JP2007134860A - Encryption program and decryption program - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a technology capable of reducing the time required for processings, while keeping security and confidentiality. <P>SOLUTION: In an encryption program disclosed herein for encrypting data, the data are compression-coded data. The encryption program generates each of random number sequences, each length of which is shorter than the length of the compression-coded data. The encryption program sequentially segments a random number sequence in length, depending on the compression rate of the data. The encryption program uses each segmented random number sequence to encrypt a prescribed position of the compression-coded data, corresponding to each segmented random number sequence. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to encryption and decryption techniques.

  In recent years, it has become increasingly important to ensure safety and confidentiality when transmitting and receiving various types of data via a network such as the Internet. Therefore, various encryption algorithms have been proposed and standardized as a method of transmitting the data while hiding the contents of the data from a third party.

  In general, the encryption algorithm is divided into a block cipher and a stream cipher. Among these, the block cipher is a collective term for a predetermined number of bits, that is, a general term for an encryption algorithm for processing a block at a time, and the number of bits of a block is called a block length. FIG. 16A is a diagram for explaining a block cipher method according to the prior art. A plaintext having a block length that is an object of encryption in the block cipher algorithm is referred to as a “plaintext block”. A ciphertext obtained by encrypting a plaintext block using this block cipher algorithm is referred to as a “ciphertext block”.

  The other stream cipher is a general term for a cipher algorithm that sequentially processes a data flow, that is, a stream. In the stream cipher, for example, encryption processing is executed in units of 1 bit, 8 bits, 32 bits, or the like. FIG. 16B is a diagram for explaining a stream encryption method according to the prior art. In the stream cipher according to the prior art, a random number sequence having the same length as the plaintext, that is, a stream cipher is calculated based on the initial value, and an exclusive OR is calculated between the calculated random number sequence and the plaintext (image code). take.

  By the way, especially with regard to encryption of JPEG (Joint Photographic Experts Group) image codes used for compressing images in a storage device such as a digital camera or a hard disk, it is scrambled so that only a part can be seen by a third party. There is a method for performing encryption or a method for encrypting the entire image. In order to encrypt the entire image, a method of applying an encryption algorithm in the process of applying JPEG compression from an uncompressed original image has been conventionally used.

When encrypting an entire image, the amount of image data is larger than that of document data. Therefore, when encrypting image data such as JPEG image code, it takes time to calculate ciphertext having the same length as plaintext. Will be required. Therefore, a technique for reducing the processing required for encryption by encrypting a predetermined portion in plain text using a stream cipher shorter than plain text is disclosed (for example, Patent Documents 1 and 2).
JP 2002-175008 A JP-A-6-125553

  For example, even for data with a large capacity, such as JPEG image code, etc., it is important to encrypt the part that should be encrypted, that is, the part that requires the data contents not to be known by a third party. Therefore, it is expected that the speed of the encryption process will be increased while maintaining the same security and confidentiality as when the entire code is encrypted.

  The present invention reduces the time required for processing while encrypting data that has been compressed and encoded, and decrypts the data thus encrypted. It aims at providing the technology that can do.

  In order to solve the above problems, the present invention provides an encryption program for encrypting data, wherein the data is a compression code, and the encryption program has a length shorter than the length of the compression code. A random number sequence having a length corresponding to the compression ratio of the data, and sequentially cutting out the random number sequence, and using the extracted random number sequence, the extracted random number sequence in the compression code The computer is configured to execute a process for encrypting a corresponding predetermined portion. Using a random number sequence having a shorter length than the compression code, an encryption process is executed for a predetermined portion to be encrypted in the compression code.

  The predetermined portion may be determined based on the cut random number. Furthermore, the data may be image data, and the compression code may be an image code generated by compression encoding according to a JPEG method.

  The present invention is also a decryption program for decrypting encrypted data, wherein the data is a compression code, and the decryption program uses a predetermined initial value used in the encryption process. Using a random number sequence having a length shorter than the length of the compression code, sequentially cutting out the random number sequence at a length according to the compression rate of the data, and using each of the extracted random number sequences, The computer is configured to execute a process of decoding a predetermined portion corresponding to each extracted random number sequence in the compressed code.

  According to the present invention, it is possible to perform higher-speed encryption / decryption processing while maintaining safety and confidentiality.

DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described in detail with reference to the drawings.
FIG. 1 is a principle diagram of an encryption and decryption method according to the present invention. The encryption method and the decryption method according to the present invention are used when, for example, a JPEG (Joint Photographic Experts Group) image code obtained by compressing an image is encrypted / decrypted in a storage device such as a digital camera or a hard disk. This is realized by executing an encryption / decryption program stored in the memory of the computer.

