CN107292184B - Image encryption method, device and key stream generating method and key stream generator - Google Patents
Image encryption method, device and key stream generating method and key stream generator Download PDFInfo
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- CN107292184B CN107292184B CN201710530241.0A CN201710530241A CN107292184B CN 107292184 B CN107292184 B CN 107292184B CN 201710530241 A CN201710530241 A CN 201710530241A CN 107292184 B CN107292184 B CN 107292184B
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- G—PHYSICS
- G06—COMPUTING; CALCULATING OR COUNTING
- G06F—ELECTRIC DIGITAL DATA PROCESSING
- G06F21/00—Security arrangements for protecting computers, components thereof, programs or data against unauthorised activity
- G06F21/60—Protecting data
- G06F21/602—Providing cryptographic facilities or services
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- H04L9/00—Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols
- H04L9/06—Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols the encryption apparatus using shift registers or memories for block-wise or stream coding, e.g. DES systems or RC4; Hash functions; Pseudorandom sequence generators
- H04L9/0618—Block ciphers, i.e. encrypting groups of characters of a plain text message using fixed encryption transformation
Abstract
The present invention relates to a kind of image encryption method, image encrypting apparatus, key stream generating method and key stream generator, wherein image encryption method comprises the following steps：Based on initial value tectonic coupling image grid sequence, become by symmetrical matrix and change commanders coupled map lattice systems series processing into first key stream, flowed into line replacement to first key by codon substitutions box and obtain the second key stream；Line shuffle is entered to original image using first key stream；Block encoding, the view data encrypted are carried out to the image after scramble using the second key stream.The encryption method of shuffle operation and block encoding is combined by the present invention, reduce the computation complexity of cryptographic operation well while cryptographic security is improved, and what is used in scramble and block encoding is different key streams, relative to using for single key stream, the security of encryption is higher.
Description
The application is a divisional application entitled "image encryption method and device", and the original application date is 2016.9.28 and the application number is 201610856467.5.
Technical Field
The present invention relates to the field of image encryption technologies, and in particular, to an image encryption method, an image encryption device, a key stream generation method, and a key stream generator.
Background
With the continuous development of information technology and internet, information security becomes a first problem to be considered in the information transmission process. The image is used as a main carrier of information transmission, so that the image encryption technology is also one of hot spots of research in the field of information security.
In recent years, a number of image encryption methods have been proposed in succession. One of the most widely used image encryption methods at present is the image encryption method based on pixel scrambling, i.e. scrambling of the rows and columns of the image. However, two points are generally considered in the image encryption process: first, whether the computational overhead is within an acceptable range; second, whether randomness meets the requirements. The existing image encryption algorithm has yet to be improved in the above two aspects. There is therefore still a need to develop an image encryption method that reduces computational overhead while preserving randomness.
Disclosure of Invention
The technical problem to be solved by the present invention is to provide an image encryption method and an image encryption apparatus, which combine image scrambling with block encoding, and a corresponding key stream generation method and a key stream generator, aiming at the defects that the randomness and the computational overhead of the existing image encryption method need to be optimized.
In a first aspect of the present invention, an image encryption method is provided, which includes the following steps:
(1) Constructing a coupling mapping lattice sequence based on the initial value, processing the coupling mapping lattice sequence into a first key stream through symmetric matrix transformation, and replacing the first key stream through a password replacement box to obtain a second key stream; wherein, a coupled mapping lattice model based on two-dimensional dynamic mapping is established, and the coupled mapping lattice model is adopted based on an initial value x 0 ,y 0 Constructing a coupled mapping trellis sequence (x, y), where x 0 ,y 0 ∈(0,1](ii) a The coupled map trellis model is:
where ε is the coupling strength of the coupling mapping grid, f 1 Is a lower tent mapping function, f 2 Is a logical mapping function;
(2) Scrambling the original image using a first keystream;
(3) Using a second key stream to perform block coding on the scrambled image to obtain encrypted image data; wherein, the scrambled image is divided into columns to obtain the pixel value of the ith column as P i (j) I =1,2.. N; j =1,2.. Times, M; n and M are pixels of each row and pixels of each column respectively; performing XOR operation on the first column of pixel values and the second key stream to obtain a first column of ciphertext sequences; and after carrying out XOR operation on the pixel values of the ith row and the second key stream, carrying out XOR operation on the pixel values of the ith row and the ciphertext sequence of the previous row to obtain the ciphertext sequence of the ith row, wherein i =2,3.
In the image encryption method according to the present invention, the step of processing the coupled-map trellis sequence into the first key stream by symmetric matrix transformation comprises:
transforming the initial interval (0,1) of the coupling mapping grid sequence (x, y) to a specified interval by using symmetric matrix transformation, and outputting (x ', y');
the lower bound of (x ', y') is taken as the first keystream.
In the image encryption method according to the present invention, the step of replacing the first key stream by the cryptographic replacement box to obtain the second key stream includes:
the second keystream is calculated by the following formula:
wherein SBox (c) i ,d i ) For subscript (c) found in the two-dimensional table SBox i ,d i ) A corresponding numerical value; (c) i ,d i ) Is a parameter value of the first keystream; x is the number of 1_b ,y 1_b Are respectively an initial value x 0 ,y 0 The first 8 bits of; i =2,3 i_b ,y i_b And respectively carrying out bit left shift operation and bit right shift operation on the key values respectively output in the previous step.
In the image encryption method according to the present invention, the number of bits to be shifted in the bit shift left and right operations is 2 to 4 bits.
