Non-Repudiation

The process of creating a cryptographically secure record that binds an entity's identity to a specific action or data. This evidence must be verifiable by a third party without relying on the claimant's cooperation.

• Digital Signatures: The primary mechanism, where a private key creates a unique signature over the data
• Trusted Timestamps: A Timestamping Authority (TSA) binds the exact time of the action to the evidence
• Audit Logs: Secure, append-only records of all actions, often implemented with Merkle Trees for tamper-evidence

The evidence package typically includes the signed data, the signature itself, the signer's certificate chain, and a trusted timestamp token.

The independent process of validating the generated evidence to confirm both the integrity of the data and the identity of the originator. Verification does not require the originator's participation.

• Signature Validation: The verifier uses the signer's public key (from a trusted PKI) to confirm the signature is mathematically valid
• Certificate Path Validation: The verifier walks the certificate chain to a trusted root CA to confirm the signing certificate was valid at the time of signing
• Timestamp Verification: The verifier confirms the TSA's signature on the timestamp token to establish the data existed before a specific moment

Successful verification produces a high-assurance, auditable conclusion that a specific entity performed a specific action.

The critical link between a cryptographic key and a real-world legal entity. Without strong identity binding, non-repudiation collapses into mere authentication.

• Public Key Infrastructure (PKI): A Registration Authority (RA) validates the entity's identity before a Certificate Authority (CA) issues a digital certificate binding the public key to that identity
• Qualified Certificates: Under eIDAS regulations, these provide the highest level of legal assurance, requiring in-person identity verification
• Hardware Token Binding: Storing private keys in Hardware Security Modules (HSMs) or Trusted Platform Modules (TPMs) ensures the key cannot be exported or copied, strengthening the assertion that only the authorized entity could have signed

The ultimate purpose of non-repudiation is to provide legally admissible evidence that can resolve disputes in a court or arbitration setting. The evidence must survive adversarial scrutiny.

• Long-Term Validation: Evidence must remain verifiable for years or decades, requiring mechanisms like Evidence Record Syntax (ERS) to preserve proof even as cryptographic algorithms weaken
• Trusted Archive Services: Specialized services that maintain the evidentiary value of signed documents over time by periodically re-timestamping with stronger algorithms
• Non-Repudiation of Delivery: A distinct service proving that a specific recipient received and acknowledged a message, often implemented with signed return receipts

The evidence chain must demonstrate: who did what, to which data, and precisely when.

While related, these are distinct security services with different threat models and evidentiary value.

• Authentication answers: "Is this entity who they claim to be right now?" It protects against impersonation during a session.
• Non-Repudiation answers: "Can this entity later deny they performed this specific action?" It protects against post-hoc repudiation.
• Key Distinction: A system can have strong authentication (e.g., biometrics) but weak non-repudiation if the audit logs are mutable or the signing keys are shared. Non-repudiation requires asymmetric cryptography where the verifier cannot forge the signer's signature, a property not offered by symmetric MACs or simple passwords.

Advanced protocols that ensure neither party can gain an advantage by aborting a transaction early. This prevents a situation where one party receives the evidence of the other's commitment without providing their own.

• Trusted Third Party (TTP): A neutral entity that mediates the exchange, ensuring both parties receive their evidence or neither does
• Gradual Exchange Protocols: Both parties release their commitments in small, probabilistic increments, ensuring computational fairness without a TTP
• Blockchain-Based Approaches: Using smart contracts and Verifiable Delay Functions (VDFs) to enforce atomic swaps of evidence without a central intermediary

These protocols are critical for contract signing, certified email, and e-commerce transactions where mutual non-repudiation is required.