JPH0573299B2 - - Google Patents

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention uses cryptographic operations for encrypting information signals such as computers or encoded voices for the purpose of preventing data eavesdropping on data communication lines. The present invention relates to a sequence generation method for carrying out such operations.

(Prior art) In recent years, in order to prevent wiretapping or tampering of communication information carried out via data lines, etc., data has been encrypted using an encryption key known only to the communicating party, so that the information content cannot be easily decoded. The need for ways to do this has increased.

The first requirement for an encryption method is that the statistical properties of the raw data in the encrypted data are destroyed, resulting in pseudo-randomization. Second, even if the plaintext and the corresponding ciphertext are obtained by a third party, they cannot be easily decrypted, that is, the encryption attack strength rating is high, and furthermore, the encryption algorithm is known and the ciphertext is known. There is no decryption method other than checking all the encryption keys, and the total number of encryption keys must be as large as possible.

A common encryption method is to randomize a raw information signal sequence to be transmitted, such as a bit sequence of a digital signal, according to a required encryption key. As shown in FIG. 2a, the device used was a linear shift register that produced a maximum length sequence (sequence length 2 ⁿ -1, where n was the number of flip-flops in the shift register).

This inputs each flip-flop output of shift register 1 to switch 2, extracts exclusive OR 3 from this output, uses it as an output, feeds this output 4 back to shift register 1, and outputs it as a desired encrypted signal. be.

At this time, by making the ON-OFF pattern of the switch 2 correspond to the coefficients of the primitive irreducible polynomial, the maximum length sequence output can be obtained.

This method can convert data into pseudo-random encrypted data with a relatively simple device configuration, but on the other hand, it is very easy to convert the switch corresponding to the encryption key using linear simultaneous equations against plane text attacks. There was a problem that the ON-OFF pattern of 2 was deciphered.

In addition, the number of primitive irreducible polynomials, that is, the number of encryption keys, is determined by the number n of shift registers as shown in Figure 2b.
In order to reduce the number of keys in DES, which is an officially recognized encryption system, to about ¹⁰¹⁶ , the number n of shift registers must be enormous, which is not practical.

In order to eliminate this drawback, the number of required shift registers has been significantly reduced by providing a device for generating a full period sequence with a sequence length of ²ⁿ , as shown in Figure 2c. In this method, as shown in the figure, the output of the shift register 7 is input to the RAM 8, and the output of the RAM is fed back to the shift register 7. Storage device 10 and input device 11
calls the feedback function values of a plurality of full period sequences stored in the storage device into the control device according to the signal from the input device, outputs data and a clock signal to the shift register, and outputs the feedback function values to the RAM. It is configured so that functions are written via an address bus and a data bus. The control device also has the additional function of providing an initial value to the shift register.

In this method, the feedback function is based on nonlinear operations, so linear simultaneous equations cannot be used against plane text attacks, and the number of encryption keys is 2 ^n-1 _2-o.
This is an extremely excellent method that satisfies each of the above-mentioned requirements as an encryption method.

In this method, it is clear that in general, the storage device 10 may store not only full-period sequences, but also ring function values of sequences generated by a nonlinear generation method such that 0 and 1 are halved. .

However, in the conventional methods described above, in order to create a sequence corresponding to an encryption key, it is generally necessary to
2 It is necessary to store all the circular function values of a sequence of length ⁿ in memory, but if the capacity is limited, there is a problem that many sequences cannot be stored in memory and it is not possible to increase the number of encryption keys.

That is, in this method, in order to obtain a desired encryption key, the capacity of the storage device must be increased accordingly, making the device complicated and expensive.

(Objective and Summary of the Invention) The present invention has been made in view of the above-mentioned circumstances, and automatically generates a sequence corresponding to an encryption key using a certain algorithm by simply storing a few parameters. It is an object of the present invention to provide a sequence generation method that requires less information to be stored and can result in an inexpensive cryptographic device.

(Examples) Hereinafter, the present invention will be described in detail based on illustrated examples.

FIG. 1a is a block diagram for explaining the principle of the sequence generation device according to the present invention, in which the internal state x of the shift register 12 is passed through the feedback circuit 13, and the output (x) is sent to the shift register 12. It is configured to provide feedback.

An example of the relationship between the state x of the return shift register and the feedback function (x) in this case is shown in FIG. 1b. still,
This example is for a 3-bit series. FIG. 1C is a diagram illustrating changes in the state of the return shift register based on FIG. 1B, and is generally called a state transition diagram. In this figure, each node in the transition diagram indicates a respective state, and the numbers next to the arrows indicate feedback function values.

Figure 1 d is the complement of the feedback function value (010) = 0, (100) = 1 when x = (010) x = (100) for state x in Figure 1 b. 010)=1 (100)=0.

FIG. 1e is a state transition diagram of FIG. 1d. Comparing FIG. 1c and FIG. 1e, it will be clear that different sequences can be generated by transforming the feedback function according to some rules.

That is, it is sufficient to memorize the rules for series conversion and convert the feedback function values shown in FIG. 1b according to the input parameters.

