FIG. 1, the same key is input to the CBC mode encryption unit 12 and the OFB mode encryption unit 13.

  The number of bytes of each of the n−1 and below plaintext blocks is a fixed number N (for example, 16; the same applies hereinafter). On the other hand, the number of bytes of the nth plaintext block is an arbitrary number equal to or less than N. Thereby, the total number of bytes of the plaintext block can be set to an arbitrary number (may be a number other than a multiple of N).

(Encryption method)
Next, an encryption method performed by the encryption device 1 will be described.

  First, the encryption unit 11 encrypts the plaintext block 10n. Next, the CBC mode encryption unit 12 encrypts each plaintext block 101,..., 10n-1 in the CBC mode using the ciphertext obtained by the encryption unit 11 as an initial vector. As a result, the (n−1) th and subsequent ciphertext blocks 201,..., 20n−1 are obtained. Next, the OFB mode encryption unit 13 encrypts the plaintext block 10n in the OFB mode using the encryption result (ciphertext block 20n-1) of the plaintext block 10n-1 as an initial vector. As a result, the nth ciphertext block 20n is obtained.

  FIG. 2 is a diagram showing the above-described encryption mechanism by the CBC mode encryption unit 12.

  The CBC mode encryption unit 12 first performs an XOR (exclusive OR) operation on the initial vector 111 which is the ciphertext obtained by the encryption unit 11 and the plaintext block 101, and encrypts the operation result with the key 501. To do. Thereby, the ciphertext block 201 is obtained.

  Next, the CBC mode encryption unit 12 performs an XOR operation on the ciphertext block 201 and the plaintext block 102 and encrypts the operation result with the key 501. Thereby, the ciphertext block 202 is obtained.

  The CBC mode encryption unit 12 sequentially performs the same thing, and performs an XOR operation on the ciphertext block obtained at the end and the plaintext block 10n-1 by the number of bytes constituting the plaintext block 10n from the beginning, The calculation result is encrypted with the key 501. Thereby, the ciphertext block 20n-1 is obtained.

  FIG. 3 is a diagram showing the above-described encryption mechanism by the OFB mode encryption unit 13.

  The OFB mode encryption unit 13 encrypts the ciphertext block 20n-1 with the key 501 and performs the XOR operation on the obtained ciphertext and the plaintext block 10n by the number of bytes constituting the plaintext block 10n from the top. Thereby, the ciphertext block 20n which is a calculation result is obtained.

  As a result of the above encryption, the number of bytes in each of the (n−1) th and subsequent ciphertext blocks is a fixed number N. On the other hand, the number of bytes of the nth ciphertext block is the same as the number of bytes of the nth plaintext block, that is, an arbitrary number equal to or less than N. As a result, the total number of bytes in the ciphertext block is the same as the total number of bytes in the plaintext block, that is, an arbitrary number (may be a number other than a multiple of N).

  As described above, according to the encryption device 1 and the encryption method according to the first embodiment, by obtaining each initial vector required for encryption in each CBC mode and OFB mode from the plaintext block, The field for the initial vector is not required, and the initial vector for the CBC mode and the OFB mode can be made different every time, and the initial vector for the CBC mode can be made unpredictable. Therefore, the obtained ciphertext block can satisfy the Appendix C standard of Non-Patent Document 2 and can be applied to the encryption of the layer 2 frame.

  Further, by using OFB mode for encryption of the nth plaintext block, the number of bytes of the nth plaintext block may be an arbitrary number equal to or less than N, and the total number of bytes of the plaintext block is an arbitrary number (a multiple of N). Any number other than. Also, padding for setting the number of bytes to N can be eliminated, the total number of bytes in the plaintext block is reduced, and the plaintext block in the Ethernet (registered trademark) frame can be used for encryption.

  Further, by not performing encryption in the ECB mode, the possibility that the obtained ciphertext block is decrypted by a third party against the intention can be reduced.

[Second Embodiment]
FIG. 4 is a configuration diagram of the decoding device 2 according to the second embodiment.

  The decryption device 2 decrypts the plaintext block obtained by the encryption device 1 according to the first embodiment, that is, n (where n ≧ 2) ciphertext blocks 201,..., 20n− 1 is a decoding device for decoding 1 and 20n.

