JPH08251156A - Method and system for ciphering electronic mail - Google Patents

Abstract

PURPOSE: To provide an electronic mail ciphering method which can be used with a normal mail together without specifying the mail system and the kind of device, reduces calculation processing and the burden of memory for attaining a security function for persons both transmitting electronic mail and receiving it, and is provided with high safety against tapping and aunauthorized persons who set up to be authorized without any need to worry about the coincidence of users' keys. CONSTITUTION: Each user of a communication network prepares key information of each and opens only an open key. Next, the transmitter A and the receiver B of the electronic mail share master keys KA, B and KB, A from his own secret key and the open key of the opposite side and the transmitter ciphers the body sentences P of the mail by a data ciphering key (k) through the use of the secret key cipher. Then the data ciphering key is ciphered by the master key. Then, they are sent to the receiver and the receiver deciphers the data ciphering key (k) through the use of the master key KB, A and deciphers the mail body sentence P from the data ciphering key (k) in addition.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electronic mail communication method for transmitting and receiving a document via a communication network, and an electronic mail encryption method with improved security against eavesdropping on the network and spoofing by an unauthorized person. Regarding

[0002]

2. Description of the Related Art For the purpose of enhancing the security of electronic mail on the Internet, the document "Linn, J., et.
al., “Privacy Enhancement for Internet Electronic
Mail: Part I, II, III, IV ", RFC-1421-1424, 1993", PEM (Privacy Enhanced Mail) has been proposed. The PEM email encryption technique is briefly described below.

In PEM, public key encryption RSA is used for key management.
(Reference "RL Rivest, A. Shamir, L. Adleman," A Meth
od for Obtaining Digital Signatures and Public Key
Cryptosystems ”, Communications of the ACM, Vol.21,1
978 ”) and uses a secret key encryption DES (reference“ Data Encryption Standar ”, which is an American encryption standard for data encryption).
d, FIPS-PUB-46, 1977 ”).

The sender A sends an electronic mail to the recipient B via the network.

Step 1. A randomly creates a data encryption key K and uses the key K to encrypt the mail text P by DES. That is, the ciphertext C ₁ of the mail body P is

[0006]

[Equation 63] Let C ₁ = E (K; P).

Step 2. A uses the public key (e _B , n _B ) of _B to encrypt the key K by RSA. That is, the ciphertext C ₂ of the key K is

[0008]

Let C ₂ = exp (K: e _B ) (mod n _B ). However, exp (a: x) represents a value of a raised to the power x.

Step 3. A uses the hash function f, which is public information, to compress the mail body P, and uses the private key d _A and public key n _A of _A to perform an RSA signature on f (P). That is, the signature sentence sgn _A (P) for f (P) is

[0010]

[Expression 65] sgn _A (P) = exp (f (P): d _A ) (mod n _A ).

Step 4. A sends C ₁ , C ₂ , and sgn _A (P) to B.

Step 5. B uses his private key d _B ,
From C ₂ ,

[0013]

The data encryption key K is decrypted by K = exp (C ₂ : d _B ) (mod n _B ).

Step 6. B uses the key K,

[0015]

The mail body P is decrypted by P = D (K; C ₁ ).

Step 7. B uses the hash function f and A's public key (e _A , n _A ) to confirm that the mail text P is sent from _A ,

[0017]

[Number 68] f (P) = exp (sgn A (P): e A) (mod n A), to make sure.

The PEM may have the following problems.

(1) From the viewpoint of security, it is desirable to change the data encryption key frequently. In the case of PEM, even if the partner has a lot of chances to perform e-mail communication, a new data encryption key must be encrypted by RSA encryption using the public key of the partner every time the data encryption key is changed and sent. . In particular, when it is desired to send mail to a large number of parties at the same time, the RSA encryption has a relatively large amount of calculation for encryption, and therefore the calculation processing load for encryption of the data encryption key is large.

(2) In the key distribution using the RSA encryption, the key is decrypted only by the recipient's private key, and therefore there is no authentication function. Therefore, in PEM, an authentication function is realized by adding a digital signature using RSA.
However, the digital signature has a drawback that the burden of calculation processing for creation and verification is large and that the header information added to the mail ciphertext becomes large.

(3) In PEM, since the keys of the users of the network are created by the users themselves, there is a possibility that the keys of different users will coincide with each other. For example, if the keys of user A and user B match, A
When A notices from B's public key, A can impersonate B or eavesdrop the e-mail communication between B and other users.

[0022]

SUMMARY OF THE INVENTION An object of the present invention is to provide an electronic mail communication system for sending and receiving mails on a communication network, which has a feature that it can be used together with ordinary mails without specifying a mail system or a model, and , E-mail sender and receiver both have less computational processing and memory burden to implement security functions,
Another object of the present invention is to propose an electronic mail encryption method that is highly secure against eavesdropping and impersonation by an unauthorized person without worrying about the user's key agreement.

[0023]

DISCLOSURE OF THE INVENTION The present invention proposes a highly efficient and highly secure e-mail encryption method in an e-mail communication system for transmitting and receiving data on a communication network.

As one of the concrete implementation methods, the sender A
Is an electronic mail encryption method for encrypting a mail text to be transmitted to the recipient B, in which the preparation processing center publishes a prime number p, a composite number n = p ₁ · p ₂ and an integer a, and Keep the prime factorization and the positive number α secret.

The sender A has an integer x _{A such} that 0 <x _A <p-1.
At random,

[0026]

Y _A = exp (a: x _A ) (mod p), the receiver _B randomly selects an integer x _{B such} that 0 <x _B <p−1,

[0027]

[Mathematical formula-see original document] y _B = exp (a: x _B ) (mod p), is calculated. However, exp (a: x) represents a value of a raised to the power x.

Registration Process to Center The sender A and the receiver B register their ID information in the center, and the center uses the sender A's ID information I _A to identify a key (s _A , peculiar to the sender _A) . v _A ) using a positive number e,

[0029]

Equation 71] _{s A = exp (I A:} e) (mod lcd (p 1 -1, p 2
_{-1)), v A = exp} (α: s A) (mod n), calculated in, the ID information I _B of the receiver B as well, the key unique to the recipient B (s _B, v _B ),

[0030]

Equation 72] _{s B = exp (I B:} e) (mod lcd (p 1 -1, p 2
−1)), v _B = exp (α: s _B ) (mod n), and each is safely distributed to sender A and receiver B. However, l. c. d (x, y) represents the least common multiple of integers x and y.

At this time, if the ID information is different, the corresponding secret key is also different.

The sender A registers (y _A , v _A ) as the public key of the sender A, and the receiver B registers (y _B , v _B ) as the receiver B.
Register as the public key of.

Master Key Sharing Process The sender A uses his own secret key (x _A , s _A ) and the public key of the receiver B by using the function g that is commonly held with the receiver B. From a certain (y _B , v _B ) the master key K _{A, B}

[0034]

(73) K _{A, B} = g (exp (y _B : x _A ) (mod p), exp (v _B :
s _A ) (mod n)), and the recipient B has his or her private key (x _B ,
s _B ) and the sender A's public key (y _A , v _A ) to generate a master key K _{B, A} ,

[0035]

(74) K _{B, A} = g (exp (y _A : x _B ) (mod p), exp
(v _A : s _B ) (mod n)),

At this time, K _{A, B} = K _{B, A} holds.

In particular, the sender A and the sender A send the recipients who often send e-mail.
From the set S _A consisting of each master key in between, or the secret key (x _A , s _A ) of the sender _A and the key generation function θ,

[0038]

[Number 75] K (A) = θ (x A, s A), by encrypting the set S _A of the master key using the key K (A), which was created by

[0039]

[Equation 76] C (A) = E _A (K (A); S _A ), is stored, and when the master key is needed for updating the data encryption key in the subsequent e-mail communication, the master key For those that are stored, the stored master key is used without creating a new master key from the other party's public key and own private key. However, E _A (k; M) is A
The plaintext M as a key k by the private key cryptographic algorithm possessed by
Represents the result of encryption.

Encryption / decryption processing of mail text Sender A randomly selects a data encryption key k and uses a secret key cryptosystem algorithm that is commonly held with receiver B to send mail text P as a data Receives by mail the ciphertext E (k; P) encrypted with the encryption key k and the ciphertext E (K _{A, B} ; k) obtained by encrypting the data encryption key k with the master key K _{A, B.} Person B.

Recipient B uses master key K _{B, A} to obtain E (K
_{A, B} ; k) decrypts the data encryption key k, and further, using the data encryption key k, E (k; P) to the mail text P
To decrypt.

[0042]

In the method of encrypting an electronic mail according to the present invention, the sender and the recipient of the electronic mail share the master key from their own private key and the public key of the other party, and use the private key encryption to encrypt the mail text. By implementing encryption and data encryption key delivery, the computational load on both senders and receivers for realizing security functions was reduced, and sender authentication was realized without adding additional information such as digital signatures. In particular, if only the master key is stored as the key information for the partner who frequently performs mail communication, the public key of the receiver is not required and the master key is generated each time the data encryption key is updated. There is no need to do it, and high-speed encryption processing is possible.

Further, since the ID information is different for each user (sender and receiver of the electronic mail), the center and the user cooperate to generate the key of the user. Since the keys do not match, high security against eavesdropping and impersonation by fraudsters is realized.

[0044]

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a diagram showing a system configuration of an embodiment of the present invention. The system shown in FIG.
It is composed of 100 and a center-side device 200. The user side devices 100 are connected to each other via a communication line 300.

FIG. 2 is a conceptual diagram of the electronic mail encryption method according to the present invention.

FIG. 3 shows the internal structure of the user-side device 100 in the first to ninth embodiments. The user-side device 100 includes a random number generator 101, a power multiplier 102, a key generator 103, a calculator 104, an encryption / decryption device 105, a hash calculator 106, a memory 107, and a communication device 108.

