Final answer:
MACs lack accountability, meaning they cannot irrefutably link a message to the sender's identity, which is essential for non-repudiation. This is because the key used in MACs is shared and does not uniquely identify a single sender.
Step-by-step explanation:
The reason MACs (Message Authentication Codes) cannot achieve non-repudiation is because they lack accountability. Non-repudiation refers to the assurance that someone cannot deny the validity of something they have digitally signed. MACs are designed to verify the integrity and authenticity of a message but do not provide a means to irrefutably link a message to the identity of the sender as digital signatures do. With MACs, the secret key is shared between the parties, so any party with the key could generate a valid MAC, making it impossible to prove who among the parties with the shared secret actually sent the message.
Encryption is not directly related to non-repudiation, but rather to confidentiality. Authentication is achieved with MACs, but since the key is shared, it does not pinpoint one specific sender. Timestamping also does not provide non-repudiation, though it can be used as an additional security measure in various protocols when combined with other mechanisms such as digital signatures.