  As shown in FIG. 1A, in the encryption method according to the present invention, first, a random number sequence is generated based on the calculated initial value (initialization vector). The initial value used here is composed of a random bit string from which a different value is calculated for each encryption. The generated random number sequence has a shorter length than plain text such as a JPEG image code to be encrypted. Next, using the generated random number sequence, a number sequence having the same length as the plaintext is generated. In the generated number sequence, the previously generated random number sequence is arranged in a distributed manner so that an exclusive OR (XOR) can be obtained with a portion to be encrypted in the plaintext. When an initial value is given to the head of the ciphertext obtained by taking the exclusive OR, the ciphertext is output.

  The ciphertext encrypted by the method shown in FIG. 1A is decrypted by the method shown in FIG. As shown in FIG. 1B, first, an initial value given to the beginning of the ciphertext is cut out, and a random number sequence is generated based on the cut out initial value. Since the random number sequence generated here is generated based on the initial value given to the ciphertext, it corresponds to the random number sequence generated for use in encryption by the method of FIG. Yes. Next, using the generated random number sequence, a number sequence having the same length as the ciphertext is generated. In the generated number sequence, the previously generated random number sequence is distributed and arranged so that an exclusive OR can be obtained with a portion to be decrypted in the ciphertext. The plaintext is obtained as a result of the exclusive OR of the sequence and the ciphertext.

  FIG. 2 is a configuration diagram of the encryption device 1 according to the present embodiment. The encryption device 1 includes an initial value calculation unit 11, a random number sequence generation unit 12, a random number sequence storage unit 13, a random number sequence distribution unit 14, a distributed random number sequence storage unit 15, an image code storage unit 16, an encryption unit 17, and a ciphertext. An output unit 18 is included.

  Each time the initial value calculation unit 11 executes a process for encrypting a certain plaintext, the initial value calculation unit 11 generates a different random bit string, and gives the generated bit string to the random number string generation unit 12 as an initial value. The random number sequence generation unit 12 generates a random number sequence based on the initial value. The generated random number sequence has, for example, about half the length of plaintext to be encrypted. The random number sequence storage unit 13 stores the generated random number sequence.

  The random number sequence distribution unit 14 reads the random number sequence stored in the random number sequence storage unit 13, sequentially reads from the top, and arranges it in an empty bit sequence having the same length as the plaintext to be encrypted. How many bits are read from the random number sequence is determined according to the compression rate of the JPEG image code. The distributed random number sequence storage unit 15 stores a number sequence (hereinafter abbreviated as a number sequence) generated by the random number sequence distribution unit 14 in which the random number sequence is distributed.

  On the other hand, the JPEG image code input to the encryption device 1 is given to the random number sequence distribution unit 14 and stored in the image code storage unit 16. Specific processing will be described later. The encryption unit 17 reads out the number sequence stored in the distributed random number sequence storage unit 15 and the image code storage unit 16 and the image code as plain text, and performs exclusive OR to encrypt them. The ciphertext output unit 18 outputs a ciphertext.

  FIG. 3 is a configuration diagram of the decoding device 2 according to the present embodiment. The decryption device 2 includes a ciphertext input unit 21, an initial value cutout unit 22, a random number sequence generation unit 23, a random number sequence storage unit 24, a random number sequence distribution unit 25, a distributed random number sequence storage unit 26, a ciphertext storage unit 27, a decryption And a plaintext output unit 29.

  The ciphertext input unit 21 is an interface that receives an encrypted image code. The initial value cutout unit 22 cuts out an initial value given to the head of the input ciphertext. The random number sequence generation unit 23 generates a random number sequence based on the initial value cut out by the initial value cutout unit 22. The random number sequence storage unit 24 stores the generated random number sequence. When the image code encrypted by the encryption device 1 in FIG. 2 is decrypted, the initial value given to the input ciphertext is the initial value calculated by the initial value calculation unit 11 of the encryption device 1. Matches the value. By generating based on the same initial value, the random number sequence generation unit 23 of the decryption device 2 in FIG. 3 generates the same random number sequence as that generated in the encryption device 1 in FIG.