In a second aspect of the present invention, there is provided an image encryption apparatus comprising:
a keystream generator to generate a first keystream and a second keystream;
an image scrambling unit, configured to scramble an original image using a first key stream;
a block encoding unit, configured to perform block encoding on the scrambled image using a second key stream to obtain encrypted image data; wherein, the scrambled image is divided into columns to obtain the pixel value of the ith column as P i (j) Wherein i =1,2.. N; j =1,2.. Times, M; n and M are pixels of each row and pixels of each column respectively; performing XOR operation on the first column of pixel values and the second key stream to obtain a first column of ciphertext sequences; performing exclusive-or operation on the ith row of pixel values and the second key stream, and performing exclusive-or operation on the ith row of pixel values and a previous row of ciphertext sequences to obtain an ith row of ciphertext sequences, wherein i =2,3.. Once, N;
wherein the keystream generator comprises:
a sequence generating unit for constructing a coupling map lattice sequence based on the initial values; the sequence generation unit establishes a coupled mapping grid model based on two-dimensional dynamic mapping, and adopts the coupled mapping grid model based on an initial value x 0 ,y 0 Constructing a coupled mapping trellis sequence (x, y), where x 0 ,y 0 ∈(0,1](ii) a The coupling map trellis model is:
where ε is the coupling strength of the coupling mapping grid, f 1 Is a lower tent mapping function, f 2 Is a logical mapping function;
a matrix transformation unit for processing the coupled mapping trellis sequence into a first keystream by symmetric matrix transformation;
and the password replacement unit is used for replacing the first key stream through the password replacement box to obtain a second key stream.
In the image encryption device according to the present invention, the step of the matrix transformation unit processing the coupled map lattice sequence into the first key stream by symmetric matrix transformation includes:
transforming the initial interval (0,1) of the coupling mapping grid sequence (x, y) to a specified interval by using symmetric matrix transformation, and outputting (x ', y');
the lower limit value of (x ', y') is taken as the first keystream.
In the image encryption device according to the present invention, the step of replacing the first key stream by the password replacement box by the password replacement unit to obtain the second key stream is:
the second keystream is calculated by the following formula:
wherein SBox (c) i ,d i ) For subscript (c) found in the two-dimensional table SBox i ,d i ) A corresponding numerical value; (c) i ,d i ) Is a parameter value of the first keystream; x is the number of 1_b ,y 1_b Are respectively an initial value x 0 ,y 0 The first 8 bits of; i =2,3 i_b ,y i_b And respectively carrying out bit left shift operation and bit right shift operation on the key values respectively output in the previous step.
In a third aspect of the present invention, a method for generating a key stream is provided, which includes the following steps:
a. constructing a coupling mapping grid sequence based on the initial values; in the step, a coupled mapping lattice model based on two-dimensional dynamic mapping is established, and the coupled mapping lattice model is adopted based on an initial value x 0 ,y 0 Constructing a coupled mapping trellis sequence (x, y), where x 0 ,y 0 ∈(0,1](ii) a The coupled map trellis model is:
where ε is the coupling strength of the coupling mapping grid, f 1 Is a lower tent mapping function, f 2 Is a logical mapping function;
b. processing the coupled mapping trellis sequence into a first keystream by symmetric matrix transformation; in the step, an initial interval (0,1) of a coupling mapping lattice sequence (x, y) is transformed into a specified interval by using symmetric matrix transformation, and (x ', y') is output; taking the lower limit value of (x ', y') as a first key stream;
c. replacing the first key stream through a password replacement box to obtain a second key stream; in this step the second keystream is calculated by the following equation:
wherein SBox (c) i ,d i ) For subscript (c) found in the two-dimensional table SBox i ,d i ) A corresponding numerical value; (c) i ,d i ) Is a parameter value of the first keystream; x is a radical of a fluorine atom 1_b ,y 1_b Are respectively an initial value x 0 ,y 0 The first 8 bits of; i =2,3 i_b ,y i_b And respectively carrying out bit left shift operation and bit right shift operation on the key values respectively output in the previous step.
In a fourth aspect of the present invention, a keystream generator is provided, comprising:
a sequence generating unit for constructing a coupling map lattice sequence based on the initial values; the sequence generation unit establishes a coupled mapping lattice model based on two-dimensional dynamic mapping, and adopts the coupled mapping lattice model based on an initial value x 0 ,y 0 Constructing a coupled mapping trellis sequence (x, y), where x 0 ,y 0 ∈(0,1](ii) a The coupled mapping trellis model is:
where ε is the coupling strength of the coupling mapping grid, f 1 Is a lower tent mapping function, f 2 Is a logical mapping function;
a matrix transformation unit for processing the coupled mapping trellis sequence into a first keystream by symmetric matrix transformation; the matrix transformation unit transforms an initial interval (0,1) of the coupling mapping grid sequence (x, y) to a specified interval by using symmetric matrix transformation, and outputs (x ', y'); taking the lower limit value of (x ', y') as a first key stream;
the password replacing unit is used for replacing the first key stream through the password replacing box to obtain a second key stream; the cryptographic replacement unit calculates a second keystream by:
wherein SBox (c) i ,d i ) For subscript (c) found in the two-dimensional table SBox i ,d i ) A corresponding numerical value; (c) i ,d i ) Is a parameter value of the first keystream; x is the number of 1_b ,y 1_b Are respectively an initial value x 0 ,y 0 The first 8 bits of; i =2,3 i_b ,y i_b And respectively carrying out bit left shift operation and bit right shift operation on the key values respectively output in the previous step.
The technical scheme of the invention has the following advantages: the invention combines the scrambling operation and the block encoding encryption method, improves the encryption security, well reduces the computation complexity of the encryption operation, uses different key streams in the scrambling and the block encoding, and has higher encryption security compared with the use of a single key stream.
Drawings
FIG. 1 is a flow chart of an image encryption method according to a preferred embodiment of the present invention;
FIG. 2 is a functional block diagram of an image encryption method according to a preferred embodiment of the present invention;
FIG. 3 is a logic diagram of a block encoding step in an image encryption method according to a preferred embodiment of the present invention;
FIG. 4 is a flow chart of a method for keystream generation in accordance with a preferred embodiment of the present invention;
FIG. 5 is a logical operation diagram of a keystream generation method in accordance with a preferred embodiment of the invention;
FIG. 6 is a block diagram of an image encryption apparatus according to a preferred embodiment of the present invention;
FIG. 7 is a block diagram of a keystream generator in accordance with a preferred embodiment of the invention;
FIG. 8 is a flowchart of an image decryption method according to a preferred embodiment of the present invention;
FIG. 9 is a block diagram of an image decryption apparatus according to a preferred embodiment of the present invention;
FIGS. 10 a-10 d are diagrams of an original image and an encrypted image and corresponding histograms according to the present invention;
fig. 11a to 11f are graphs illustrating correlation analysis between an original image and an encrypted image according to the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. All other embodiments, which can be obtained by a person skilled in the art without any inventive step based on the embodiments of the present invention, are within the scope of the present invention.
