Cryptography – Communication system using cryptography – Symmetric key cryptography
Reexamination Certificate
1997-04-17
2002-05-07
Decady, Albert (Department: 2132)
Cryptography
Communication system using cryptography
Symmetric key cryptography
C380S285000, C380S030000, C713S185000
Reexamination Certificate
active
06385318
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an encrypting method, a deciphering method and a certifying method, and more particularly to an encrypting method, a deciphering method and a certifying method adapted for use in various information services.
2. Related Background Art
Ciphers can be generally classified into (A) common ciphers and (B) public key ciphers.
The common cipher (A) employs one and the same key secretly owned by the transmitter and the receiver, and is also called a common key cipher or a secret key cipher.
In the public key cipher (B), the enciphering key and the deciphering key are mutually different, and the encrypting key is made publicly open while the deciphering key is held secret. In the following there will be given an explanation of the public key cipher, with respect to (a) features, (b) protocol, (c) a representative example, and (d) the RSA cipher as a specific example thereof.
(a) Features of the Public Key Cipher
1. Since the encrypting key and the deciphering key are different and the encrypting key can be made public, it is not necessary to deliver the encrypting key in secret, and thus the key delivery is made easier.
2. Since the encrypting key of each user is made public, each user is only required to maintain the deciphering key secret.
3. There can be realized a certifying function allowing the receiver to confirm that the transmitter of the transmitted message is not false and that the transmitted message has not been tampered with.
(b) Protocol of the Public Key Cipher
For a message M to be communicated, with a public encrypting key k
P
(hereinafter called public key) for defining an encrypting operation E(k
P
, M) and a secret deciphering key k
S
for defining a deciphering operation D(k
S
, M), the public key cipher algorithm in the first place satisfies the following two conditions:
(1) If the public key k
P
is known, the encrypting operation E(k
P
, M) can be easily calculated. Also if the secret key k
S
is known, the deciphering operation D(k
S
, M) can be easily calculated.
(2) In a case where the secret key k
S
is not known, even if the above-mentioned public key k
P
and the calculating procedure C=E(k
P
, M) for the above-mentioned enciphering operation E are known, the determination of the message M is difficult in consideration of the amount of calculation.
The secret communication can be realized by satisfying the following condition (3), in addition to the foregoing conditions (1) and (2):
(3) The encrypting operation E(k
P
, M) can be defined for all the messages (plain texts) M, and there stands a relation:
D
(
k
S
, E
(
k
P
, M
))=
M
Thus, since the k
P
is made public, anybody can execute the calculation of the encrypting operation E(k
P
, M), but the restoration of the message M through the deciphering operation D(k
S
, E(k
P
, M)) can only be made by the person who has the secret key k
S
. On the other hand, the certified communication can be realized by satisfying the following condition (4), in addition to the foregoing conditions (1) and (2):
(4) D(k
S
, M) can be defined for all the messages (plain texts) M, and there stands a relation:
E
(
k
P
, D
(
k
S
, M
))=
M
The deciphering operation D(k
S
, M) can be calculated only by the proper holder of the secret key k
S
, and, even if another person pretends to be such proper holder of the secret key k
S
by calculating D(k
S′
, M) with a false secret key k
S′
, the receiver can confirm that the received information is false since E(k
P
, D(k
S′
, M))≠M. Also if D(k
S
, M) is tampered with, there results E(k
P
, D(k
S
, M)′)≠M, so that the receiver can confirm that the received information is improper.
In the following there will be shown the protocols of secret communication, certified communication and secret communication with signature from a transmitter A to a receiver B by the public key cipher, wherein the transmitter A is assumed to have a secret key k
S
A
and a public key k
P
A
, and the receiver B is assumed to have a secret key k
S
B
and a public key k
P
B
.
Secret Communication
The secret communication of a message (plain text) M from the transmitter A to the receiver B is executed in the following procedure.
At first, in a step 1, the transmitter A encrypts the message M with the public key k
P
B
of the receiver B and sends the cipher text C to the receiver B, wherein:
C=E
(
k
P
B
, M
).
Then, in a step 2, the receiver B deciphers the received cipher text C with his own secret key k
S
B
to obtain the original plain text M by:
M=D
(
k
S
P
, C
).
Since the public key k
P
B
of the receiver B is made public to unspecified plural persons, the secret communication to the receiver B can be made not only by the transmitter A but also by any other person.
Certified Communication
The certified communication of a message (plain text) M from the transmitter A to the receiver B is executed in the following procedure.
At first, in a step 1, the transmitter A generates a transmission text S with his secret key k
S
A
of the receiver A and sends it to the receiver B, wherein:
S=D
(
k
S
A
, M
).
The transmission text S mentioned above is called a signature text, and the operation of obtaining such signature text S is called signing.
Then, in a step 2, the receiver B executes the restoring conversion of the signature text S with the public key k
P
A
of the transmitter A, thereby obtaining the original plain text M by:
M=E
(
k
P
A
, S
)
By the confirmation that the restored plain text M mentioned above constitutes a meaningful message, it is certified that the above-mentioned plain text M has certainly been transmitted from the transmitter A.
Since the public key of the transmitter A is made public to the unspecified plural persons, the signature text of the transmitter A can be certified not only by the receiver B but also any other person. Such certification is called digital signature.
Signed Secret Communication
The signed secret communication of a message (plain text) M from the transmitter A to the receiver B is executed in the following procedure.
At first, in a step 1, the transmitter A prepares a signed text S by signing the message M with the secret key k
S
A
of the transmitter A, wherein:
S=D
(
k
S
A
, M
).
Then the transmitter A encrypts the signed text S with the public key k
P
B
of the receiver B and sends the cipher text C to the receiver B, where:
C=E
(
k
P
B
, S
).
Then, in a step 2, the receiver B deciphers the cipher text C with the secret key k
S
B
of the receiver B to obtain the signed text S by:
S=D
(k
S
B
, C
).
Also the receiver B executes the restoring conversion of the signed text S with the public key k
P
A
of the transmitter A, thereby obtaining the original plain text M by:
M=E
(
k
P
A
, S
).
By the confirmation that the restored plain text M mentioned above constitutes a meaningful message, it is certified that the above-mentioned plain text M has certainly been transmitted from the transmitter A.
The order of applications of the functions in the foregoing steps of the signed secret communication may also be inverted. More specifically, in addition to the above-mentioned procedure:
C=E
(
k
P
B
, D
(
k
S
A
, M
)) Step 1
M=E
(
k
P
, D
(
k
S
B
, C
)) Step 2
the signed secret communication can also be realized by the following procedure:
C=D
(
k
S
A
, E
(
k
P
B
, M
)) Step 1
M=D
(
k
S
B
, E
(
k
P
A
, C
)) Step 2
(c) Specific Example of Public Key Cipher
As it is difficult to explain the individual cipher systems, in the following there will be explained the RSA cipher system as a specific example. The RSA cipher was invented by Rivest, Shamir and Adleman of MIT and was named after them.
The RSA cipher is presently one of the most promising public key ciphers. In the following, there will be explained the basic principle of the RSA cipher, in the order (1) key generation, (2) encrypting and (3) deciph
Callahan Paul E.
Canon Kabushiki Kaisha
De'cady Albert
Fitzpatrick ,Cella, Harper & Scinto
LandOfFree
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