  The operations of the random number sequence distribution unit 25 and the distributed random number sequence storage unit 26 are the same as those of the random number sequence distribution unit 14 and the distributed random number sequence storage unit 15 in FIG. The encrypted JPEG image code input to the decryption device 2 is given to the initial value cutout unit 22 via the ciphertext input unit 21 and stored in the ciphertext storage unit 27. The decryption unit 28 reads out the number sequence and ciphertext stored in the distributed random number sequence storage unit 26 and the ciphertext storage unit 27, respectively, and performs exclusive OR to decrypt them. The plaintext output unit 29 outputs an image code that is plaintext obtained by decoding.

  A compression code such as a JPEG image code is encrypted by the encryption apparatus 1 shown in FIG. 2, and the ciphertext is decrypted by the decryption apparatus 2 shown in FIG. The decryption process is the same as the encryption process except that the exclusive OR is performed on the ciphertext based on the random number generated using the initial value used for encryption. Hereinafter, the encryption method according to the present embodiment will be mainly described.

  FIG. 4 is a configuration diagram of the random number sequence distribution unit 14 in the encryption device 1 according to the present embodiment. The random number sequence distribution unit 14 includes a compression rate calculation unit 41, a random number sequence cutout unit 42, a partial random number sequence storage unit 43, a start position calculation unit 44, a sequence calculation unit 45, and a reference table (lookup table, hereinafter referred to as LUT) 46. including.

  As shown in the drawing explaining the compression rate calculation method in FIG. 5, the compression rate calculation unit 41 includes the image code input to the encryption device 1 and the original image obtained from the header information of the image code. The compression ratio is calculated from the ratio of the sizes of. The random number sequence cutout unit 42 cuts out values from the random number sequence by the number of bits corresponding to the calculated compression rate. The extracted random number is hereinafter referred to as a partial random number sequence. The partial random number sequence storage unit 43 stores a partial random number sequence.

  Based on the partial random number sequence and the compression rate of the image code, the start position calculation unit 44 and the sequence calculation unit 45 calculate the start position and the post-arrangement sequence where the partial random number sequence is arranged in an empty bit sequence having the same length as the plaintext, respectively. To do. The process of calculating the numerical sequence by these units is performed by referring to the LUT 46.

  By the random number sequence distribution unit 14 shown in FIG. 4, the random number sequence is cut out from the beginning and sequentially arranged in the empty bit sequence, and when performing the encryption process, a sequence for taking an exclusive OR with the plaintext is obtained. Created. The plain text is, for example, a JPEG image code, and in the present embodiment, the position to be encrypted is intensively encrypted in the JPEG image code so that safety and confidentiality are maintained.

  FIG. 6 is a diagram for explaining codes to be encrypted in the encryption method according to the present embodiment. The JPEG image is first subjected to discrete cosine transform (hereinafter abbreviated as DCT) in units of blocks, with 8 × 8 64 pixels (64 bytes) as one block. Next, the DCT coefficients obtained by DCT are arranged so that the frequency decreases in the horizontal and vertical frequency components in the left and upper directions in the figure, respectively, as shown in FIG. When encoding each value of the DCT coefficient of the 8 × 8 pixel block, zigzag scanning is performed. The larger the quantization step value in the quantization table, the higher the compression ratio, the smaller the total data amount. Note that each DCT component in the case where the compression rate shown in FIG. 6 is relatively high and low is shown to be almost equal in length for the sake of simplicity. Actually, the DCT coefficient having a lower frequency has a longer bit length.

It is known that among the compression codes encoded by entropy, the DC component DC and the AC components AC0, AC1, and AC2 account for about 80% of the entire code. Each compression code is composed of a Huffman code and additional bits.
Both the Huffman code and the number of load bits are set for the difference value of the DC coefficient (difference value with the DC coefficient of the adjacent block) and the result of grouping the AC coefficient values. As for the Huffman code, a code corresponding to the appearance probability of each group is specified in the code table. However, in the case of the AC coefficient Huffman code, it is set by a combination of the appearance probability and the run length (run length) of the invalid coefficient in the block.

  The number of bits is set for each group as described above, and indicates which coefficient the previous Huffman code indicates belongs to the same group. Since this appears with an equal probability with the number of bits fixed, it is relatively difficult to estimate by a third party.

  Thus, in the present embodiment, by encrypting these JPEG compressed image codes, that is, additional bits of DC, AC1, AC2, and AC3, the processing required for encryption is shortened while maintaining safety. At this time, according to the compression rate of JPEG, the position of the corresponding additional bit is determined according to the length of each different DCT coefficient. Hereinafter, with reference to FIGS. 7 to 11, in the encryption method according to the present embodiment, a method for realizing the encryption of the additional bits in the JPEG image code will be specifically described.

