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Non-repudiation refers to 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".
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. Here, 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. This prevents the owner of the account from denying actions performed by the account.
Proof of data integrity is typically the easiest of these requirements to accomplish. A data hash such as SHA2 usually ensures that the data will not be changed undetectably. Even with this safeguard, it is possible to tamper with data in transit, either through a man-in-the-middle attack or phishing. Because of this, data integrity is best asserted when the recipient already possesses the necessary verification information.
Common methods to provide non-repudiation in the context of digital communications or storage are Message Authentication Codes (MAC), useful when the communicating parties have arranged to use a shared secret that they both possess, and Digital Signatures, a more powerful tool that provides non-repudiation in a publicly verifiable manner. 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. If confidentiality is also required, then an encryption scheme can be combined with the digital signature, or some form of authenticated encryption could be used. Verifying the digital origin means that the certified/signed data likely came from someone who possesses the private key corresponding to the signing certificate. If the key used to digitally sign a message is not properly safeguarded by the original owner, digital forgery can occur.
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 only TTP is the repository for public key certificates. This lets the recipient verify the origin of an item even if the public information has ever been directly exchanged. The digital signature, however, is forensically identical in both legitimate and forged uses - if someone possesses the private key they can create a "real" signature. Protecting the private key is the idea behind the United States Department of Defense's Common Access Card (CAC), which never lets the key leave the card. This means that, to use the card for encryption and digital signatures, a person needs the personal identification number (PIN) code necessary to unlock it.