FIG. 4 shows the internal structure of the center-side device 200. The center device 200 is a prime number generator 201 and a primitive root generator.
202, a private key generation device 203, and a computing unit 204.

FIG. 5 shows the internal structure of the card 400. The card 400 includes a random number generator 401, a power multiplier 402, and a key generator 4
03, arithmetic unit 404, encryption / decryption device 405, hash calculator 40
6, a memory 407 and an output device 408 are provided.

FIG. 6 is a user side apparatus in the tenth embodiment.
The internal structure of 500 is shown. The user device 500 includes a card reading device 501 and a communication device 502.

The user of the network is the user device 100 (card 400 and user device) connected to the communication line.
Consider a case where another user who uses another user side device 100 (card 400 and user side device 500) is used to perform e-mail communication using another device (500).

At this time, an electronic mail encryption method will be described in detail in the following embodiment.

Z represents an integer ring, and Z / (n) represents a remainder ring modulo the integer n. Especially when n is a prime number, Z /
(n) becomes a remainder. For xεZ, x represents a coset whose representative element is x in the corresponding coset. Also, exp (a: x) represents a value of a raised to the power x.

When all network user ID information is represented by t bits or less, the user ID information is Z.
It can be considered as an element of / (exp (2: t) -1). At this time, a set Λ of user ID information is Λ = Z / (exp (2:
t) -1).

(Example 1) Preparation Each user λ of the network creates the following information by using the random number generator 101 and the power multiplier 102 in the user side device 100 of his / her own, and discloses only the public key. ·to register.

Private key:

[0057]

[Number 77] _{· x λj ∈Z st 0 <x} λj <p-1 for j = 1, ..., n. Public key:

[0058]

Y y _λj = exp (a: x _λj ) (mod p) for j = 1, ..., n. Here, p is a prime number, a is 0 <a <p, and is an integer that is a primitive root of Z / (p), and n is a security parameter, which is given in advance.

Further, in the user side device 100 connected to the network, there is an encryption / decryption device 104 having the same secret key cryptosystem algorithm built therein, and the plaintext M is converted into the key K using this secret key cryptosystem algorithm. The encrypted result and the decrypted result are represented by E (K; M) and D (K; M), respectively.

Master Key Sharing Regarding A and B who are users of the network, A wants to send an electronic mail to B. Under this purpose, A, B
Perform the following steps:

A is a power multiplier in the user side device 100A of A
102A and the arithmetic unit 104A are used to obtain the master key K _{A, B} from the private key x _{Aj of the user} and the public key y _Bj (j = 1, ..., N) of _B ,

[0062]

K _{A, B} = Π {exp (y _Bj : x _Aj ) (mod p) | j =
1, ..., N} are calculated. However, for the symbol 100A, etc., the user is identified by the alphabet after the number.

Similarly, B is in B's user side device 100B.
Use your power multiplier 102B and calculator 104B to keep your secret
Key x_BjAnd A's public key y_Aj(J = 1, ..., n) to master
Key K _{B, A}To

[0064]

(80) K _{B, A} = Π {exp (y _Aj : x _Bj ) (mod p) | j =
1, ..., N} are created. At this time, obviously,

[0065]

[Formula 81] K _{A, B} = K _{B, A} holds.

Further, for users who frequently send e-mails, A is a set S _A of each master key between those users and _A , or secret information x _Aj (j = 1 of A). ，
, N), the key generator 103A in the user-side device 100A creates a key K (A), and the encryption / decryption device 105A is used to generate the key K (A).
Encrypted S _A by (A)

[0067]

[Equation 82] C (A) = E (K (A); S _A ) is stored in the memory 107A. In subsequent e-mail communications, if a master key is required for updating the data encryption key, etc., if the master key is stored, create a new master key from your private key and the public key of the other party. Without using the master key stored in the memory 107A.

For encryption A of the mail text, a random number r is selected using the random number generator 101A in the user side device 100A, and the data encryption key k is created from the key generator 103A by using r as an input. Email text P encryption / decryption device 105A
Ciphertext C ₁ encrypted with the data encryption key k using
To

[0069]

[Equation 83] C ₁ = E (k; P), is created. Further, a ciphertext C obtained by encrypting the data encryption key k with the master keys K _{A and B} using the encryption / decryption device 105A.
₂

[0070]

[Equation 84] C ₂ = E (K _{A, B} ; k), is created.

A uses the communication device 108A to connect C ₁ and C ₂ to B
Send to

Decryption of mail body B is a device on the user side of B for C ₁ and C ₂ sent.
Using the encryption / decryption device 105B in 100B, first, the master key K
_{From B and A} ,

[0073]

[Equation 85] The data encryption key k is decrypted by k = D (KB _{, A} ; C ₂ ), and then the data encryption key k is used to obtain

[0074]

The mail body P is decrypted with P = D (k; C ₁ ),

(Embodiment 2) Embodiment 2 describes an electronic mail encryption method having a high level of security in which the key information of the user does not accidentally match assuming the existence of a center.

Preparation The center creates the following information using the prime number generator 201, the primitive root generator 202 and the arithmetic unit 204 in the center side device 200.

Public information: p ₁ ; prime number, p ₂ = 2q ₁ q ₂ +1; prime number, a ₁ εZ st 0 <a ₁ <p ₁ , and a ₁ is Z / (p ₁ ). Primitive root, ・ a ₂ ∈Z st 0 <a ₂ <p ₂ , and a ₁ is Z / (p ₁ ) and primitive root, secret information: ・ q ₁ , q ₂ ∈Z; prime number. Each user λ of the network uses the random number generator 101 and the power multiplier 102 in the user side device 100 to create the next key.

Private key:

[0079]

X _λ εZ st 0 <x _λ <p ₁ −1. Public key:

[0080]

Y _λ = exp (a ₁ : x _λ ) (mod p ₁ ). Registration to the center The network user λ has the ID information I _{λ at the} center.
To register.

The center uses the secret key generating device 203 in the center side device 200 to

[0082]

[Formula 89] ed ≡ ₁ (mod lcd (q ₁ -1, q ₂ -1)), e, d ∈ Z is created, and for the ID information I _{λ of the} user λ,

[0083]

[Mathematical formula-see original document] s _λ = exp (I _λ : e) (mod p ₂ −1), is calculated, and s _λ is safely distributed to the user λ.

At this time,

[0085]

Note that s _λ ≠ s _μ if I _λ ≠ I _μ εΛ = Z / (p ₂ −1), holds.

A master key sharing network user λ registers a vector (y _λ , v _λ ) as a public key of λ. However,

[0087]

[Expression 92] v _λ = exp (a ₂ : s _λ ) (mod p ₂ ). Further, in the user side device 100 connected to the network, there is an encryption / decryption device 105 having the same secret key cryptosystem algorithm built therein, and the plaintext M is encrypted with the key K using this secret key cryptosystem algorithm. The result and the decoded result are represented by E (K; M) and D (K; M), respectively.

Regarding network users A and B, A
Wants to send an email to B. Under this purpose,
A and B execute the following procedure: A is a power multiplier 102A and an arithmetic unit in the user side device 100A of A.
Using 104A and the hash calculator 106A, from A's private key (x _A , s _A ) and B's public key (y _B , v _B ) to the master key K
_{A, B}

[0089]

(93) K _{A, B} = g (φ (exp (y _B : x _A ) (mod p ₁ ), exp
(v _B : s _A ) (mod p ₂ ))) ,. Similarly, B uses B's private key (x _B , s _B ) and A's public key (y _A by using the power multiplier 102B, the calculator 104B, and the hash calculator 106B in the user device 100B of _B. ，
v _A ) to the master key K _{B, A} ,

[0090]

(94) K _{B, A} = g (φ (exp (y _A : x _B ) (mod p ₁ ), exp
(v _A : s _B ) (mod p ₂ ))), However, g represents the hash function used in the hash calculator, and φ is

[0091]

[Expression 95] φ: Z / (p ₁ ) × Z / (p ₂ ) → Z / (n) ((x, x) → x), which represents an isomorphic map (n = p ₁ p ₂ ). Here, x represents a residue class whose representative element is x in each residue ring.

At this time, it is clear that K _{A, B} = K _{B, A.}

Furthermore, for users who frequently send e-mail, A refers to the set of master keys S _A between those users and _A , or the secret information x _{A of A} from the user side. The key generator 103A in the device 100A creates the key K (A), and the encryption / decryption device 105A encrypts S _A with the key K (A) C (A) = E (K (A); S _A ), the memory 107A
To memorize. In subsequent e-mail communications, if a master key is required for updating the data encryption key, etc., if the master key is stored, create a new master key from your private key and the public key of the other party. Without using the master key stored in the memory 107A.

Encryption of the mail text A selects a random number r using the random number generator 101A in the user-side device 100A, and inputs the r to create a data encryption key k from the key generator 103A. Email text P encryption / decryption device 105A
Ciphertext C ₁ encrypted with the data encryption key k using
With C ₁ = E (k; P). Furthermore, the data encryption key k is transferred to the master key K _{A, B} using the encryption / decryption device 105A.
A ciphertext C ₂ encrypted by is created by C ₂ = E (KA _{, B} ; k).

A uses the communication device 108A to connect C ₁ and C ₂ to B
Send to

Decryption of mail text B is a device on the user side of B for C ₁ and C ₂ sent.
Using the encryption / decryption device 105B in 100B, first, the master key K
_{From B and A} , the data encryption key k is k = D (KB _{, A} ; C ₂ ).
, And then using the data encryption key k, P = D
At (k; C ₁ ), the mail text P is decrypted.

(Third Embodiment) In the third embodiment, the information (s _λ , for the user λ created by the center in the second embodiment is
An example of creating v _λ ) by another function different from that of the second embodiment will be given.

