In law, non-repudiation is a situation where a statement's author cannot successfully dispute its authorship or the validity of an associated contract.
[3] In digital security, non-repudiation means:[4] Proof of data integrity is typically the easiest of these requirements to accomplish.
[7] Message Authentication Codes (MAC), useful when the communicating parties have arranged to use a shared secret that they both possess, does not give non-repudiation.
Note that the goal is not to achieve confidentiality: in both cases (MAC or digital signature), one simply appends a tag to the otherwise plaintext, visible message.
[8][9][10] To mitigate the risk of people repudiating their own signatures, the standard approach is to involve a trusted third party.
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.