JPH06259011A - Enciphering device and decoder for open key - Google Patents

Abstract

PURPOSE:To make a ciphertext hard to be deciphered by excluding specific plaintexts 0, 1, and n-1 which become the same texts as the original plaintext even after RSA ciphering. CONSTITUTION:A plaintext block division part 1 divides an input plaintext into plaintext blocks of integers between 0 and n-3, a enciphering preprocessing part 2 adds two to the respective plaintext blocks, and an RSA enciphering processing part 3 enciphers only blocks of integers between 2 and n-2 by utilizing an key (e, n). On a decoding side, an RSA decoding processing part 8 deciphers the respective ciphertext blocks by utilizing a decoding key (d, n), a decoding postprocessing part 10 substracts two from the decoding plaintexts, and the results are connected in order to obtain the original plaintext.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an encryption device and a decryption device that use a public key cryptographic algorithm that uses a power and a modular exponentiation known in RSA cryptography.

[0002]

2. Description of the Related Art The RSA cryptosystem is known as a representative public key cryptographic algorithm that uses a power and a modular exponentiation. (For a detailed explanation of RSA encryption, see Chapter 6 etc. of Contemporary Cryptography, published by The Institute of Electronics, Information and Communication Engineers). RS
In the A-cipher, the encryption side encrypts the plaintext M represented by an integer by using the encryption exponent e and the modulus n. On the other hand, the decryption side restores the ciphertext D to the original plaintext M using the decryption index d and the modulus n. In the RSA cryptosystem, the plaintext M is limited to an integer of 0 ≦ M ≦ n−1 due to the characteristics of the algorithm.

It has been pointed out that there are always nine sets of plaintext M whose encrypted result matches the original plaintext, that is, plaintext M which cannot be encrypted (Reference: Contemporary Cryptography, Chapter 6 of the Institute of Electronics, Information and Communication Engineers, Chapter 6). pp. 114-116). However, conventionally, in a real system, since n is about the 512th power, the probability that the above-mentioned 9 sets will appear is extremely small, 9/2 ^512, and therefore it has not been particularly emphasized.

[0004]

However, three of these nine sets are 0, 1, n-1 in specific plaintext. The values 0 and 1 do not depend on the encryption index e, the decryption index d, and the modulus n, and n-1 is a value dependent only on the modulus n. Therefore,
No matter what encryption index e, decryption index d, or modulus n is selected, a malicious eavesdropper can use these three sets of plaintext M.
Can be partially decoded.

For example, in the case of encrypting plaintext of modulo n or more, when a material in which a certain figure or character is written is read as binary data by using a scanner (image reader), continuous 0s continue for several kilobits. Often also. Even if such data is encrypted using RSA encryption,
In the ciphertext, the same sequence of 0s as the original plaintext becomes several kilobits.

The object of the present invention is to provide a public-key cryptographic device in which there is no combination in which the encryption result is always the same as the original plaintext, no matter what key (encryption index e, decryption index d, modulus n) is selected. And to provide a decoding device therefor.

[0007]

According to the first aspect of the present invention, the plaintext represented by an integer of 0 or more and n-3 or less is added by 2 by the correction constant adding means, and the added plaintext is plaintext encrypted. It is encrypted by means of exponentiation and modulo n modulo operation. According to the second aspect of the present invention, the ciphertext represented by an integer of 2 or more and n-1 or less is exponentiated and modulo n by the ciphertext decryption means.
Is decrypted into a plaintext by using the remainder operation of, and 2 is subtracted from the decrypted plaintext by the correction constant subtraction means to obtain a regular plaintext.

[0008]

The plaintext is expressed by an integer of 0 or more and n-3 or less, and 2 is added to this, that is, the plaintext becomes an integer of 2 or more and n-1. Since the added plaintext is encrypted, it is encrypted. The immediately preceding plaintext does not include 0 and 1, that is, the plaintexts 0 and 1 that are the original plaintext even if encrypted are excluded. that's all,
Even if it is encrypted, it is possible to avoid encrypting the specific plaintext that is the original plaintext, and to avoid partial decryption of the specific plaintext. To exclude the specific plaintext n-1 for encryption, the plaintext may be first expressed by an integer of 0 or more and n-4 or less.

[0009]

Embodiments of the present invention will be outlined. At first,
An integer of n-3 or more is divided into a plurality of data blocks of 0 or more and n-4 or less in advance. In this way, all positive integers are divided into integers from 0 to n-4, and then 2 is added to all data blocks. Next, 2
By dividing the data block into n-2 or less, all positive integers can be encrypted while avoiding specific plaintext.

FIG. 1 shows a public key encryption system using the encryption device and the decryption device of the present invention. The part newly added to the conventional device to implement the present invention is only the pre-encryption processing unit 2 and the post-decryption processing unit 10, and other than that is exactly the same as the conventional RSA encryption device. Since the digital signal is usually expressed by a binary number, this embodiment also uses 2
Described on the basis of a base number.