Fig. 1 is a flowchart illustrating an image encryption method according to a preferred embodiment of the invention. As shown in fig. 1, the image encryption method of this embodiment includes the steps of:
first, in step S101, a Key stream generation step of generating a first Key stream Key1 and a second Key stream Key2 is performed. In the invention, the first key stream and the second key stream are different key streams. The method specifically comprises the following steps: and constructing a coupled mapping lattice sequence based on the initial value, processing the coupled mapping lattice sequence into a first key stream through symmetric matrix transformation, and replacing the first key stream through a password replacement box to obtain a second key stream.
Subsequently, an image scrambling step, i.e., scrambling the original image using the first Key stream Key1, is performed in step S102. The first keystream Key1 serves as a Key for cyclic shifting in a scrambling operation. Specifically, the scrambling step may obtain a row shift array and a column shift array respectively after performing value expansion rounding (E-R) based on the one-dimensional array of the first key stream, and perform row shift and column shift on the gray value matrix of the original image. The image scrambling process is reversible so that the original image can be restored in a subsequent decryption process.
Finally, in step S103, a block encoding step is performed, that is, the scrambled image is block-encoded using the second Key stream Key2, so as to obtain encrypted image data. Let the pixel size of the original image be M × N, where N and M are each row of pixels and each column of pixels, respectively. The image size after the scrambling is still M × N. In step S103, the scrambled image is subjected to block processing, pixels of the image are blocked in a row, a column or other form, and then the second Key stream Key2 is used to encode the gray level of each block of image. For example, the scrambled image pixels are subjected to a column processing, i.e. each column is xored with a keystream of equal length, while, in order to ensure randomness, each column of ciphertext is xored with the ciphertext of the previous column starting from the second column.
Referring to fig. 2, a schematic block diagram of an image encryption method according to a preferred embodiment of the invention is shown. As shown in fig. 2, in the image encryption method, the aforementioned Key stream generation step S101 is first executed, and the first Key stream Key1 and the second Key stream Key2 are generated by the Key stream generator 601.
Subsequently, the foregoing image scrambling step S102 is performed, including: the method comprises the steps of obtaining a gray value matrix of an input original image 202 through matrix mapping 203, and performing row shifting and column shifting 204 on the gray value matrix of the original image by using a first Key stream Key1 to obtain a scrambled image 205.
Specifically, assume that the pixel size of the original image is M × N; the first Key stream Key1 generated by the Key stream generator may be denoted as (c) i ,d i ) I ∈ max { M, N }, as a key for cyclic shifting. For c i Take i = M, c 1 ,c 2 ……,c M Form a one-dimensional array (c) 1 ,c 2 ……,c M ) Then, the value of the one-dimensional array is processed by E-R, i.e. the spread value is rounded, to obtain a row shift array, which is recorded as Rshift = floor [ N (c) ] 1 ,c 2 ……,c M )]And shifting rows of a gray value matrix of the original image by using Rshift, namely controlling the number of right-shifting bits of a first row of the image by using a first parameter of the array Rshift, controlling the number of right-shifting bits of a second row of the image by using a second parameter, and so on. In other embodiments of the present invention, the right shift operation may be replaced with a left shift operation. For d, the same principle i Take i = N, d 1 ,d 2 ……,d N Form a one-dimensional array (d) 1 ,d 2 ……,d N ) The values are E-R processed to yield a column shift array, denoted Cshift = floor [ M (d) 1 ,d 2 ……,d N )]Each column of the gray value matrix of the original image is shifted down by Cshift. In other embodiments of the present invention, the aforementioned shift-down operation may be replaced with a shift-up operation.
Finally, the block encoding step S103 is executed, in which the scrambled image 205 is block encoded using the second Key stream Key2, and the encrypted image data 206 is obtained. Referring to fig. 3, a logic operation diagram of block encoding in the image encryption method according to the preferred embodiment of the invention is shown. As shown in fig. 3, the block encoding step specifically includes:
1. the scrambled image 205 is divided into rows to obtain the pixel value of the ith row as P i (j) Wherein i =1,2...... Cndot.n; j =1,2.. Times, M; n and M are each row and column of pixels of the original image, respectively.
2. And performing XOR operation on the first column of pixel values and the second key stream to obtain a first column of diffused pixel values, namely a first column of ciphertext sequences. Let the second Key stream Key2 be denoted asi =1,2. The resulting encrypted image data 206 may be represented as a ciphertext sequence C i (j) I =1,2.. And N, j =1,2.. And M. The first column of ciphertext sequences
3. Starting from the second column, performing exclusive-or operation on the pixel values of the ith column and a second key stream, and then performing exclusive-or operation on the pixel values of the ith column and a ciphertext sequence of the previous column to obtain diffused pixel values of the ith column, namely the ciphertext sequence of the ith column, wherein i =2,3.
Is provided withThen the
That is, the ith column ciphertext sequence may be calculated in this step by the following formula (1):
although the image is block-coded in the form of columns in the above embodiment, the present invention is not limited to this, and coding may also be performed in a line-division or other block-division manner.
The invention combines scrambling operation and block coding encryption method, improves the security of the encryption process and well reduces the computational complexity of encryption operation. In addition, different key streams are used in scrambling and block encoding, and compared with the use of a single key stream, the encryption security is higher.
