JP2007193137A - Encryption device, encryption method, decryption device, and decryption method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enhance encryption strength while suppressing the load imposed on processing of encryption and decryption. <P>SOLUTION: When communication from a server 1 to a terminal device 2 is carried out, processing to reduce autocorrelation is applied to plaintext data first, and encryption processing is performed to the secondary data which is the data obtained in the result. Then, a ciphertext generated through the encryption is transmitted via a network 3. The terminal device 2 can take out the secondary data by performing decryption processing to the received ciphertext, and performs processing (post processing) to restore the autocorrelation to the taken-out secondary data to obtain plaintext data. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an encryption device for encrypting plaintext data, an encryption method, a decryption device for decrypting encrypted data, and a decryption method, and in particular, while suppressing a load on encryption and decryption processing, The present invention relates to an encryption device, an encryption method, a decryption device, and a decryption method.

  Conventionally, communication encryption has been performed to conceal communication contents from a third party. For example, Patent Document 1 discloses a technique of compressing audio data and then encrypting the data, and Patent Document 2 discloses a technique of encrypting data by substitution processing. Patent Document 3 discloses encryption using a specific table, and Patent Document 4 discloses setting of a conversion table for converting input data to a predetermined value when performing encryption.

JP-A-8-316951 Japanese Patent No. 262223 JP-A-9-114373 Japanese Patent No. 3029381

  By the way, now that computers are widely used and their processing power is remarkably improved, the classical encryption method can be easily deciphered. Therefore, when strong encryption strength is required, advanced encryption using algebraic curve theory is often employed.

  However, advanced cryptography requires a very large amount of computation even for legitimate users compared to classical cryptography. For this reason, there is a problem that it is difficult to perform encryption for all communications in terminals such as in-vehicle terminals and portable terminals that have only a limited computing capability (such as a low-speed CPU).

  On the other hand, for illegal decryption of encryption, there is known a technique that uses a bias of communication data (a state in which autocorrelation is high, such as a specific bit string repeatedly appearing) as a clue. In particular, when there is a large bias in plain text, more advanced cryptographic processing is required to eliminate such bias by encryption and ensure sufficient cryptographic strength, causing a further increase in the amount of calculation.

  The present invention has been made to solve the above-described problems in the prior art and to solve the problems, and an encryption device that improves the encryption strength while suppressing the load on encryption and decryption processing, It is an object to provide an encryption method, a decryption device, and a decryption method.

  In order to solve the above-described problems and achieve the object, an encryption method according to the invention of claim 1 is an encryption method for encrypting plaintext data, and performs processing for reducing autocorrelation with the plaintext data. And an autocorrelation reducing step for outputting the secondary data as a secondary data and an encryption processing step for encrypting the secondary data.

  According to the first aspect of the present invention, the encryption method performs processing for reducing autocorrelation on plaintext data, outputs it as secondary data, and encrypts the secondary data.

  The encryption method according to claim 2 is the encryption method according to claim 1, wherein the autocorrelation reducing step truncates a specific bit string from the plaintext data and outputs it as the secondary data. And

  According to the second aspect of the present invention, the encryption method truncates a specific bit string from the plaintext data and outputs it as secondary data to encrypt the secondary data.

  An encryption method according to a third aspect of the present invention is the encryption method according to the first aspect, wherein the autocorrelation reducing method truncates the bit string when the plaintext data includes a specific bit string, and When the plaintext data is compressed data, the plaintext data is output as it is as the secondary data. When the plaintext data does not include the specific bit string and is not compressed data, the plaintext data is output. The data is subjected to a predetermined compression process and output as the secondary data.

  According to the third aspect of the present invention, when the plaintext data includes a specific bit string, the encryption method truncates the bit string and outputs it as secondary data. When the plaintext data is compressed data, the encryption method outputs the plaintext data. Output as secondary data as it is, and when the plaintext data does not contain a specific bit string and is not compressed data, the plaintext data is subjected to predetermined compression processing and output as secondary data, and the secondary data is encrypted .

  According to a fourth aspect of the present invention, there is provided a decryption device for decrypting encrypted data, the decryption processing means for performing decryption processing on the encrypted data, and the decryption And autocorrelation restoring means for performing plain autocorrelation restoration processing on the data obtained by the conversion processing and outputting plaintext data.

  According to the fourth aspect of the present invention, the decryption device performs decryption processing on the encrypted data, and then performs predetermined autocorrelation restoration processing to output plaintext data.

