JP4763972B2 - Encrypted information transmission method, computer program, and communication system - Google Patents

Description

The present invention relates to an encryption method, and more particularly to a ciphertext delivery method in which a secret key in a common key encryption method is difficult to be decrypted .

  Since the common key cryptosystem has a small amount of calculation for encryption and decryption, it has an advantage that processing by a computer can be performed at high speed. However, since the secret key is exchanged between the sender and the receiver, a method for safely delivering the secret key is desired.

  For example, there is a method in which both transmission and reception hold a common master key, transmission data is encrypted with a work key, this work key is encrypted with the master key, and the ciphertext and the encrypted work key are transmitted to the other party.

  In this case, decryption can be attempted using any combination of random numbers instead of the work key. Since one of every combination is a correct work key, the ciphertext can be decrypted by exhaustive key search. Therefore, if the original plaintext has redundancy, the work key corresponding to the meaningful information obtained by the decryption attempt is the correct work key.

Once the correct work key is obtained, the master key can be obtained by treating the encrypted work key as ciphertext and decrypting it. Therefore, when such a conventional method is used, the master key must be frequently changed even if it takes a long time to try all combinations of random numbers for work key decryption.
JP 2000-101564 A JP2002-300151

When the master key is frequently changed, there is a possibility that a difference occurs between the sending side and the receiving side. There is a method for solving this, but the process is complicated. Therefore, in order to use one master key for a long time, a ciphertext delivery method in which the secret key is difficult to decipher is required.

The present invention has been developed in view of the above-described problems, and an object thereof is to realize ciphertext delivery in which a secret key is difficult to decrypt . Specifically, the present invention is a common key cryptosystem, in which a transmission side and a reception side share a first secret key and a second secret key, and a transmission side uses a first secret key as a random number. Converting to generate a first key; converting a random number with a second secret key to generate a second key; encrypting the object with the second key; A transmission method for transmitting an object encrypted with a key and a first key is provided.
In the case of the above method according to the present invention, even if the second key is known as a result of attempting to decrypt using any combination of random numbers instead of the second key, the second key Is the result of converting the random number with the second secret key, and the random number used for the second secret key cannot be known from only one decryption result. Further, even if an attempt is made to select a first secret key candidate from the decrypted second key and the encrypted first key, if the used random number is not specified, the first secret key candidate It is also not possible to select. The random number can be changed frequently, and most often at each transmission. Therefore, communication security can be maintained only by concealing the first and second secret keys.

  The present invention is also a common key cryptosystem, in which a transmitting side and a receiving side share a first secret key and a second secret key, and a receiving side encrypts an object encrypted with the second key. Receiving a first key, reversely converting the received first key with a first secret key to obtain a random number, and using the random number and the second secret key, a second key And a receiving method comprising: decrypting an object encrypted using a second key. Thus, even if the random number is changed frequently, it can be easily decoded.

  It is also effective to perform the above processing for three or more secret keys. This makes the calculation more complicated and makes it possible to perform encrypted communication that is difficult to decipher. Moreover, since the above method is performed on a computer, a program for realizing these methods is also provided.

  FIG. 1 is a diagram showing processing on the transmission side of the present invention. First, both the sender and the receiver have a first secret key a1 and a second secret key b2 as a common secret key (so-called master key). The common key is preferably a random number, but may be other than a random number.

  Next, the sender A encrypts the plain text. First, a random number R3 is generated. As this random number, for example, a true random number generated using physical noise or the like is desirable, but a pseudo-random number can also be used. That is, a random number with no reproducibility is preferable. Next, this random number R3 is converted by the first secret key a1 and the second secret key b2, respectively, 4 and 6, thereby generating the first key Ra5 and the second key Rb7, respectively. Note that the conversion 4 and the conversion 6 may be a common process or different processes. In this embodiment, since encryption is performed using the second key Rb7, the second key Rb7 is a so-called work key.

  Encryption is performed using the second key Rb7. As shown in the figure, the plaintext D8 is encrypted 9 with the second key Rb7 to obtain a ciphertext D (Rb) 10. Thereafter, the first key Ra5 and the ciphertext D (Rb) 10 are transmitted to the other party.

  Next, processing after reception will be described with reference to FIG. When the receiver B receives the first key Ra5 and the ciphertext D (Rb) 10, the receiver B obtains a random number R3 by performing an inverse transformation 11 on the first key Ra5 with the first secret key a1 held in advance. . Next, this random number R3 is converted with a second secret key b2 held in advance to obtain a second key Rb7.

  As described above, since the plaintext D8 is encrypted with the second key Rb7, the receiver B can decrypt it with the second key Rb7 obtained by calculation. The plaintext D8 can be obtained by decrypting 12 the ciphertext D (Rb) 10 with the second key Rb7.

  If the first and second secret keys are shared in this way, the random number R3 can always be decrypted on the receiving side even if the random number R3 for encryption is frequently changed. By changing the random number, the encryption key is frequently changed, and secure communication can be performed.

Here, the safety of the present invention will be examined. Here, the conversion between the first and second secret keys a and b and the random number R is considered by exclusive OR (xor). The key length is 2 bits. Table 1 shows the conversion table.

  First, let the first and second secret keys a and b be a = 1, b = 2, and the random number R = 3. From the conversion table, Ra = 2 and Rb = 1. Assume that the ciphertext Y obtained by encrypting plaintext with Rb is decrypted by exhaustive search and the work key Rb is found. Now, Ra is known because it is sent with the ciphertext. That is, since Ra = (R xor a) and Rb = (R xor b), since the indefinite equation of three variables (R, a, b) and two equations (Ra and Rb are known), the secret key key a, b cannot be specified.

