Needham-Schroeder Public-Key Protocol (NSPK)

What:

Another key-exchange protocol developed in 1978.

Given:

• Alice (initiator)
• Bob (receiver)
• $K_A$ : Alice’s public key
• $K_B$ : Bob’s public key
• Message $\{M{\}}_K$ is encrypted with $K$

Protocol Steps:

1. Alice sends Bob an encrypted message (containing a secret unique number) with his public key:
   1. $\{A,N_A{\}}_{K_B}$
2. Bob responds back to the message, with his own secret unique number. He encrypts the message with Alice’s public key:
   1. $\{N_A,N_B{\}}_{K_A}$ (In the safer NSL version he also sends $\{N_A,N_B,B{\}}_{K_A}$)
   2. He sends Alice’s number to confirm he got it
3. Alice responds with $\{N_B{\}}_{K_B}$ to confirm she’s received the message.
4. After all this, they can confirm they’re both talking with each-other securely.

Problem:

• It’s susceptible to a Man-In-The-Middle attacker. Imagine inserting yourself in between Alice and Bob. Since there’s no authentication, you could have a secure chat between both of them, but simply act as proxy. Boom!