24 Needham-Schroeder Key Establishment Protocol

$$\begin{array}{rl}1.& A\to S:A,B,N_a\\ 2.& S\to A:\{N_a,B,K_{ab},\{K_{ab},A{\}}_{K_{bs}}{\}}_{K_{as}}\\ 3.& A\to B:\{K_{ab},A{\}}_{K_{bs}}\\ 4.& B\to A:\{N_b{\}}_{K_{ab}}\\ 5.& A\to B:\{N_b+1{\}}_{K_{ab}}\end{array}$$

How It Works :

• $S$ is a Trusted Third Party that helps establish a shared key $K_{ab}$.
• $S$ encrypts the session key separately for Alice and Bob.
• Alice and Bob mutually authenticate using nonces $N_a$ and $N_b$.
• Ensures that only Alice and Bob know $K_{ab}$.