  The decryption device 2 decrypts the n-th ciphertext block 20n by the OFB mode decryption unit 21 that decrypts the n-th ciphertext block 20n in the OFB mode with the n-1-th ciphertext block 20n-1 as an initial vector. The plaintext block 10n is encrypted, and the ciphertext blocks 201,..., 20n-1 in the CBC mode are decrypted in the CBC mode using the ciphertext obtained by the encryption unit 22 as an initial vector. A CBC mode decoding unit 23.

  Although not shown in FIG. 4, the same key is input to the OFB mode decryption unit 21 and the CBC mode decryption unit 23. This key needs to be the same as the key 501 used in the encryption device 1. The encryption unit 22 needs to be the same as the encryption unit 11 of the encryption device 1.

  The number of bytes of each of the (n−1) th and following ciphertext blocks is a fixed number N. On the other hand, the number of bytes of the nth ciphertext block is the same as the number of bytes of the nth plaintext block, that is, an arbitrary number equal to or less than N. That is, the total number of bytes of the ciphertext block is the same as the total number of bytes of the plaintext block, that is, an arbitrary number (may be a number other than a multiple of N).

(Decryption method)
Next, a decoding method performed by the decoding device 2 will be described.

  First, the OFB mode decryption unit 21 decrypts the ciphertext block 20n using the ciphertext block 20n-1 as an initial vector. As a result, the nth plaintext block 10n is obtained. Next, the encryption unit 22 encrypts the plaintext block 10n decrypted from the ciphertext block 20n. The ciphertext obtained as a result is the same as the initial vector 111 of the first embodiment. Next, the CBC mode decryption unit 23 decrypts each ciphertext block 201,..., 20n-1 in the CBC mode using the ciphertext obtained by the encryption unit 22 as an initial vector. As a result, n−1th and subsequent plaintext blocks 101,..., 10n−1 are obtained.

  FIG. 5 is a diagram illustrating the above-described decoding mechanism by the OFB mode decoding unit 21.

  The OFB mode decryption unit 21 encrypts the ciphertext block 20n-1 with the key 501, and performs the XOR operation on the obtained ciphertext and the ciphertext block 20n by the number of bytes constituting the ciphertext block 20n from the beginning. Thereby, the plaintext block 10n which is a calculation result is obtained.

  FIG. 6 is a diagram illustrating the above-described decoding mechanism by the CBC mode decoding unit 23.

  The CBC mode decryption unit 23 first encrypts the ciphertext block 201 with the key 501 and performs an XOR operation on the obtained ciphertext and the initial vector 111. Thereby, the plaintext block 101 which is a calculation result is obtained.

  Next, the CBC mode decryption unit 23 encrypts the ciphertext block 202 with the key 501 and performs an XOR operation between the obtained ciphertext and the ciphertext block 201. As a result, a plaintext block 102 which is a calculation result is obtained.

  The CBC mode decryption unit 23 sequentially performs the same thing, and finally encrypts the ciphertext block 20n-1 with the key 501 and performs an XOR operation on the obtained ciphertext and the (n-2) -th ciphertext block. Do. Thereby, the plaintext block 10n-1 which is a calculation result is obtained.

  As a result of the above decryption, the number of bytes in each of the n−1 and below plaintext blocks is a fixed number N. On the other hand, the number of bytes of the nth plaintext block is the same as the number of bytes of the nth ciphertext block, that is, an arbitrary number equal to or less than N. As a result, the total number of bytes in the plaintext block is the same as the total number of bytes in the ciphertext block, that is, an arbitrary number (may be a number other than a multiple of N).

  As described above, according to the decryption apparatus 2 and the decryption method thereof according to the second embodiment, by obtaining each initial vector necessary for decryption in each CBC mode and OFB mode from the ciphertext block, the initial vector The initial vector for the CBC mode and the OFB mode can be made different each time, and the initial vector for the CBC mode can be made unpredictable. Also, the ciphertext block can satisfy the Appendix C standard of Non-Patent Document 2, and can be applied to layer 2 frame encryption.

[Third Embodiment]
FIG. 7 is a configuration diagram of the encryption device 3 according to the third embodiment.

  The encryption device 3 is, for example, an encryption device that encrypts plaintext that is document data including secret information and the like, and n (where n ≧ 2) plaintext blocks 101,. For example, encryption is performed using AES (Advanced Encryption Standard).