  According to a fifth aspect of the present invention, there is provided a decryption method for decrypting encrypted data, the decryption process step for performing decryption processing on the encrypted data, and the decryption And an autocorrelation restoring step of performing plain autocorrelation restoration processing on the data obtained by the conversion processing and outputting plaintext data.

  According to the fifth aspect of the present invention, the decryption method performs decryption processing on the encrypted data and then performs predetermined autocorrelation restoration processing to output plaintext data.

  According to a sixth aspect of the present invention, there is provided an encryption apparatus for encrypting plaintext data, wherein the plaintext data is subjected to processing for reducing autocorrelation and output as secondary data. It is characterized by comprising a reducing means and an encryption processing means for encrypting the secondary data.

  According to the sixth aspect of the present invention, the encryption apparatus performs processing for reducing autocorrelation on the plaintext data, outputs it as secondary data, and encrypts the secondary data.

  According to the first aspect of the present invention, the encryption method performs processing for reducing autocorrelation on plaintext data, outputs it as secondary data, and encrypts the secondary data. It is possible to obtain an encryption method that improves the encryption strength while suppressing the load on the device.

  According to the second aspect of the invention, the encryption method truncates a specific bit string from the plaintext data and outputs it as secondary data to encrypt the secondary data. It is possible to obtain an encryption method that suppresses the load on the device and improves the encryption strength.

  According to the invention of claim 3, the encryption method is such that when the plaintext data includes a specific bit string, the bit string is truncated and output as secondary data, and when the plaintext data is compressed data, the plaintext data is output. Is output as secondary data, and if the plaintext data does not contain a specific bit string and is not compressed data, the plaintext data is subjected to a predetermined compression process and output as secondary data, and the secondary data is encrypted. Therefore, there is an effect that it is possible to obtain an encryption device that improves the encryption method while suppressing the load on encryption and decryption processing according to the type of data.

  According to the invention of claim 4, the decryption device performs decryption processing on the encrypted data, and then performs predetermined autocorrelation restoration processing to output plaintext data. There is an effect that it is possible to obtain a decryption device that performs strong encrypted communication while suppressing a load on processing.

  According to the invention of claim 5, the decryption method performs decryption processing on the encrypted data and then performs predetermined autocorrelation restoration processing to output plaintext data. There is an effect that it is possible to obtain a decryption method that performs strong encrypted communication while suppressing a load on processing.

  According to the invention of claim 6, since the encryption device performs processing for reducing autocorrelation on the plaintext data and outputs it as secondary data, and encrypts the secondary data. It is possible to obtain an encryption device that improves the encryption strength while suppressing the load on the process.

  Exemplary embodiments of an encryption device, an encryption method, a decryption device, and a decryption method according to the present invention will be described below in detail with reference to the accompanying drawings.

  FIG. 1 is an explanatory diagram for explaining the concept of cryptographic communication according to an embodiment of the present invention. In the figure, a server 1 and a terminal device 2 are connected via a network 3. Here, the terminal device is an in-vehicle information terminal or a mobile phone.

  As shown in the figure, when communication is performed from the server 1 to the terminal device 2, first, processing for reducing autocorrelation is performed on plaintext data, and secondary data that is obtained as a result is processed. Perform encryption processing. Then, the ciphertext generated by the encryption is transmitted via the network 3.

  Accordingly, the terminal device 2 can extract secondary data by performing decryption processing on the received ciphertext, and performs processing (post-processing) for restoring autocorrelation on the extracted secondary data. To obtain plaintext data.

  Similarly, when communication is performed from the terminal device 2 to the server 1, the terminal device 2 performs processing for reducing autocorrelation on plaintext data, and encrypts the secondary data that is obtained as a result. The process is performed. Then, the ciphertext generated by the encryption is transmitted to the server 1 via the network 3.

  Thereafter, the server 1 extracts the secondary data by performing a decryption process on the received ciphertext, and further performs a process (post-processing) for restoring the autocorrelation on the extracted secondary data. Get.

  In this way, by performing the process of reducing the autocorrelation in advance before encryption, the processing load is relatively small, and even when an algorithm with a low encryption strength obtained by the encryption process itself is adopted, The autocorrelation of the finally obtained ciphertext can be reduced to reduce the data bias and increase the difficulty of decryption.

