Cryptography – Key management
Reexamination Certificate
1999-09-16
2004-09-07
Morse, Gregory (Department: 2134)
Cryptography
Key management
C380S279000
Reexamination Certificate
active
06788788
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a cryptographic communication method, encryption method, and cryptographic communication system that afford a high level of safety in encryption and communication of information by utilizing information encrypted in such a manner not to be understood by anyone but the intended persons.
2. Description of the Related Art
In today's society, sometimes called the advanced information society, documents and graphic information that are important for business are transmitted and processed in the form of electronic information, using a computer network as a platform. By its nature, this electronic information is easy to duplicate, making it hard to tell an original from a copy, and information security has therefore become an important issue. In particular, building a computer network that satisfies the requirements of “shared computer resources,” “multi-access,” and “broad area networking” is essential to the establishment of a true highly sophisticated information society, but this includes factors that are in conflict with the goal of information security between involved parties. Encryption technology, which has been used primarily for military and diplomatic purposes in past human history, is attracting attention as an effective means for resolving this conflict.
Cryptography deals with exchanging information in such a way that the meaning thereof cannot be understood by anyone but the intended recipient. In cryptography, converting original text that can be understood by anyone (plaintext) into text that is meaningless to a third party (ciphertext) is called encryption, while returning the ciphertext to the original plaintext is called decryption, and the overall process of this encryption and decryption is called a cryptosystem. Secret encryption and decryption keys are used in the process of encryption and decryption, respectively. Since a secret decryption key is necessary during decryption, only someone who knows this decryption key can decrypt a ciphertext, so encryption allows the confidentiality of information to be preserved.
An encryption key may be the same as or different from a decryption key. A cryptosystem in which the two keys are the same is called a common or shared key cryptosystem, and the DES (Data Encryption Standards) employed by the Bureau of Standards of the US Department of Commerce is a typical example of this. Public key cryptosystems are proposed as an example of a cryptosystem in which the two keys are different. With a public key cryptosystem, each user (entity) that utilizes the cryptosystem generates a pair of keys, i.e., an encryption key and a decryption key, discloses the encryption key on a public key list, and keeps just the decryption key secret. An advantage of a public key cryptosystem is that the paired encryption key and decryption key are different and a one-way function is utilized, which makes it impossible for someone to deduce the decryption key from the encryption key.
A public key cryptosystem is a revolutionary cryptosystem in which the encryption key is disclosed, and satisfies the above three requirements necessary for the establishment of a sophisticated information society. A great deal of research has gone into these systems in an effort to utilize them in such fields as information communication technology, and a typical public key cryptosystem that has been proposed is the RSA cryptosystem. This RSA cryptosystem makes use of the difficulty of factoring large prime numbers using one-way functions. There are also public key cryptosystems that make use of the difficulty of solving discrete logarithm problems.
There is also a cryptosystem that makes use of personal ID (identity) information, such as the name or address of each entity. With this cryptosystem, a shared encryption key is generated between the sender and recipient on the basis of ID information. Cryptographic methods based on this ID information include (1) those that require pre-communication between the sender and recipient prior to the transmission of the ciphertext, and (2) those that do not require pre-communication between the sender and recipient prior to the transmission of the ciphertext. Since the second type of method does not require any pre-communication, it is very convenient for an entity, and is expected to become a mainstay of cryptosystems in the future.
The second type of scheme is called ID-NIKS (an ID-based non-interactive key sharing scheme), in which an encryption key is shared, without any pre-communication being performed, by using ID information about the communicating party. ID-NIKS does not require that a public key and secret key be exchanged between the sender and recipient, nor does it require a key list or service by a third party, allowing secure communication between any entities.
FIG. 9
of the accompanying drawings is a diagram showing the principle behind this ID-NIKS system The existence of a center that can be trusted is assumed, and a common key generation system is built around this center. In this diagram, ID information such as the name, address, and telephone number of an entity X, which is personal information of entity X, is expressed as h(ID
x
) using a hash function h(·). The center calculates secret information S
xi
as follows on the basis of public center information {PC
i
}, secret center information (SC
i
}, and entity X ID information h(ID
x
), and secretly distributes this calculated information to entity X.
S
xi
=F
i
({
SC
i
}, {PC
i
}, h
(
ID
x
))
For communication with any other entity Y, entity X uses his own secret information {S
Xi
}, public center information {PC
i
}, and the other entity Y ID information h(ID
Y
) to generate a common key K
XY
for encryption and decryption as follows.
K
XY
=f
({
S
Xi
}, {PC
i
}, h
(
ID
Y
))
Similarly, entity Y also generates a common key K
YX
for entity X. As long as the relationship K
XY
=K
YX
holds true, then these keys K
XY
and K
YX
can be used as an encryption key and decryption key between entities X and Y.
With the public key cryptosystems discussed above, in the case of an RSA cryptosystem, for instance, this public key is over ten times as long as current telephone numbers, and is therefore far from simple. In contrast, with an ID-NIKS system, if each set of ID information is registered in the form of a roster, then a public key can be generated between itself and any other entity by referring to this roster. Therefore, if the ID-NIKS illustrated in
FIG. 9
could be safely implemented, it would be possible to construct a convenient cryptosystem over a computer network to which many entities subscribe. It is for this reason that an ID-NIKS system is expected to be at the forefront of future cryptosystems.
It is preferable for ID-NIKS, in which common keys that serve as encryption and decryption keys are mutually shared by using the ID information of the communicating parties without any pre-communication being performed, to be sufficiently secure against attack involving a collusion of a plurality of entities, for example. However, this ID-NIKS has the problem that the secret parameters of the center can be revealed if enough people (entities) are in collusion since such attack method has been studied. Whether a cryptologically safe ID-NIKS system can be constructed is an important question for a sophisticated information society, and a search is underway for a more ideal encryption system.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a novel cryptographic communication method and cryptographic communication system involving ID-NIKS system, with which secret key generation functions and key sharing functions are not separable, key sharing is probabilistically possible, and high degree of security is realized.
According to the first aspect of the present invention, there is provided a cryptographic communication method for communication of inf
Kasahara Masao
Murakami Yasuyuki
Brown Christopher J
Hogan & Hartson LLP
Morse Gregory
Murata Kikai Kabushiki Kaisha
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