Which concept ensures users cannot deny their actions on a system?

• A. Digital signature
• B. Key agreement
• C. Data encipher
• D. Decipher only

Answer: (A)

The concept that ensures users cannot deny their actions on a system is a digital signature. A digital signature provides a means of verifying the authenticity and integrity of a message, document, or transaction, making it possible to confirm that the sender is indeed who they claim to be and that the message has not been altered in transit.

When a user signs a document or message with their private key, the unique properties of the digital signature allow any recipient to verify this signature using the sender's public key. This process not only confirms the identity of the sender but also binds the sender to the content of the signed message, preventing them from denying their involvement later. This principle is crucial in various legal and financial contexts, where non-repudiation is required to ensure accountability.

Other options, though they pertain to essential security protocols, do not specifically address the concept of non-repudiation. For instance, key agreement relates to the secure sharing of cryptographic keys, while data encipher and decipher refer to the processes of encrypting and decrypting data for confidentiality. None of these directly prevent a user from denying their actions in the way that a digital signature does.