  FIG. 7 is a diagram for explaining the operation of the random number sequence distribution unit 14. The random number sequence R read from the random number sequence storage unit 13 has a length that is half or less of the plaintext (JPEG image code) to be encrypted. Specifically, for example, it has a length of 30 to 50% of plain text. The length of the random number sequence R is appropriately set according to the compression rate of the plaintext JPEG image code. Processing for determining the length of the random number sequence R will be described later.

The random number sequence R is cut out in order from the head, and partial random number sequences R ₁ , R ₂ ,... (Hereinafter referred to as partial random number sequence R ₁ etc.) are generated. The partial random number sequence R ₁ or the like is arranged at a predetermined location of an encryption number sequence C (hereinafter abbreviated as number sequence C) based on information held by itself. In this way, the random number sequence R is divided into a number sequence C and arranged as shown in FIG.

It should be noted that the number sequence C stores “0” before the partial random number sequence R _{1 and the} like are arranged. That is, when the number sequence C in which the divided random number sequence R (partial random number sequence R _{1 and the} like) is arranged is ORed with the plaintext, the portion where the random number sequence R is arranged becomes a ciphertext. As for the other parts, the same bit string as the plaintext is obtained as a result of taking the exclusive OR.

FIG. 8 is a diagram for further explaining the operation of the random number sequence distribution unit 14. Based on the calculated compression rate, a predetermined number of bits are cut out from the beginning of the random number sequence R. In the example of FIG. 8, it is assumed that a predetermined number of bits is a designated number of bits and n bits (n is a positive integer of 8 or less). As shown in FIG. 8, the kth partial random number sequence R _k (k is a positive integer) cut out from the n-bit random number sequence is stored in an 8-bit empty bit sequence in the present embodiment. Based on the partial random number sequence R _k , the location for storing (arranging) the random numbers in the sequence C and the value of the stored random number are determined.

  FIG. 9 is a diagram for explaining generation of a distributed random number sequence in the present embodiment. According to the compression rate of the JPEG image code that takes the exclusive OR with the sequence C, for example, the case classification as shown in (a) to (d) of FIG. A sequence C is generated. The four examples shown in FIG. 9 are shown in order from the case where the compression rate of the JPEG image code is small to large. In this embodiment, (a) when the compression rate is 12% or more, (b) when the compression rate is 6% or more and less than 12%, (c) when the compression rate is 3% or more and less than 6%, and (d) the compression rate Divided when less than 3%.

First, with reference to FIG. 9A, a process of generating a sequence C using the partial random number sequence R _k when the compression rate is approximately 16% will be described. A partial random number sequence R _k obtained by cutting out a specified number of 8 bits from the random number sequence is shown on the left side of FIG. Of the 8 bits, the first 4 bits are used to determine where to store the latter 4 bits. The first 4-bit random number sequence is used as a positioning random number. The latter half of the 4-bit random number sequence is used as the XOR random number. With reference to the LUT 46 in FIG. 10, a method for determining the location where the random number for XOR is arranged using the random number for positioning will be described.

  FIG. 10 is an example of the LUT 46. 10 (a) to 10 (d) show that when the compression rate is (a) 12% or more, (b) When the compression rate is 6% or more and less than 12%, (c) The compression rate is 3% or more and less than 6%, respectively. And (d) the LUT 46 when the compression rate is less than 3%.

Specified bit string n = 8, that is, when the partial random number sequence R _k is composed of 8 bits, based on the 4 bits from the beginning of the partial random number sequence R _k, or to place the remaining 4 bits where on the sequence C To decide. For example, if the positioning random number is “0000”, the XOR random number is stored in the first bit to the fourth bit of the empty bit string, and if the positioning random number is “0001”, the second bit of the empty bit string is 5 The XOR random number is stored in the bit. In the example of FIG. 9A, since the positioning random number is composed of 4 bits, there are 2 ⁴ = 16 arrangement methods.

The XOR random number whose arrangement position on the empty bit is determined by the positioning random number is the remaining 4 bits of the extracted random number sequence, and its content is 2 ⁴ = 16. Therefore, 2 ⁴ × 2 ⁴ = 256 combinations of the sequence C generated by arranging the 4-bit XOR random numbers in the 8-bit partial random number sequence R _k in the 16-bit empty bit sequence for encryption. become.

FIG. 9 (b), for example, the compression ratio is a partial random number sequence R _k and the partial random number sequence R _k sequences generated from C in the case of approximately 8%. In this case, since the designated bit string n for cutting out the random number is 4 bits, as shown on the left side of FIG. 9B, the random number string R _k composed of 8 bits is indicated by shading. A value cut out from the random number sequence R is stored only in the 4 bits from the beginning, and “0” is stored in the rest.