  Next, a specific configuration example of the server 1 will be described with reference to the schematic configuration diagram shown in FIG. As shown in the figure, the server 1 includes therein a preprocessing unit 10, an encryption processing unit 20, a transmission processing unit 31, an authentication ID creation unit 32, a terminal information database 33, an authentication processing unit 34, a reception processing unit 35, It has a decryption processing unit 40, a post-processing unit 50, a rule change processing unit 60, and a rule storage unit 61.

  The preprocessing unit 10 is a processing unit that generates secondary data by performing processing for reducing autocorrelation on plaintext data, and outputs the generated secondary data to the encryption processing unit 20. The encryption processing unit 20 is a processing unit that performs encryption processing on the secondary data to create encrypted data, and outputs the created encrypted data to the transmission processing unit 31 and the authentication ID creation unit 32.

  The terminal information database 33 stores identification information unique to each terminal device regarding the terminal device group (including the terminal device 2) that communicates with the server 1. The authentication ID creating unit 32 creates an authentication ID to be used at the time of communication using the identification information of the terminal device that is the data transmission destination and the encrypted data created by the encryption processing unit 20. Then, the transmission processing unit 31 performs processing for adding the authentication ID created by the authentication ID creating unit 32 to the encrypted data created by the encryption processing unit 20 and transmitting it.

  When receiving data from the outside, the reception processing unit 35 outputs the authentication ID to the authentication processing unit 34 and the encrypted data (the body of the received data) to the authentication processing unit 34 and the decryption processing unit 40, respectively.

  The authentication processing unit 34 performs an authentication process using the authentication ID, the encrypted data, and the terminal information database 33, and performs a process of confirming that the data transmission source is an authorized user.

  The decryption processing unit 40 is a processing unit that performs decryption processing on the encrypted data output from the reception processing unit 35. By this decryption processing, secondary data (data obtained by subjecting plaintext data to autocorrelation reduction processing) can be obtained from the encrypted data. The post-processing unit 50 performs processing for restoring the autocorrelation on the secondary data output from the decryption processing unit 40, and outputs it as plain text data.

  The rule storage unit 61 stores specific contents of autocorrelation reduction processing by the preprocessing unit 10, encryption processing by the encryption processing unit 20, and authentication ID creation processing by the authentication ID creation unit 32. Further, the authentication processing unit 34 executes authentication processing by referring to the authentication ID creation rule during the authentication processing, and the decryption processing unit 40 executes decryption processing by referring to the processing content of the encryption processing. The processing unit 50 executes the autocorrelation restoration process with reference to the processing content of the autocorrelation reduction process.

  The rule change processing unit 60 performs processing for changing these processing contents stored in the rule storage unit 61. Further, the processing content of the change processing itself by the rule change processing unit 60 is also stored in the rule storage unit 61, and the content of the change processing itself can be changed.

  As shown in FIG. 3, the specific configuration of the terminal device 2 includes terminal identification information 36 instead of the terminal information database 33 included in the server 1, and the authentication ID creating unit 37 and the authentication processing unit 38 include the terminal identification information. 36 is the same as the server 1 except that an authentication ID creation process and an authentication process are performed using 36, the same components are denoted by the same reference numerals, and description thereof is omitted.

  The terminal identification information 36 is identification information of information unique to the own terminal. The authentication ID creating unit 37 creates an authentication ID used at the time of communication using the terminal identification information 36 and the encrypted data created by the encryption processing unit 20. Further, the authentication processing unit 38 performs an authentication process using the received authentication ID, encrypted data, and terminal identification information 36, and performs a process of confirming that the data transmission source is an authorized user.

  Next, a detailed configuration and specific processing contents of each processing unit will be described. First, the preprocessing unit 10 includes a plaintext receiving unit 11, a classification processing unit 12, a specific bit truncation unit 13, and a compression processing unit 14 therein.

  The plaintext receiving unit 11 is a processing unit that receives plaintext data to be encrypted and transmitted, and the classification processing unit 12 performs a process of classifying the plaintext data received by the plaintext receiving unit 11. Specifically, the classification processing unit 12 converts the plaintext data into “data including a specific bit string”, “compressed data in which some compression is performed”, “not including a specific bit string and being not compressed. Classify as “data”.

  The specific bit truncation unit 13 performs a process of truncating a specific bit string on the plaintext data classified as “data including a specific bit string” by the classification processing unit 12. For example, as shown in FIG. 4, when the plaintext data is a set of data whose upper 3 bits are 0, the common upper 3 bits are discarded and the remaining data are concatenated, and the truncation condition, Identification information indicating that the upper 3 bits are 0 is added and output as secondary data.

