Nonrepudiation
Assurance that a sender cannot credibly deny having sent a message.
Non-repudiation ties an action to an identity in a way the actor can’t later disown. Digital signatures provide it because only the holder of the private key could have produced the signature; symmetric MACs do not, because both parties share the same key and either could have generated it. In operations, signed actions plus tamper-evident logging give a defensible audit trail.
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