As in the case of FIG. 9A, the first half of the random number of the designated number of bits is the positioning random number, and the second half is the XOR random number. In FIG. 9B, in the random number sequence R _k , the first 2 bits are positioning random numbers, and the subsequent 2 bits are XOR random numbers.

  Since there are 2 × 2 = 4 combinations of positioning random numbers, there are four combinations of positions arranged on an 8-bit number sequence. Furthermore, since there are 2 × 2 = 4 combinations of the XOR random numbers themselves, 4 × 4 = 16 combinations are prepared in the LUT 46 as shown in FIG.

FIG. 9C shows a partial random number sequence R _k and a number sequence C generated thereby when the compression rate is approximately 4%. At this time, the designated bit string n = 4. Unlike the above two examples, in the example of FIG. 9C, the entire partial random number sequence R _k is used as a random number for XOR. For example, the XOR random numbers are arranged at predetermined intervals in an empty bit string, and the positioning random numbers are not used.

As shown on the right side of FIG. 9 (c), when the compression rate is relatively small at about 4%, a partial random number sequence is stored every 2 bits on the 8-bit number sequence C. Unlike the cases (a) and (b) above, the positioning random numbers are not used. The random number storage position is determined in advance, and since the XOR random number is composed of 4 bits, 2 ⁴ = 16 combinations are prepared in the LUT 46.

FIG. 9D shows a partial random number sequence R _k and a generated number sequence C when the compression rate is approximately 2%. At this time, the designated bit string n = 4. Compared with the case of the compression rate of about 4% in FIG. 9C, it is different in that the random number for XOR is divided into four and stored in the sequence C every other bit when being distributed on the sequence C. .

  FIG. 11 is a diagram for explaining the operation of the encryption unit 17. The JPEG image code to be encrypted is read one word at a time from the image code storage unit 16 in FIG. The number sequence C for encrypting the image code is read word by word from the fractional random number sequence storage unit 15 and given to the encryption unit 17. In the encryption unit 17, an exclusive OR is performed between the image code and the sequence C to obtain a ciphertext. Here, one word is generally composed of 32 bits or more. (A) to (d) of FIG. 11 partially represent 16 bits or 8 bits of one word length = 32 bits. Further, the numerical sequence C shown in FIGS. 11A to 11D corresponds to the numerical sequence C generated in FIGS. 9A to 9D, respectively.

First, the encryption process shown in FIG. 11A when the compression rate is approximately 16% will be described. Of the plaintext, the DC component is composed of approximately 8 bits each for the Huffman code and the additional bits. The length of the subsequent AC component becomes shorter as compared with the DC component. As already described, in the present embodiment, the number sequence C generated when the compression rate is approximately 16% is 16 bits, and the partial random number sequence R _k is 4 bits. That is, although the bit length of each component is not constant, the sequence C is constant at 16 bits. Therefore, even if the sequence C and the plaintext are each given to the encryption unit 17 one word at a time, the top of the sequence C and each The timing with the beginning of the component does not match. However, since the lengths of the Huffman codes and additional bits in the DC component and the AC component are both determined based on the compression rate, and the length of the sequence C is also calculated based on the compression rate, the additional bits are generally XORed. Random numbers can be superimposed.

  Similarly, when the compression rate in FIG. 11B is approximately 8%, the length of the sequence C and the lengths of the DC component and the AC component are all determined based on the compression rate of the image code. The XOR random number in C can be superimposed on the additional bits in each component of the image code.

  Next, an encryption process when the compression rate is approximately 4% will be described with reference to FIG. At this time, the DC component and AC component of the image code are all about 4 bits, while the sequence C is 8 bits. As already described with reference to FIG. 9C, the XOR random numbers cut out from the random number sequence are stored in the sequence C every 2 bits. As a result, the XOR random numbers stored every two bits can be superimposed on the additional bits of each component.

  Similarly, when the compression rate is about 2% in FIG. 11D, the sequence C stores a random number for XOR for each bit. In each component of the image code, a Huffman code and an additional bit are stored. Appears at about every 1 bit, so that the random number for XOR can be almost superimposed on the additional bits.

As described with reference to FIGS. 11A to 11D, in the present embodiment, the position where the additional bits appear in the JPEG image code changes according to the compression rate of the image code. Is classified according to the compression rate, and the partial random number sequence R _k is stored at the position where the additional bit appears, and encryption is performed intensively.