  The encryption device 3 encrypts one plaintext block from the 2nd to the nth, and the CFB (Cipher Feed Back) mode using the ciphertext obtained by the encryption unit 31 as an initial vector. A CFB mode encryption unit 32 for encrypting each of the first to nth plaintext blocks. Although omitted in FIG. 7, a key is input to the CFB mode encryption unit 32.

  The number of bytes of each of the n−1 and below plaintext blocks is a fixed number N. On the other hand, the number of bytes of the nth plaintext block is an arbitrary number equal to or less than N. Thereby, the total number of bytes of the plaintext block can be set to an arbitrary number (may be a number other than a multiple of N).

(Encryption method)
Next, an encryption method performed by the encryption device 3 will be described.

  First, the encryption unit 31 encrypts one plaintext block from the second to the nth. Here, it is assumed that the second plaintext block 102 is encrypted. Next, the CFB mode encryption unit 32 encrypts each plaintext block 101, 102,..., 10n in the CFB mode using the ciphertext obtained by the encryption unit 31 as an initial vector. Thereby, the 1st to nth ciphertext blocks 201, 202,..., 20n are obtained.

  FIG. 8 is a diagram showing the above-described encryption mechanism by the CFB mode encryption unit 32.

  First, the CFB mode encryption unit 32 encrypts the initial vector 311 that is the ciphertext obtained by the encryption unit 31 with the key 601, and performs an XOR operation on the obtained ciphertext and the plaintext block 101. Thereby, the ciphertext block 201 which is a calculation result is obtained.

  Next, the CFB mode encryption unit 32 encrypts the ciphertext block 201 with the key 601 and performs an XOR operation between the obtained ciphertext and the plaintext block 102. Thereby, the ciphertext block 202 which is a calculation result is obtained.

  The CFB mode encryption unit 32 sequentially performs the same thing, encrypts the ciphertext block obtained at the end with the key 601, and uses the obtained ciphertext and the plaintext block 10n to convert the plaintext block 10n from the top. XOR operation is performed for the number of bytes to be configured. Thereby, the ciphertext block 20n which is a calculation result is obtained.

  As a result of the above encryption, the number of bytes in each of the (n−1) th and subsequent ciphertext blocks is a fixed number N. On the other hand, the number of bytes of the nth ciphertext block is the same as the number of bytes of the nth plaintext block, that is, an arbitrary number equal to or less than N. As a result, the total number of bytes in the ciphertext block is the same as the total number of bytes in the plaintext block, that is, an arbitrary number (may be a number other than a multiple of N).

  As described above, according to the encryption device 3 and the encryption method according to the third embodiment, each initial vector necessary for encryption in the CFB mode is obtained from the plaintext block, so that The CFB mode initial vector can be made different every time, and the CFB mode initial vector can be made unpredictable. Therefore, the obtained ciphertext block can satisfy the Appendix C standard of Non-Patent Document 2 and can be applied to the encryption of the layer 2 frame.

  Further, by using the CFB mode for encryption of the nth plaintext block, the number of bytes of the nth plaintext block may be an arbitrary number equal to or less than N, and the total number of bytes of the plaintext block is an arbitrary number (a multiple of N). Any number other than. Also, padding for setting the number of bytes to N can be eliminated, the total number of bytes in the plaintext block is reduced, and the plaintext block in the Ethernet (registered trademark) frame can be used for encryption.

  Further, by not performing encryption in the ECB mode, the possibility that the obtained ciphertext block is decrypted by a third party against the intention can be reduced.

[Fourth Embodiment]
FIG. 9 is a configuration diagram of the decoding device 4 according to the fourth embodiment.

  The decrypting device 4 decrypts the plaintext block obtained by the encrypting device 3 of the third embodiment, that is, n (where n ≧ 2) ciphertext blocks 201, 202,. This is a decoding device for decoding 20n.

  The decryption device 4 includes a first CFB mode decryption unit 41 that decrypts each of the second to nth ciphertext blocks 202,..., 20n in the CFB mode using the first ciphertext block 201 as an initial vector, and the second to An encryption unit 42 for encrypting a plaintext block decrypted from one ciphertext block in the n-th ciphertext block 202,..., 20n, and 1 as an initial vector for the ciphertext obtained by the encryption unit 42 A second CFB mode decrypting unit 43 for decrypting the ciphertext block 201 in the CFB mode.

  Although omitted in FIG. 9, the same key is input to the first CFB mode decryption unit 41 and the second CFB mode decryption unit 43. This key needs to be the same as the key 601 used in the encryption device 3. The encryption unit 42 needs to be the same as the encryption unit 31 of the encryption device 3.