Preparation The center creates the following information using the prime number generator 201, the primitive root generator 202 and the arithmetic unit 204 in the center side device 200.
However, it is assumed that the function f is stored in the secret key generation device 203.

Public information :-p ₁ ; prime number-p ₂ = 2q ₁ q ₂ +1; prime number-a ₁ εZ st 0 <a ₁ <p ₁ and a ₁ is Z / (p ₁ ). Primitive root, ・ a ₂ ∈Z st 0 <a ₂ <p ₂ , and a ₁ is Z / (p ₁ ) and primitive root, secret information: ・ q ₁ , q ₂ ∈Z; prime number. Each user λ of the network uses the random number generator 101 and the power multiplier 102 in the user side device 100 to create the next key.

Private key:

[0101]

X _λ ∈ Z st 0 <x _λ <p ₁ −1. Public key:

[0102]

Y _λ = exp (a ₁ : x _λ ) (mod p ₁ ). Registration to the center The network user λ has the ID information I _{λ at the} center.
To register.

The center uses the secret key generating device 203 in the center-side device 200, and (1) I _λ + i _λ (mod p ₂ -1) is the quadratic remainder for the ID information I _{λ of the} user λ. (2) I _λ + i _λ (mod p ₂ −1) ≠ I _μ + i _μ (mod
p ₂ −1) if positive _λ i _λ such that if λ ≠ μ,

[0104]

S _λ = exp (I _λ + i _λ ): 1/2) (mod p ₂ −1) is calculated and s _λ is safely distributed to the user λ.

At this time, obviously, s _λ ≠ s _μ if λ
≠ μ holds.

Master Key Sharing The network user λ registers the vector (y _λ , v _λ ) as the public key of λ. However, v _λ = exp (a
₂ : s _λ ) (mod p ₂ ). Further, in the user side device 100 connected to the network, there is an encryption / decryption device 105 having the same secret key cryptosystem algorithm built therein, and the plaintext M is encrypted with the key K using this secret key cryptosystem algorithm. The result of decoding and the result of decoding are E (K; M) and D, respectively.
It is represented by (K; M).

Regarding network users A and B, A
Wants to send an email to B. Under this purpose,
A and B execute the following procedure: A is a power multiplier 102A and an arithmetic unit in the user side device 100A of A.
Using 104A and the hash calculator 106A, from A's private key (x _A , s _A ) and B's public key (y _B , v _B ) to the master key K
_{A, B}

[0108]

(99) K _{A, B} = g (φ (exp (y _B : x _A ) (mod p ₁ ), exp
(v _B : s _A ) (mod p ₂ ))) ,. Similarly, B uses B's private key (x _B , s _B ) and A's public key (y _A by using the power multiplier 102B, the calculator 104B, and the hash calculator 106B in the user device 100B of _B. ，
v _A ) to the master key K _{B, A} ,

[0109]

[Equation 100] K _{B, A} = g (φ (exp (y _A : x _B ) (mod p ₁ ), ex
p (v _A : s _B ) (mod p ₂ ))), However, g represents the hash function used in the hash calculator, and φ is

[0110]

[Expression 101] φ: Z / (p ₁ ) × Z / (p ₂ ) → Z / (n) ((x, x) → x), which represents an isomorphism (n = p ₁ p ₂ ). Here, x represents a residue class whose representative element is x in each residue ring.

At this time, it is clear that K _{A, B} = K _{B, A.}

Furthermore, for users who frequently send e-mail, A refers to the set of master keys S _A between those users and _A , or the secret information x _{A of A} from the user side. The key generator 103A in the device 100A creates the key K (A), and the encryption / decryption device 105A encrypts S _A with the key K (A) C (A) = E (K (A); S _A ), the memory 107A
To memorize. In subsequent e-mail communications, if a master key is required for updating the data encryption key, etc., if the master key is stored, create a new master key from your private key and the public key of the other party. Without using the master key stored in the memory 107A.

Encryption of the mail text A uses the random number generator 101A in the user side device 100A to select a random number r, and inputs the r to create a data encryption key k from the key generator 103A. Email text P encryption / decryption device 105A
Ciphertext C ₁ encrypted with the data encryption key k using
With C ₁ = E (k; P). Furthermore, the data encryption key k is transferred to the master key K _{A, B} using the encryption / decryption device 105A.
A ciphertext C ₂ encrypted by is created by C ₂ = E (KA _{, B} ; k).

A uses the communication device 108A to connect C ₁ and C ₂ to B
Send to

Decryption of mail text B is a device on the user side of B for C ₁ and C ₂ sent.
Using the encryption / decryption device 105B in 100B, first, the master key K
_{From B and A} , the data encryption key k is k = D (KB _{, A} ; C ₂ ).
, And then using the data encryption key k, P = D
At (k; C ₁ ), the mail text P is decrypted.

(Fourth Embodiment) In the fourth embodiment, the information (s _λ ,
An example of creating v _λ ) by another function different from those in the second and third embodiments will be given.

Preparation The center creates the following information using the prime number generator 201, the primitive root generator 202 and the arithmetic unit 204 in the center side device 200.

Public information: p ₁ ; prime number, p ₂ ; prime number, a ₁ εZ st 0 <a ₁ <p ₁ , and a ₁ is a primitive root of Z / (p ₁ ), a ₂ ΕZ st 0 <a ₂ <p ₂ and a ₁ is a primitive root with Z / (p ₁ ), secret information: secret key cryptographic algorithm and key r that outputs the element of Z / (p ₂ −1) ΕZ. Results of encrypting and decrypting the plaintext M with the key k by the secret key encryption algorithm which is the secret information of the center are represented by E ₀ (k; M) and D ₀ (k; M), respectively.

Each user λ of the network is a user-side device
A random number generator 101 and a power multiplier 102 in 100 are used to generate the next key.

Private key:

[0121]

[Number 102] _{· x λ ∈Z st 0 <x} λ <p 1 -1. Public key:

[0122]

Y _λ = exp (a ₁ : x _λ ) (mod p ₁ ). Registration to the center The network user λ has the ID information I _{λ at the} center.
To register.

The center uses the secret key generating device 203 in the center-side device 200 for the ID information I _{λ of the} user λ,

[0124]

S _λ = E ₀ (r; I _λ ) εZ / (p ₂ −1), is calculated, and s _λ is safely distributed to the user λ.

At this time,

[0126]

Note that s _λ ≠ s _μ if I _λ ≠ I _μ εΛ = Z / (p ₂ −1), holds.

Master Key Sharing A network user λ registers a vector (y _λ , v _λ ) as a public key of λ. However, v _λ = exp (a
₂ : s _λ ) (mod p ₂ ). Further, in the user side device 100 connected to the network, there is an encryption / decryption device 105 having the same secret key cryptosystem algorithm built therein, and the plaintext M is encrypted with the key K using this secret key cryptosystem algorithm. The result of decoding and the result of decoding are E (K; M) and D, respectively.
It is represented by (K; M).

Regarding network users A and B, A
Wants to send an email to B. Under this purpose,
A and B execute the following procedure: A is a power multiplier 102A and an arithmetic unit in the user side device 100A of A.
Using 104A and the hash calculator 106A, from A's private key (x _A , s _A ) and B's public key (y _B , v _B ) to the master key K
_{A, B}

[0129]

K _{A, B} = g (φ (exp (y _B : x _A ) (mod p ₁ ), ex
p (v _B : s _A ) (mod p ₂ ))). Similarly, B uses B's private key (x _B , s _B ) and A's public key (y _A by using the power multiplier 102B, the calculator 104B, and the hash calculator 106B in the user device 100B of _B. ，
v _A ) to the master key K _{B, A} ,

[0130]

K _{B, A} = g (φ (exp (y _A : x _B ) (mod p ₁ ), ex
p (v _A : s _B ) (mod p ₂ ))), However, g represents the hash function used in the hash calculator, and φ is

[0131]

Φ: Z / (p ₁ ) × Z / (p ₂ ) → Z / (n) ((x, x) → x), which represents the isomorphic mapping (n = p ₁ p ₂ ). Here, x represents a residue class whose representative element is x in each residue ring.

At this time, it is clear that K _{A, B} = K _{B, A.}

Furthermore, for users who frequently send e-mail, A refers to the set S _A of each master key between those users and _A , or the secret information x _{A of A} from the user side. The key generator 103A in the device 100A creates the key K (A), and the encryption / decryption device 105A encrypts S _A with the key K (A) C (A) = E (K (A); S _A ), the memory 107A
To memorize. In subsequent e-mail communications, if a master key is required for updating the data encryption key, etc., if the master key is stored, create a new master key from your private key and the public key of the other party. Without using the master key stored in the memory 107A.

Encryption of the mail text A selects a random number r using the random number generator 101A in the user side device 100A, and inputs the r to create a data encryption key k from the key generator 103A. Email text P encryption / decryption device 105A
Ciphertext C ₁ encrypted with the data encryption key k using
With C ₁ = E (k; P). Furthermore, the data encryption key k is transferred to the master key K _{A, B} using the encryption / decryption device 105A.
A ciphertext C ₂ encrypted by is created by C ₂ = E (KA _{, B} ; k).

A uses the communication device 108A to connect C ₁ and C ₂ to B
Send to

Decryption of mail text B is a device on the user side of B for C ₁ and C ₂ sent.
Using the encryption / decryption device 105B in 100B, first, the master key K
_{From B and A} , the data encryption key k is k = D (KB _{, A} ; C ₂ ).
, And then using the data encryption key k, P = D
At (k; C ₁ ), the mail text P is decrypted.

(Embodiment 5) Embodiment 5 is an expanded version of Embodiment 2, in which the value of α is kept secret to further improve security.

Preparation The center creates the following information using the prime number generator 201, the primitive root generator 202 and the arithmetic unit 204 in the center side device 200.