  FIG. 12 is a flowchart showing encryption processing according to the present embodiment. The process illustrated in FIG. 12 is started when a JPEG image code to be encrypted is input to the encryption apparatus 1.

  First, in step S1, a partial random number sequence is generated, and in step S2, the compression rate of the JPEG image code to be encrypted is calculated. In step S3, it is determined whether or not the compression rate is equal to or greater than a first threshold value TH0. In the above embodiment, TH0 = 0.12. That is, when the compression rate is 12% or more, the process proceeds to step S4, where 4 bits are extracted from the partial random number sequence generated in step S1, and the encryption start position, that is, the above-described implementation is performed based on the extracted random number sequence in step S5. In the embodiment, the position for storing the XOR random number is calculated, and the process proceeds to step S10.

  If the compression rate is less than 12% in step S3, the process proceeds to step S6, and it is further determined whether or not the compression rate of the image code is equal to or greater than the second threshold value TH1. In the above embodiment, TH1 = 0.06, that is, the compression rate is 6%. When the compression rate of the image code is 6% or more in step S6, the process proceeds to step S7, where 2 bits are extracted from the partial random number sequence, and in step S8, the encrypted position is calculated based on the extracted random number sequence, Proceed to step S10.

  If the compression rate is less than 6% in step S6, the process further proceeds to step S9 to determine whether or not the compression rate of the image code is equal to or greater than the third threshold value TH2. In the above embodiment, TH2 = 0.03, that is, the compression rate is 3%. In step S9, after determining whether or not the compression rate of the image code is 3% or more, the process proceeds to step S10.

  In step S10, based on the results determined in steps S3, S6, and S9, the XOR random number is calculated according to the compression rate of the image code, for example, based on the obtained encryption start position. Alternatively, it is calculated by cutting out at a predetermined bit interval, and the process ends.

  FIG. 13 is a flowchart showing a decoding process according to this embodiment. The process illustrated in FIG. 13 is started when an encrypted JPEG image code is input to the decryption apparatus 2.

  First, in step S21, an initial value is cut out. In step S22, a partial random number sequence is generated based on the cut out initial value. In step S23, the compression rate of the input JPEG image code is calculated. In step S24, it is determined whether or not the compression rate is equal to or greater than the first threshold value TH0 (for example, 0.12). When the calculated compression rate is 12% or more, the process proceeds to step S25, where 4 bits are extracted from the partial random number sequence generated in step S1, and in step S26, the decoding start position, that is, the random number is extracted based on the extracted random number sequence. The position for storing the column is calculated, and the process proceeds to step S31.

  If the compression rate is less than 12% in step S24, the process proceeds to step S27, and it is further determined whether or not the compression rate is equal to or higher than the second threshold value TH1 (for example, 0.06). If the compression rate is 6% or more in step S27, the process proceeds to step S28, where 2 bits are extracted from the partial random number sequence. In step S29, the decoding position is calculated based on the extracted random number sequence, and the process proceeds to step S31. .

  If the compression rate is less than 6% in step S27, the process further proceeds to step S30 to determine whether or not the compression rate is equal to or greater than the third threshold value TH2 (for example, 0.03), and then step S31. Proceed to

  In step S31, based on the result determined in steps S24, S27, and S30, the random number is calculated in accordance with the compression rate of the encrypted image code, for example, based on the obtained decoding start position. Alternatively, calculation is performed by cutting out at predetermined bit intervals, and the process is terminated.

  Incidentally, the encryption device 1 in FIG. 2 and the decryption device 2 in FIG. 3 can be configured using, for example, an information processing device (computer) as shown in FIG. 14 includes a CPU (central processing unit) 101, a memory 102, an input device 103, an output device 104, an external storage device 105, a medium driving device 106, and a network connection device 107, which are connected to each other by a bus 108. It is connected.

  The memory 102 includes, for example, a read only memory (ROM), a random access memory (RAM), and the like, and stores programs and data used for processing. The CPU 101 performs necessary processing by executing a program using the memory 102.

The encryption device 1 in FIG. 2 and the decryption device 2 in FIG. 3 correspond to functions realized by executing a program stored in the memory 102.
The input device 103 is, for example, a keyboard, a pointing device, a touch panel, and the like, and is used for inputting instructions and information from an operator or the like. The output device 104 is, for example, a display, a printer, a speaker, and the like, and is used for outputting an inquiry to the operator, an alarm, a processing result, and the like.

