20 Needham-Schroeder-Lowe Public Key Protocol

Protocol Steps

$$\begin{array}{rl}1.& A\to B:E_B(N_a,A)\\ 2.& B\to A:E_A(N_a,N_b,B)\\ 3.& A\to B:E_B(N_b)\\ 4.& B\to A:\{M{\}}_{key(N_a,N_b)}\end{array}$$

Why is this secure ?

• A and B mutually authenticate each other.
• The attack where Elvis intercepts and impersonates Bob is no longer possible
• Mutual Authentication - Alice and Bob both verify each other.
• Protection Against MITM - Attackers cannot impersonate Bob.

Question

Can they read the message encrypted with key ($N_a$, $N_b$) ?

Ans : Secure against the standard attacker : Intercept, replay, delete, alter.

Question

What about governments ?

Ans : After the protocol runs, government can legally force people to handover their private keys.