Session Keys

• Diffie-Hellman exponential key exchange allows a secret key to be generated by public exchange of information between two people
• A and B exchange two integers n, m with 1 < n < m, m large
• Each decides on a secret number a and b
• A transmits the number n^a (mod m)
• B transmits the number n^b (mod m)
• Each can then calculate (n^a)^b = (n^b)^a = n^(a*b) (mod m)
• This is the private key, for use in, say, DES
• It is hard to calculate this from the public information
• This does not authenticate the other user