  The external storage device 105 is, for example, a magnetic disk device, an optical disk device, a magneto-optical disk device, a tape device, or the like. The information processing apparatus stores the program and data in the external storage device 105 and loads them into the memory 102 for use as necessary.

  The medium driving device 106 drives a portable recording medium 109 and accesses the recorded contents. The portable recording medium 109 is an arbitrary computer-readable recording medium such as a memory card, a flexible disk, a CD-ROM (compact disk read only memory), an optical disk, and a magneto-optical disk. The operator stores the program and data in the portable recording medium 109 and loads them into the memory 102 for use as necessary.

  The network connection device 107 is connected to an arbitrary communication network such as a local area network (LAN) or the Internet, and performs data conversion accompanying communication. The information processing apparatus receives the program and data from an external apparatus via the network connection apparatus 107 as necessary, and loads them into the memory 102 for use.

  FIG. 15 shows a computer-readable recording medium that can supply a program and data to the information processing apparatus of FIG. Programs and data stored in the portable recording medium 109 and the database 113 of the server 111 are loaded into the memory 102 of the information processing apparatus 112. The server 111 generates a carrier signal that carries the program and data, and transmits the carrier signal to the information processing apparatus 112 via an arbitrary transmission medium on the network. The CPU 101 executes the program using the data and performs necessary processing.

  As described above, according to the encryption method and the decryption method according to the present embodiment, when encrypting / decrypting a JPEG image code obtained by compressing an image, according to the compression rate, Executes the process to encrypt / decrypt the information to be encrypted / decrypted, specifically, the information that is not necessary to hide the additional bits (eg, Huffman code). To do. By performing encryption / decryption processing with emphasis on the parts necessary to maintain the safety and confidentiality of information, the entire process is prevented while being decrypted by a third party. It can be shortened.

(Appendix 1)
An encryption program for encrypting data,
The data is a compression code;
The encryption program is
Generating a random number sequence having a length shorter than the length of the compression code;
The random number sequence is sequentially cut out with a length corresponding to the compression ratio of the data,
An encryption program that causes a computer to execute processing for encrypting a predetermined portion corresponding to each of the extracted random number sequences in the compression code using each of the extracted random number sequences.
(Appendix 2)
The encryption program according to appendix 1, wherein the predetermined portion is determined based on a cut random number.
(Appendix 3)
The encryption program according to appendix 2, wherein the predetermined portion is further determined based on a compression rate of the data.
(Appendix 4)
The encryption program according to appendix 2, wherein the data is image data, and the compression code is an image code generated by compression encoding according to a JPEG method.
(Appendix 5)
The predetermined portion is included in an additional bit in the image code that is a JPEG method,
The encryption program according to appendix 4, wherein the encryption process is performed by taking an exclusive OR between the extracted random number sequence and the additional bits.
(Appendix 6)
The random number sequence is calculated based on a predetermined initial value,
The encryption program according to appendix 5, wherein the encryption obtained as a result of the encryption process is obtained by adding the initial value to the result of exclusive OR.
(Appendix 7)
Each of the extracted random number sequences is arranged on a number sequence composed of empty bits and having the same length as the image code,
The encryption program according to appendix 4, wherein the image code is encrypted by taking an exclusive OR with the number sequence.
(Appendix 8)
A decryption program for decrypting encrypted data,
The data is a compression code;
The decryption program is
Using a predetermined initial value used in the encryption process, generate a random number sequence having a length shorter than the length of the compression code,
The random number sequence is sequentially cut out with a length corresponding to the compression ratio of the data,
A decoding program that causes a computer to execute a process of decoding a predetermined portion corresponding to each extracted random number sequence in the compression code using each extracted random number sequence.
(Appendix 9)
The compression code is
Generating a random number sequence having a length shorter than the length of the compression code;
The random number sequence is sequentially cut out with a length corresponding to the compression ratio of the data,
The appendix 8 is characterized in that each extracted random number sequence is encrypted by a process of taking an exclusive OR for a predetermined portion corresponding to the extracted random number sequence in the compressed code. Decryption program.
(Appendix 10)
The decoding program according to appendix 9, wherein the data is image data, and the compression code is an image code generated by compression encoding according to a JPEG method.
(Appendix 11)
The decryption program according to appendix 10, wherein the random number sequence used in the decryption process is calculated based on a predetermined initial value.
(Appendix 12)
An encryption method for encrypting data,
The data is a compression code;
The encryption method is:
Generating a random number sequence having a length shorter than the length of the compression code;
The random number sequence is sequentially cut out with a length corresponding to the compression ratio of the data,
An encryption method comprising: encrypting a predetermined portion corresponding to each extracted random number sequence in the compression code using each extracted random number sequence.
(Appendix 13)
A decryption method for decrypting encrypted data, comprising:
The data is a compression code;
The decoding method is:
Using a predetermined initial value used in the encryption process, generate a random number sequence having a length shorter than the length of the compression code,
The random number sequence is sequentially cut out with a length corresponding to the compression ratio of the data,
A decoding method, wherein a predetermined portion corresponding to each extracted random number sequence in the compression code is decoded using each extracted random number sequence.
(Appendix 14)
An encryption device for encrypting data,
The data is a compression code;
The encryption device is:
Generating means for generating a random number sequence having a length shorter than the length of the compression code;
Cutout means for sequentially cutting out the random number sequence with a length according to the compression rate of the data;
An encryption apparatus comprising: an encryption unit that encrypts a predetermined portion corresponding to each extracted random number sequence in the compressed code using each extracted random number sequence.
(Appendix 15)
A decoding device for decoding data,
The data is a compression code;
The decoding device
Generating means for generating a random number sequence having a length shorter than the length of the compression code using a predetermined initial value used in the encryption process;
Cutout means for sequentially cutting out the random number sequence with a length according to the compression rate of the data;
A decoding apparatus comprising: decoding means for decoding a predetermined portion corresponding to each of the extracted random number sequences in the compressed code using each of the extracted random number sequences.

