A transmission apparatus performs a one-way operation on plaintext to generate a first value and transmits the first value, generates first additional information, performs an invertible operation on the plaintext and first additional information to generate connected information, encrypts the connected information using an encryption algorithm to generate ciphertext, and transmits the ciphertext. A reception apparatus receives the first value and the ciphertext, generates second additional information identical to the first additional information, decrypts the ciphertext using a decryption algorithm, which is an inverse-conversion of the encryption algorithm, to generate decrypted connected information, decrypts the decrypted connected information and the second additional information according to an inverse of the invertible operation to generate decrypted text, performs the one-way operation on the decrypted text to generate a second value, compares the first and second values, and judges that the decrypted text is valid only when the first and second values match.

This application is based on an application No. 2000-384835 filed in Japan, the content of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to an encryption technology used as an information security technology, and especially to a technology for detecting errors that occur in decrypting.

(2) Description of the Related Art

As data communication using a computer technology or a communication technology becomes widespread, cryptocommunication systems are becoming prevalent. Cryptocommunication enables data communication without revealing the communications to a third party who is not an intended party.

Cryptosystems are used for realizing the cryptocommunication systems. In cryptosystems, an authentic encryption key is used in applying an encryption algorithm to plaintext for generating ciphertext, and an authentic decryption key is used in applying a decryption algorithm to the ciphertext for generating decrypted text. In some cryptosystems, there is a possibility of generating decrypted text which is different from original plaintext. Hereinafter, the phenomena in which generated decrypted text is different from its original plaintext is referred to as a “decryption error,” and the cryptosystem in which this decryption error occurs is referred to as a “decryption error vulnerable cryptosystem.”

One example of the above-mentioned decryption error vulnerable cryptosystems is a NTRU cryptosystem. The NTRU cryptosystem, simply put, generates ciphertext by encrypting plaintext by using random numbers as parameters and an encryption key, and generates decrypted text by decrypting the ciphertext by using a decryption key. This cryptosystem, which uses random numbers as parameters, has a chance of obtaining different ciphertext from the same plaintext.

For a detailed description of the NTRU cryptosystem, please refer to Jeffrey Hoffstein, Jill Pipher, and Joseph H. Silverman, “NTRU: Aringbasedpublic key cryptosystem,” Lecture Notes in Computer Science, 1423, pp. 267–288, Springer-Verlag, 1998.

In the cryptocommunication system using the NTRU cryptosystem, there is a possibility of obtaining different decrypted text from the original plaintext. Therefore, intended information is not insured to be transmitted to the receivers.

FIRST CONVENTIONAL EXAMPLE

In order to overcome the above stated problem, the following cryptocommunication system using the NTRU cryptosystem has been proposed. This cryptocommunication system consists of an encrypting apparatus and a decrypting apparatus. The encrypting apparatus and the decrypting apparatus are connected to each other through a communication channel.

The encrypting apparatus generates n random numbers r1, r2, . . . ,rn, and encrypts plaintext m by using an encryption key Kp stored in advance and the mentioned random numbers as parameters, in order to obtain n pieces of ciphertext c1, c2, . . . ,cn.
c1=E(m,Kp,r1)
c2=E(m,Kp,r2)

Here, the equation C=E (M, K, R) shows that the ciphertext C is generated by encrypting the plaintext M by using the encryption key K and the random number R as parameters.

Next, the encrypting apparatus transmits, to the decrypting apparatus, the generated n pieces of ciphertext c1, c2, . . . , cnthrough the communication channel.

Next, the decrypting apparatus considers that a decryption error has occurred if a single component in the decrypted text m′1, m′2, . . . , m′nis different from its corresponding original plaintext.

This cryptocommunication system is inefficient in that the system increases communications, even if the system is capable of detecting the occurrence of decryption errors. In addition, there is a possibility of degrading the security for this cryptocommunication system, in which different random numbers are used as parameters for transmitting a plurality of pieces of ciphertext based on the same plaintext.

This is due to the possibility that, from n equations,
c1=E(m,Kp,r1)
c2=E(m,Kp,r2)

. . .
cn=E(m,Kp,rn),
the information on the plaintext m or on the random numbers r[1], r[2], . . . , r[n] are likely to be revealed to third parties. The encryption attack using this inherent disadvantage in the NTRU system is called “multiple transmission attack.”

Specifically, it is known that the security is endangered when the decryption error detection is performed in the NTRU cryptosystem which is one of the detection error vulnerable cryptosystems. Please refer, for a detailed description about the multiple transmission attack, to Jeffrey Hoffstein, Jill Pipher, and Joseph H. Silverman, “NTRU: A ring based public key cryptosystem,” Lecture Notes in Computer Science, 1423, pp. 267–288, Springer-Verlag, 1998.

As described above, the decryption error detection performed in the NTRU cryptosystem has a problem of increasing communications and lowering the security level.

SECOND CONVENTIONAL EXAMPLE

A Japanese Laid-Open Publication No. 2000-216773 discloses the following technology for the purpose of providing a method and an apparatus for judging the correctness of the encrypted information in which receivers of the encrypted information can judge whether or not the decrypted information is correct.

In this technology, a sender calculates a first hash value of plaintext by using a predetermined hash value generation algorithm, and sends the first hash value with ciphertext resulting from encrypting the plaintext by using an encryption algorithm. Then, a receiver receives the ciphertext with the first hash value, generates decrypted text by decrypting the ciphertext, calculates a second hash value of the decrypted text by using the same hash value generation algorithm that was used for calculating the first hash value, compares the first hash value and the second hash value, and judges that the decrypted text is correct only when the first and the second hash values match.

