Digital signatures provide non-repudiation because they use cryptographic techniques to prove the origin and integrity of a message, document, or transaction. Once a digital signature is attached, the signer cannot credibly deny signing the document or message, because the digital signature is unique to both the signer and the document. Audit logs, while useful for tracking activities, do not tie an action irrefutably to an initiator. Time stamps indicate when an action took place but do not necessarily tie that action to an individual. Usernames can be repudiated as they can sometimes be shared or stolen, whereas a digital signature is secured and much harder to forge or repudiate.