It is a principle diagram of the encryption and decryption method according to the present invention. It is a block diagram of the encryption apparatus which concerns on this embodiment. It is a block diagram of the decoding apparatus which concerns on this embodiment. It is a block diagram of the random number sequence dispersion | distribution part in the encryption apparatus which concerns on this embodiment. It is a figure explaining the compression rate calculation method. It is a figure for demonstrating the code | symbol encrypted in the encryption method which concerns on this embodiment. It is a figure for demonstrating operation | movement of a random number sequence dispersion | distribution part. It is a figure for further explaining operation of a random number sequence distribution part It is a figure for demonstrating the production | generation of the distributed random number sequence in this embodiment. It is an example of a lookup table. It is a figure for demonstrating operation | movement of an encryption part. It is the flowchart which showed the process of the encryption which concerns on this embodiment. It is the flowchart which showed the process of the decoding which concerns on this embodiment. It is a block diagram of information processing apparatus (computer). It is the figure which showed the computer-readable recording medium which can supply a program and data to the information processing apparatus of FIG. It is a figure explaining the block encryption method and stream encryption method which concern on a prior art.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Encryption apparatus 2 Decryption apparatus 11 Initial value calculation part 12 Random number sequence generation part 13 Random number sequence storage part 14 Random number sequence distribution part 15 Distributed random number sequence storage part 16 Image code storage part 17 Encryption part 18 Ciphertext output part 21 Encryption Sentence input unit 22 Initial value cutout unit 23 Random number sequence generation unit 24 Random number sequence storage unit 25 Random number sequence distribution unit 26 Distributed random number sequence storage unit 27 Ciphertext storage unit 28 Decryption unit 29 Plaintext output unit

Claims (5)

An encryption program for encrypting data,
The data is a compression code;
The encryption program is
Generating a random number sequence having a length shorter than the length of the compression code;
The random number sequence is sequentially cut out with a length corresponding to the compression ratio of the data,
An encryption program that causes a computer to execute processing for encrypting a predetermined portion corresponding to each of the extracted random number sequences in the compression code using each of the extracted random number sequences. The encryption program according to claim 1, wherein the predetermined portion is determined based on a cut random number. The encryption program according to claim 2, wherein the data is image data, and the compression code is an image code generated by compression encoding according to a JPEG method. A decryption program for decrypting encrypted data,
The data is a compression code;
The decryption program is
Using a predetermined initial value used in the encryption process, generate a random number sequence having a length shorter than the length of the compression code,
The random number sequence is sequentially cut out with a length corresponding to the compression ratio of the data,
A decoding program that causes a computer to execute a process of decoding a predetermined portion corresponding to each extracted random number sequence in the compression code using each extracted random number sequence. The compression code is
Generating a random number sequence having a length shorter than the length of the compression code;
The random number sequence is sequentially cut out with a length corresponding to the compression ratio of the data,
The encrypted code is encrypted by a process of taking an exclusive OR for a predetermined portion corresponding to the extracted random number sequence in the compression code, using each extracted random number sequence. The decryption program described.