However, even when the above-mentioned conventional technologies are used, it is difficult to completely avoid third parties' attacks. A more secured cryptocommunication system is therefore desired.

SUMMARY OF THE INVENTION

The object of the present invention, in order to solve the above problem, is to provide a cryptocommunication system, a transmission apparatus, a reception apparatus, a method of cryptocommunication, a program for a cryptocommunication, and a recording medium on which the program is recorded, that are more secure.

The object of the present invention is achieved by a cryptocommunication system including a transmission apparatus and a reception apparatus. The transmission apparatus encrypts plaintext to generate ciphertext, performs a one-way operation on the plaintext to generate a first value, and transmits the ciphertext and the first value to the reception apparatus. The reception apparatus receives the ciphertext and the first value, decrypts the ciphertext to generate decrypted text, performs the one-way operation on the decrypted text to generate a second value, and judges that the decrypted text matches the plaintext when the second value and the first value match. The transmission apparatus includes: a first generating unit for generating first additional information; a first operation unit for performing an invertible operation on the plaintext and the first additional information to generate connected information; an encrypting unit for encrypting the connected information according to an encryption algorithm so as to generate the ciphertext; and a transmitting unit for transmitting the ciphertext. The reception apparatus comprises: a receiving unit for receiving the ciphertext; a second generating unit for generating second additional information which is identical to the first additional information; a decrypting unit for decrypting the ciphertext according to a decryption algorithm, which is an inverse-conversion of the encryption algorithm, so as to generate decrypted connected information; and a second operation unit for performing an inverse operation of the invertible operation on the decrypted connected information and the second additional information so as to generate the decrypted text.

According to this structure, the transmission apparatus enables connected information to be generated by performing an invertible operation on the plaintext and on the first additional information, encrypted connected information to be generated by encrypting the connected information, and the encrypted connected information to be transmitted. The reception apparatus enables the connected information to be received, decrypted connected information to be generated by decrypting the received encrypted connected information, and decrypted text to be generated by performing an inverse operation of the invertible operation on the decrypted connected information and on the second additional information. This realizes a more secure cryptocommunication system than the conventional systems.

Here, in the cryptocommunication system of the present invention, the second generating unit synchronizes with the first generation unit so as to generate the second additional information which is identical to the first additional information.

According to this structure, the second generating unit synchronizes with the first generating unit in order to generate second additional information which is identical to the first additional information, thereby enabling decrypted connected information to be obtained which has the same content as the connected information.

Here, in the cryptocommunication system of the present invention, the first generating unit generates a random number, and sets the generated random number as the first additional information.

According to this structure, the first generating unit can generate first additional information by using a random number, thereby generating different additional information for each communication. This makes it difficult to infer first additional information from encrypted connected information. Here, in the cryptocommunication system of the present invention, the invertible operation unit bit-connects the plaintext with the first additional information so as to generate the connected information, and the second operation unit deletes the second additional information from the decrypted connected information so as to generate the decrypted text.

According to this structure, the invertible operation unit can generate connected information by bit-connecting the plaintext with the first additional information, and the inverse invertible operation unit can generate decrypted text by deleting the second additional information from the decrypted connected information. Therefore, correct decrypted text is assured to be obtained from the decrypted connected information.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following is a description of a cryptocommunication system1which is one embodiment pertaining to the present invention.

1.1 The Structure of the Cryptocommunication System1

A cryptocommunication system1is a system in which decryption error detection is enabled for the decryption error vulnerable cryptocommunication systems. As shown inFIG. 1, the cryptocommunication system1consists of a transmission apparatus10and a reception apparatus20, both of which are connected to each other through an internet30.

The cryptocommunication system1uses the NTRU cryptosystem which is one of the decryption error vulnerable cryptosystems. Please refer to Jeffrey Hoffstein, Jill Pipher, and Joseph H. Silverman, “NTRU: A ring based public key cryptosystem,” Lecture Notes in Computer Science, 1423, pp. 267–288, Springer-Verlag, 1998 for a detailed description about a method of generating NTRU ciphertext, and a method of generating an encryption key and a decryption key for the NTRU cryptosystem.

The transmission apparatus10generates ciphertext by applying, to the plaintext stored in advance, the encryption algorithm according to the NTRU cryptosystem, and transmits the generated ciphertext to the reception apparatus20. The reception apparatus20, in turn, receives the ciphertext, and generates decrypted text by applying, to the received ciphertext, the decryption algorithm according to the NTRU cryptosystem.

1.2 The Structure of the Transmission Apparatus10

The transmission apparatus10consists of a plaintext storage101, an additional information generation unit102, an information adding unit103, a one-way operation unit104, an encrypting unit105, and a transmitting unit106. The transmission apparatus10is concretely a computer system composed of a microprocessor, ROM, RAM, a hard disk unit, a display unit, a key board, a mouse, a communication unit, and the like. The RAM or the hard disk unit stores a computer program. The transmission apparatus10realizes its function by making the microprocessor work according to the computer program.

The plaintext storage101stores plaintext m in advance. The plaintext m is composed of information with a fixed length.

(2) The Additional Information Generation Unit102

The additional information generation unit102generates additional information Ra which is a random number with a predetermined bit length of rLen, and outputs the generated additional information Ra to the information adding unit103.

