Modified ID-Based Public key Cryptosystem using Double Discrete Logarithm Problem

— In 1984, Shamir [1] introduced the concept of an identity-based cryptosystem. In this system, each user needs to visit a key authentication center (KAC) and identify him self before joining a communication network. Once a user is accepted, the KAC will provide him with a secret key. In this way, if a user wants to communicate with others, he only needs to know the “identity” of his communication partner and the public key of the KAC. There is no public file required in this system. However, Shamir did not succeed in constructing an identity based cryptosystem, but only in constructing an identity-based signature scheme. Meshram and Agrawal [4] have proposed an id - based cryptosystem based on double discrete logarithm problem which uses the public key cryptosystem based on double discrete logarithm problem. In this paper, we propose the modification in an id based cryptosystem based on the double discrete logarithm problem and we consider the security against a conspiracy of some entities in the proposed system and show the possibility of establishing a more secure system.


INTRODUCTION
In a network environment, secret session key needs to be shared between two users to establish a secret communication.While the number of users in the network is increasing, key distribution will become a serious problem.In 1976, Diffie and Hellman [6] introduced the concept of the public key distribution system (PKDS).In the PKDS, each user needs to select a secret key and compute a corresponding public key stored in the public directory.The common secrete session key, which will be shared between two users can then be determined by either user, based on his own secret key and the partner"s public key.Although the PKDS provides an elegant way to solve the key distribution problem, the major concern is the authentication of the public keys used in the cryptographic algorithm.
Many attempts have been made to deal with the public key authentication issue.Kohnfelder [7] used the RSA digital signature scheme to provide public key certification.His system involves two kinds of public key cryptography: one is in modular p, where p is a large prime number; the other is in modular n, where n = p q, and p and q are large primes.Blom [11] proposed a symmetric key generation system (SKGS based on secret sharing schemes.The problems of SKGS however, are the difficulty of choosing a suitable threshold value and the requirement of large memory space for storing the secret shadow of each user. In 1984, Shamir [1] introduced the concept of an identity-In this system; each user needs to visit a based cryptosystem.Key authentication center (KAC) and identify him self before joining the network.Once a user is accepted, the KAC will provide him with a secret key.In this way, a user needs only to know the "identity" of his communication partner and the public key of the KAC, together with his secret key, to communicate with others.There is no public file required in this system.However, Shamir did not succeed in constructing an identity-based cryptosystem, but only in constructing an identity-based signature scheme.Since then, much research has been devoted, especially in Japan, to various kinds of IDbased cryptographic schemes.Okamoto et al. [10] proposed an identity-based key distribution system in 1988, and later, Ohta [12] extended their scheme for user identification.These schemes use the RSA public key cryptosystem [18] for operations in modular n, where n is a product of two large primes, and the security of these schemes is based on the computational difficulty of factoring this large composite number n. Tsujii and Itoh [2] have proposed an ID-based cryptosystem based on the discrete logarithm problem with single discrete exponent which uses the ElGamal public key cryptosystem.Meshram and Agrawal [5] have proposed an ID-based cryptosystem based on the integer factoring and double discrete logarithm problem which uses the public key cryptosystem based on integer factoring and double discrete logarithm problem.Meshram and Agrawal [4] have also proposed an ID-based cryptosystem based on double discrete logarithm problem which uses the public key cryptosystem based on double discrete logarithm problem.Now we Modified this cryptosystem for discrete logarithm problem with distinct double discrete exponent because we face the problem of solving double and triple distinct discrete logarithm problem at the same time in the multiplicative group of finite fields as compared to the other public key cryptosystem where we face the difficulty of solving the traditional discrete logarithm problem in the common group.
In this paper , we present modification in an ID based cryptosystem based on the double discrete logarithm problem with distinct discrete exponent (the basic idea of the proposed http://ijacsa.thesai.org/system comes on the public key cryptosystem based on double discrete logarithm problem) here we describe further considerations such as the security of the system, the identification for senders.etc. our scheme does not require any interactive preliminary communications in each message transmission and any assumption except the intractability of the discrete logarithm problem.(thisassumption seems to be quite reasonable)thus the proposed scheme is a concrete example of an ID -based cryptosystem which satisfies Shamir"s original concept [1] in a strict sense.