Public information :-p; prime number, -n = q-q ',-a? Zst 0 <a <p, and a is a primitive root in Z / (p), secret information: -q, q '∈ Z; prime number st q-1 = 2ξ ₁ ξ ₂ , q'-1
= 2η ₁ η ₂ (ξ ₁ , ξ ₂ , η ₁ , η ₂ ; prime number), α ∈ Z st α is a primitive root with Z / (q) and Z / (q '). Each user λ of the network uses the random number generator 101 and the power multiplier 102 in the user side device 100 to create the next key.

Private key:

[0141]

[Number 109] _{· x λ ∈Z st 0 <x} λ <p-1. Public key:

[0142]

Y _λ = exp (a: x _λ ) (mod p). Registration to the center The network user λ has the ID information I _{λ at the} center.
To register.

The center uses the secret key creating device 203 in the center-side device 200 to create e and d such that ed≡1 (mod M), and s _λ for the ID information I _λ of the user _λ. = Exp
(I _λ : e) (mod N),

[0144]

Calculate v _λ = exp (α: s _λ ) (mod n), and securely distribute (s _λ , v _λ ) to the user λ. However, M = lcd (ξ ₁ -1, ξ ₂ -1, η ₁ -1,
η ₂ −1), N = lcd (q−1, q′−1).

At this time,

[0146]

S _λ ≠ s _μ and v _λ ≠ v _μ if I _λ ≠ I _μ
Note that εΛ = Z / (N), holds.

Master Key Sharing A network user λ registers a vector (y _λ , v _λ ) as a public key of λ.

Further, in the user side device 100 connected to the network, there is the encryption / decryption device 105 having the same secret key cryptosystem algorithm built therein, and the plaintext M is converted into the key K using this secret key cryptosystem algorithm. The encrypted result and the decrypted result are represented by E (K; M) and D (K; M), respectively.

Regarding network users A and B, A
Wants to send an email to B. Under this purpose,
A and B execute the following procedure: A is a power multiplier 102A and an arithmetic unit in the user side device 100A of A.
Using 104A and the hash calculator 106A, from A's private key (x _A , s _A ) and B's public key (y _B , v _B ) to the master key K
_{A, B}

[0150]

[Equation 113] K _{A, B} = g (φ (exp (y _B : x _A ) (mod p), exp
(v _B : s _A ) (mod n))) ,. Similarly, B uses his / her own private key (x _B , s _B ) and A's public key (y _A by using the power multiplier 102B, the arithmetic unit 104B and the hash calculator 106B in the user side device 100B of _B. ，
v _A ) to the master key K _{B, A} ,

[0151]

[Equation 114] K _{B, A} = g (φ (exp (y _A : x _B ) (mod p), exp
(v _A : s _B ) (mod n))), However, g represents the hash function used in the hash calculator, and φ is

[0152]

Φ: Z / (p) × Z / (n) → Z / (m) ((x, x) → x), which represents an isomorphic mapping (m = pn). Here, x represents a residue class whose representative element is x in each residue ring.

At this time, it is clear that K _{A, B} = K _{B, A.}

Furthermore, for users who frequently send e-mail, A refers to the set S _A of each master key between those users and A or the secret information x _{A of A} from the user side. The key generator 103A in the device 100A creates the key K (A), and the encryption / decryption device 105A encrypts S _A with the key K (A) C (A) = E (K (A); S _A ), the memory 107A
To memorize. In subsequent e-mail communications, if a master key is required for updating the data encryption key, etc., if the master key is stored, create a new master key from your private key and the public key of the other party. Without using the master key stored in the memory 107A.

Encryption of mail text A uses the random number generator 101A in the user side device 100A to select a random number r, and inputs the r to create a data encryption key k from the key generator 103A. Email text P encryption / decryption device 105A
Ciphertext C ₁ encrypted with the data encryption key k using
With C ₁ = E (k; P). Furthermore, the data encryption key k is transferred to the master key K _{A, B} using the encryption / decryption device 105A.
A ciphertext C ₂ encrypted by is created by C ₂ = E (KA _{, B} ; k). Furthermore, the ciphertext C ₂ obtained by encrypting the data encryption key k with the master keys K _{A and B} using the encryption / decryption device 105A is
₂ = E (K _{A, B} ; k).

A uses the communication device 108A to connect C ₁ and C ₂ to B
Send to

Decryption of mail body B is a device on the user side of B for C ₁ and C ₂ sent.
Using the encryption / decryption device 105B in 100B, first, the master key K
_{From B and A} , the data encryption key k is k = D (KB _{, A} ; C ₂ ).
, And then using the data encryption key k, P = D
At (k; C ₁ ), the mail text P is decrypted.

(Embodiment 6) Embodiment 6 is an expanded version of Embodiment 3 in which the value of α is kept secret to further improve security.

Preparation The center creates the following information using the prime number generator 201, the primitive root generator 202 and the arithmetic unit 204 in the center side device 200.
However, it is assumed that the function f is stored in the secret key generation device 203.

Public information :-p; prime number, -n = q-q ',-a? Zst 0 <a <p, and a is a primitive root in Z / (p), secret information: -q, q '∈ Z; prime number st q-1 = 2ξ ₁ ξ ₂ , q'-1
= 2η ₁ η ₂ (ξ ₁ , ξ ₂ , η ₁ , η ₂ ; prime number), α ∈ Z st α is a primitive root with Z / (q) and Z / (q '). F: pseudo-random function. Each user λ of the network uses the random number generator 101 and the power multiplier 102 in the user side device 100 to create the next key.

Private key:

[0162]

[Number 116] _{· x λ ∈Z st 0 <x} λ <p-1. Public key:

[0163]

Y _λ = exp (a: x _λ ) (mod p). Registration to the center The network user λ has the ID information I _{λ at the} center.
To register.

The center uses the secret key generating device 203 in the center side device 200, and (1) I _λ + i _λ (mod N) is the quadratic remainder, and (2) is the ID information I _{λ of the} user λ. I _λ + i _λ (mod N) ≠ I _μ + i _μ (mod N)
choose a positive integer i _λ so that if λ ≠ μ,

[0165]

S _λ = exp (I _λ + i _λ : 1/2) (mod N), v _λ = exp (α: s _λ ) (mod n), is calculated, and (s _λ , v _λ ) Is safely distributed to user λ.

At this time, obviously, s _λ ≠ s _μ , v _λ ≠
v _μ if λ ≠ μ holds.

Master Key Sharing A network user λ registers a vector (y _λ , v _λ ) as a public key of λ.

Further, in the user side device 100 connected to the network, there is the encryption / decryption device 105 having the same secret key cryptosystem algorithm built therein, and the plaintext M is converted into the key K using the secret key cryptosystem algorithm. The encrypted result and the decrypted result are represented by E (K; M) and D (K; M), respectively.

Regarding network users A and B, A
Wants to send an email to B. Under this purpose,
A and B execute the following procedure: A is a power multiplier 102A and an arithmetic unit in the user side device 100A of A.
Using 104A and the hash calculator 106A, from A's private key (x _A , s _A ) and B's public key (y _B , v _B ) to the master key K
_{A, B}

[0170]

K _{A, B} = g (φ (exp (y _B : x _A ) (mod p), exp
(v _B : s _A ) (mod n))) ,. Similarly, B uses his / her own private key (x _B , s _B ) and A's public key (y _A by using the power multiplier 102B, the arithmetic unit 104B and the hash calculator 106B in the user side device 100B of _B. ，
v _A ) to the master key K _{B, A} ,

[0171]

[Equation 120] K _{B, A} = g (φ (exp (y _A : x _B ) (mod p), exp
(v _A : s _B ) (mod n))), However, g represents the hash function used in the hash calculator, and φ is

[0172]

Φ: Z / (p) × Z / (n) → Z / (m) ((x, x) → x), which represents an isomorphic mapping (m = pn). Here, x represents a residue class whose representative element is x in each residue ring.

At this time, it is clear that K _{A, B} = K _{B, A.}

Furthermore, for users who frequently send e-mail, A refers to the set S _A of each master key between those users and _A , or the secret information x _{A of A} from the user side. The key generator 103A in the device 100A creates the key K (A), and the encryption / decryption device 105A encrypts S _A with the key K (A) C (A) = E (K (A); S _A ), the memory 107A
To memorize. In subsequent e-mail communications, if a master key is required for updating the data encryption key, etc., if the master key is stored, create a new master key from your private key and the public key of the other party. Without using the master key stored in the memory 107A.

Encryption of mail text A uses the random number generator 101A in the user side device 100A to select a random number r, and inputs the r to create a data encryption key k from the key generator 103A. Email text P encryption / decryption device 105A
Ciphertext C ₁ encrypted with the data encryption key k using
With C ₁ = E (k; P). Furthermore, the data encryption key k is transferred to the master key K using the encryption / decryption device 105A.
_A ciphertext C ₂ encrypted with _{A and B is} created with C ₂ = E (K _{A, B} ; k). Furthermore, a ciphertext C ₂ obtained by encrypting the data encryption key k with the master keys K _{A and B} using the encryption / decryption device 105A,
Create with C ₂ = E (K _{A, B} ; k).

A uses the communication device 108A to connect C ₁ and C ₂ to B
Send to

Decryption of mail text B is a device on the user side of B for C ₁ and C ₂ sent.
Using the encryption / decryption device 105B in 100B, first, the master key K
_{From B and A} , the data encryption key k is k = D (KB _{, A} ; C ₂ ).
, And then using the data encryption key k, P = D
At (k; C ₁ ), the mail text P is decrypted.

(Embodiment 7) Embodiment 7 is an expanded version of Embodiment 4, in which the value of α is kept secret to further improve security.

Preparation The center creates the following information using the prime number generator 201, the primitive root generator 202 and the arithmetic unit 204 in the center side device 200.