In a preferred embodiment of the present invention, a new keystream generation method is provided that utilizes a coupled image trellis (CML) and cryptographic transpose box (SBox) to generate keys. Referring to fig. 4, a flow chart of a method for generating a keystream according to a preferred embodiment of the invention is shown. As shown in fig. 4, the aforementioned key stream generation step S101 further includes the following steps:
first, in step S401, a sequence generation step is performed to construct a coupled map trellis sequence based on the initial values. Referring to fig. 5, a logic operation diagram of a key stream generating method according to a preferred embodiment of the invention is shown. As shown in fig. 5, the step of constructing a coupled map trellis sequence based on the initial values includes:
1. establishing a coupling mapping grid model based on two-dimensional dynamic mapping:
where ε is the coupling strength of the coupling mapping grid, f 1 Is a lower tent mapping function, f 2 Is a logical mapping function; i ∈ max { M, N } N and M are each row and column of pixels of the original image, respectively.
Lower tent mapping function f 1 The following formula:
wherein x is the initial state of the system, x belongs to (0,1), p is the control parameter of the system, and p belongs to (0,1).
Logical mapping function f 2 The following formula:
f 2 (x)＝ax(1-x),a∈(0,4] (4)
where a is a control parameter, and when the value of a is close to 4, the periodicity is negligible and chaos is achieved, so a =3.99973 is preferable.
2. Using the aforementioned coupled mapping trellis model based on the initial value x 0 ,y 0 Constructing a coupled mapping trellis sequence (x, y), where x 0 ,y 0 ∈(0,1]Corresponding to CML operation 501 in fig. 5.
Subsequently, in step S402, a matrix transformation step is performed, i.e., the coupled image lattice sequence is processed into the first Key stream Key1 by symmetric matrix transformation. The matrix transformation step is shown in SMT operation 502 in fig. 5, and specifically includes:
1. and transforming the initial interval (0,1) of the coupled mapping grid sequence (x, y) into a specified interval by using symmetric matrix transformation, and outputting (x ', y'). In a preferred embodiment of the present invention, x ', y' e (0,15 ], is transformed as follows:
where K is the transformation parameter.
Let the symmetric matrixThen K =5 and the signal is transmitted,
the above symmetric matrix a is only for converting the value range of (x, y), and the symmetric matrix a may also be implemented by other matrices, for example:orBecause the transformed value interval is determined, different symmetric matrixes only correspond to one unique transformation parameter value.
2. Taking the lower bound of (x ', y') as the output of SMT, i.e., the first keystream Key1, may be represented as (c) i ,d i )。c i ＝floor(x i ')，d i ＝floor(y i ')，i∈max{M,N}。
Finally, in step S403, a password replacement step is performed, in which the first Key stream Key1 is replaced by the password replacement box to obtain a second Key stream Key2.
In this step, the second Key stream Key1 can be calculated by the following formula:
wherein SBox (c) i ,d i ) For subscript (c) found in the two-dimensional table SBox i ,d i ) A corresponding numerical value; (c) i ,d i ) Is the parameter value of the first Key stream Key 1; x is the number of 1_b ,y 1_b Are respectively an initial value x 0 ,y 0 The first 8 bits of (a); i =2,3 i_b ,y i_b And respectively carrying out bit left shift operation and bit right shift operation on the key values respectively output in the previous step.
As shown in fig. 5, the password replacing step specifically includes:
1. (c) is processed by SBox operation 503 i ,d i ) Substitution to SBox (c) i ,d i )；
The SBox is preferably a 16 × 16 two-dimensional table, corresponding to 16 × 16 decimal numbers (0,255), by subscript c i ,d i The corresponding number in the SBox can be found as the SBox output.
2. For x of the initial value 0 ,y 0 Operations 504 and 505 are performed separately, i.e., the first 8 bits are extracted to obtain x 1_b ,y 1_b Then, the XOR operation 506 is performed to obtain:
3. when i =2,3 i_b ,y i_b . In this embodiment willLeft shift by 3 bits to x 2_b Will beRight shift by 3 bits to y 2_b And so on. In other embodiments of the present invention, the invention can also be applied toRight shift by 3 bits to x i+1_b To, forLeft shift by 3 bits to y i+1_b . Also, the number of bits shifted left and right may take other values, such as 2 bits or 4 bits shifted. The shifted value is then subjected to an exclusive or operation 506 to obtain:
through the steps above, the product is obtainedAs the second Key stream Key2.
The key stream generation method can generate key streams of different levels, wherein the first key stream is relatively simple, the security level of the second key stream is higher, if the first key stream is adopted for encryption in scrambling operation and block coding in the image encryption method, the encryption is too simple, and the security is not high; the computational overhead is too large if both are encrypted with the second keystream. Therefore, the first key stream and the second key stream are combined for use, so that the requirement of the secondary key on the safety after encryption is ensured, the calculation overhead of the key is well saved, and the encryption efficiency of the image is directly improved.
Fig. 6 is a block diagram of an image encryption apparatus according to a preferred embodiment of the present invention. The image encryption apparatus 600 includes:
the Key stream generator 601 is configured to generate a first Key stream Key1 and a second Key stream Key2. The function and implementation of the key stream generator 601 are identical to the key stream generating step S101 in the aforementioned image encryption method.
The image scrambling unit 602 is connected to the Key stream generator 601 for scrambling the original image using the first Key stream Key1. The function and implementation procedure of the image scrambling unit 602 are the same as those of the image scrambling step S102 in the aforementioned image encryption method. Specifically, the image scrambling unit 602 may obtain a row shift array and a column shift array, respectively, after performing value expansion rounding (E-R) based on the one-dimensional array of the first key stream, for performing row shift and column shift on the gray value matrix of the original image. The image scrambling process is reversible so that the original image can be restored in a subsequent decryption process.
