Which of the following ensures that a sender cannot deny having sent a message?

Non-repudiation is the principle that ensures a sender cannot deny having sent a message. This is achieved through mechanisms such as digital signatures and certificates that provide proof of the origin of the message. When a sender digitally signs a message, they are creating a unique cryptographic signature that is tied to both the message and the sender's private key. This makes it very difficult for the sender to later claim they did not send the message, as the digital signature can be verified using their public key, affirming their identity as the sender.

In the context of security, ensuring non-repudiation is critical for accountability. It serves legal and compliance purposes, particularly in transactions where it’s important to prove who sent a message or completed a transaction. This is crucial in various domains, such as banking and legal communications, to prevent disputes regarding the authenticity and origination of the message.

Authenticity deals with verifying the identity of the sender but does not in itself ensure that the sender cannot later deny having sent the message. Confidentiality focuses on keeping information secret and only accessible to authorized parties, whereas integrity ensures that the information has not been altered during transmission. Neither confidentiality nor integrity directly addresses the issue of non-repudiation.