Public information :-p; prime number-n = q-q '(q, q'; prime number) -a? Zst 0 <a <p, and a is Z / (p) and a primitive root , Confidential information: ・ Secret key encryption algorithm and key r that outputs the element of Z / (N) ・ α ∈ Z st α is a primitive root with Z / (q) and Z / (q '). Here, N = lcd (q-1, q'-1). Also,
Results of encrypting and decrypting the plaintext M with the key k by the secret key encryption algorithm which is the secret information of the center are represented by E ₀ (k; M) and D ₀ (k; M), respectively.

Each user λ of the network is a user-side device
A random number generator 101 and a power multiplier 102 in 100 are used to generate the next key.

Private key:

[0183]

[Number 122] _{· x λ ∈Z st 0 <x} λ <p-1. Public key:

[0184]

Y _λ = exp (a: x _λ ) (mod p). Registration to the center The network user λ has the ID information I _{λ at the} center.
To register.

The center uses the secret key generating device 203 in the center-side device 200 to obtain the ID information I _{λ of the} user λ,

[0186]

S _λ = E ₀ (r; I _λ ) εZ / (N), v _λ = exp (α: s _λ ) (mod N), is calculated to obtain (s _λ , v _λ ). Distribute safely to user λ.

At this time, s _λ ≠ s _μ and v _λ ≠ v _μ i
Note that f I _λ ≠ I _μ εΛ = Z / (N), holds.

Master Key Sharing A network user λ registers a vector (y _λ , v _λ ) as a public key of λ.

Further, in the user side device 100 connected to the network, there is the encryption / decryption device 105 having the same secret key cryptosystem algorithm built therein, and the plaintext M is converted into the key K using this secret key cryptosystem algorithm. The encrypted result and the decrypted result are represented by E (K; M) and D (K; M), respectively.

Regarding network users A and B, A
Wants to send an email to B. Under this purpose,
A and B execute the following procedure: A is a power multiplier 102A and an arithmetic unit in the user side device 100A of A.
Using 104A and the hash calculator 106A, from A's private key (x _A , s _A ) and B's public key (y _B , v _B ) to the master key K
_{A, B}

[0191]

[Equation 125] K _{A, B} = g (φ (exp (y _B : x _A ) (mod p), exp
(v _B : s _A ) (mod n))) ,. Similarly, B uses his / her own private key (x _B , s _B ) and A's public key (y _A by using the power multiplier 102B, the arithmetic unit 104B and the hash calculator 106B in the user side device 100B of _B. ，
v _A ) to the master key K _{B, A} ,

[0192]

K _{B, A} = g (φ (exp (y _A : x _B ) (mod p), exp
(v _A : s _B ) (mod n))), However, g represents the hash function used in the hash calculator, and φ is

[0193]

Φ: Z / (p) × Z / (n) → Z / (m) ((x, x) → x), which represents an isomorphic mapping (m = pn). Here, x represents a residue class whose representative element is x in each residue ring.

At this time, it is clear that K _{A, B} = K _{B, A.}

Furthermore, for users who frequently send e-mail, A refers to the set S _A of each master key between those users and _A , or the secret information x _{A of A} from the user side. The key generator 103A in the device 100A creates the key K (A), and the encryption / decryption device 105A encrypts S _A with the key K (A) C (A) = E (K (A); S _A ), the memory 107A
To memorize. In subsequent e-mail communications, if a master key is required for updating the data encryption key, etc., if the master key is stored, create a new master key from your private key and the public key of the other party. Without using the master key stored in the memory 107A.

Encryption of mail text A uses the random number generator 101A in the user side device 100A to select a random number r, and inputs the r to create a data encryption key k from the key generator 103A. Email text P encryption / decryption device 105A
Ciphertext C ₁ encrypted with the data encryption key k using
With C ₁ = E (k; P). Furthermore, the data encryption key k is transferred to the master key K using the encryption / decryption device 105A.
_A ciphertext C ₂ encrypted with _{A and B is} created with C ₂ = E (K _{A, B} ; k).

A uses the communication device 108A to connect C ₁ and C ₂ to B
Send to

Decryption of mail text B is a device on the user side of B for C ₁ and C ₂ sent.
Using the encryption / decryption device 105B in 100B, first, the master key K
_{From B and A} , the data encryption key k is k = D (KB _{, A} ; C ₂ ).
, And then using the data encryption key k, P = D
At (k; C ₁ ), the mail text P is decrypted.

(Embodiment 8) In Embodiments 1 to 7, the mail text is encrypted and the mail text is decrypted as follows.

Encryption of mail text A uses the random number generator 101A in the user side device 100A to select a random number r, and inputs the r to create a data encryption key k from the key generator 103A. Email text P encryption / decryption device 105A
Ciphertext C ₁ encrypted with the data encryption key k using
With C ₁ = E (k; P). Further, an exclusive OR of the data encryption key k and the master key K _{A, B} is created by using an arithmetic unit with C ₂ = k * K _{A, B.}

A uses the communication device 108A to send C ₁ and C ₂ to B.

・ Decryption of mail body B: For C ₁ and C ₂ sent, first, the arithmetic unit 104B
Is used to decrypt the data encryption key k by calculating the exclusive OR of the master keys K _{B, A} and C ₂ . That is,
k = C ₂ * K _{A, B.} Next, the encryption / decryption device 105B is used to decrypt the mail text P from the data encryption key k with P = D (k; C ₁ ).

(Embodiment 9) In the master key agreement of Embodiments 2 to 8, A is A using the power multiplier 102A, the arithmetic unit 104A and the hash calculator 106A in the user side device 100A of A. Private key (x _A , s _A ) and B's public key (y _B , v _B )
To the master key K _{A, B} ,

[0204]

K _{A, B} = g ((exp (y _B : x _A ) (mod p)) * (ex
p (v _B : s _A ) (mod n))), Similarly, B uses his / her own private key (x _B , s _B ) and A's public key (y _A by using the power multiplier 102B, the arithmetic unit 104B and the hash calculator 106B in the user side device 100B of _B. ，
v _A ) to the master key K _{B, A} ,

[0205]

K _{B, A} = g ((exp (y _A : x _B ) (mod p)) * (ex
p (v _A : s _B ) (mod n))), However, g represents a hash function used in the hash calculator, and x * y represents an exclusive OR of x and y.

(Embodiment 10) Each user of the network possesses the card 400, and inserts the card 400 into the card reader 501 in the user-side device 500 at the time of electronic mail communication. Random number generator 101, exponentiator 102, key generator 103, calculator 10 in the user-side device 100
4. The random number generator 4 in the card 400 performs the processes using the encryption / decryption device 105, the hash calculator 106, and the memory 107.
01, exponentiator 402, key generator 403, arithmetic unit 404, encryption / decryption device 405, hash calculator 406, memory 407, the transmission to the recipient using the output device 408 in the card 400 Then, it is output to the card reading device 501 in the user side device 500, and is further transmitted via the communication line 300 using the communication device 502 in the user side device 500.

[0207]

According to the electronic mail encryption method of the present invention, the electronic mail encryption method has a feature that it can be used in combination with ordinary mail without specifying the mail system or model, and therefore can be applied regardless of the form of the communication network. In addition, the sender and the recipient of the electronic mail share the master key from their own private key and the public key of the other party, and use the private key encryption to encrypt the mail text and deliver the data encryption key. Therefore, both the sender and the receiver can perform high-speed encryption processing with less calculation processing load. In addition, since the sender can be authenticated without adding additional information such as a digital signature, the header information of the mail ciphertext is small, and the processing time for realizing the e-mail security function is greatly reduced. Furthermore, since the center and the user cooperate to generate the key of the user by utilizing the fact that the ID information is different for each user, there is no fear of the user's key matching, and there is no fear of eavesdropping or unauthorized persons. Achieved high security against impersonation.

[Brief description of drawings]

FIG. 1 is a block diagram showing a system configuration according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing the concept of the electronic mail encryption method of the present invention.

FIG. 3 is a block diagram showing an internal configuration of a user-side device in the system configurations of Examples 1 to 9.

FIG. 4 is a block diagram showing an internal configuration of a center-side device in the system configurations of Embodiments 1 to 9.

FIG. 5 is a block diagram showing an internal configuration of a card in the system configuration of the tenth embodiment.

FIG. 6 is a block diagram showing the internal configuration of a user-side device in the system configuration of Example 10.

[Explanation of symbols]

 100 ... User device, 101 ... Random number generator in user device 100, 102 ... Power multiplier in user device 100, 103 ... Key generator in user device 100, 104 ... User Operation unit in the side device 100, 105 ... Encryption / decryption device in the user side device 100, 106 ... Hash calculator in the user side device 100, 107 ... Memory in the user side device 100, 108 ... User Communication device in the side device 100, 200 ... Center side device.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location H04L 12/58

Claims (13)

