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Legal term
In law, non repudiation is a situation where a statement's author cannot successfully dispute its authorship or the validity of an associated contract. The term is often seen in a legal setting when the authenticity of a signature is being challenged. In such an instance, the authenticity is being "repudiated". For example, Mallory buys a cell phone for $100, writes a paper cheque as payment, and signs the cheque with a pen. Later, she finds that she can't afford it, and claims that the cheque is a forgery. The signature guarantees that only Mallory could have signed the cheque, and so Mallory's bank must pay the cheque. This is non repudiation; Mallory cannot repudiate the cheque. In practice, pen and paper signatures aren't hard to forge, but digital signatures can be very hard to break.

In general, non repudiation involves associating actions or changes with a unique individual. For example, a secure area may use a key card access system where non repudiation would be violated if key cards were shared or if lost and stolen cards were not immediately reported. Similarly, the owner of a computer account must not allow others to use it, such as by giving away their password, and a policy should be implemented to enforce this.

To mitigate the risk of people repudiating their own signatures, the standard approach is to involve a trusted third party. The two most common TTPs are forensic analysts and notaries. A forensic analyst specializing in handwriting can compare some signature to a known valid signature and assess its legitimacy. A notary is a witness who verifies an individual's identity by checking other credentials and affixing their certification that the person signing is who they claim to be. A notary provides the extra benefit of maintaining independent logs of their transactions, complete with the types of credentials checked, and another signature that can be verified by the forensic analyst. This double security makes notaries the preferred form of verification. For digital information, the most commonly employed TTP is a certificate authority, which issues public key certificates. A public key certificate can be used by anyone to verify digital signatures without a shared secret between the signer and the verifier. The role of the certificate authority is to authoritatively state to whom the certificate belongs, meaning that this person or entity possesses the corresponding private key. However, a digital signature is forensically identical in both legitimate and forged uses. Someone who possesses the private key can create a valid digital signature. Protecting the private key is the idea behind some smart cards such as the United States Department of Defense's Common Access Card (CAC), which never lets the key leave the card. That means that to use the card for encryption and digital signatures, a person needs the personal identification number (PIN) code necessary to unlock it.