What is the difference between non-repudiation and plausible deniability

Question

Both non-repudiation and plausible deniability seem to deal with proving or denying actions, but their definitions appear to conflict. Non-repudiation ensures that a party cannot deny their involvement in an action, while plausible deniability allows someone to deny knowledge or involvement in something. Could someone explain the core differences between these two concepts, preferably with examples to help me understand their practical implications?

Answer 1

Non-repudiation and plausible deniability are two distinct concepts in information security, each serving different purposes regarding the attribution of actions or communications.

Non-Repudiation

• Definition: Non-repudiation ensures that a party involved in a communication or transaction cannot deny their participation or the authenticity of their actions. This is typically achieved through mechanisms that provide verifiable evidence linking an individual to a specific action.

• Purpose: To provide undeniable proof of the origin and integrity of data, thereby preventing any party from falsely denying their involvement.

• Example: Digital signatures are a common tool for non-repudiation. When a person signs a digital document using their private key, the signature can be verified by others using the corresponding public key. This process ensures that the signer cannot later deny having signed the document, as only they possess the private key used to create the signature.

Plausible Deniability

• Definition: Plausible deniability allows an individual to deny knowledge of or involvement in a particular action, even if evidence exists, because the evidence cannot irrefutably link them to the action.

• Purpose: To protect individuals by ensuring that actions cannot be definitively attributed to them, thereby allowing them to deny involvement if necessary.

• Example: Certain secure messaging protocols, like those used in Signal, are designed to provide plausible deniability. While messages are authenticated between sender and receiver to prevent tampering, the cryptographic methods used (such as HMAC with shared secrets) ensure that third parties cannot conclusively prove who sent a particular message. This design means that, if a message is leaked, the alleged sender can plausibly deny having sent it, as the message lacks a verifiable signature that uniquely identifies them.

Key Differences

• Attribution: Non-repudiation seeks to ensure that actions can be definitively attributed to a specific individual, preventing denial. In contrast, plausible deniability aims to prevent definitive attribution, allowing individuals to deny involvement.

• Use Cases: Non-repudiation is crucial in legal and financial transactions where accountability is essential. Plausible deniability is important in scenarios where privacy and the ability to deny involvement protect individuals, such as in whistleblowing or private communications.

• Implementation: Non-repudiation often employs asymmetric cryptography (e.g., digital signatures) to create verifiable links to individuals. Plausible deniability may use symmetric cryptography or protocols designed to prevent the creation of verifiable evidence linking actions to individuals.