[Claims] 1. An electronic mail communication method for sending and receiving mails on a communication network, which is an electronic mail encryption method for encrypting a mail text sent from a sender A to a recipient B, which is a preparation process. , Sender A creates private key x _A and public key v _A corresponding to private key x _A ,
Receiver B creates a public key v _B corresponding to the private key x _B and secret key x _B, only expose the respective public keys, as the master key sharing process, the sender A is received with its own secret key x _A The master key K _{A, B} is created from the public key v _{B of the} person B, and the recipient B uses the private key x _B of the person B and the public key v _A of the recipient B to obtain the master key K _{A.
B, A} is created, and at this time, K _{A, B} = K _{B, A} holds. Especially, for the recipients who often send the e-mail, the sender A sends them together with those recipients. Storing information obtained by encrypting each master key between persons A or each master key with a key created from the secret key of the sender A, for updating the data encryption key in the subsequent e-mail communication, etc. If a master key is required in, if the master key is stored, the stored master key is used without creating a new master key from the other party's public key and own private key, As the encryption / decryption processing of the mail text, sender A
Randomly selects the data encryption key k and uses the secret key cryptosystem algorithm commonly held with the recipient B,
Ciphertext E ₁ (k; P) obtained by encrypting the mail text P with the data encryption key k, and ciphertext E ₂ (K _{A, B} obtained by encrypting the data encryption key k with the master key K _{A, B B} ; k) to the recipient B, who uses the master key K _{B, A} to generate E ₂ (K _{A, B} ;
An electronic mail encryption method characterized by decrypting the data encryption key k from k), and further decrypting the mail text P from E ₁ (k; P) using the data encryption key k. 2. An electronic mail communication method for sending and receiving mail on a communication network, which is an electronic mail encryption method for encrypting a mail text sent from a sender A to a recipient B, which is a preparation process. , The center creates a one-to-one function f as the secret information of the center, the sender A creates the private key x _A and the public key y _A corresponding to the private key x _A , and the receiver B creates the private key x _B and A public key y _B corresponding to the private key x _B is created, and as registration processing to the center, sender A and recipient B register their ID information in the center,
The center uses the function f, which is the secret information of the center, to obtain the secret key s _A unique to the sender _A from the ID information I _A of the sender A.
s _A = f (I _A ), and similarly, the secret key s _B unique to the recipient _B from the ID information I _B of the recipient _B is s _B = f (I _B ),
The private key s _A and the public key v corresponding to s _A
Set of _A (s _A, v _A) safely distributed to the sender A, similar to the public key v _B of the set corresponding to the secret key s _B and _{s B (s B, v B} )
Is safely distributed to the receiver B. At this time, if I _A ≠ I _B, then s _A ≠ s _B and v _A ≠ v _B , then the sender A (y _A ,
v _A a), the recipient B is registered as (y _B, v _B) your public information, respectively, as a shared processing of the master key, the sender A is receiving his private key (x _A, s _A) and The master key K _{A, B} is created from the public key (y _B , v _B ) of the person B, and the receiver B own private key (x _B , s _B ) and the public key (y _A , B of the receiver _B ).
v _A ) to create a master key K _{B, A} , where K _{A, B} =
K _{B, A} is established, and in particular, the sender A sends each master key between the receiver and the sender A or each master key to the receiver who often has the opportunity to send an electronic mail. The information encrypted by using the key created from the private key of the person A is stored, and when the master key is necessary for updating the data encryption key in the subsequent e-mail communication, the master key is stored. If the master key is stored, the stored master key is used without creating a new master key from the other party's public key and its own private key. An encryption key k is randomly selected, and a ciphertext E ₁ (k; which is obtained by encrypting the mail text P with the data encryption key k using a secret key cryptosystem algorithm commonly held with the recipient B. P) and the data encryption key k
Ciphertext E ₂ (K _{A, B} ； k) that is encrypted with master key K _{A, B}
To the receiver B, and the receiver B uses the master key K _{B, A} to decrypt the data encryption key k from E ₂ (K _{A, B} ; k) and further uses the data encryption key k. A method of encrypting an electronic mail, characterized by decrypting a mail text P from E ₁ (k; P). 3. An electronic mail communication method for sending and receiving mail on a communication network, which is an electronic mail encryption method for encrypting a mail text sent from a sender A to a recipient B, which is a preparation process. , Sender A is 0
<X _Aj <p−1, an integer x _Aj (j = 1, ..., N) is randomly selected, and an integer a and a prime number p, which are public information, are used to obtain y _Aj = exp (a: x _Aj ) (mod p) for j = 1, ..., N, ... ( _Equation 1) is calculated, y _Aj (j = 1, ..., n) is registered as the public key of the sender A, and the recipient B is , 0 <x _Bj <p−1, an integer x _Bj (j = 1,
, N) is randomly selected, and y _Bj = exp (a: x _Bj ) (mod p) for j = 1, ..., n, (Equation 2) is calculated, and y _Bj (j = 1, ..., N) is registered as the public key of the receiver B (however, exp (a: x) represents a value of a raised to the power x), and the sender A shares its secret as master key sharing processing. Using the integer x _Aj , which is the key, and y _Bj (j = 1, ..., N), which is the public key of the recipient B, the master key K _{A, B} is expressed as follows: K _{A, B} = Π {exp (y _Bj : x _Aj ) (mod p) | j = 1, ..., N}, and the receiver B has an integer x _Bj , which is his private key, and y, which is the public key of the sender A. _{Using Aj} (j = 1, ..., N), the master key K _{B, A} is expressed as follows: K _{B, A} = Π {exp (y _Aj : x _Bj ) (mod p) | j = 1 , N}, and at this time, K _{A, B} = K _{B, A} holds, and in particular, sender A sends an e-mail. For recipients who often have a chance to do so, a set S _A of each master key between those recipients and sender _A , or an integer x _Aj (j =
1, ..., N) and the key generation function θ, using the key K (A) created by the following equation, K (A) = θ (x _A1 , ..., x _An ), the master key set S encrypts the _{a C (a) = E a} (K (a); S a), and stores, in an electronic mail communication thereafter, when the master key, etc. for the data encryption key update is required, the master For keys that have been stored, the stored master key is used without creating a new master key from the other party's public key and own private key (provided that E _A (k;
(M) represents the result of encrypting the plaintext M with the key k by the secret key encryption algorithm possessed by A), and the sender A randomly selects the data encryption key k as the encryption / decryption process of the mail body. , The ciphertext E (k; which is obtained by encrypting the mail text P with the data encryption key k, using the secret key cryptosystem algorithm commonly held with the recipient B.
P) and a ciphertext E (K _{A, B} ; k) obtained by encrypting the data encryption key k with the master key K _{A, B,} to the mail recipient B,
Recipient B uses master key K _{B, A} to decrypt data encryption key k from E (K _{A, B} ; k), and uses data encryption key k to send email from E (k; P) An electronic mail encryption method characterized by decrypting a body P. 4. An electronic mail communication method for sending and receiving mail on a communication network, which is an electronic mail encryption method for encrypting a mail text sent from a sender A to a recipient B, which is a preparation process. , The center is a prime number p ₁ and a prime number p _{2 in} which p ₂ −1 has two large prime numbers as factors, and integers a ₁ , a
₂ is disclosed, the prime factorization of p ₂ −1 is kept secret, and the sender A
Randomly selects an integer x _{A such} that 0 <x _A <p ₁ −1, and calculates y _A = exp (a ₁ : x _A ) (mod p ₁ ), and the receiver B An integer x _B of 0 <x _B <p ₁ -1 is randomly selected, and y _B = exp (a ₁ : x _B ) (mod p ₁ ), is calculated (where exp (a: x ) Represents a value obtained by multiplying a by the power of x.) As a registration process to the center, sender A and receiver B
Registers its own ID information in the center, and the center calculates I _A + i _A (mod p ₂ −1) is a quadratic remainder from the ID information I _A of the sender A, and I _A ≠ I _B , I _A + I _A (mod p ₂ -1)
_A positive integer i _A is selected so that ≠ I _B + i _B (modp ₂ −1), and the secret key s _A unique to the sender _A is expressed as follows: s _A = exp (I _A + i _A : 1 / _{2) (mod p 2 -1)} , was calculated at, from the ID information I _B similarly recipient B, and unique secret key s _B to the sender B, Equation 9] s _B = exp (I _B + I _B : 1/2) (mod p ₂ -1), and each is safely distributed to sender A and receiver B. At this time, if the ID information is different, the corresponding secret key is also different, the sender a and y _a, the public key [number 10] corresponding to the secret key _{s a v a = exp (a} 2: s a) (mod p 2), sending the pair (y _a, v _a) of the Registered as the public key of person A,
Receiver B y _B and, public key [number 11] corresponding to the secret key _{s B v B = exp (a} 2: s B) (mod p 2), the combination of the (y _B, v _B) receive a Registered as the public key of person B,
In the master key sharing process, the sender A uses his / her own function g to be shared with the receiver B, which is (x _A , s _A ), which is his private key, and the public key of the receiver B. From (y _B , v _B ), the master key K _{A, B} is calculated as follows: K _{A, B} = g (exp (y _B : x _A ) (mod p ₁ ), exp
(v _B : s _A ) (mod p ₂ )), the recipient B has his / her private key (x _B ,
s _B ) and the public key (y _A , v _A ) of sender A, the master key K _{B, A} is expressed as follows: K _{B, A} = g (exp (y _A : x _B ) (mod p ₁ ), exp
(v _A : s _B ) (mod p ₂ )), and at this time, K _{A, B} = K _{B, A} holds, and in particular, the sender A receives an e-mail with many opportunities to receive it. From the set S _{A of} the master keys between the receiver and the sender _A , or the secret key (x _A , s _A ) of the sender _A and the key generation function θ, K (A) = θ (x _A , s _A ), the master key set S _A is encrypted by using the key K (A) created by the following equation: C (A) = E _A (K (A); S _A ), is stored, and when the master key is required for the purpose of updating the data encryption key in the subsequent e-mail communication, if the master key is stored, the master key is newly stored. Instead of creating a master key from the public key and your own private key, use the stored master key (however, E _A (k;
