Digital Signature

Digital signatures cryptographically bind the identity of the signer to the signed data. The signature is created using the signer's private key, and anyone with the corresponding public key can verify that the signature was generated by that specific key holder.

• Prevents impersonation in AI decision logs
• Relies on the secrecy of the private key
• Often combined with Public Key Infrastructure (PKI) for identity binding
• Enables attribution of model inference decisions to specific operators

Any modification to the signed data, even a single bit, will cause signature verification to fail. This is because the signature is computed over a cryptographic hash of the message.

• Detects tampering in audit trails immediately
• Uses hash functions like SHA-256 to create a fixed-size digest
• Ensures AI model inputs and outputs remain unaltered
• Forms the basis for tamper-evident logging architectures

The signer cannot plausibly deny having signed the data, as only they possess the private key required to generate the valid signature. This is critical for legal accountability in automated systems.

• Creates a non-repudiation token for each AI decision
• Prevents entities from disclaiming model inference actions
• Essential for regulatory compliance under frameworks like the EU AI Act
• Strengthened by combining with Timestamping Authority (TSA) services

Digital signatures use a mathematically linked key pair: a private key kept secret for signing, and a public key distributed openly for verification. This eliminates the need for shared secrets.

• Common algorithms include ECDSA, Ed25519, and RSA-PSS
• Enables scalable verification without key distribution problems
• Quantum-safe cryptography variants like CRYSTALS-Dilithium are emerging
• Public keys can be embedded in Decentralized Identifiers (DIDs)

Verification involves three steps: computing the hash of the original message, decrypting the signature with the signer's public key, and comparing the results. A match confirms both authenticity and integrity.

• Fully automated and computationally efficient
• Can be performed independently by any auditor
• Failed verification indicates either tampering or key mismatch
• Forms the basis for Sigstore and keyless signing ecosystems

Digital signatures are validated through a hierarchical trust model. A signature is trusted because the public key is certified by a Certificate Authority (CA) or anchored in a Verifiable Data Registry.

• X.509 certificates bind identities to public keys in traditional PKI
• Blockchain anchoring provides decentralized trust for audit logs
• Verifiable Credentials (VCs) use BBS+ signatures for selective disclosure
• Enables end-to-end verifiability from AI inference to compliance report

Every prediction made by a high-risk AI system must be logged immutably. A digital signature, generated using the model's unique private key, is applied to the inference hash (a digest of the input, output, and model version). This creates a non-repudiation token that proves a specific model version produced a specific output at a specific time, preventing an operator from later denying the system's automated decision.

An AI Bill of Materials is a structured inventory of all components in an AI supply chain. Developers digitally sign the AI BOM to assert its integrity and provenance. Auditors then verify this signature against the developer's public key in a Public Key Infrastructure (PKI) to ensure:

• The BOM has not been tampered with since publication.
• The listed dependencies and training datasets are authentic. This is critical for supply chain security and vulnerability management.

For audit logs to be admissible as evidence, a verifiable chain of custody must be maintained. Each entity that handles a log—from the AI system to a security information and event management (SIEM) system to an archivist—digitally signs the record upon receipt and transfer. This creates a sequential, cryptographically verifiable trail that documents exactly who possessed the log and when, eliminating gaps in accountability.

In privacy-preserving compliance, an auditor may need to verify a specific claim (e.g., 'the model was trained on an approved dataset') without seeing all underlying data. BBS+ signatures support selective disclosure. A signed credential containing multiple attributes can be used to generate a zero-knowledge proof that reveals only the required attribute while maintaining the cryptographic integrity of the original signature. This enables privacy-respecting regulatory audits.

To provide an independent, publicly verifiable integrity proof, the hash of a batch of signed audit logs can be embedded into a public blockchain transaction. This process, known as blockchain anchoring, leverages the blockchain's immutability to create an unalterable timestamp and existence proof. Even if the internal audit system is compromised, the anchored hash proves the logs existed in a specific state at a specific time.

The framework of hardware, software, policies, and standards that creates, manages, distributes, uses, stores, and revokes digital certificates. PKI establishes a chain of trust by binding public keys to verified identities through Certificate Authorities (CAs).

• Enables scalable identity verification across distributed AI systems
• Manages the full certificate lifecycle: issuance, renewal, and revocation
• Critical for establishing non-repudiation in audit trails

A sequential application of a cryptographic hash function where each link incorporates the hash of the previous entry. This creates a tamper-evident sequence where altering any single record invalidates all subsequent hashes.

• Forms the backbone of append-only audit logs
• Enables efficient verification without storing full history
• Used in conjunction with digital signatures for temporal integrity

A trusted third-party service that issues cryptographic timestamps proving specific data existed at a particular point in time. The TSA applies its own digital signature to a hash of the data combined with the current time.

• Establishes verifiable chronology for AI decision logs
• Prevents backdating or post-dating of audit records
• Essential for regulatory compliance under frameworks like eIDAS and ESIGN

A dedicated physical computing device that safeguards and manages digital keys for strong authentication. HSMs provide tamper-resistant hardware for secure key generation, storage, and cryptographic operations.

• Protects private signing keys from extraction or compromise
• Enforces dual control and key usage policies
• Achieves FIPS 140-2 Level 3 certification for compliance environments

A piece of cryptographic evidence that prevents an entity from plausibly denying its involvement in a specific action. A digital signature combined with a trusted timestamp creates a legally binding non-repudiation token.

• Binds identity, action, and time into a single verifiable artifact
• Critical for legal accountability in automated AI decisions
• Survives long-term archival with cryptographic longevity measures

The process of embedding a cryptographic hash of an audit log into a public blockchain transaction. This leverages the blockchain's distributed consensus and immutability to provide an external, independent integrity proof.

• Creates a tamper-proof witness that exists outside organizational control
• Enables public verifiability without exposing sensitive log contents
• Commonly used with Merkle trees to anchor large datasets efficiently