A. Implementation of the ID -Based Cryptosystem
Preparation for the center and each entity Step 1.Each entity generates a k-dimensional binary vector for his ID .We denote entity A"s ID by Each entity registers his ID with the center, and the center stores it in a public file.
Step 2.: The center generate two random prime number p and q and compute pq N  is the numbers of bits of N .
Step 3. Center's secrete information: -The center chooses an arbitrary large prime number p and q and compute pq N  and also generated n-dimensional vector a and m- dimensional vector b over Where I and J are n-dimensional binary vector and stores it as the centers secret information.The condition of equation ( 5) is necessary to avoid the accidental coincidence of some entities secrete key.A simple ways to generate the vectors a and b is to use Merkle and Hellmans scheme [19].


Remark 1: it is clear that the vector and defined by ( 9) satisfies ( 4)-(5) the above scheme is one method of generating an n and m dimensional vectors and satisfies (4)- (5).In this paper, we adopt the above scheme.However, another method might be possible.
Step 5: The center also chooses an arbitrary integer t such that    1) ( , ,..... )(1 ) Since j D and k D are one to one system.
Step 5 Center public information: The center chooses two arbitrary generators  and  of * ) ( N Z  and computes n- dimensional vector h using generator  & m-dimensional http://ijacsa.thesai.org/vector g using generator  corresponding to the vector a and b .

 
The center informs each entity   , , , , N h g  as public information.
Step 6.Each entity secrete key: Entity  The security of the proposed ID based cryptosystem is based on the intractability of the discrete logarithm problem.It is very difficult to give formal proofs for the security of a http://ijacsa.thesai.org/cryptosystem, in the following; we analyze some possible attacks against the above schemes and show that the security of these attacks is based on the DLP assumption. 1.
An intruder should solve a discrete logarithm problem twice to obtain the private key given the public as following: In this encryption the public key is given by  

IV. CONCLUSION
In this paper present the modification in an ID-based cryptosystem based on double discrete logarithm problem with distinct discrete exponents in the multiplicative group of finite fields.The proposed scheme satisfies Shamir"s original concepts in a strict sense, i.e. it does not require any interactive preliminary communications in each data transmission and has no assumption that tamper free modules are available.This kind of scheme definitely provides a new scheme with a longer and higher level of security than that based on a double discrete logarithm problem with distinct discrete exponents.The proposed scheme also requires minimal operations in encryption and decryption algorithms and thus makes it is very efficient.The present paper provides the special result from the security point of view, because we face the problem of solving double and triple distinct discrete logarithm problem at the same time in the multiplicative group of finite fields as compared to the other public key cryptosystem, where we face the difficulty of solving the traditional discrete logarithm problem in the common groups.
, where x   also denote the floor function which implies the largest integer smaller than compute x .

s
are given by inner product of a and b (the centre"a : n -dimensional vector and b m-dimensional vector {see (8)-(9)} Center public information h : n -dimensional vector & g m-dimensional vector {see eqn.(12-13)} p and q :large prime numbers, e : random integers , two generator  and  of * ) ( N Z  .Entity s A' secrete keys a s and b s = entity s A' public information = A ID ,k-dimensional vector.C.Protocol of the proposed cryptosystemWithout loss of generality suppose that entity B wishes to send message to entity A.

1  and 2 
in Public key cryptosystem based on double discrete logarithm problem.be entity B"s message to be transmitted.Entity B select two random integer u and v such that

 2 .s
This information is equivalent to computing the discrete logarithm problem over multiplicative cyclic group* ) ( N Z and corresponding secrete key a s and b s will never be revealed to the public.An attacker might try to impersonate user A however trying to obtain w from  and  is equivalent to compute the discrete logarithm problem.