  Here, the case where the upper 3 bits are all 0 has been described as an example, but the same applies to the case where, for example, the lower bits are aligned, or there is a bit string common to a specific internal position. It can be handled. Further, the common bit string is not limited to all 0s or all 1s, but can be handled in the same manner when any bit string such as “00101” is shared.

  In this way, the autocorrelation of plaintext data can be easily reduced by truncating the common bit string.

  On the other hand, the data classified as “compressed data subjected to some kind of compression” by the classification processing unit 12 is considered to have reduced autocorrelation due to the compression, and thus is directly output as secondary data.

  Furthermore, the compression processing unit 14 performs a predetermined compression process on the plaintext data classified as “data that does not include a specific bit string and is not compressed by the classification processing unit 12, thereby providing autocorrelation. Reduced and output as secondary data. Here, in the normal compression algorithm, information indicating the data compression method is added to the beginning of the compressed file. However, the compression processing unit 14 eliminates the information indicating the compression method, thereby restoring the data by a third party. Inhibits.

  Thus, by classifying plaintext data and performing processing according to the classification result, the autocorrelation of plaintext data can be reduced easily and effectively. Note that information indicating which classification has been performed on the secondary data output by the preprocessing unit 10 may be added.

  Further, the classification processing content by the classification processing unit 12 is stored in the rule storage unit 61 as a classification rule, the processing content by the specific bit truncation unit 13 is stored as a bit truncation rule, and the processing content by the compression processing unit 14 is stored as a compression rule. Yes.

  Next, the configuration and processing of the encryption processing unit 20 will be described. The encryption processing unit 20 includes a block division processing unit 21, a block replacement processing unit 22, a camouflage processing unit 23, and a substitution processing unit 24 therein.

  The block division processing unit 21 divides the secondary data output from the preprocessing unit 10 into a plurality of fragments (blocks) as shown in FIG. Here, it is preferable to divide the size of one block so as to be unequal intervals within a range of, for example, 1 to 256 bytes. Further, no division is performed when the size of the secondary data itself is, for example, 4 bytes or less.

  Similarly, as illustrated in FIG. 5, the block replacement processing unit 22 performs a process of replacing (rearranging) the blocks generated by the block division processing unit 21. The order of the blocks after the replacement needs to appear random with respect to the appearance order on the original secondary data.

  The camouflage processing unit 23 embeds the block sequence output by the block replacement processing unit 22 in the camouflaged data as shown in FIG. 6 and outputs it as tertiary data. The impersonation data is executable binary data, image data, audio data, etc. that are unrelated to the original plaintext data. Further, it is necessary to prevent inconsistency in the camouflaged data itself even after the block string is embedded in the camouflaged data. Therefore, the place where the block is embedded is an area where an arbitrary bit pattern is allowed to appear in the camouflaged data.

  For example, in an executable binary, an arbitrary bit pattern is allowed to appear in sections such as an instruction sequence, a data initial value, and fixed data. If a block sequence is embedded in the header section or relocation information section, the embedded block is suspicious when the impersonation data (tertiary data) after embedding is validated as an executable binary. May be detected as incorrect data.

  Furthermore, pointers pointing to other sections and data in the file are embedded in the original impersonation data at various places. For this reason, when embedding a block, the values of various pointers are corrected to maintain the consistency of the relationship between the pointer and index destination data.

  In addition, information indicating the type of data is added to the beginning of the camouflaged data that is originally executable binary, image data, audio data, or the like. If this is left behind, it may become a clue for decryption (identifying the tertiary data from the encrypted data after substitution) when the tertiary data is encrypted by the substitution method in the next stage. . Therefore, information indicating the type originally provided in the camouflage data is removed by the camouflage processing unit 23.

  By the way, the data used as the camouflaged data is more difficult to specify the embedded block as the size is larger than the size of the embedded block, and the camouflage effect is enhanced. However, if the size of the camouflaged data is large, the subsequent processing and the load during communication also increase.

  Therefore, the camouflage processing unit 23 includes a camouflaged data selection unit 23a, and the camouflaged data selection unit 23a selects the camouflaged data based on the importance level and data size of the original data. For example, when transmitting highly important data, use large-sized impersonation data as the cipher strength highest priority mode, and when transmitting less important data, use small amount of impersonation as the traffic priority mode. Use the data. Further, it is preferable to set one or more intermediate modes between these modes and select them appropriately.

