Non-repudiation is the ability for a system to prove that a specific user and only that specific user sent a message and that it hasn’t been modified.
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".[citation needed]
For example, Mallory buys a cell phone for $100 and 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. Unfortunately, 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.)