In this embodiment, the series conversion rule is such that the complement of the specified feedback function value is calculated, and the input parameters are the value and number of x.

According to the conversion of this embodiment, for example, when the initial value of the state is x = (111), the series outputs in Figure 1 a are all 1
It is.

This fact means that the following restrictions are required when using this sequence generation method as an encryption method.

FIG. 1 is a block diagram showing an embodiment of a sequence generation device using a synchronous feedback shift register system.

The transmitting device 14 inputs the output 16 of the shift register 15 to the feedback circuit 17, feeds back the output of the circuit to the shift register 15, and combines this output and input data with the exclusive OR circuit 1.
8, and the output of this circuit is output as an encrypted signal, but in order to synchronize with the receiving side, a synchronizing signal from a synchronizing signal generator 19 is superimposed on the encrypted signal output in a mixer 20 and transmitted. Signal.

On the other hand, in the receiving side device 21, the shift register 22
to the input of the shift register 22 of a closed loop circuit configured to feed back the output of the shift register 22 to the shift register 22 via the feedback circuit 23;
A separation circuit 24 separates the synchronization signal from the encrypted signal sent from the transmitting device, inputs only the received data signal, and inputs the separated synchronization signal to the synchronization signal control device 25 . Further, the shift register 22 is controlled by an initial value and a synchronization signal generated by a synchronization signal control device to achieve synchronization with the transmitting device. Furthermore, an exclusive OR circuit 26 is inserted between the separation circuit 24 and the input of the shift register 22, and the exclusive OR of the encrypted data 27 and the output of the circular return circuit 23 is determined, and this is the demodulated signal output. becomes.

In this method, if the plaintext data sequence is Pi, the feedback shift register output sequence is Si, and the encrypted data sequence is Ci, then encryption: Ci = Pi + Si (mod 2) decryption: Pi = Ci + Si (mod 2), so the sequence is If Si is an initial value, for example, all 1, it cannot be used as an encryption device.

Therefore, in this embodiment, it is used as a self-synchronized feedback shift register as shown in FIG. 1g.

In the configuration shown in FIG. 1g, in the transmitting side device 28, a shift register 29, a feedback circuit 30, and an exclusive logic circuit 31 are connected in the same manner as in the above example, and the receiving side device 32 is also connected with a shift register 29, a feedback circuit 30, and an exclusive logic circuit 31. 33, a feedback circuit 34, and an exclusive logic circuit 35, which differs from that in FIG. 1() in that a synchronizing signal is not transmitted.

Figure 1g shows a case where a self-synchronized feedback shift register is used; in this case, the shift register does not close by itself, and the sum of the external signal is input, so the state of the shift register is determined by the external signal. It changes from moment to moment. At this time, if the feedback function value has 1/2 of 1 and 0, the encrypted data is pseudo-randomized and has good properties as encrypted data.

When a sequence generation device is constructed based on the sequence generation principle of this embodiment, it becomes as shown in FIG. 1h.

The output of the shift register 36 is input to the switching device 37, and the output of the switching device 37 is input to the temporary storage device 38. The output and input data 39 are input to an exclusive OR circuit 40, output as the output series 41, and fed back to the shift register 36. The control device 42 is connected to an input device 43, a storage device 44, and an arithmetic device 45, and outputs a clock 46 and data 47 to a shift register 37, an address 48 and a switching signal 49 to a switching device 37, and a temporary storage device 38. Data 45' is output to.

Next, to explain the operation of this configuration, the control device 42
inputs the sequence transformation parameters from the input device 43, and uses the arithmetic unit 45 to generate the sequence using the sequence generation algorithm stored in the control device 42 together with the feedback function value of only one sequence stored in the storage device 44. Convert. Next, the feedback function value of the generated series is written into the temporary storage device 38 through the data 45 with the switching device 37 set to the address 48 side. Finally, the control device 42 switches the switching device 37 to the shift register 36.
Switch to side, shift register 3 using data 47
An initial value of 6 is input, a clock 46 is supplied to the shift register 36, and a sequence 41 is created by encrypting the input data 39.

The decoding circuit is configured as described above using the feedback shift register shown on the receiving side of FIG. 1g.

The sequence generation algorithm stored in the control device is shown in FIG. 1i.

The example described above was a general theory of sequence generation, but by making the feedback function value and input parameters of the sequence to be stored special, it is possible to create a sequence that can be used for both synchronous and asynchronous feedback shift registers. Generation methods can be configured.

This will be explained in detail below using the drawings.

In FIG. 1i, the state x of the shift register 50 is input to the feedback circuit 51, and the output of the feedback circuit 51 is fed back to the shift register 50. FIG. 1j shows an example of the state x of the shift register and the feedback function value (x), which is one of the full period sequences (sequence length 2 ⁿ , where n is the number of flip-flops of the shift register).

