5.1 INTRODUCTION
In the real world , key management is the hardest part of cryptology .Cryptanalysts often attack cipher system through their key management .One of the problems facing symmetric cipher systems is key distribution Keys must be distributed in secret , since knowledge of the key gives knowledge of the massage .From the other side, keys are shared by pairs , and could work well in small networks , but raised tremendously as network grows up , since every pair of users must exchange keys .
The total numbers of key exchanges required in n-person network is n(n-1)/2 .In six person network , 15 key exchange are required , in 1000 – person network ,nearly 500,000 key exchanges are requied . Public key system are invented to over come the problems of key distribution faced by symmetric (one - key) cipher systems .We summarize the mentioned problems as follows :
· First key must be distributed in secret .
· Second : If key is compromised , then stranger can share the system as member .
· Third : number of key increases rapidly as users increased .
Public key cryptography based on one-way hash function (trap-door function),that is depend on two keys public key used for encryption process and it is available for every one in the network, the second key is the secret key which is used for decryption processes , and every person in the network has his own secret keys .In another meaning users have their own secret keys and share others with public keys