(3) The Information Adding Unit103

The information adding unit103reads out the plaintext m from the plaintext storage101, and receives the additional information Ra from the additional information generation unit102.

Next, the information adding unit103connects the read plaintext m with the received additional information Ra by a bit-connecting method, so as to obtain resulting connected information F (m, Ra)=m||Ra.

Here, the operator “||” signifies a bit-connecting. The bit-connecting represents a single value which is a result from uniting two values, each being represented as a bit row. In an example assuming that m=10, rLen=5, and Ra=7, the bit row representation for the plaintext m is “1010”, and the bit row representation for the additional information Ra with a length of rLen is “00111”. Thus, the result from the bit-connecting is “101000111,” which means 327 in decimal notation.

Next, the information adding unit103outputs the generated connected information F(m, Ra) to the encrypting unit105.

The one-way operation unit104stores a hash function h which is a one-way operation.

Here, the one-way operation is a function which is designed to calculate a value from an inputted value, and which makes it difficult to calculate the originally inputted value from the value. Further, an assumption is made about the hash function h used here that it is assured to be difficult enough to obtain a value for the plaintext m by using the value h(m), and it is collide-free. For the details of the one-way operation, the hash function, the security of the hash function, and the collision-free characteristic of the hash function, refer to Tatsuaki Okamoto, Hirosi Yamamoto, “Gendai Ango” (Modern cryptography), Series/Mathematics in Information Science, Sangyo-Tosho, 1997,pp. 56, and pp. 189–195.

The one-way operation unit104reads out the plaintext m from the plaintext storage101, calculates a value h(m) from the read plaintext m by using the hash function h, and outputs the calculated value h(m) to the transmitting unit106.

As shown inFIG. 2, the encrypting unit105consists of a random number generation unit1051, an encryption key storage1052, and an encryption function unit1053.

a. The Encryption Key Storage Unit1052

The encryption key storage unit1052stores an encryption key Kp in advance.

b. The Random Number Generation Unit1051

The random number generation unit1051generates a random number r, using a rand ( ) which is a library function for the C language, for example, and outputs the generated random number r to the encryption function unit1053.

c. The Encryption Function Unit1053

The encryption function unit1053includes an encryption algorithm dedicated to the NTRU encryption cryptosystem in advance.

The encryption function unit1053receives a connected information F (m, Ra) from the information adding unit103, receives a random number r from the random number generation unit1051, and reads out an encryption key Kp from the encryption key storage1052.

Next, the encryption function unit1053, by using the random number r and the read encryption key Kp, encrypts the received connected information F(m,Ra) according to the encryption algorithm, so as to generate encrypted connected information E (F(m, Ra), Kp, r), and outputs the generated encrypted connected information E(F(m, Ra), Kp, r) to the transmitting unit106.

The transmitting unit106receives the encrypted connected information E (F(m, Ra), Kp, r) and the value h(m), and transmits the received encrypted connected information E (F(m, Ra), Kp, r) and value h(m) to the reception apparatus20through the internet30.

1.3 The Structure of the Reception Apparatus20

The reception apparatus20consists of a receiving unit201, a decrypting unit202, an information removing unit203, a one-way operation unit204, a comparison (comparing) unit205, a decrypted text storage206, and a comparison result storage207. The reception apparatus20is specifically the same computer system as the transmission apparatus10.

(1) The Receiving Unit201

The receiving unit201receives, from the transmission apparatus10, the encrypted connected information E (F(m, Ra), Kp, r) and the value h(m) through the internet30, and outputs the received encrypted connected information E to the decrypting unit202, and outputs the received value h (m) to the comparison unit205.

As shown inFIG. 3, the decrypting unit202consists of a decryption key storage2021and a decryption function unit2022.

a. The Decryption Key Storage2021

The decryption key storage unit2021stores a decryption key Ks in advance.

b. The Decryption Function Unit2022

The decryption function unit2022stores a decryption algorithm which is an inversed form of the encryption algorithm which is included in the encryption function unit1053.

The decryption function unit2022receives the encrypted connected information E(F(m, Ra), Kp, r) from the receiving unit201, and reads out the decryption key Ks from the decryption key storage2021.

Next, the decryption function unit2022, by using the read decryption key Ks, decrypts the received encrypted connected information E(F(m,Ra),Kp,r) according to the decryption algorithm, so as to generate a decrypted connected information D (E(F(m, Ra), Kp, r), Ks), and outputs the decrypted connected information to the information removing unit203.

(3) The Information Removing Unit203

The information removing unit203stores a bit length rLen in advance.

The information removing unit203receives the decrypted connected information D(E(F(m, Ra), Kp, r), Ks) from the decrypting unit202, removes the additional information Ra from the decrypted connected information, by removing a bit row of the rLen bit length from the end of the received decrypted connected information D(E(F(m, Ra), Kp, r), Ks), generates decrypted text from the remaining information after the additional information Ra is removed from the decrypted connected information, and outputs the generated decrypted text m′ to the one-way operation unit204. The information removing unit203also writes the generated decrypted text m′ on the decrypted text storage206.

The one-way operation unit204stores, in advance, the same hash function h which is included in the one-way operation unit104.

The one-way operation unit204receives the decrypted text m′ from the information removing unit203, hashes the received decrypted text m′ according to the hash function h so as to generate a functional value h(m′), and outputs the value h(m′) to the comparison unit205.

(5) The Comparison Unit205

The comparison unit205receives the value h(m) from the receiving unit201, and the value h(m′) from the one-way operation unit204.