FIG. 1k is a state transition diagram of FIG. 1j. The full period sequence is the maximum length sequence created from the shift register, has 2 ⁿ states, and the state transition diagram is expressed as a single cycle. In other words, no matter what initial value it starts from, the state is always in one cycle, and the feedback function values are 1/2 each, 0 and 1, so it can not only be used as a cryptographic circuit but also as an asynchronous feedback shift register. It can also be used as a synchronous feedback shift register.

Next, an algorithm for generating another all-periodic sequence from an all-periodic sequence will be explained.

In FIG. 1, the state x of the shift register 50 is input to the feedback circuit 51, the feedback value (x) is fed back to the shift register 50, and a series output 52 is obtained. FIG. 1j is a table of one state in the total periodic sequence and the feedback function (x). Figure 1 k is the first
FIG. 6 is a state transition diagram of FIG. The figure shows an example of creating another full-period sequence using k. First, as state x _i
Take 001 (represents 1 in decimal). For this x _i
Calculate x _i ^* , x _i ^* =x _i +N/2 (where N=2 ⁿ n is the number of flip-flops in the shift register).

In this case, 3+2 ³ /2=7. Next, by taking the complements of the feedback functions (x _i ) and (x _i ^* ), we get (x _i )=0 (x _i ^* )=1. By converting the feedback functions in this way, the state transition diagram becomes 001, 010, 100, 000, 001 and 101,
It is separated into two circuits: 011, 111, 110, and 101. Next, x _j and x _j ^* (x _j ^* = x _j + N/2) are respectively
Select so that it is on a closed path. For example, if x _j = 2 x _j ^* = 2 + 2 ³ /2 = 6, then (x _j ) = 1 (x _j ^* ) = 0, if the two cycles are transformed in this way, the relationship between state x and feedback function (x) becomes as shown in FIG. 1l, and the state transition diagram becomes one cycle again, that is, a full period series, as shown in FIG. 1m. The above algorithm can be summarized as shown in FIG.

The configuration of the self-periodic feedback shift register based on the principle of the second embodiment is basically the same as that in FIG. 1h in the first embodiment.

Furthermore, in order to obtain a synchronous feedback shift register, a configuration as shown in FIG. 1(o) may be adopted. That is, inputting the output of the shift register 52 to the switching device 53,
The output of the switching device 53 is input to the temporary storage device 54. The temporary storage device output and input data 55 are input to an exclusive OR 56 to form an output series 57. Further, the output of the temporary storage device 54 is fed back to the shift register 52. The control device 58 is connected to a storage device 59, an input device 60, an arithmetic device 61, and a synchronization circuit 62, and the output of the synchronization circuit 62 is superimposed on the output series signal by a mixer and transmitted as an encrypted signal.

The operation and operation of the above structure are almost the same as those shown in FIG. 1h described above.

(Effects of the Invention) Since the present invention is configured and functions as explained above, 0 and 1 are reduced to 1/2 each, not only for full-cycle sequences or full-cycle sequences for encryption. Since such a nonlinear sequence signal can be generated extremely efficiently with a simple configuration, it is extremely effective in providing an excellent cryptographic device with an extremely inexpensive and simple configuration.

[Brief explanation of the drawing]

1a to 1p are diagrams showing an embodiment of the present invention, in which a, g, h, o, and i are block diagrams of a part of the cryptographic generation device, and b, d, j, and l are block diagrams of a shift register. A diagram showing an example of the relationship between the content and the feedback function, c, e, k, and m are state transition diagrams p and n are flowcharts explaining the algorithm, and Figure 2 a, b, and c are conventional cryptographic generation methods. FIG. 2 is a block diagram showing the relationship between the contents of a flip-flop and a feedback function. 12, 15, 22, 29, 33, 36, 50...
...Shift register, 13, 30, 34, 17, 2
3...Feedback circuit, 43...Input device.

Claims (1)

[Scope of Claims] 1. A first storage device that stores a recursive function sequence of a full periodic series serving as at least one seed, an n-stage shift register, and a necessary return function according to the content value of the shift register. It includes a return function generating means for generating a ring function and feeding it back to the input of the shift register, a second storage device for storing a numerical value selected and determined in advance, and a control device for controlling the entire system. , the two state values x _i and x _j in the seed all-periodic series, and x ^* _i ≡x _i +N/2 for these.
Find the state values of the species all-periodic series such that x ^* _j ≡x _j +N/2 (where N = 2 ⁿ ), and calculate the recursive functions f(x _i ), f(x j ), f(x _j ) for the four state values, f(x ^* _i , f(x ^* _j )
When finding the complements of (x _i ), (x _j ), (x ^* _i ), (x ^* _j ) and replacing them with a part of the recursive function of the all periodic series, the recursive function becomes Two state values x _i such that a new total periodic sequence is formed,
A pair group of x _j is stored in the second storage device, and when generating a new full-period sequence, one pair of the pair group of state values is used as a key, and the complements (x _i ), ( By finding x _j ), (x ^* _i ), and (x ^* _j ) and using them as part of a new recursive function, we have found a total periodic sequence that is different from the species all-periodic sequence. Featured sequence generation method.