The block encoding unit 603 is connected to the image scrambling unit 602 and the Key stream generator 601, and is configured to perform block encoding on the scrambled image by using the second Key stream Key2 to obtain encrypted image data. The function and implementation of the block encoding unit 603 are identical to those of the block encoding step S103 in the aforementioned image encryption method. The block encoding unit 603 divides the scrambled image into rows to obtain the ith row having a pixel value of P i (j) Wherein i =1,2.. Anner, N; j =1,2.. Times, M; n and M are pixels of each row and pixels of each column respectively; performing XOR operation on the first column of pixel values and the second key stream to obtain a first column of ciphertext sequences; and after carrying out XOR operation on the pixel values of the ith row and the second key stream, carrying out XOR operation on the pixel values of the ith row and the ciphertext sequence of the previous row to obtain the ciphertext sequence of the ith row, wherein i =2,3.
Fig. 7 is a block diagram of a key stream generator according to a preferred embodiment of the invention. As shown in fig. 7, the key stream generator 601 includes:
a sequence generation unit 701 for constructing a coupled map trellis sequence based on the initial values. The function and implementation of the sequence generating unit 701 are identical to those of the sequence generating step S201 in the aforementioned key stream generating method. The sequence generating unit 701A coupled image lattice model based on two-dimensional dynamic mapping, namely formula (1), is established. The sequence generation unit 701 then uses the coupled-map trellis model based on the initial value x 0 ,y 0 Constructing a coupled mapping trellis sequence (x, y), where x 0 ,y 0 ∈(0,1]。
The matrix transformation unit 702 is connected to the sequence generation unit 701 and is configured to process the coupled image trellis sequence into the first Key stream Key1 through symmetric matrix transformation. The function and implementation of the matrix transformation unit 702 are identical to the matrix transformation step S202 in the aforementioned key stream generation method. The sequence generation unit 701 converts the initial section (0,1) of the coupled mapping lattice sequence (x, y) into a designated section by symmetric matrix transformation, outputs (x ', y'), and takes the lower limit value of (x ', y') as the first Key stream Key1.
The cipher replacement unit 703 is connected to the matrix transformation unit 702 and the sequence generation unit 701, and is configured to replace the first keystream Key1 with a cipher replacement box to obtain a second keystream Key2. The function and implementation procedure of the cipher replacement unit 703 are the same as those of the cipher replacement step S203 in the aforementioned key stream generation method. The cipher substitution unit 703 calculates a second key stream by equation (6).
Fig. 8 is a flowchart of an image decryption method according to a preferred embodiment of the invention. As shown in fig. 8, the image decryption method includes the steps of:
step S801 is to extract the first Key stream Key1 and the second Key stream Key2 from the image encrypted data.
And S802, extracting a ciphertext sequence from the image encrypted data, carrying out XOR on the ciphertext sequence and a previous block from the last block according to image blocks, and carrying out XOR on the ciphertext sequence and a second Key stream Key2 to obtain an image matrix. This step S802 is the reverse of the block encoding step S103. Taking the column division form for blocking as an example, ciphertext sequences are extracted, each column of ciphertext sequences is numbered, each column is subjected to exclusive or with the previous column from the Nth column, then each column is subjected to exclusive or with each column in the second key stream, and the exclusive or results form an M × N matrix.
Step S803, using the row shift array Rshift and the column shift array Cshift based on the first Key stream Key1 to shift rows and columns of the image matrix to obtain an original image. This step S803 is the reverse of the image scrambling step S102. For example, the columns of the image matrix are shifted up by using the column shift array Cshift, and then the columns of the image matrix are shifted right by using the row shift array Rshift, so as to obtain the original image.
Fig. 9 is a block diagram of an image decryption apparatus according to a preferred embodiment of the present invention. As shown in fig. 9, the image decryption apparatus 900 includes:
a Key extraction unit 901, configured to extract the first Key stream Key1 and the second Key stream Key2 from the image encrypted data.
The first decryption unit 902, connected to the Key extraction unit 901, is configured to extract a ciphertext sequence from the image encrypted data, perform xor on the ciphertext sequence and a previous block, starting from a last block according to image partitioning, and perform xor on the ciphertext sequence and the second Key stream Key2 to obtain an image matrix.
The second decryption unit 903 is configured to shift rows and columns of the image matrix to obtain an original image by using the row shift array Rshift and the column shift array Cshift based on the first Key stream Key1.
The present invention has studied the feasibility of the image encryption method of the present invention through the following experiments. Inputting Lena gray image with 256 multiplied by 256 as original image, selecting x as initial value of key stream generating step 0 ＝0.27，y 0 ＝0.8370，a＝3.99973。
1. Key space analysis
The size of the key space of an encryption algorithm represents the total number of different keys that the encryption method can use for encryption. The image encryption method of the present invention has four initial values, x 0 ,y 0 P ∈ (0,1), a ∈ (3.57,4). According to the IEEE 754 standard, the accuracy of a 64-bit double precision number is 10 -15 The key space of the encryption method can be as large as 2 197 . Thus, the key space is well resistant to brute force attacks.
2. Entropy of source
The source entropy is an index used to describe the degree of random occurrence of source symbols. The source entropy can be defined as
Wherein P (m) i ) Is a source symbol m i The probability of occurrence. For an ideal random image, the source entropy can theoretically reach 8. The information entropy of the original image and the encrypted image adopted by the invention is shown in table 1:
table 1
| Image of a person | Original image | Encrypted image |
| Lena (Lina image) | 7.4532 | 7.9843 |
| Rice (Rice image) | 5.7596 | 7.9895 |
| Barbara (Barbara image) | 7.5838 | 7.9890 |
3. Histogram analysis
A histogram reflects the distribution of pixels in an image at the same gray level. A good encryption system is provided, the histogram of the encryption map of the good encryption system should be uniformly distributed so as to resist external mathematical statistics attacks. Fig. 10a is a lina image (Lena) of the original image, fig. 10b is an encrypted image, fig. 10c is a histogram of the original image, and fig. 10d is a histogram of the encrypted image. The histogram shows that the histogram of the image before encryption is irregularly changed, and the histogram of the image after encryption is basically uniformly distributed, which shows that the ciphertext can not provide effective information for illegal users, thereby effectively resisting external statistical attack.