  The number of bytes of each of the (n−1) th and following ciphertext blocks is a fixed number N. On the other hand, the number of bytes of the nth ciphertext block is the same as the number of bytes of the nth plaintext block, that is, an arbitrary number equal to or less than N. That is, the total number of bytes of the ciphertext block is the same as the total number of bytes of the plaintext block, that is, an arbitrary number (may be a number other than a multiple of N).

(Decryption method)
Next, a decoding method performed by the decoding device 4 will be described.

  First, the first CFB mode decrypting unit 41 decrypts each ciphertext block 202,..., 20n in the CFB mode using the ciphertext block 201 as an initial vector. As a result, the second to nth plaintext blocks 102,..., 10n are obtained. Next, the encryption unit 42 encrypts the plaintext block decrypted from one ciphertext block in the plaintext blocks 102,. The decrypted ciphertext block needs to be a ciphertext block obtained by encrypting the plaintext block encrypted by the encryption unit 31 of the encryption device 3 by the CFB mode encryption unit 32. Here, it is assumed that the encryption unit 42 has encrypted the second plaintext block 102 decrypted from the second ciphertext block 202. The ciphertext thus obtained is the same as the initial vector 311 of the third embodiment.

  Next, the second CFB mode decryption unit 43 decrypts the ciphertext block 201 in the CFB mode using the ciphertext obtained by the encryption unit 42 as an initial vector. As a result, the first plaintext block 101 is obtained.

  FIG. 10 is a diagram showing a mechanism of the decoding by the first CFB mode decoding unit 41. As shown in FIG.

  The first CFB mode decryption unit 41 first encrypts the ciphertext block 201 with the key 601 and performs an XOR operation on the obtained ciphertext and the ciphertext block 202. As a result, a plaintext block 102 which is a calculation result is obtained.

  Next, the first CFB mode decryption unit 41 encrypts the ciphertext block 202 with the key 601 and performs an XOR operation on the obtained ciphertext and the ciphertext block 203. As a result, a plaintext block 103 which is a calculation result is obtained.

  The first CFB mode decryption unit 41 sequentially performs the same process, encrypts the ciphertext block obtained at the end with the key 601, and encrypts the ciphertext and ciphertext block 20 n obtained from the beginning. XOR operation is performed for the number of bytes constituting the sentence block 20n. Thereby, the plaintext block 10n which is a calculation result is obtained.

  FIG. 11 is a diagram showing a mechanism of the decoding by the second CFB mode decoding unit 43. As shown in FIG.

  The second CFB mode decrypting unit 43 encrypts the initial vector 311 with the key 601 and performs an XOR operation with the obtained ciphertext and the ciphertext block 201. Thereby, the plaintext block 101 which is a calculation result is obtained.

  As a result of the above decryption, the number of bytes in each of the n−1 and below plaintext blocks is a fixed number N. On the other hand, the number of bytes of the nth plaintext block is the same as the number of bytes of the nth ciphertext block, that is, an arbitrary number equal to or less than N. As a result, the total number of bytes in the plaintext block is the same as the total number of bytes in the ciphertext block, that is, an arbitrary number (may be a number other than a multiple of N).

  As described above, according to the decryption device 4 and the decryption method thereof according to the fourth embodiment, each initial vector necessary for decryption in the CFB mode is obtained from the ciphertext block, whereby the field for the initial vector is obtained. Is not required, and the initial vector in the CFB mode can be made different every time, and the initial vector in the CFB mode can be made unpredictable. Also, the ciphertext block can satisfy the Appendix C standard of Non-Patent Document 2, and can be applied to layer 2 frame encryption.

  The computer program for causing the encryption method and the decryption method of the present embodiment to be executed by a computer and a computer program for causing the decryption device to execute are a semiconductor memory, a magnetic disk, an optical disk, a magneto-optical disk, and a magnetic tape. The program may be stored in a computer-readable recording medium such as a display medium and distributed, or the computer program may be transmitted via a communication network such as the Internet.

