Non-repudiation
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Non-repudiation is the concept of ensuring that a contract cannot later be denied by either of the parties involved. Non-repudiation is the opposite of plausible deniability.
In regard to digital security, non-repudiation means that it can be verified that the sender and the recipient were, in fact, the parties who claimed to send or receive the message, respectively. In other words, non-repudiation of origin proves that data has been sent, and non-repudiation of delivery proves it has been received.
Traditional methods such as seals or signatures are vulnerable to forgery. Digital transactions are also potentially subject to fraud, such as when computer systems are broken into or infected with trojan horses or viruses, or the network connecting the systems allows for a man-in-the-middle attack. Participants can potentially claim such fraud to attempt to repudiate a transaction.