Electrical computers and digital processing systems: support – Multiple computer communication using cryptography – Particular communication authentication technique
Reexamination Certificate
1999-02-17
2003-02-18
Smithers, Matthew (Department: 2132)
Electrical computers and digital processing systems: support
Multiple computer communication using cryptography
Particular communication authentication technique
C713S168000
Reexamination Certificate
active
06523115
ABSTRACT:
This application is based on application No. 10-036029 filed in Japan, the content of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to cryptography techniques, especially a correctness verification technique for decrypted data, that are used in secret communication.
2. Description of the Related Art
Cryptographic techniques are used as one way to transmit important information securely without the information reaching third parties. Such techniques are used, for example, by e-mail (electronic mail) encryption systems that have broadcast functions, in which e-mail messages are encrypted and sent to a plurality of recipients.
The following describes, with reference to
FIG. 1
, the operation of an example e-mail encryption system that has a broadcast function,
Mail sender
1501
encrypts message
1531
written in a plaintext form using public key
1521
to generate a ciphertext. The generated ciphertext is sent to mail recipients
1502
-
1505
in mail recipient group
1541
. Mail recipients
1502
-
1505
then decrypt the received ciphertext using secret key
1523
to obtain the message in the plaintext form.
Mail sender
1501
also encrypts another message
1532
using another public key
1522
to generate another ciphertext. The generated ciphertext is sent to mail recipients
1505
-
1507
in another mail recipient group
1542
. Mail recipients
1505
-
1507
then decrypt the received ciphertext using another secret key
1524
to obtain the message in the plaintext form.
Encrypted communication is generally achieved thorough the following steps: (a) encryption of a plaintext, (b) transmission of a ciphertext and other data, (c) decryption of the ciphertext, and (d) verification of a decrypted plaintext.
In the above e-mail encryption system, a message addressed to a plurality of recipients is encrypted once to generate a single ciphertext that is broadcast to the recipients. However, should mail recipient
1505
mistakenly use secret key
1524
, instead of secret key
1523
, to decrypt a message that has been encrypted with public key
1521
, the encrypted message will not be correctly decrypted. In other cases, errors during transmission can result in a partial loss of the ciphertext or in mistransmission of its content. Here also, the encrypted message will not be correctly decrypted. In this way, a mail recipient having two or more secret keys can't know, whether a failure to correctly decrypt a ciphertext is due to the use of the wrong secret key or an error during transmission.
SUMMARY OF THE INVENTION
In view of the above problem, the present invention aims to provide an encryption device, a decryption device, an encryption method, a decryption method, a cryptography system, a computer-readable recording medium that stores an encryption program, and a computer-readable recording medium that stores a decryption program, wherein verification data is generated with which it can be verified, when a ciphertext is not correctly decrypted, whether encrypted communication has been performed without errors up to the step of “(b) transmission of a ciphertext and other data” in the steps (a)-(d) for encrypted communication that are described above.
The encryption device encrypts a plaintext to generate a ciphertext and outputs the ciphertext to a decryption device, the encryption device including: a plaintext storage unit for storing a plaintext; a first generating unit for generating first verification data by performing a first message digest algorithm for the plaintext, the plaintext having been read from the plaintext storage unit; an encryption unit for encrypting the plaintext using a predetermined encryption algorithm to generate a ciphertext, the plaintext having been read from the plaintext storage unit; a second generating unit for generating second verification data by performing a second message digest algorithm for a combination of the first verification data and the ciphertext; and a data outputting unit for outputting the ciphertext, the first verification data, and the second verification data to the decryption device.
With the construction of this encryption device, the first verification data is generated by performing the first message digest algorithm for the plaintext, and the second verification data is generated by performing the second message digest algorithm for the combination of the first verification data and the ciphertext. As a result, the encryption device can send to the decryption device information with which it can be verified, when the ciphertext is not correctly decrypted by the decryption device, whether encrypted communication has been performed without errors up to the step of “(b) transmission of a ciphertext and other data” in the steps (a)-(d) for encrypted communication.
Here, the first generating Unit may include a digital signature generation algorithm in addition to the first message digest algorithm and generate first verification data by performing the first message digest algorithm for the plaintext to produce a message digest and then performing the digital signature generation algorithm for the message digest.
With the construction of the above encryption device, the first verification data is generated by performing the digital signature generation for the message digest generated by performing the first message digest algorithm, and the second verification data is generated by performing the second message digest algorithm for the combination of the first verification data and the ciphertext. As a result, this encryption device has an advantage of being able to send to the decryption device information that the decryption device can use to confirm the sender of a message, in addition to the advantage, which was described earlier, of being able to send verification information
Here, the second generating unit may have a digital signature generation algorithm in addition to the second message digest algorithm and generate second verification data by performing the second message digest algorithm for the combination of the first verification data and the ciphertext to produce a message digest and then performing the digital signature generation algorithm for the message digest.
With the construction of this encryption device, the first verification data is generated by performing the first message digest algorithm, and the second verification data is generated by performing the digital signature generation for the message digest generated by performing the second message digest algorithm for the combination of the first verification data and the ciphertext. As a result, this encryption device has an advantage of being able to send information that the decryption device can use to confirm the sender of a message, in addition to the advantage, which was described earlier, of being able to send verification information.
The encryption device encrypts a plaintext to generate a ciphertext and outputs the ciphertext to a decryption device, the encryption device including: a plaintext storage unit for storing a plaintext; a first generating unit for generating first verification data by performing a first message digest algorithm for the plaintext, the plaintext having been read from the plaintext storage unit; an encryption unit for encrypting the plaintext using a predetermined encryption algorithm to generate a ciphertext, the plaintext having been read from the plaintext storage unit; a second generating unit for generating second verification data by performing a second message digest algorithm for the ciphertext; and a data outputting unit for outputting the ciphertext, the first verification data, and the second verification data to the decryption device.
For the construction of this encryption device, the first verification data is generated by performing the first message digest algorithm for the plaintext, and the second verification data is generated by performing the second message digest algorithm for the ciphertext. Since data sizes of the
Ono Takatoshi
Saijo Takeshi
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