  For example, from Table 1, when Ra = 2, there are four types of combinations of R and a: (R, a) = (0, 2) (1, 3) (2, 0) (3, 1). Similarly, when Rb = 1, (R, b) = (0, 1) (1, 0) (2, 3) (3, 2). That is, there are four possible combinations of (R, a, b): (0, 2, 1) (1, 3, 0) (2, 0, 3) (3, 1, 2).

  From the above, even if the second key Rb is found by exhaustive search or the like, it is safe because there is no information for specifying the first and second secret keys a and b.

  Next, another embodiment of the secret key distribution method will be described. As described above, the sender A and the receiver B have the first and second secret keys in common. Next, the sender A generates two random numbers R1 and R2. The two random numbers R1 and R2 are converted with the first and second secret keys to obtain R1a and R1b. Further, R12 is obtained from R1 and R2. This can be performed by, for example, exclusive OR of R1 and R2, or hashing. The plaintext D is encrypted with R12, and R1a, R2b, and ciphertext D (R12) are transmitted.

  On the receiving side, after receiving R1a, R2b, and D (R12), R1a and R1b are inversely converted from the first and second secret keys a and b to obtain R1 and R2. Next, R12 is calculated from R1 and R2. Finally, plaintext D can be obtained by reversely converting (decrypting) ciphertext D (R12) with R12.

  Cryptographic communication can be performed safely by the method as described above. By realizing each of these processes as a program, a system capable of safely communicating important information in a communication network such as the Internet or a mobile phone can be constructed. The present invention can also be applied to three or more secret keys. If the sum of the number of random numbers used for encryption and the number of secret keys is larger than the total number of keys obtained when the ciphertext is broken and the number of key information passed during transmission and reception, Therefore, the secret key cannot be specified and is secure. Therefore, even if the ciphertext is decrypted, the secret key can be safely used for subsequent encryption without updating the secret key.

  As described above, according to the present invention, the key for decryption is obtained by the random number R and the first or second secret key. In transmission, the result of encrypting the random number with the first secret key is sent. By changing the random number for each transmission, the key can be delivered safely, and encryption communication that is difficult to decipher can be performed.

FIG. 1 is a block diagram showing encryption processing before transmission according to the present invention. FIG. 2 is a block diagram showing a decoding process after reception according to the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 1st secret key 2 2nd secret key 3 Random number 4 Conversion key 5 1st key 6 Conversion sentence 7 2nd key 8 Plaintext 9 Encryption 10 Ciphertext 11 Reverse conversion 12 Decomposition

Claims (3)

At least a receiving unit is excluded from at least a transmitting unit configured to include a random number generating unit, one or more calculating units that calculate exclusive OR , an encrypting unit, and a transmitting unit. A method of transmitting encrypted information to a receiving device comprising one or a plurality of arithmetic means for calculating a logical OR and a decryption means,
Generating a random number to be executed in the transmitting device;
Generating a first key by calculating an exclusive OR of the random number and the first secret key;
Generating a second key by calculating an exclusive OR of the random number and a second secret key;
Encrypting the object with a second key;
Transmitting only the object encrypted with the second key and the first key;
Receiving the object encrypted with the second key and the first key, executed at the receiving device;
Obtaining a random number by calculating an exclusive OR of the received first key and the first secret key;
Generating a second key by calculating an exclusive OR of the random number and a second secret key;
Decrypting the object encrypted with the second key,
A transmission method characterized in that a first secret key and a second secret key are shared between a transmission device and a reception device. At least a means for generating a random number, one or a plurality of calculating means for calculating an exclusive OR, encrypting means and configured transmission unit and a transmitting unit, at least a receiving unit, exclusive A first secret key and a second secret key provided with another communication device, the receiving unit comprising one or a plurality of calculation means for calculating a logical OR, and a decoding means A computer program for operating a communication device sharing
Generating a random number executed in the transmitting unit;
Generating a first key by calculating an exclusive OR of the random number and the first secret key;
Generating a second key by calculating an exclusive OR of the random number and a second secret key;
Encrypting the object with a second key;
Transmitting only the object encrypted with the second key and the first key;
Receiving the object encrypted with the second key and the first key, executed at the receiving unit;
Obtaining a random number by calculating an exclusive OR of the received first key and the first secret key;
Generating a second key by calculating an exclusive OR of the random number and a second secret key;
Decrypting the object encrypted using the second key. And at least a means for generating a random number, one or a plurality of arithmetic means for calculating an exclusive OR , an encryption means, and a transmission means.
Generating a random number;
Generating a first key by calculating an exclusive OR of the random number and the first secret key;
Generating a second key by calculating an exclusive OR of the random number and a second secret key;
Encrypting the object with a second key;
Transmitting a target encrypted with a second key and a step of transmitting only the first key;
At least a receiving means, one or a plurality of calculating means for calculating exclusive OR, and a decoding means,
Receiving the object encrypted with the second key and the first key;
Obtaining a random number by calculating an exclusive OR of the received first key and the first secret key;
Generating a second key by calculating an exclusive OR of the random number and a second secret key;
A receiving device for performing the step of decrypting the object encrypted using the second key,
A communication system, wherein a first secret key and a second secret key are shared between a transmission device and a reception device.

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