Next, the comparison unit205compares the value h(m) with the value h(m′), judges whether the two values match, and generates a comparison result j which shows whether these values match or do not match. Specifically, the comparison unit205, when these values match, generates a comparison result which shows j=1, and when these values do not match, generates a comparison result which shows j=0. The comparison unit205writes the generated comparison result j on the comparison result storage207.

(6) The Decrypted Text Storage206

The decrypted text storage206has an area for storing decrypted text.

(7) The Comparison Result Storage207

The comparison result storage207has an area for storing the comparison result j.

1.4 The Action of the Transmission Apparatus10

The following is a description of the action (operation) that the transmission apparatus10performs, with reference to the flowcharts shown inFIG. 4andFIG. 5.

The additional information generation unit102generates additional information Ra, and outputs the generated additional information Ra to the information adding unit103(Step S101).

Next, the information adding unit103reads out the plaintext m from the plaintext storage101(step S102), receives the additional information Ra from the additional information generation unit102(step S103), generates connected information F(m, Ra) by uniting the plaintext m with the additional information Ra, and outputs the generated connected information F(m, Ra) to the encrypting unit105(step S104).

Next, the encrypting unit105receives the connected information F(m, Ra), generates encrypted connected information E(F(m, Ra), Kp, r) by applying the encrypting algorithm E to the received connected information F(m, Ra)(step S105), and outputs the generated encrypted connected information E(F(m,Ra), Kp, r) to the transmitting unit106(step S106).

Next, the one-way operation unit104reads out the plaintext m from the plaintext storage101(step S107), calculates a value h (m) from the plaintext m by using the hash function h (step S108), and outputs the calculated value h(m) to the transmitting unit106(step S109).

The transmitting unit106receives the encrypted connected information E(F(m,Ra), Kp, r) and the value h(m), and transmits, through the internet30, the received encrypted connected information and the value h(m) to the reception apparatus (step S110).

1.5 The Action that the Reception Apparatus20Performs

The following is a description of the action that the reception apparatus20performs, with reference to the flow-chart shown inFIG. 6.

The receiving unit201receives, from the transmission apparatus10, the encrypted connected information E(F(m,Ra), Kp,r) and the value h(m) through the internet30(step S151), outputs the received encrypted connected information to the decrypting unit202, and outputs the received value h(m) to the comparison unit205(step S152).

The decrypting unit202receives the encrypted connected information E(F(m, Ra), Kp, r), generates decrypted connected information D(E(F(m,Ra), Kp, r)) by applying the decryption algorithm D to the received encrypted connected information E(F(m,Ra), Kp,r)(step S153), and outputs the decrypted connected information to the information removing unit203(step S154).

The information removing unit203receives the decrypted connected information D(E(F(m,Ra),Kp,r)), removes the additional information Ra from the received decrypted connected information so as to generate decrypted text m′ (step S155), outputs the generated decrypted text m′ to the one-way operation unit204, and writes the generated decrypted text m′ on the decrypted text storage206(step S156).

The one-way operation unit204receives the decrypted text m′, hashes the received decrypted text m′ according to the hash function h so as to calculate a value h(m′), and outputs the calculated value h(m′) to the comparing unit205(step S157).

The comparing unit205receives the value h(m) and the value h(m′), compares the two values to judge whether the two values match, generates a comparison result j either showing that the values match or do not match, and writes the generated comparison result j on the comparison result storage207(step S158).

1.6 The Comparison of Action Between the Embodiment and the Conventional Examples

The following is a description of decryption error detection according to the embodiment of the present invention. The decryption error detection of the present invention is then compared with those used in the conventional technologies.

When there is not a decryption error, the comparison result j which is to be outputted from the comparison unit205of the reception apparatus20is always 1.

The possibility that the comparison result j is 1, that is, the possibility that h(m′) generated from the one-way operation unit204of the reception apparatus20happens to be equal to (i.e. match) h(m) generated from the one-way operation unit104of the transmission apparatus10is as follows:

For the one-way operation unit104and the one-way operation unit204using the hash function outputting a hash value of the length of k bits, there are 2kways of hash value with k bits. Therefore, the possibility thereof is 2−k.

Therefore, if there is actually a decryption error, the possibility that the decryption error is detected by examining the comparison result j generated by the reception apparatus20is 1–2−k.

For example, when it is assumed that a hash function is SHA-1, the SHA-1 has at least 160 bits of output. Therefore, the possibility will be 1–2−160, which means that almost all the detection errors can be detected.

Moreover, the communications through the internet30is a sum of the bit length of the ciphertext outputted from the encrypting unit105and the bit length of the hash value h′ (m) outputted from the one-way operation unit104. Generally speaking, the output bit length for a hash function is smaller than that for inputted data. Therefore, it is unlikely that the communications in this example is more than twice as many as the output bit length for the ciphertext.

For example, when the hash function to be used is SHA-1, this holds true since most cryptosystems including the NTRU cryptosystem use a ciphertext length of 160 bits or more.

The communications in the data cryptosystem according to the first conventional has several times as many as the output bits length of the ciphertext. Thus, it can be concluded that the communications is reduced for the present embodiment, therefore enhancing the communication efficiency.

Further, as for security considerations, the present embodiment makes it difficult to infer the inputted value from the outputted value. Moreover, the present embodiment is not designed, unlike the first conventional example, to transmit the same plaintext more than one time. Therefore, an adequate security level is insured in the present embodiment. In addition, in a case in which the protocol is adopted for re-transmitting the same data again by a re-transmission request, after the decryption error detection is performed, the present embodiment is more resistant to the multiple transmission attack than the data cryptosystem described in the first conventional example, since the present embodiment adds a random number to plaintext before encrypting.

