What is the difference between authenticity and non-repudiation

Question

Authenticity and non-repudiation are terms I often encounter together in discussions about cybersecurity, but their precise meanings and differences seem unclear to me. If authenticity ensures that a message or data comes from a verified source, how is it different from non-repudiation? Are there scenarios where one is achieved without the other, and if so, how?

Answer 1

Authenticity and non-repudiation are fundamental concepts in cybersecurity, each addressing different aspects of secure communications and data integrity.

Authenticity

• Definition: Authenticity ensures that a message, transaction, or piece of data genuinely originates from the claimed sender. It verifies the legitimacy of the source, confirming that the entity is who it professes to be.

• Purpose: To prevent impersonation and ensure that communications or data exchanges occur between legitimate parties.

• Implementation: Common methods include the use of digital certificates, cryptographic keys, and authentication protocols that validate the identity of users or systems.

Non-Repudiation

• Definition: Non-repudiation ensures that a party involved in a communication or transaction cannot deny their participation or the authenticity of their signature on a document. It provides undeniable proof of involvement, preventing any party from falsely denying their actions.

• Purpose: To provide verifiable evidence of the origin and integrity of data, ensuring accountability and trust in digital communications.

• Implementation: Digital signatures are a primary tool for non-repudiation. When a sender signs a document with their private key, the signature can be verified by others using the corresponding public key, ensuring the sender cannot later deny having signed the document.

Key Differences

• Scope of Assurance: Authenticity focuses on verifying identity at the moment of communication, ensuring the sender is legitimate. Non-repudiation extends this by providing enduring proof that can be presented to third parties, ensuring the sender cannot deny their involvement even after the fact.

• Proof to Third Parties: Authenticity assures the recipient of the sender's identity but may not provide transferable proof to others. Non-repudiation offers evidence that can be independently verified by third parties, establishing accountability beyond the immediate communication.

Scenarios Illustrating the Difference

1. Authenticity without Non-Repudiation:

   Alice sends Bob an authenticated message over a secure channel. Bob is confident the message is from Alice. However, if Bob later needs to prove to a third party (e.g., Charlie) that Alice sent the message, he lacks the necessary evidence, as the authentication was only valid within the context of their secure session.

2. Non-Repudiation without Authenticity

   This situation is less common, as non-repudiation mechanisms typically include authenticity. However, if a system records actions with verifiable evidence (e.g., digital signatures) but does not actively verify identities during the transaction, it could be argued that non-repudiation exists without real-time authenticity checks.