M) represents the result of encrypting the plaintext M with the key k by the secret key encryption algorithm possessed by A. ), The sender A randomly selects the data encryption key k as the encryption / decryption process for the mail text, and uses the secret key cryptosystem algorithm commonly held with the receiver B to send the mail text P. Ciphertext E (k; encrypted with the data encryption key k
P) and a ciphertext E (K _{A, B} ; k) obtained by encrypting the data encryption key k with the master key K _{A, B} , and the recipient B receives the master key K _{B. , A} is used to decrypt the data encryption key k from E (K _{A, B} ; k), and further the data encryption key k is used to decrypt the mail text P from E (k; P). And the email encryption method. 5. An electronic mail communication method for transmitting and receiving mail on a communication network, which is an electronic mail encryption method for encrypting a mail text sent from a sender A to a recipient B, which is a preparation process. , The center is a prime number p ₁ and a prime number p _{2 in} which p ₂ −1 has two large prime numbers as factors, and integers a ₁ , a
₂ is disclosed, the prime factorization of p ₂ −1 is kept secret, and the sender A
Randomly selects an integer x _{A such} that 0 <x _A <p ₁ −1 and calculates y _A = exp (a ₁ : x _A ) (mod p ₁ ), and the receiver B An integer x _{B with} 0 <x _B <p ₁ −1
Is randomly selected, and y _B = exp (a ₁ : x _B ) (mod p ₁ ), is calculated, and sender A and receiver B have their own ID information as registration processing to the center. Is registered in the center, and the center uses the secret information s _A peculiar to the sender _A from the ID information I _A of the sender A using a positive number e as follows: s _A = exp (I _A : e) (Mod p ₂ −1), and similarly, from the ID information I _B of the receiver B, the secret key s _B unique to the sender B is calculated using a positive number e as follows: s _B = _{exp (I B: e) (} mod p 2 -1), was calculated at each was securely distribute the sender a and recipient B (however, exp (a: x) is raised to the power x of a value At this time, if the ID information is different, the corresponding secret key is also different, and the sender A has y _A and the public key corresponding to the secret key s _A is as follows: v _A = exp (a ₂ : s _A ) (mod p _2), set (y _a, v _a Registered as a public key of the sender A, receiver B y _B and, public key [number 21] corresponding to the secret key _{s B v B = exp (a} 2: s B) (mod p 2), of The set (y _B , v _B ) is registered as the public key of the receiver B, and the sender A uses the function g that is commonly held with the receiver B as the master key sharing process. From the secret key (x _A , s _A ) and the public key of the recipient B (y _B , v _B ), the master key K _{A, B} is calculated as follows: K _{A, B} = g (exp ( y _B : x _A ) (mod p ₁ ), exp
(v _B : s _A ) (mod p ₂ )), and the recipient B has his / her private key (x _B ,
s _B ) and the public key (y _A , v _A ) of the sender A, the master key K _{B, A} is expressed as follows: K _{B, A} = g (exp (y _A : x _B ) (mod p ₁ ), exp
(v _A : s _B ) (mod p ₂ )), and at this time, K _{A, B} = K _{B, A} holds, and in particular, the sender A receives an e-mail with many opportunities to send. To the sender, a set S _A of each master key between the receiver and the sender _A , or a secret key (x _A ,
s _A ) and the key generation function θ, the set of master keys S _A is encrypted using the key K (A) created by K (A) = θ (x _A , s _A ), [Equation 25] C (A) = E _A (K (A); S _A ), is stored, and when the master key is necessary for updating the data encryption key in the subsequent e-mail communication, the master key For those stored, the stored master key is used without creating a new master key from the other party's public key and own private key (provided that E _A (k;
(M) represents the result of encrypting the plaintext M with the key k by the private key encryption algorithm possessed by A). , The ciphertext E (k; which is obtained by encrypting the mail text P with the data encryption key k, using the secret key cryptosystem algorithm commonly held with the recipient B.
P) and a ciphertext E (K _{A, B} ; k) obtained by encrypting the data encryption key k with the master key K _{A, B,} to the mail recipient B,
Recipient B uses master key K _{B, A} to decrypt data encryption key k from E (K _{A, B} ; k), and uses data encryption key k to send email from E (k; P) An electronic mail encryption method characterized by decrypting a body P. 6. An electronic mail communication method for transmitting and receiving mail on a communication network, which is an electronic mail encryption method for encrypting a mail text sent from a sender A to a recipient B, as preparatory processing. , The center discloses the prime number p, the composite number n = p ₁ · p ₂ and the integer a, keeps the prime factorization of n and the positive number α secret, and the sender A sets 0 <x _A <p-
The integer x _A of 1 is randomly selected, and y _A = exp (a: x _A ) (mod p), is calculated, and the receiver B receives the integer x _{B of} 0 <x _B <p−1. Is randomly selected, and y _B = exp (a: x _B ) (mod p), is calculated, and sender A and receiver B register their ID information as center registration processing. The ID information of the sender _A , I _A + i _A (mod lcd (p
₁ −1, p ₂ −1)) is a quadratic residue, and I _A ≠ I _B , I _A + i _A (mod lcd (p ₁ -1, p ₂ −1)) ≠ I _B + i _B
A positive integer i _A is selected so that (mod lcd (p ₁ -1, p ₂ -1)), and the key (s _A , v _A ) unique to the sender A is given by s _A = exp _{(I A + i A: 1/2} ) (mod lcd (p 1 -
_{1, p 2 -1)),} v A = exp (α: s A) (mod n), calculated in, the ID information I _B similarly recipient B, the key unique to the recipient B (s _B, v and _B), [number 29] _{s B = exp (I B +} i B: 1/2) (mod lcd (p 1 -
1, p ₂ −1)), v _B = exp (α: s _B ) (mod n), and each is safely distributed to sender A and receiver B (however, exp (a: x) represents a value of a raised to the power of x). At this time, if the ID information is different, the corresponding key is also different, and the sender A registers (y _A , v _A ) as the public key of the sender A,
The receiver B registers (y _B , v _B ) as the public key of the receiver B, and as a master key sharing process, the sender A uses the function g held in common with the receiver B, From the private key (x _A , s _A ) and the public key (y _B , v _B ) of the receiver B, the master key K _{A, B} is expressed as follows: K _{A, B} = g ( exp (y _B : x _A ) (mod p), exp (v _B :
s _A ) (mod n)), and the recipient B has (x _B ,
s _B ) and the public key (y _A , v _A ) of the sender _A , the master key K _{B, A} is expressed as follows: K _{B, A} = g (exp (y _A : x _B ) ( mod p), exp (v _A :
s _B ) (mod n)), and at this time, K _{A, B} = K _{B, A} holds, in particular,
For the recipients who often send e-mails, the sender _A sets S _{A of} the master keys between the recipients and the sender _A , or the sender A's private key (x _A , s _A )
And the key generation function θ, the master key set S _A is encrypted using the key K (A) created by K (A) = θ (x _A , s _A ). C (A) = E _A (K (A); S _A ), and the master key is stored when the master key is needed for updating the data encryption key in the subsequent electronic mail communication. The master key that has been stored is used without creating a new master key from the other party's public key and its own private key (provided that E _A (k;
(M) represents the result of encrypting the plaintext M with the key k by the secret key encryption algorithm possessed by A), and the sender A randomly selects the data encryption key k as the encryption / decryption process of the mail body. , The ciphertext E (k; which is obtained by encrypting the mail text P with the data encryption key k, using the secret key cryptosystem algorithm commonly held with the recipient B.
P) and a ciphertext E (K _{A, B} ; k) obtained by encrypting the data encryption key k with the master key K _{A, B} , and the recipient B receives the master key K _{B. , A} is used to decrypt the data encryption key k from E (K _{A, B} ; k), and further the data encryption key k is used to decrypt the mail text P from E (k; P). And the email encryption method. 7. An electronic mail communication method for sending and receiving mail on a communication network, which is an electronic mail encryption method for encrypting a mail text sent from a sender A to a recipient B, as a preparation process. , The center is the prime number p and the composite number n = p ₁ · p
₂ and the integer a are disclosed, the prime factorization of n and the positive number α are kept secret, and the sender _A randomly selects an integer x _{A such} that 0 <x _A <p−1, and y _A = exp (a: x _A ) (mod p), the receiver _B randomly selects an integer x _{B such} that 0 <x _B <p−1, and y _B = exp (a: x _B ) (Mod p), and the sender A and the recipient B register their ID information in the center as a registration process to the center, and the center uses the sender A's ID information I _{A as} the sender. The key (s _A , v _A ) peculiar to _A is expressed by using a positive number e as follows: s _A = exp (I _A : e) (mod lcd (p ₁ -1, p _{2
-1)), v A = exp} (α: s A) (mod n), calculated in, the ID information I _B of the receiver B as well, the key unique to the recipient B (s _B, v _B ), and Equation 37] _{s B = exp (I B:} e) (mod lcd (p 1 -1, p 2
−1)), v _B = exp (α: s _B ) (mod n), and each is safely distributed to sender A and receiver B (where exp (a: x) is a Represents the value raised to the power of x). At this time, if the ID information is different, the corresponding secret key is also different, the sender A registers (y _A , v _A ) as the public key of the sender A, and the receiver B (Y _B , v _B ) is registered as the public key of the receiver B, and the master key sharing process is performed by the sender A.
Uses the function g that is commonly held with the recipient B,
From the private key (x _A , s _A ) and the public key (y _B , v _B ) of the receiver B, the master key K _{A, B} is calculated as follows: K _{A, B} = g ( exp (y _B : x _A ) (mod p), exp
(v _B : s _A ) (mod n)), the recipient B has his / her private key (x _B ,
s _B ) and the public key (y _A , v _A ) of the sender _A , the master key K _{B, A} is calculated as follows: K _{B, A} = g (exp (y _A : x _B ) ( mod p), exp
(v _A : s _B ) (mod n)), where K _{A, B} = K _{B, A} holds, in particular,
For the recipients who often send e-mails, the sender _A sets S _{A of} the master keys between the recipients and the sender _A , or the sender A's private key (x _A , s _A )
From the key generation function θ and the key generation function θ, the master key set S _A is encrypted using the key K (A) created by K (A) = θ (x _A , s _A ). C (A) = E _A (K (A); S _A ), and the master key is stored when the master key is needed for updating the data encryption key in the subsequent electronic mail communication. The master key that has been stored is used without creating a new master key from the other party's public key and its own private key (provided that E _A (k;