In the encryption device, the data (plaintext) to be encrypted is put into the plaintext blocking unit 1. The plaintext blocking unit 1 first divides the data into 0 to n-4 data blocks. The processing operation of the plaintext blocking unit 1 will be described with reference to FIG. First, j is set to 1 (S ₁ ), then k bits are extracted from the beginning of the input plaintext and set to M _j (S ₂ ). It is checked whether or not M _j is smaller than n-3 (S ₃ ). If it is smaller than n-3, M _j is set as the j-th plaintext block (S ₄ ). If M _j is not smaller than n−3, the least significant bit of that M _j is returned to the beginning of the remaining plaintext (S ₅ ), and M _j is shifted one bit lower (to the right), that is, M _j. Is set to one half (S ₆ ), and the process proceeds to step S ₄ to set M _j as the j-th plaintext. Next, j is incremented by 1 and the process returns to step S ₂ (S ₇ ).

The plaintext block M of k bits, which is an integer of 0 to n-4, obtained in this way is added with 2 to all the data blocks in the encryption preprocessing unit 2, and as shown in FIG. 3A. The value 2 of each plaintext block M is shifted. As a result, the plaintext block M (2 ≦ M ≦ n−2) is completed. This plaintext block M is encrypted by the RSA encryption processing unit 3 using the encryption key (e, n) by the same method as the conventional RSA encryption, and the encrypted text D (2 ≦ D ≦ n−2) is obtained. obtain.

At this time, the number of bits of the ciphertext D has a different bit length from 2 bits to k bits. If a plurality of ciphertexts D are continuously transmitted, the data cannot be decrypted because the receiving side does not know where the ciphertext is a break. Therefore, in order to align the bits with the longest bit length of k bits, 0 is added to the upper bits so that the ciphertext with less than k bits has k bits. By doing so, the ciphertext D becomes all k-bit ciphertext blocks. This ciphertext block has k bits, but since only the high-order bits are filled with 0s, the ciphertext block can also be represented as the ciphertext block D (2 ≦ D ≦ n−2), which is the same as the ciphertext D. it can.

The ciphertext block synthesizing unit 5 adjusts the bit length of the encrypted data to k bits to form a ciphertext block D, synthesizes the ciphertext block D, and transmits the ciphertext block D to the receiving side via the network 6. The decryption device on the receiving side fetches the received ciphertext into the ciphertext blocking unit 7. The ciphertext block forming unit 7 divides the received ciphertext into ciphertext blocks of k bits. The ciphertext block is decrypted by the RSA decryption processing unit 8 using the decryption key (d, n) 9 in exactly the same manner as the conventional decryption.

Next, the post-decryption processing unit 10 subtracts 2 from the decrypted plaintext block, and outputs the integer 2 as shown in FIG. 3B.
Shift each n-2 by a smaller value by 2. Finally,
The plaintext block synthesis unit 11 synthesizes the plaintext blocks to restore the original plaintext. The correction constant 2 is
Even if it is known to an eavesdropper or the like, it does not affect the encryption strength at all. Therefore, there is no problem even if anyone uses 2 as the correction constant at any time. Of course, the encryption side may add 2 as a header before transmitting the ciphertext or disclose that 2 is used. In the above, the plaintext is first made into an integer block of 0 to n-4, but the plaintext is set to 0 to n-
Even if the block is divided into an integer block of 3, normally, n-3 plaintext blocks do not continue, so the block obtained by adding 2 to this is n-1, and the ciphertext is also n-1. ,
There is no danger of being deciphered as a whole.

[0016]

As one of the weak points common to the conventional public-key cryptographic algorithms that use the modular exponentiation, there is a property that some ciphertexts can be easily solved no matter what key is selected. However, according to the present invention, by encrypting such a part of ciphertexts, it is possible to prevent a part of the ciphertexts from being easily decrypted no matter what key is selected.

[Brief description of drawings]

FIG. 1 is a block diagram showing an entire embodiment of the present invention.

FIG. 2 is a flowchart showing a processing example for blocking sufficiently long plain text.

3A is a diagram specifically illustrating an internal operation of a pre-encryption processing unit 2 in FIG. 1, and FIG. 3B is a diagram specifically illustrating an internal operation of a post-decryption processing unit 10 in FIG. .

Claims (2)

[Claims] 1. A public-key cryptographic apparatus that uses a power of exponentiation and a modular exponentiation, and pre-encryption means for adding 2 to a plaintext represented by an integer of 0 or more and n-3 or less. Plaintext 2 or more n added by the processing means
-A public key encryption device comprising: a plaintext encryption means for encrypting a ciphertext represented by an integer of -2 or less. 2. A public-key decryption apparatus that uses a power operation and a modulo n remainder operation, and a ciphertext decryption unit that restores a ciphertext represented by an integer of 2 or more and n-2 or less to plaintext, and the ciphertext decryption unit. A public key decryption apparatus comprising: post-decryption processing means for obtaining a regular plaintext by subtracting 2 from the plaintext decrypted by the means.