Diffie-Hellman Key Exchange Algorithm

• The Diffie-Hellman algorithm is widely known as key exchange algorithm or key agreement algorithm developed by Whitfield Diffie and Martin Hellman in 1976. Diffie-Hellman is used to generate same (symmetric) private cryptographic key at sender as well as a receiver and so that there is no need to transfer this key from sender to receiver.
• Remember that Diffie-Hellman algorithm is used only for a key agreement, not for encryption or decryption of the message. If sender and receiver want to communicate with each other they first agree on the same key generated by a Diffie-Hellman algorithm, later on, they can use this key for encryption or decryption.

Steps for Diffie-Hellman Algorithm:

1. If A wants to communicate with B, they first must agree on two large prime numbers p and q (q < p).
2. A selects another secret large random integer number X_A, and calculate Y_A such that 
Y_A = q^X_Amod p
3. A sends this Y_A to B.
4. B independently selects another secret large random integer number X_B, and calculate Y_B such that,
Y_B = q^X_Bmod p
5. B sends this number Y_B to A.
6. Now, A is calculating his secret key by using,
A_K = (Y_B)^X_Amod p
7. Similarly, B calculates his secret key Y_K by using,
B_K = (Y_A)^X_Bmod p
8. If A_K = B_K, then A and B can agree for future communication called as key agreement algorithm.

Fig. Diffie-Hellman Key exchange algorithm