Moreover, the conventional technology encrypts plaintext from its intact condition. This increases the possibility of being decrypted by a third party who intercepts the communication channel, when the sender resends, upon request from the receiver, ciphertext that is generated from the same plaintext. That is, there is a possibility that a third party can intercept and decrypt the several pieces of ciphertext into the plaintext. (This phenomenon is called multiple transmission attack as is mentioned in the first conventional example.)

On the contrary, the present embodiment is able to set different additional information Ra for each communication. This enables to create a different m||Ra value for same plaintext each time the sender has to resend ciphertext. This reduces the possibility of being illegally decrypted by a third party attempting to perform a multiple transmission attack.

Moreover, the low transmission quality of the transmission channel enables a difference to be detected between the original plaintext and the decrypted text, when the bit is lost or garbled, just as mentioned in the above.

2. Modifications of the Cryptocommunication System1

The following is a description of modifications for the cryptocommunication system1.

2.1 Modifications on the Additional Information

In the cryptocommunication system1, the additional information generation unit102is to generate additional information Ra which is a random number. However, it is also possible to replace the additional information Ra with time stamp information or counter information. To summarize, the additional information that is generated by the additional information generation unit102can be any type of information as long as such information yields a different value every time it is used.

The time stamp information represents a current time when the additional information generation unit102generates a piece of additional information, and is specifically composed of information showing year, month, day, hour, minute, second, and millisecond, in a fixed length.

The counter information is numerical information in fixed digits, and is designed to add 1 every time it is used.

2.2 Modifications for Calculating the Connected Information F(m, Ra)

In the cryptocommunication system1, the information adding unit103calculates connected information F(m, Ra)=m||Ra, by uniting the plaintext m with the additional information Ra. However, the calculation method can be other than this method if it is invertible in such a way that m can be converted in the reverse direction based on the additional information.

Examples of the other calculation methods are explained below including the calculation method of the embodiment.

In order to extract only the plaintext by removing the additional information from the connected information, an inverse operation is performed.

The calculation method 1 is expressed as connected information F(m,Ra)=m||Ra, where “||” signifies a bit-connecting. This is a calculation for obtaining plaintext for the embodiment.

Note that an expression “connected information F(m,Ra)=Ra||m” can be alternatively used for the expression described above.

Further, the plaintext m is divided into several pieces of partial plaintext information, each having length of 4 bits. In the same way, the additional information is also divided into several pieces of partial additional information, each having length of 4 bits. Then, connected information may be obtained by uniting the pieces of partial plaintext information and the pieces of partial additional information alternately. Generally speaking, a length of plaintext m is greater than a length of additional information. Therefore, the connected information usually ends with partial plaintext information.

The calculation method 2 is expressed as “connected information F(m,Ra)=m(+)Ra,” where “(+)” signifies an exclusive OR, with its inverse operation being expressed as “decrypted text m′=connected information F(+) Ra.”

The calculation method 3 is expressed as “connected information F(m,Ra)=m+Ra,” with its inverse operation being expressed as “decrypted text m′=connected information F-Ra.”

The calculation method 4 is expressed as “connected information F(m,Ra)=m×Ra mod p” where p is a prime number greater than m.

The inverse operation is performed as follows:

Decrypted text m′=connected information F/Ra mod p

The calculation method 5 is expressed as “connected information F(m,Ra)=BitPerm [Ra] (m),” where BitPerm [Ra] (m) is an operation for replacing the bit expression m based on Ra.

The specific operation methods are shown in the following:

This expression is to bit-rotate m by Ra bits.

For example, if m is assumed to be “1111000011110000”, and Ra is assumed to be Ra=3(in decimal notation), then the m after replacement can be expressed as m=1000011110000111.

Here, the reverse bit rotation is also possible.

The inverse operation is performed by rotating the connected information F in a reverse direction by Ra bits.

In this method, m is replaced according to the calculation algorithm. In other words, an operation is performed first by making Ra an inputted value, and then m is replaced based on the calculation result.

The above-described two calculation method is described by using the following examples.

EXAMPLE

Ra is assumed to be 128-bit-length. The hash value of 16-bit-length is calculated from Ra by using a hash function. Next, m is bit-rotated by the obtained hash value as shown in the calculation method 5-1.

The inverse operation is performed as follows:

The connected information F is replaced according to the calculation algorithm. In other words, the operation is performed by making the Ra an inputted value. Then, the connected information F is replaced based on the operation result, in order to obtain decrypted text m′.

EXAMPLE

Ra is assumed to be 128-bit-length. A 16-bit-length hash value is calculated from Ra by using a hash function. Next, as the calculation method 5-1, the connected information F is bit-rotated in a reverse direction by the obtained hash value.

In the calculation method 5-3, several pieces of partial information are generated by dividing m into 4 bit length. Next, each piece of partial information is replaced by using the replacement table for 4 input-output bit length corresponding to Ra.

Here, the replacement table includes 16 sets before-conversion bit row with 4 bit length, and the corresponding after conversion bit row with 4 bit length.

In the above fashion, more than one type of replacement table is made possible for each value of Ra.

The inverse operation is performed as follows.

Connected information F is divided into 4 bits, in order to generate several pieces of partial connected information. Next, the replacement in a reverse direction is performed for each piece of partial connected information by using the replacement table for 4 input-output bit length corresponding to a Ra.