It is a block diagram of the encryption apparatus 1 which concerns on 1st Embodiment. It is a figure which shows the mechanism of the encryption by the CBC mode encryption part. It is a figure which shows the mechanism of the encryption by OFB mode encryption part 13. FIG. It is a block diagram of the decoding apparatus 2 which concerns on 2nd Embodiment. It is a figure which shows the mechanism of the decoding by OFB mode decoding part 21. FIG. It is a figure which shows the mechanism of the decoding by the CBC mode decoding part 23. FIG. It is a block diagram of the encryption apparatus 3 which concerns on 3rd Embodiment. It is a figure which shows the mechanism of the encryption by the CFB mode encryption part 32. FIG. It is a block diagram of the decoding apparatus 4 which concerns on 4th Embodiment. It is a figure which shows the mechanism of the decoding by the 1st CFB mode decoding part 41. FIG. It is a figure which shows the mechanism of the decoding by the 2nd CFB mode decoding part 43. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 3 ... Encryption apparatus 2, 4 ... Decryption apparatus 11, 22, 31, 42 ... Encryption part 12 ... CBC mode encryption part 13 ... OFB mode encryption part 21 ... OFB mode decryption part 23 ... CBC mode decryption part 32 ... CFB mode encryption unit 41 ... first CFB mode decryption unit 43 ... second CFB mode decryption unit

Claims (7)

An encryption method performed by an encryption device that encrypts n (where n ≧ 2) plaintext blocks,
In the plaintext block, each of the 1st to (n-1) th plaintext blocks has a certain number of bytes N (where N is a positive integer), and the nth plaintext block has a number of bytes M (wherein M is a positive integer of M ≦ N),
An encryption unit configured in the encryption device encrypts the nth plaintext block;
A CBC (Cipher Block Chaining) mode encryption unit configured in the encryption device encrypts each of the n−1 and subsequent plaintext blocks in the CBC mode using the ciphertext obtained by the encryption unit as an initial vector. ,
An OFB (Output Feed Back) mode encryption unit configured in the encryption device encrypts the n-th plaintext block in the OFB mode using an encryption result of the n−1-th plaintext block as an initial vector; An encryption method characterized by the above. A decryption method performed by a decryption device that decrypts n (where n ≧ 2) ciphertext blocks,
In the ciphertext block, the 1st to (n-1) th ciphertext blocks each have a certain number of bytes N (where N is a positive integer), and the nth ciphertext block has a number of bytes M. Where M is a positive integer M ≦ N,
An OFB (Output Feed Back) mode decryption unit configured in the decryption device decrypts the n-th ciphertext block in the OFB mode using the n−1-th ciphertext block as an initial vector,
An encryption unit configured in the decryption device encrypts a plaintext block decrypted from the nth ciphertext block;
A CBC (Cipher Block Chaining) mode decryption unit configured in the decryption device decrypts each ciphertext block of the (n−1) th or less in the CBC mode using the ciphertext obtained by the encryption unit as an initial vector; A decoding method characterized by the above. An encryption method performed by an encryption device that encrypts n (where n ≧ 2) plaintext blocks,
In the plaintext block, each of the 1st to (n-1) th plaintext blocks has a certain number of bytes N (where N is a positive integer), and the nth plaintext block has a number of bytes M (wherein M is a positive integer of M ≦ N),
An encryption unit configured in the encryption device encrypts one plaintext block from the second to the nth;
The CFB (Cipher Feed Back) mode encryption unit configured in the encryption device encrypts each of the first to nth plaintext blocks in the CFB mode using the ciphertext obtained by the encryption unit as an initial vector. An encryption method characterized by: A decryption method performed by a decryption device that decrypts n (where n ≧ 2) ciphertext blocks,
In the ciphertext block, each of the 1st to (n-1) th ciphertext blocks has a certain number of bytes N (where N is a positive integer), and the nth ciphertext block has a number of bytes M. (Where M is a positive integer with M ≦ N),
A first CFB (Cipher Feed Back) mode decryption unit configured in the decryption device decrypts each of the second to nth ciphertext blocks in CFB mode using the first ciphertext block as an initial vector,
An encryption unit configured in the decryption device encrypts a plaintext block decrypted from one ciphertext block in the second to nth;
A second CFB mode decryption unit configured in the decryption device decrypts the first ciphertext block in a CFB mode using the ciphertext obtained by the encryption unit as an initial vector. .   A computer program for causing an encryption apparatus configured by a computer to execute the encryption method according to claim 1 or 3.   5. A computer program for causing a decoding device constituted by a computer to execute the decoding method according to claim 2 or 4.   A computer-readable recording medium on which the computer program according to claim 5 is recorded.

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