Diffie-Hellman
Definition
Key exchange protocol by Diffie, Hellman and Merkle (1976) that lets two parties agree on a shared secret over a public channel. Security rests on the discrete logarithm problem.
- Originators
- Whitfield Diffie, Martin Hellman, Ralph Merkle
- Year
- 1976
- Function
- Key exchange over a public channel
- Security basis
- Discrete logarithm problem
Common questions
Why is Diffie-Hellman described as a key exchange rather than an encryption method?+
Diffie-Hellman does not encrypt messages itself; it only lets two parties who have never met derive a shared secret value over a channel an eavesdropper can observe, and that shared secret is then typically used as a key for a separate symmetric encryption algorithm to protect the actual data.
What is the practical weakness Diffie-Hellman is vulnerable to without additional authentication?+
Basic Diffie-Hellman does not verify the identity of either party, so it is vulnerable to a man-in-the-middle attack where an intermediary establishes separate shared secrets with each side and relays traffic while decrypting it, which is why real-world protocols combine it with digital signatures or certificates for authentication.
Related terms
- Brute Force
- Exhaustive search over the key space. The benchmark every other attack must beat. AES-128 brute force is computationally infeasible on classical hardware;...
- Cryptanalysis
- The study of breaking cryptographic systems. Complement to cryptography. Goals include recovering the key, recovering the plaintext, or distinguishing a cipher's output...
- Padding Oracle
- An attack that uses an error-message distinction (or a timing distinction) between 'padding invalid' and 'authentication invalid' to recover plaintext from CBC-mode...
- Perfect Forward Secrecy (PFS)
- Property of a key exchange where compromise of a long-term private key does not let the attacker decrypt past sessions. Achieved by...
- Post-Quantum Cryptography (PQC)
- Algorithms designed to resist attacks by sufficiently capable quantum computers. NIST standardised CRYSTALS-Kyber as ML-KEM and CRYSTALS-Dilithium as ML-DSA in 2024.
- Side-Channel Attack
- Attack that exploits information leaked by a correct implementation: timing, power consumption, electromagnetic emanation, cache state, acoustic noise. Independent of the algebra...