The calculation method 6 is expressed as “connected information F(m,Ra)=Tab[Ra](m),” where Tab[Ra](m) means to convert m according to the conversion table Tab.

For example, when m is assumed to have 8-bit-length, each m is converted according to the table Tab as shown inFIG. 7which is stored for each Ra. The conversion table Tab includes 256 sets of 8-bit value and 8-bit value.

For an example in which m=1, plaintext m is converted into 39 according to the conversion table Tab shown inFIG. 7.

The inverse operation is performed as follows:

Connected information F is converted in the reverse direction to the above, according to the conversion table Tab.

2.3 Modification Examples of the Cryptocommunication System1in Which Additional Information is Shared

The following is a description on modification examples for the cryptocommunication system1in which additional information is shared.

(1) A first Modification Example

As a first modification example, a cryptocommunication system1bis described which is a modified form of the cryptocommunication system1.

(A Structure of the Cryptocommunication System1b)

The cryptocommunication system1bconsists of a transmission apparatus10band a reception apparatus20b, as shown inFIG. 8.

The transmission apparatus10band the reception apparatus20beach have the same structure as the transmission apparatus10and the reception apparatus20, respectively, that constitute the cryptocommunication system1. The following is a description of the transmission apparatus10band the reception apparatus20b, with an emphasis on the difference between the transmission apparatus10and the reception apparatus20.

The transmission apparatus10bis further equipped with a synchronizing unit107. In addition, the transmission apparatus10bis equipped with an additional information generation unit102binstead of the additional information generation unit102which the cryptocommunication system1has. In addition, the reception apparatus20bis further equipped with a synchronizing unit208and an additional information generation unit209. The synchronizing unit107and the synchronizing unit208are connected to each other through the dedicated line40b.

The synchronizing unit107generates a random number XR, and outputs the generated random number XR through the dedicated line40bto the synchronizing unit208. The synchronizing unit107further outputs the generated random number XR to the additional information generation unit102b.

The additional information generation unit102b, upon receiving the random number XR from the synchronizing unit107, generates additional information Ra by using the received random number XR, and outputs the generated additional information Ra to the information adding unit103. Here, an assumption is made that the random number XR is used as the additional information Ra without being processed, which is one example of generating additional information Ra from the random number XR.

The synchronizing unit208receives the additional information XR through the dedicated line40b, and outputs the received additional information XR to the additional information generation unit209.

The additional information generation unit209, upon receiving the random number XR from the synchronizing unit208, generates additional information Ra by using the received random number XR, and outputs the generated additional information Ra to the information removing unit203. Here, an assumption is made that the random number XR is used as the additional information Ra without being processed, which is one example of generating additional information Ra from the random number XR.

(Action of the Cryptocommunication System1b)

The action that the cryptocommunication system1bperforms is described in the following with reference to the flowchart shown inFIG. 9.

Note that the focus here is on the differences between the cryptocommunication systems1band1, since most of the action is the same between the two systems.

The synchronizing unit107generates a random number XR (Step S201), and outputs the generated random number XR through the dedicated line40bto the synchronizing unit208(Step S202). The synchronizing unit107further outputs the generated random number XR to the additional information generation unit102b(step S203).

The additional information generation unit102b, upon receiving the random number XR from the synchronizing unit107, generates additional information Ra by using the received random number XR, and outputs the generated additional information Ra to the information adding unit103(step S203).

The synchronizing unit208receives the random number through the dedicated line40b, and outputs the received random number XR to the additional information generation unit209(step S202).

The additional information generation unit209, upon receiving the random number XR, generates additional information Ra by using the received random number XR (step S204), and outputs the generated additional information Ra to the information removing unit203(step S205). The information removing unit203receives the additional information Ra (step S205), and generates decrypted text m′ from decrypted connected information by using the received additional information Ra (step S206).

(2) A Second Modification Example

A cryptocommunication system1cis described which is a second modification example of the cryptocommunication system1.

(A Structure of the Cryptocommunication System1c)

The cryptocommunication system1cconsists of a transmission apparatus10cand a reception apparatus20c, as shown inFIG. 10.

The transmission apparatus10cand the reception apparatus20ceach have the same structure as the transmission apparatus10and the reception apparatus20for the cryptocommunication system1.

The transmission apparatus10c, instead of the additional information generation unit102and the transmitting unit106, is equipped with an additional information generation unit102cand a transmitting unit106c. The reception apparatus20c, instead of the information removing unit203and the receiving unit201, is equipped with an information removing unit203cand a receiving unit201c.

The additional information generation unit102c, the transmitting unit106c, the information removing unit203c, and the receiving unit201ceach have the same structure as the additional information generation unit102, the transmitting unit106, the information removing unit203, and the receiving unit201, respectively. Therefore, the focus in the following description will be on the differences there between.

The additional information generation unit102coutputs the generated additional information Ra to the transmitting unit106c.

The transmitting unit106creceives the additional information Ra from the additional information generation unit102c, and transmits the received additional information Ra to the reception apparatus20cthrough the internet30.

The receiving unit201creceives the additional information Ra through the internet30from the transmission apparatus10c, and outputs the received additional information Ra to the information removing unit203c.

The information removing unit203creceives the additional information Ra from the receiving unit201, and generates decrypted text m′ from decrypted connected information by using the received additional information Ra.

(Action of the Cryptocommunication System1c)

The action that the cryptocommunication system1cperforms is described in the following with reference to the flowchart shown inFIG. 11.

