Non-repudiation
Proof that an action cannot be denied later
Non-repudiation is the assurance that a sender cannot later deny performing an action, such as sending a message or signing a document.
If Person A signs a document with their private key, they can't later claim "I never signed that" — the signature is mathematical proof tied only to them.
How It's Achieved
One way non-repudiation is achieved is through digital signatures, which rely on three things working together:
- Private key → proves the sender's identity.
- Hashing → the recipient recalculates the hash and compares it against the signed hash to verify integrity.
- Public key verification → confirms the signature is valid.
How a Digital Signature Provides Non-repudiation
Private Key = Identity
Only the sender holds their private key. Signing something with it is like a unique, unforgeable fingerprint tied directly to them.
Hashing = Integrity
The recipient recalculates the hash and compares it against the signed hash. If even one character changes, the hash changes completely — revealing tampering.
Digital signatures combine a private key (proves who signed it) with hashing (proves it wasn't altered) — together they create non-repudiation.