4. Pixel correlation analysis
For a normal image, its neighboring pixels, including horizontal, vertical, and diagonal directions, are theoretically highly correlated. The correlation of adjacent pixels of the encrypted image is an important index for measuring an encryption system, and the lower the correlation of the adjacent pixels is, the better the encryption effect is. The invention selects 5000 pairs of adjacent pixel points to test the correlation of the adjacent pixels, and the correlation of the original image and the encrypted image is shown in figures 11a-11 f. In which fig. 11a and 11b are horizontal direction pixel correlations of the original image and the encrypted image, respectively, fig. 11c and 11d are vertical direction pixel correlations of the original image and the encrypted image, respectively, and fig. 11e and 11f are diagonal direction pixel correlations of the original image and the encrypted image, respectively.
Pixel dependence r xy The calculation formula of (a) is as follows:
cov(x,y)＝E[(x-E(x))(y-E(y))] (9)
wherein x and y are gray values of adjacent pixels of the image. The correlation coefficients of the original image and the encrypted image are shown in table 2:
table 2
| Direction | Original drawing | Encryption map |
| Level of | 0.9642 | -0.02023 |
| Is perpendicular to | 0.9309 | 0.00933 |
| Diagonal line | 0.9061 | -0.00586 |
5. Differential attack analysis
The differential attack is a chosen plaintext attack, and in order to resist the chosen plaintext attack, a small number of pixels in an input image are required to be changed, so that a large number of pixels of a ciphertext are required to be changed. The pixel number change rate (NPCR) and the uniform average change strength (UACI) are often used as analysis indexes of the characteristics, and the encryption effect is better when the NPCR value is closer to 100 and the UACI value is closer to 34. NPCR and UACI are defined as follows:
wherein, C 1 、C 2 Respectively two ciphertext images with only one pixel difference, C 1 (r,c)、C 2 (r, C) are each C 1 、C 2 The pixel value at point (r, c), size (D), is the size of the corresponding image. The simulation results of the NPCR and UACI for this encryption method are shown in table 3. In table 3, the conventional method 1 is a diffusion strategy (diffusion strategy), and the conventional method 2 is a two-dimensional coupled-mapping trellis (2D CML). It can be seen that the invention can basically achieve a more ideal effect through one round of encryption, and the NPCR can be ensured after two rounds of iteration>, 0.996 and UACI>, 33.4, while the other two encryption methods can be realized only by iterating at least twice to obtain the same effect.
Table 3
| Method | First wheel | Second wheel | ||
| NPCR | UACI | NPCR | UACI | |
| The method of the invention | 99.3982 | 32.4453 | 99.6264 | 33.4086 |
| Prior method 1 | 99.3046 | 32.2430 | 99.6135 | 33.3104 |
| Prior method 2 | 46.6524 | 17.1732 | 99.4036 | 33.4023 |
In summary, the present invention designs a new fast image encryption method based on the secondary key encryption based on the security and complexity problems of the conventional image encryption method. The method uses the key stream twice, the first key stream is taken in the pixel scrambling process, the second key stream is taken in the pixel diffusion process, and the computational complexity of the algorithm is well reduced while the algorithm safety is improved. Simulation results show that the method also has a large key space and has better capability of resisting exhaustive attacks; the information source entropy of the encrypted image is close to an ideal value, which shows that the gray distribution of the encrypted image is relatively uniform; the encrypted histograms are basically uniformly distributed, the correlation of adjacent pixels can meet the expected requirement, and the statistical attack can be effectively resisted; the NPCR value and the UACI value are close to ideal values, and differential attacks can be well resisted. In order to enhance the anti-attack capability of the encryption method, the encryption method can be iterated for multiple times to ensure the security of the encryption method.
Finally, it should be noted that: the above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it should be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.
Claims (10)
1. An image encryption method, comprising the steps of:
(1) Constructing a coupling mapping lattice sequence based on the initial value, processing the coupling mapping lattice sequence into a first key stream through symmetric matrix transformation, and replacing the first key stream through a password replacement box to obtain a second key stream;
the step (1) of constructing the coupled mapping trellis sequence based on the initial values comprises the following steps:
establishing a coupled mapping grid model based on two-dimensional dynamic mapping, and adopting the coupled mapping grid model based on an initial value x 0 ,y 0 Constructing a coupled mapping trellis sequence (x, y), where x 0 ,y 0 ∈(0,1](ii) a The coupled map trellis model is:
where ε is the coupling strength of the coupling mapping grid, f 1 Is a lower tent mapping function, f 2 Is a logical mapping function; i belongs to max { M, N } N and M are pixels of each row and each column of the original image respectively;
lower tent mapping function f 1 The following formula:
wherein x is the initial state of the system, x belongs to (0,1), p is the control parameter of the system, and p belongs to (0,1);
logical mapping function f 2 The following formula:
f 2 (x)＝ax(1-x),a∈(0,4]
wherein a is a control parameter;
the step of replacing the first key stream by the password replacing box in the step (1) to obtain the second key stream comprises the following steps:
the second keystream is calculated by the following formula:
wherein SBox (c) i ,d i ) For subscript (c) found in the two-dimensional table SBox i ,d i ) A corresponding numerical value; (c) i ,d i ) Is a parameter value of the first keystream; x is the number of 1_b ,y 1_b Are respectively an initial value x 0 ,y 0 The first 8 bits of; i =2,3 i_b ,y i_b Respectively carrying out bit left shift operation and bit right shift operation on the key value output previously;
(2) Scrambling the original image using a first keystream; the first key stream is used as a key for cyclic shift in image scrambling operation, and the image scrambling operation is based on a one-dimensional array of the first key stream, and after value expansion and rounding, a row shift array and a column shift array are respectively obtained and are used for carrying out row shift and column shift on a gray value matrix of an original image; the image scrambling operation is reversible;
(3) Using a second key stream to perform block coding on the scrambled image to obtain encrypted image data; setting the pixel size of an original image as M multiplied by N, wherein N and M are pixels of each row and pixels of each column respectively, and the size of the scrambled image is still M multiplied by N; in the step, the scrambled image is subjected to blocking processing, image pixels are blocked in a column form, and then a second key stream is used for coding each image gray value; wherein, the pixel value of the ith column obtained by dividing the scrambled image into columns is P i (j) I =1,2.. N; j =1,2.. Times, M; n and M are pixels of each row and pixels of each column respectively; performing XOR operation on the first column of pixel values and the second key stream to obtain a first column of ciphertext sequencesAnd after carrying out XOR operation on the pixel values of the ith row and the second key stream, carrying out XOR operation on the pixel values of the ith row and the ciphertext sequence of the previous row to obtain the ciphertext sequence of the ith row, wherein i =2,3.