Note that the focus will be on the differences between the two systems, since the most of the action of the cryptocommunication system1cis the same as the cryptocommunication system1.

The additional information generation unit102cgenerates additional information Ra, and outputs the generated additional information Ra to the transmitting unit106c(step S221).

The transmitting unit106creceives the additional information Ra from the additional information generation unit102c, and transmits the received additional information Ra through the internet30to the reception apparatus20c(step S222).

The receiving unit201creceives the additional information Ra through the internet30from the transmission apparatus10c, and outputs the received additional information Ra to the information removing unit203c(step S222).

The information removing unit203creceives the additional information Ra from the receiving unit201c(step S223), and generates decrypted text m′ from the decryption connected information by using the received additional information Ra (step S224).

(3) A Third Modification Example

The following is a description of a cryptocommunication system1dwhich is a third modification example of the cryptocommunication system1.

(A Structure of the Cryptocommunication System1d)

The cryptocommunication system1dconsists of a transmission apparatus10dand a reception apparatus20d, as shown inFIG. 12.

The transmission apparatus10dand the reception apparatus20deach have the same structure as the transmission apparatus10and the reception apparatus20that compose the cryptocommunication system1.

The transmission apparatus10d, instead of the additional information generation unit102, the encrypting unit105, and the transmitting unit106, is equipped with an additional information generation unit102d, an encrypting unit105d, and a transmitting unit106d. The reception apparatus20d, instead of the decrypting unit202, the information removing unit203, and the receiving unit201, is equipped with a decrypting unit202d, an information removing unit203d, and a receiving unit201d.

The additional information generation unit102d, the encrypting unit105d, the transmitting unit106d, the decrypting unit202d, the information removing unit203d, and the receiving unit201d, each have the same structure as the additional information generation unit102, the encrypting unit105, the transmitting unit106, the decrypting unit202, the information removing unit203, and the receiving unit201, respectively. The following description focuses on the differences therebetween.

The additional information generation unit102dgenerates additional information Ra, and outputs the generated additional information Ra to the encrypting unit105d.

The encrypting unit105d, upon receiving the additional information Ra from the additional information generation unit102d, applies an encryption algorithm to the received additional information Ra, so as to generate encrypted additional information E(Ra,Kp,r2). Here, r2 is a random number as r. Next, the encrypting unit105doutputs the generated encrypted additional information E(Ra, Kp, r2) to the transmitting unit106d.

The transmitting unit106dreceives the encrypted additional information E(Ra,Kp,r2) from the encrypting unit105d, and transmits the received encrypted additional information E (Ra,Kp,r2) through the internet30to the reception apparatus20d.

The receiving unit201dreceives, through the internet30, the encrypted additional information E(Ra,Kp,r2) from the transmitting unit106d, and outputs the received encrypted additional information E(Ra,Kp,r2) to the decrypting unit202d.

The decrypting unit202dreceives the encrypted additional information E(Ra,Kp,r2) from the receiving unit201d, and generates decrypted additional information D(E(Ra,Kp,r2),Ks) by applying a decryption algorithm to the received encrypted additional information E(Ra,Kp,r2). Next, the decrypting unit202doutputs the generated decrypted additional information D(E(Ra,Kp,r2),Ks) to the information removing unit203d.

The information removing unit203dreceives the decrypted additional information D(E(Ra,Kp,r2),Ks) from the decrypting unit202d, and generates decrypted text m′ from decrypted connected information D(E(F(m,Ra),Kp,r),Ks) by using the received decrypted additional information D(E(Ra,Kp,r2),Ks).

(Action of the Cryptocommunication System1d)

The action that the cryptocommunication system1dperforms is described in the following with reference to the flowchart shown inFIG. 13.

Since the action of the cryptocommunication system1dis mostly the same as that of the cryptocommunication system1, the focus in the following description is on their differences.

The additional information generation unit102dgenerates additional information Ra, and outputs the generated additional information Ra to the encrypting unit105d(step S241).

The encrypting unit105dreceives the additional information Ra from the additional information generation unit102d, generates encrypted additional information E(Ra,Kp,r2) by applying an encryption algorithm to the received additional information Ra, and outputs the generated encrypted additional information E(Ra,Kp,r2) to the transmitting unit106d(step S242).

The transmitting unit106dreceives the encrypted additional information E(Ra,Kp,r2) from the encrypting unit105d, and transmits the received encrypted additional information E(Ra,Kp,r2) through the internet30to the reception apparatus20d(step S243).

The receiving unit201dreceives, from the transmitting unit106d, the encrypted additional information E(Ra,Kp,r2) through the internet30, and outputs the received encrypted additional information E(Ra,Kp,r2) to the decrypting unit202d(step S243).

The decrypting unit202dreceives the encrypted additional information E(Ra,Kp,r2) from the receiving unit201d, and generates decrypted additional information D(E(Ra,Kp,r2),Ks) by applying a decryption algorithm to the received encrypted additional information E(Ra,Kp,r2). Then, the decrypting unit202doutputs the generated decrypted additional information D(E(Ra,Kp,r2),Ks) to the information removing unit203d(step S244).

The information removing unit203dreceives, from the decrypting unit202d, the decrypted additional information D(E(Ra,Kp,r2),Ks) (step S245), and generates decrypted text m′ from the decrypted connected information D(E(F(m,Ra),Kp,r),Ks) by using the received encrypted additional information D(E(Ra,Kp,r2),Ks) (Step S246).

