Non-repudiation is a concept in cryptography that ensures the sender of a message cannot deny having sent it. It is an important security measure used to protect against fraud and other malicious activities. Non-repudiation relies on digital signatures, which are created using cryptographic algorithms and public key infrastructure (PKI). The signature acts as proof that the message was sent by the sender, and can be verified by anyone with access to the public key. This provides a secure way for parties to communicate without fear of repudiation or tampering.