Key Agreement Protocol

Tags

security networking

Is an implementation of perfect forward secrecy where both the client and recipient contribute to the creation of a secure session key.

The general algorithm for a sender/reciever pair of Alice/Bob is:

1. Alice & Bob declare and share two numbers between themselves.

   • A large prime number $p$, and
   • A number $g,1<g<p$.

   These don't need to be secret so they can be sent over an unsafe channel.

2. Both Alice and Bob now independently pick some random number $a$ and $b$ respectively.

3. Alice sends Bob $g^a(\text{mod}p)$

4. Bob sends Alice $g^b(\text{mod}p)$

5. Both Alice and Bob now calculate $S=(g^b\mathrm{mod}p{)}^a=(g^a\mathrm{mod}p{)}^b=g^{ab}\mathrm{mod}p$. $S$ is the secret session key used to secure correspondences between Alice and Bob.

This algorithm is secure so long as the discrete log problem is maintained.

Vulnerabilities

This protocol is still [see page 39, vulnerable] to man in the middle attacks.