  The counterfeit data may be selected from those prepared in advance in the server 1 or the terminal device 2, or may be created when performing the forgery process. Moreover, you may select from the data which can be used as camouflaged data among the other data which the server 1 and the terminal device 2 have.

  The substitution processing unit 24 performs an encryption process using the substitution method on the tertiary data output by the camouflage processing unit 23, and outputs the processing result as encrypted data.

  A division rule indicating the processing content of the division by the block division processing unit 21, a replacement rule indicating the processing content of the block replacement by the block replacement processing unit 22, an embedding processing content by the impersonation processing unit 23, and an impersonation data selection by the impersonation data selection unit 23a The disguise rule indicating the processing content of the above and the substitution table used for substitution by the substitution processing unit 24 are stored in the rule storage unit 61, respectively.

  As shown in FIG. 7, the authentication ID creation unit 32 acquires terminal identification information of a terminal device that is a data transmission destination from the terminal information database 33, and creates an authentication ID using the terminal identification information and encrypted data. To do. For example, processing such as extracting data from encrypted data and terminal identification information according to a predetermined rule and combining them, or calculating a hash value using a one-way hash function and creating its exclusive logical ring To create an authentication ID.

  In this way, by creating an authentication ID from the encrypted data and the terminal identification information, a different authentication ID is used for each connection, and fraud due to impersonation, etc., compared to using the terminal identification information itself as a public key. Connection can be prevented more effectively. Note that the processing content of the authentication ID creation processing by the authentication ID creation unit 32 is stored in the rule storage unit 61 as an ID creation rule.

  The rule change processing unit 60 performs processing for changing various rules already described. Contents of processing such as autocorrelation reduction processing, encryption processing, ID creation processing, etc. and various parameters used for them are fixed. This is because there is a possibility that the parameters may be overlooked.

  Specifically, the upper limit of the effective number is set for the processing contents and parameters, that is, the rule, and when the usage number reaches the upper limit, the rule is updated to a new rule.

  In order to realize this function, each rule holds two sets of “common rule” and “preliminary rule”. Then, a counter indicating “remaining number of usable times” is prepared in the regular rule, and updated every time it is used.

  When the “remaining number of usable times” is “0”, the rule change processing unit 60 switches the rule that has been the “preliminary rule” to the “common rule” and uses it in the subsequent processing. Furthermore, the rule change processing unit 60 prepares for the next rule switching by creating a new “preliminary rule”.

  The processing content of the change processing itself by the rule change processing unit 60 is also stored as a change rule in the rule storage unit 61, and safety can be further improved by changing the content of the change processing itself.

  The new preliminary rule is changed by the server 1, and the terminal device 2 is notified of the changed new rule. In the terminal device 2, the received preliminary rule is stored in the rule storage unit 61.

  The decryption processing unit 40 includes therein a substitution processing unit 41, a camouflage release unit 42, a block replacement processing unit 43, and a block combination processing unit 44. The substitution processing unit 41 is a processing unit that releases the substitution performed on the encrypted data using the substitution table and restores the tertiary data.

  The camouflage cancellation unit 42 performs a process of extracting a block embedded in the tertiary data based on the camouflage rule. Further, after the block replacement processing unit 43 rearranges the order of the blocks based on the replacement rule and returns to the original order, the block combining processing unit 44 performs the combining process to obtain secondary data.

  The post-processing unit 50 includes therein a classification processing unit 51, a specific bit addition unit 52, a decompression processing unit 53, and a plaintext output unit 54.

  The classification processing unit 51 is a processing unit that identifies and classifies what autocorrelation reduction processing is performed on the secondary data. As a result of this classification, when the secondary data has a specific bit truncated, the bit truncated by the specific bit adding unit 52 is restored and sent to the plaintext output unit 54.

  If the secondary data has been subjected to a predetermined compression process in the autocorrelation recommendation process, the decompression processing unit 53 performs decompression processing to restore the autocorrelation and sends it to the plaintext output unit 54. Further, if the secondary data has not been subjected to any processing in the autocorrelation reduction processing (that is, if it has been compressed data that has undergone some compression processing in advance), the classification processing unit 51 The data is sent to the plaintext output unit 54 as it is.

  The plaintext output unit 54 outputs the data output from the specific bit adding unit 52, the decompression processing unit 53, or the classification processing unit 51 as plaintext data.

