Bob, your senior colleague, has sent you a mail regarding aa deal with one of the clients. You are requested to accept the offer and you oblige. After 2 days, Bob denies that he had ever sent a mail.

Confidentiality ensures data is accessible only to authorized parties but does not create proof identifying who sent or originated the data.

Integrity ensures data has not been altered in transit but does not address whether the original sender can later deny having sent the message.

Non-repudiation is the security property that prevents a party from denying a previously performed action. In email, digital signatures provide non-repudiation by binding the message to the sender's private key, creating cryptographic proof that only the key holder could have authored the message - making it impossible for Bob to credibly deny sending it.

Authentication verifies the identity of a party at the time of access but does not create a persistent, undeniable record that prevents later denial of a completed action.