2. The image encryption method of claim 1, wherein the step of processing the coupled shadow trellis sequence into the first key stream by symmetric matrix transformation in step (1) comprises:
transforming the initial interval (0,1) of the coupling mapping grid sequence (x, y) to a specified interval by using symmetric matrix transformation, and outputting (x ', y');
the lower bound of (x ', y') is taken as the first keystream.
3. The image encryption method according to claim 2, wherein the step of transforming the initial interval (0,1) of the coupled mapping trellis sequence (x, y) to the designated interval by using symmetric matrix transformation is specifically: computingK is a transformation parameter, and A is a symmetric matrix; wherein the symmetric matrixOrOr
4. The image encryption method according to claim 1, wherein the step (2) is based on the one-dimensional array of the first key stream, and after the value expansion and rounding, a row shift array and a column shift array are respectively obtained, and the step for performing the row shift and the column shift on the gray value matrix of the original image specifically includes:
setting the pixel size of an original image as M multiplied by N; the first keystream generated by the keystream generator is denoted as (c) i ,d i ) I belongs to max { M, N }, and is used as a key of cyclic shift;
for c i Take i = M, c 1 ,c 2 ……,c M Form a one-dimensional array (c) 1 ,c 2 ……,c M ) E-R processing is carried out on the numerical value of the one-dimensional array to obtain a row shift array which is recorded as Rshift = floor [ N (c) 1 ,c 2 ……,c M )]Shifting rows of a gray value matrix of an original image by using Rshift, wherein a first parameter of an array Rshift controls the number of bits of right shift of a first row of the image, a second parameter controls the number of bits of right shift of a second row of the image, and the like, and the right shift operation can be replaced by left shift operation;
for d i Take i = N, d 1 ,d 2 ……,d N Form a one-dimensional array (d) 1 ,d 2 ……,d N ) The values are E-R processed to yield a column shift array, denoted Cshift = floor [ M (d) 1 ,d 2 ……,d N )]Each column of the gradation value matrix of the original image is subjected to a shift-down or shift-up operation with Cshift.
5. The image encryption method according to claim 1, wherein SBox in the step (1) is a 16 x 16 two-dimensional table, corresponding to 16 x 16 (0, 255) decimal numbers.
6. An image encryption apparatus characterized by comprising:
a keystream generator to generate a first keystream and a second keystream;
an image scrambling unit, configured to scramble an original image using a first key stream; the first key stream is used as a key for cyclic shift in the image scrambling operation, and the image scrambling operation is based on a one-dimensional array of the first key stream, and after the one-dimensional array is subjected to value expansion and rounding, a row shift array and a column shift array are respectively obtained and are used for carrying out row shift and column shift on a gray value matrix of an original image; the image scrambling operation is reversible;
a block encoding unit, configured to perform block encoding on the scrambled image using a second key stream to obtain encrypted image data; setting the pixel size of an original image as M multiplied by N, wherein N and M are pixels of each row and pixels of each column respectively, and the size of the scrambled image is still M multiplied by N; in the step, the scrambled image is subjected to blocking processing, image pixels are blocked in a column form, and then a second key stream is used for coding each image gray value; wherein, the pixel value of the ith column obtained by dividing the scrambled image into columns is P i (j) Wherein i =1,2.. N; j =1,2.. Times, M; n and M are pixels of each row and pixels of each column respectively; performing XOR operation on the first column of pixel values and the second key stream to obtain a first column of ciphertext sequences; performing exclusive-or operation on the ith row of pixel values and the second key stream, and then performing exclusive-or operation on the ith row of pixel values and the previous row of ciphertext sequences to obtain an ith row of ciphertext sequences, wherein i =2,3.. Once, N;
wherein the keystream generator comprises:
a sequence generating unit for constructing a coupling map lattice sequence based on the initial values; the sequence generation unit establishes a coupled mapping grid model based on two-dimensional dynamic mapping, and adopts the coupled mapping grid model based on an initial value x 0 ,y 0 Constructing a coupled mapping trellis sequence (x, y), where x 0 ,y 0 ∈(0,1](ii) a The coupling mapping grid model is:
where ε is the coupling strength of the coupling mapping grid, f 1 Is a lower tent mapping function, f 2 Is a logical mapping function; i belongs to max { M, N } N and M are pixels of each row and each column of the original image respectively;
lower tent mapping function f 1 The following formula:
wherein x is the initial state of the system, x belongs to (0,1), p is the control parameter of the system, and p belongs to (0,1);
logical mapping function f 2 The following formula:
f 2 (x)＝ax(1-x),a∈(0,4]
wherein a is a control parameter;
a matrix transformation unit for processing the coupled mapping trellis sequence into a first keystream by symmetric matrix transformation;
the password replacing unit is used for replacing the first key stream through the password replacing box to obtain a second key stream;
the cryptographic replacement unit calculates a second keystream by:
wherein SBox (c) i ,d i ) For subscript (c) found in the two-dimensional table SBox i ,d i ) A corresponding numerical value; (c) i ,d i ) Is a parameter value of the first keystream; x is the number of 1_b ,y 1_b Are respectively an initial value x 0 ,y 0 The first 8 bits of; i =2,3 i_b ,y i_b Respectively of a preceding outputAnd the key value is obtained by respectively carrying out bit left shift operation and bit right shift operation.