  Next, the processing operation of the preprocessing unit 10 will be described with reference to the flowchart of FIG. This processing flow is started when the plaintext receiving unit 11 receives plaintext data to be encrypted.

  The classification processing unit 12 first determines whether or not a specific bit string exists in the plaintext data (step S101). As a result, if there is a specific bit string, the specific bit truncation unit 13 discards the bit string (step S105), and then outputs it as secondary data (step S104), and the process is terminated.

  On the other hand, when the specific bit string does not exist (step S101, No), the classification processing unit next determines whether the plain text data is a compressed file (step S102). As a result, if it is a compressed file (step S102, Yes), it is output as secondary data as it is (step S104), and the process is terminated. If it is not a compressed file (step S102, No), the compression process is performed. After a predetermined compression process is performed by the unit 14 (step S103), the data is output as secondary data (step S104), and the process ends.

  As described above, the server 1 and the terminal device 2 according to the present embodiment suppress the load on the encryption and decryption processes by performing the process of reducing the autocorrelation in advance before the encryption. However, the encryption strength is improved.

  Therefore, even a terminal having only a limited computing capability (such as a low-speed CPU) such as an in-vehicle terminal or a portable terminal can perform encrypted communication as necessary. Further, even in communications that require a reduction in processing load, communication contents can be concealed within the allowable processing load range.

  As described above, the encryption device, encryption method, decryption device, and decryption method according to the present invention are useful for concealing communication, and are particularly suitable for balancing the processing load of encryption strength.

It is explanatory drawing explaining the concept of the encryption communication which is embodiment of this invention. FIG. 2 is a schematic configuration diagram illustrating a schematic configuration of a server illustrated in FIG. 1. It is a schematic block diagram which shows the schematic structure of the terminal device shown in FIG. It is explanatory drawing explaining the autocorrelation reduction by truncating a specific bit. It is explanatory drawing explaining the division | segmentation and rearrangement of secondary data. It is explanatory drawing explaining the embedding to impersonation data. It is explanatory drawing explaining creation of authentication ID. It is a flowchart explaining the processing operation | movement of an autocorrelation reduction process.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Server 2 Terminal device 3 Network 10 Pre-processing part 11 Plain text reception part 12 Classification processing part 13 Specific bit truncation part 14 Compression processing part 20 Encryption processing part 21 Block division processing part 22 Block replacement processing part 23 Impersonation processing part 23a Impersonation data Selection unit 24 Substitution processing unit 31 Transmission processing unit 32, 37 Authentication ID creation unit 33 Terminal information database 34, 38 Authentication processing unit 35 Reception processing unit 36 Terminal identification information 40 Decoding processing unit 41 Substitution processing unit 42 Impersonation release unit 43 Block Replacement processing unit 44 Block combination processing unit 50 Post processing unit 51 Classification processing unit 52 Specific bit addition unit 53 Decompression processing unit 54 Plain text output unit 60 Rule change processing unit 61 Rule storage unit

Claims (6)

An encryption method for encrypting plaintext data,
An autocorrelation reduction step of performing processing to reduce autocorrelation on the plaintext data and outputting as secondary data;
An encryption process for encrypting the secondary data;
An encryption method characterized by comprising:   2. The encryption method according to claim 1, wherein the autocorrelation reducing step truncates a specific bit string from the plaintext data and outputs it as the secondary data.   In the autocorrelation reducing method, when the plaintext data includes a specific bit string, the bit string is truncated and output as the secondary data, and when the plaintext data is compressed data, the plaintext data is used as it is. Output as secondary data, and when the plaintext data does not include the specific bit string and is not compressed data, the plaintext data is subjected to predetermined compression processing and output as the secondary data. Item 2. The encryption method according to Item 1. A decryption device for decrypting encrypted data,
Decryption processing means for performing decryption processing on the encrypted data;
Autocorrelation restoration means for performing plain autocorrelation restoration processing on the data obtained by the decryption processing and outputting plaintext data;
A decoding device comprising: A decryption method for decrypting encrypted data,
A decryption processing step of performing decryption processing on the encrypted data;
An autocorrelation restoration step of performing a predetermined autocorrelation restoration process and outputting plaintext data to the data obtained by the decoding process;
The decoding method characterized by including. An encryption device for encrypting plaintext data,
Autocorrelation reducing means for performing processing for reducing autocorrelation on the plaintext data and outputting as secondary data;
An encryption processing means for encrypting the secondary data;
An encryption device comprising:

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