2.4 Feasible Combination Between Modifications

Feasible combinations between the modifications regarding the additional information, the modifications for the calculation of the connected information F(m,Ra), and the modification examples of the cryptocommunication system in which the additional information is shared are described in the following with reference to the table shown inFIG. 14.

As the table inFIG. 14shows, each modification for additional information (i.e. a random number, a time stamp, and a counter) can be used with all the modifications for calculating connected information F(m,Ra), or with the modification examples of the cryptocommunication system in which the additional information is shared.

Further, as shown inFIG. 14, the modification in which m||Ra is used for calculating the connected information is applicable to cryptocommunication systems1, and1b–1d.

3. Other Modification Examples

So far, the present invention was described based on the embodiment. The present invention is not limited to the described embodiment, and also includes other cases described below.

(1) The cryptocommunication system1may be structured in the following way. The one-way function unit104of the transmission apparatus10receives connected information F(m, Ra) from the information adding unit103, hashes the received connected information F(m,Ra) according to the hash function h to generate a functional value h(F(m, Ra)), and transmits the functional value h(F(m,Ra)), through the transmitting unit106, the internet30, and the receiving unit201, to the comparing unit205.

The one-way function unit204of the reception apparatus20receives decrypted connected information D(E(F(m,Ra),Kp,r),Ks) from the decrypting unit202, hashes the received D(E(F(m, Ra),Kp,r),Ks) according to the hash function h so as to generate a functional value h(D(E(F(m,Ra),Kp,r),Ks)), and outputs the generated functional value h(D(E(F(m,Ra),Kp,r),Ks)) to the comparing unit205. The comparing unit205compares the functional value h(F(m,Ra)) and the functional value h(D(E(F(m,Ra),Kp,r),Ks)) and judges whether the two values match.

In the above way, the judgement is performed as to whether or not the plaintext has been correctly decrypted.

(2) Moreover, the Cryptocommunication System1May be Structured in the Following Way.

The information adding unit103of the transmission apparatus10, by using G which is a different invertible operation from F, generates connected information G(m,Ra). Here, an example of G is G=Ra||m. Next, the information adding unit103outputs the generated connected information G(m,Ra) to the one-way function unit104. The one-way function unit104receives the connected information G(m,Ra) from the information adding unit103, hashes the received connected information G(m,Ra) according to the hash function h so as to generate a functional value h(G (m, Ra)), and transmits the generated functional value h(G(m,Ra)), through the transmitting unit106, the internet30, and the receiving unit201, to the comparing unit205.

Further, the information removing unit205of the reception apparatus20, by using the decrypted text m′, a random number Ra, and the connected information G, generates connected information G(m′,Ra), and transmits the generated G(m′,Ra) to the one-way function unit204. Here, the information removing unit203shares the same random number Ra with the transmission apparatus10, as shown in the first modification example. The one-way function unit204receives the connected information G(m′,Ra), hashes the received connected information G(m′,Ra) according to the hash function h so as to generate a functional value h (G (m′,Ra)), and outputs the generated functional value h(G(m′,Ra)) to the comparing unit205. The comparing unit205compares the functional value h(G(m,Ra)) and the functional value h(G(m′,Ra) to see whether the two values match.

In the above way, the judgment is performed as to whether or not the plaintext has been decrypted correctly.

(3) The encryption algorithm and the decryption algorithm are not limited to those described in the embodiment, and other crypto-algorithms are also possible. For example, ordinary cryptosystems such as the DES cryptosystem, the RSA cryptosystem, and the E1Gama1 cryptosystem can also be used.

In addition, for the one-way operation unit104, cryptosystem functions used for the ordinary cryptosystems can be also used as well as the hash functions.

For a detailed description about the DES cryptosystem, the RSA cryptosystem, and the E1Gama1 cryptosystem, please refer to Tatsuaki Okamoto, Yamamoto Hirosi, “Gendai Ango” (Modern Cryptography), Series/Mathematics in Information Science, Sangyo Tosho, 1997.

Further, it can be also arranged so that each couple of transmitting users and receiving users can have a different one-way operation, instead of all the users in one system sharing one one-way operation.

(4) In the present embodiment, the transmission apparatus10and the reception apparatus20are connected to each other through the internet30. However, the means to connect the transmission apparatus10and the reception apparatus20is not limited to the internet, and can also be a dedicated line, or by over-the-air (wireless) communication.
(5) The present invention can be the method described above, or can be a computer program enabling the method by a computer. Alternatively, the present invention can be implemented as digital signals comprised of the computer program.

Further, the present invention can be a recording medium which can be read from by using a computer, such as a flexible disk, a hard disk, CD-ROM, MO, a DVD, DVD-ROM, DVD-RAM, or semiconductor memory, which stores the computer program or the digital signals. Alternatively, the present invention can also be the computer program or the digital signals recorded on these recording media.

Further, the present invention can transmit the computer program or the digital signals through a network represented by electric communication lines, over-the-air, cable transmission lines, or the internet, for example.

Further, the present invention can be a computer system that is equipped with a microprocessor and memory, in which the memory stores a computer program, and the microprocessor can work according to the computer program.

In addition, another computer system which is independent from the described computer system can realize the tasks, by transmitting the computer program or the digital signals stored in the recording media, or by transmitting the computer program or the digital signals through the network and the like.

(6) The stated embodiment and the modifications thereof can be combined with each other.

Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless such changes and modifications otherwise depart from the scope of the present invention, they should be construed as being included therein.