7. The image encryption apparatus according to claim 6, wherein the matrix transformation unit processes the coupled shadow trellis sequence into the first key stream by symmetric matrix transformation, comprising:
transforming the initial interval (0,1) of the coupling mapping grid sequence (x, y) to a specified interval by using symmetric matrix transformation, and outputting (x ', y');
the lower bound of (x ', y') is taken as the first keystream.
8. The image encryption device according to claim 6, wherein the one-dimensional array based on the first key stream in the image scrambling unit is subjected to value expansion and rounding to obtain a row shift array and a column shift array, respectively, and the step for performing row shift and column shift on the gray-level matrix of the original image specifically comprises:
setting the pixel size of an original image as M multiplied by N; the first keystream generated by the keystream generator is denoted as (c) i ,d i ) I belongs to max { M, N }, and is used as a key of cyclic shift;
for c i Take i = M, c 1 ,c 2 ……,c M Form a one-dimensional array (c) 1 ,c 2 ……,c M ) E-R processing is carried out on the numerical value of the one-dimensional array to obtain a row shift array which is recorded as Rshift = floor [ N (c) 1 ,c 2 ……,c M )]Shifting rows of a gray value matrix of an original image by using Rshift, wherein a first parameter of an array Rshift controls the number of bits of right shift of a first row of the image, a second parameter controls the number of bits of right shift of a second row of the image, and the like, and the right shift operation can be replaced by left shift operation;
for d i Taking i = N, d 1 ,d 2 ……,d N Form a one-dimensional array (d) 1 ,d 2 ……,d N ) And E-R processing is carried out on the numerical value of the column displacement array to obtain a column displacement array which is marked as Cshift = floor [ M (d) 1 ,d 2 ……,d N )]Each column of the gradation value matrix of the original image is subjected to a shift-down or shift-up operation with Cshift.
9. A method for generating a keystream, comprising the steps of:
a. constructing a coupling mapping grid sequence based on the initial values; in the step, a coupled mapping lattice model based on two-dimensional dynamic mapping is established, and the coupled mapping lattice model is adopted based on an initial value x 0 ,y 0 Constructing a coupled mapping trellis sequence (x, y), where x 0 ,y 0 ∈(0,1](ii) a The coupled map trellis model is:
where ε is the coupling strength of the coupling mapping grid, f 1 Is a lower tent mapping function, f 2 Is a logical mapping function; i belongs to max { M, N } N and M are pixels of each row and each column of the original image respectively;
lower tent mapping function f 1 The following formula:
wherein x is the initial state of the system, x belongs to (0,1; p is the control parameter of the system, and p belongs to (0,1);
logical mapping function f 2 The following formula:
f 2 (x)＝ax(1-x),a∈(0,4]
wherein a is a control parameter;
b. processing the coupled mapping trellis sequence into a first keystream by symmetric matrix transformation; in the step, an initial interval (0,1) of the coupling mapping grid sequence (x, y) is transformed into a specified interval by using symmetric matrix transformation, and (x ', y') is output; whereinK is changedChanging parameters, wherein A is a symmetric matrix; taking the lower limit value of (x ', y') as a first key stream;
c. and replacing the first key stream by a password replacing box to obtain a second key stream, wherein the second key stream is calculated by the following formula:
wherein SBox (c) i ,d i ) For subscript (c) found in the two-dimensional table SBox i ,d i ) A corresponding numerical value; SBox is a 16 × 16 two-dimensional table, corresponding to 16 × 16 (0,255) decimal numbers; (c) i ,d i ) Is a parameter value of the first keystream; x is the number of 1_b ,y 1_b Are respectively an initial value x 0 ,y 0 The first 8 bits of (a); i =2,3 i_b ,y i_b And respectively carrying out bit left shift operation and bit right shift operation on the key values respectively output in the previous step.
10. A keystream generator, comprising:
a sequence generating unit for constructing a coupling map lattice sequence based on the initial values; the sequence generation unit establishes a coupled mapping lattice model based on two-dimensional dynamic mapping, and adopts the coupled mapping lattice model based on an initial value x 0 ,y 0 Constructing a coupled mapping trellis sequence (x, y), where x 0 ,y 0 ∈(0,1](ii) a The coupled map trellis model is:
where ε is the coupling strength of the coupling mapping grid, f 1 Is a lower tent mapping function, f 2 Is a logical mapping function; i belongs to max { M, N } N and M are pixels of each row and each column of the original image respectively;
lower tent mapping function f 1 The following formula:
wherein x is the initial state of the system, x belongs to (0,1), p is the control parameter of the system, and p belongs to (0,1);
logical mapping function f 2 The following formula:
f 2 (x)＝ax(1-x),a∈(0,4]
wherein a is a control parameter;
a matrix transformation unit for processing the coupled-map-trellis sequence into a first keystream by symmetric matrix transformation; the matrix transformation unit transforms an initial interval (0,1) of the coupling mapping grid sequence (x, y) to a specified interval by using symmetric matrix transformation, and outputs (x ', y'); whereinK is a transformation parameter, and A is a symmetric matrix; taking the lower limit value of (x ', y') as a first key stream;
the password replacing unit is used for replacing the first key stream through the password replacing box to obtain a second key stream; the cryptographic replacement unit calculates a second keystream by:
wherein SBox (c) i ,d i ) For subscript (c) found in the two-dimensional table SBox i ,d i ) A corresponding numerical value; SBox is a 16 × 16 two-dimensional table, corresponding to 16 × 16 (0,255) decimal numbers; (c) i ,d i ) Is a parameter value of the first keystream; x is the number of 1_b ,y 1_b Are respectively an initial value x 0 ,y 0 The first 8 bits of; i =2,3 i_b ,y i_b And respectively carrying out bit left shift operation and bit right shift operation on the key values respectively output in the previous step.
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