The Diffie-Hellman key exchange protocol is a cryptographic technique used to securely generate a shared secret between two parties over an insecure communication channel.
Thank you for reading this post, don't forget to subscribe!- This shared key can then be used for encrypted communication using symmetric encryption algorithms.
- It was one of the first public-key protocols and was developed by Whitfield Diffie and Martin Hellman in 1976.
- Unlike traditional encryption algorithms that exchange encrypted data, Diffie-Hellman enables two parties to establish a shared secret key that can be used for symmetric encryption—even if they have never met before.
Key Concepts of Diffie Hellman Protocol:
- Public Parameters:
- A large prime number p and a primitive root modulo p (called generator) g are chosen and made public.
- Private Keys:
- Each party selects a private key: a for Alice and b for Bob. These are kept secret.
- Public Keys:
- Alice computes A = g^a mod p and sends it to Bob.
- Bob computes B = g^b mod p and sends it to Alice.
- Shared Secret:
- Alice computes the shared secret: K = B^a mod p.
- Bob computes the shared secret: K = A^b mod p.
- Both arrive at the same value because (g^b)^a mod p = (g^a)^b mod p.
How the Diffie-Hellman Protocol Works:
- Agreement on Public Parameters:
- Choose a prime number p and a primitive root g.
- These values are public and can be known by everyone.
- Private Key Selection:
- Alice chooses a private key a (a secret number).
- Bob chooses a private key b (another secret number).
- Public Key Computation:
- Alice computes her public key as A = g^a mod p.
- Bob computes his public key as B = g^b mod p.
- Exchange of Public Keys:
- Alice sends A to Bob.
- Bob sends B to Alice.
- Computation of Shared Secret:
- Alice computes K = B^a mod p.
- Bob computes K = A^b mod p.
- Both values of K are equal: K = g^(ab) mod p.
Security of Diffie-Hellman:
- The protocol is secure because while g, p, A, and B are public, it is computationally infeasible to determine the private keys a or b or compute the shared key K from public information due to the Discrete Logarithm Problem.
Limitations:
- Vulnerable to Man-in-the-Middle (MITM) Attacks unless combined with authentication methods like digital signatures.
- It does not provide authentication—only secure key agreement.
Use Cases:
- Used in protocols like SSL/TLS, SSH, and IPsec for secure key exchange.
- Establishes session keys for secure communication channels.