(M) represents the result of encrypting the plaintext M with the key k by the secret key encryption algorithm possessed by A), and the sender A randomly selects the data encryption key k as the encryption / decryption process of the mail body. , The ciphertext E (k; which is obtained by encrypting the mail text P with the data encryption key k, using the secret key cryptosystem algorithm commonly held with the recipient B.
P) and a ciphertext E (K _{A, B} ; k) obtained by encrypting the data encryption key k with the master key K _{A, B} , and the recipient B receives the master key K _{B. , A} is used to decrypt the data encryption key k from E (K _{A, B} ; k), and further the data encryption key k is used to decrypt the mail text P from E (k; P). And the email encryption method. 8. An electronic mail communication method for sending and receiving mail on a communication network, which is an electronic mail encryption method for encrypting a mail text sent by a sender A to a recipient B, as a preparation process. , The center is a prime number p ₁ , p ₂ and an integer a ₁ ,
The secret key cryptographic algorithm that publishes a ₂ and outputs the element of the remainder ring Z / (p ₂ −1) and the key r as secret information (the plaintext M is converted into the key k by the secret key cryptographic algorithm that is the secret information of the center) The result encrypted with is represented by E ₀ (k; M)),
The sender A randomly selects an integer x _{A such} that 0 <x _A <p ₁ −1, calculates y _A = exp (a ₁ : x _A ) (mod p ₁ ), and B is an integer x _{B such} that 0 <x _B <p ₁ −1
Is randomly selected, and y _B = exp (a ₁ : x _B ) (mod p ₁ ), is calculated (where exp (a: x) represents the value of a raised to the power x), and Sender A and receiver B are registered as
Registers its own ID information in the center, and the center obtains the secret key s _A unique to the sender _A from the ID information I _A of the sender A by: s _A = E ₀ (r; I _A ) , And similarly, from the ID information I _B of the receiver B, the secret key s _B unique to the sender B is calculated by s _B = E ₀ (r; I _B ), , Each of them is safely distributed to the sender A and the recipient B. At this time, if the ID information is different, the corresponding secret key is also different, and the sender A has y _A and the public key corresponding to the secret key s _A. The pair (y _A , v _A ) of v _A = exp (a ₂ : s _A ) (mod p ₂ ), is registered as the public key of the sender A, and the receiver B secrets y _B and secret. The public key corresponding to the key s _B is expressed as follows: v _B = exp (a ₂ : s _B ) (mod p ₂ ), the set (y _B , v _B ) is registered as the public key of the receiver B, As a key sharing process, sender A and recipient B In using a function g held in common, from their a secret key is a public key of the (x _A, s _A) and the receiver _{B (y B, v B)} , master key K _A, the _B, K _{A, B} = g (exp (y _B : x _A ) (mod p ₁ ), exp
(v _B : s _A ) (mod p ₂ )), and the recipient B has his / her private key (x _B ,
s _B ) and the public key (y _A , v _A ) of the sender A, the master key K _{B, A} is expressed as follows: K _{B, A} = g (exp (y _A : x _B ) (mod p ₁ ), exp
(v _A : s _B ) (mod p ₂ )), and at this time, K _{A, B} = K _{B, A} holds, and in particular,
For the recipients who often send e-mails, the sender _A sets S _{A of} the master keys between the recipients and the sender _A , or the sender A's private key (x _A , s _A )
From the key generation function θ and the key generation function θ, the master key set S _A is encrypted using the key K (A) created by K (A) = θ (x _A , s _A ). C (A) = E _A (K (A); S _A ), and the master key is stored when the master key is needed for updating the data encryption key in the subsequent electronic mail communication. The master key that has been stored is used without creating a new master key from the other party's public key and its own private key (provided that E _A (k;
(M) represents the result of encrypting the plaintext M with the key k by the secret key encryption algorithm possessed by A), and the sender A randomly selects the data encryption key k as the encryption / decryption process of the mail body. , The ciphertext E (k; which is obtained by encrypting the mail text P with the data encryption key k, using the secret key cryptosystem algorithm commonly held with the recipient B.
P) and a ciphertext E (K _{A, B} ; k) obtained by encrypting the data encryption key k with the master key K _{A, B} , and the recipient B receives the master key K _{B. , A} is used to decrypt the data encryption key k from E (K _{A, B} ; k), and further the data encryption key k is used to decrypt the mail text P from E (k; P). And the email encryption method. 9. An electronic mail communication method for sending and receiving mail on a communication network, which is an electronic mail encryption method for encrypting a mail text sent from a sender A to a recipient B, which is a preparation process. , The center discloses the prime number p, the composite number n = p ₁ · p ₂ and the integer a, and outputs the integer α and the element of the remainder ring Z / (N) and the secret key cryptographic algorithm and the key r as secret information ( However, N = lcd (p ₁
-1, p ₂ −1), and the result of encrypting the plaintext M with the key k by the secret key encryption algorithm which is the secret information of the center is represented by E ₀ (k; M)), and the sender A <X _A <p−1, an integer x _A is randomly selected, and y _A = exp (a: x _A ) (mod p), is calculated, and the receiver B sets 0 <x _B <p. ₁ -1 integer x _B
Is randomly selected, and y _B = exp (a: x _B ) (mod p), is calculated, and the sender A and the receiver B register their ID information as center registration processing. The center registers the key (s _A , v _A ) peculiar to the sender _A from the ID information I _A of the sender A by: s _A = E ₀ (r; I _A ), v _A = exp (α: s _A ) (mod n), and the key (s _B , v _B ) unique to the recipient B is similarly calculated from the ID information I _B of the recipient _{B by} = E ₀ (r; I _B ), v _B = exp (α: s _B ) (mod n), and each is safely distributed to the sender A and the receiver B. At this time, ID information The sender A registers (y _A , v _A ) as the sender A's public key, and the recipient B registers (y _B , v _B ) as the recipient B's public key. Register and send as master key sharing process , Using function g held in common between the receiver B, and their is a private key is a public key of the (x _A, s _A) and the receiver _{B (y B, v B)} , Using the master key K _{A, B} , K _{A, B} = g (exp (y _B : x _A ) (mod p), exp (v _B :
s _A ) (mod n)), and the recipient B has his or her private key (x _B ,
s _B ) and the public key (y _A , v _A ) of the sender _A , the master key K _{B, A} is calculated as follows: K _{B, A} = g (exp (y _A : x _B ) ( mod p), exp (v _A :
s _B ) (mod n)), and at this time, K _{A, B} = K _{B, A} holds, in particular,
For the recipients who often send e-mails, the sender _A sets S _{A of} the master keys between the recipients and the sender _A , or the sender A's private key (x _A , s _A )
From the key generation function θ and K (A) = θ (x _A , s _A ), the master key set S _A is encrypted using the key K (A) created by C (A) = E _A (K (A); S _A ), and the master key is stored when the master key is needed for updating the data encryption key in the subsequent electronic mail communication. The master key that has been stored is used without creating a new master key from the other party's public key and its own private key (provided that E _A (k;
(M) represents the result of encrypting the plaintext M with the key k by the secret key encryption algorithm possessed by A), and the sender A randomly selects the data encryption key k as the encryption / decryption process of the mail body. , The ciphertext E (k; which is obtained by encrypting the mail text P with the data encryption key k, using the secret key cryptosystem algorithm commonly held with the recipient B.
P) and a ciphertext E (K _{A, B} ; k) obtained by encrypting the data encryption key k with the master key K _{A, B,} to the mail recipient B,
Recipient B uses master key K _{B, A} to decrypt data encryption key k from E (K _{A, B} ; k), and uses data encryption key k to send email from E (k; P) An electronic mail encryption method characterized by decrypting a body P. 10. The function g commonly held between the sender A and the receiver B in the master key sharing process according to claim 4, 5, 6, 7, 8 or 9. G (x ₁ , x ₂ ) = h (φ (x ₁ , x ₂ )), where h is a hash function and φ is: φ: Z / (p) × Z / (n) → Z / (pn)
((X, x) → x), represents an isomorphism, Z / (m) represents a residue ring modulo the integer m, and x represents a residue class with x as the representative element in each residue ring. . ), An email encryption method that creates a master key. 11. A function g commonly held between a sender A and a receiver B in the master key sharing process according to claim 4, 5, 6, 7, 8 or 9. G (x ₁ , x ₂ ) = h (x ₁ * x ₂ ), where h is a hash function and x ₁ *
x ₂ represents an exclusive OR of x ₁ and x ₂ ), an electronic mail encryption method for creating a master key. 12. The sender A according to claim 3, 4, 5, 6, 7, 8, 9 or 11, wherein the sender A randomly selects a data encryption key k as the encryption / decryption processing of the mail text. A ciphertext E (k; which is obtained by encrypting the mail text P with the data encryption key k using the secret key cryptosystem algorithm commonly held with B
P) and the exclusive OR k * K _{A, B} of the data encryption key k and the master key K _{A, B} to the receiver _B, who receives the master key K _{B, A} and k * K. An electronic mail encryption system that decrypts the data encryption key k by taking the exclusive OR of _{A and B} , and further decrypts the mail text P from E (k; P) using the data encryption key k . 13. An electronic mail communication method for sending and receiving mail on a communication network, which is an electronic mail encryption method for encrypting a mail text sent by a sender A to a recipient B, which comprises: The electronic device according to any one of claims 1 to 12, wherein the receiver has a storage medium with an arithmetic function having each secret key built therein, and the sender uses the computer 1 connected to the communication network and the storage medium A of the sender. According to the mail encryption method, the encryption process is performed in the storage medium A, the transmission text is output to the computer 1, and the transmission text is transmitted from the computer 1 to the computer 2 used by the recipient through the communication line. An electronic mail characterized in that a transmission text sent from a person is output from the computer 2 to the storage medium B with a calculation function of the receiver, and the data encryption key and the mail text are decrypted in the storage medium B. Le encryption method.