# Diffie-Hellman, Seo And Tseng Protocol Analysis

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Chapter-3
Diffie-Hellman, Seo and Tseng Protocol
3.1 Introduction
Whitefield Diffie and Martin Hellman in 1976 had proposed key exchange or agreement algorithm. In this algorithm, the participants i.e. the sender and receiver have to agree on a symmetric key i.e. the same key or single key can be used for encryption as well as decryption. The key is only used for key agreement purpose, not for enciphering and deciphering the message. Once the agreement of key has been taken place between participants on key, then the key can be used for encryption as well as decryption. This is the main fundamental of key exchange for secure communication over insecure medium.
3.1.1 Steps of the Algorithm
Let us assume that if A (sender) & B (receiver) want to authenticate to each other then they have to agree upon a key to be used for enciphering & deciphering the messages. So the steps are as:-
In first step, A (sender) & B (receiver) have to agree on the two large prime numbers i.e. n & g. It is not necessary that these two prime numbers have to be secret. These prime numbers are public. They can be used on insecure medium.
• Another random number i.e. select by A (sender), and calculate A such that
C=gx
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Then the key (Key1Q)Q^-1 mod n by applying Q-1 by is calculated by sender A and it must be equal to Key1 because Q.Q-1 = 1 mod (n-1). Therefore, although so we can say a wrong session key is obtained by sender and intruder cannot compute the same wrong session key, and the sender A have to trust on it. So it is proved that verification of the session key cannot be achieved using this protocol. 3.3 Tseng’s Modified Key Agreement