ap2-cryptographic-signing

AP2 Cryptographic Signing

Before writing code

Fetch live docs:

1. Fetch https://ap2-protocol.org/specification/ for cryptographic signing requirements
2. Fetch https://ap2-protocol.org/topics/privacy-and-security/ for security architecture
3. Web-search site:github.com google-agentic-commerce AP2 signature mandate for signing implementations
4. Web-search ap2 protocol VDC signing cryptographic hardware-backed for community guides

Conceptual Architecture

Why Cryptographic Signing Matters

AP2's core innovation is verifiable intent — cryptographic proof that:

• The user authorized a specific transaction
• The merchant committed to specific terms
• Neither party can deny what they agreed to (non-repudiation)
• No intermediary tampered with the mandate (integrity)

VDC Credential Format

AP2 VDCs use the SD-JWT with Key Binding (+kb) format, enabling selective disclosure and cryptographic holder binding.

Supported Signing Algorithms

AP2 supports ECDSA with the following algorithm/curve combinations:

• ES256 — ECDSA with P-256 curve
• ES384 — ECDSA with P-384 curve
• ES512 — ECDSA with P-521 curve

JSON Canonicalization (JCS)

Before signing, JSON payloads are canonicalized using JCS (RFC 8785) to produce a deterministic byte representation. This ensures that logically equivalent JSON objects produce the same signature regardless of key ordering or whitespace.

Detached JWS for Merchant Authorization

The merchant_authorization field on Cart Mandates uses Detached JWS format:

<base64url-header>..<base64url-signature>

Note the double dots — the payload is omitted from the JWS because it is the JCS-canonicalized CartContents, which the verifier already possesses.

JWT Header and Payload Requirements

JWT header MUST include:

• alg — The signing algorithm (ES256, ES384, or ES512)
• kid — Key identifier for the signing key

JWT payload for merchant_authorization includes:

• iss — Issuer (merchant identifier)
• aud — Audience
• iat — Issued-at timestamp
• exp — Expiration timestamp
• jti — Unique JWT identifier
• cart_hash — Hash of the canonicalized cart contents

Two Types of Signatures

User Signatures

• Hardware-backed device keys — Generated and stored in secure hardware (TPM, Secure Enclave)
• In-session authentication — User must authenticate (biometric, PIN) at signing time
• Attestation — Device provides cryptographic proof of the signing context
• Purpose — Proves the user explicitly authorized the transaction

Merchant Signatures

• Entity-level — Signed by the merchant organization, not by the AI agent
• Fulfillment guarantee — Commits the merchant to the stated terms
• Key management — Organizational-level key infrastructure
• Purpose — Proves the merchant committed to specific products/prices

What Gets Signed

| VDC | Signed By | What's Covered | |-----|-----------|---------------| | Cart Mandate | Merchant + User | Exact items, prices, totals, payment methods | | Intent Mandate | User | Shopping constraints, categories, intent, TTL | | Payment Mandate | User | Payment method selection, transaction amount |

Trusted Device Surface

The user signing step (especially for Cart and Payment Mandates) involves:

1. Shopping Agent triggers redirect to trusted device surface
2. User's device displays the transaction summary
3. User authenticates (biometric, PIN, passkey)
4. Device generates signature using hardware-backed key
5. Attestation object created proving the signing context
6. Signature + attestation returned to Shopping Agent

This is a load-bearing security step — the agent cannot bypass it.

Signature Verification

Verifiers check:

1. Signature validity — Cryptographic verification against the signer's public key
2. Signer identity — Public key belongs to the claimed entity
3. Content integrity — Signed content matches the mandate contents
4. Temporal validity — Signature was created within acceptable timeframe
5. Attestation validity — Device attestation is genuine (for user signatures)

Non-Repudiation

Signed mandates provide non-repudiation for disputes:

• User can't deny they authorized a purchase (their device signed it)
• Merchant can't deny their offer terms (their entity signed it)
• The cryptographic evidence is deterministic, not inferred

Key Management Considerations

• User keys: Managed by the user's device secure hardware
• Merchant keys: Managed at the organization level (HSM or key vault)
• Key rotation: Support for rotating keys without breaking verification
• Key revocation: Ability to revoke compromised keys
• Certificate chain: Trust chain from key to identity

Man-in-the-Middle Prevention

VDC signatures prevent MITM attacks:

• An attacker cannot modify mandate contents without invalidating signatures
• End-to-end integrity from creation to verification
• Digital signatures cover the complete mandate payload

Best Practices

• Always use hardware-backed keys for user signatures when available
• Never store private signing keys in agent code or config
• Implement proper key rotation procedures
• Verify all signatures before trusting mandate contents
• Store signed mandates with signatures for dispute resolution
• Use standard cryptographic libraries — don't implement crypto primitives
• Test with both valid and invalid signatures
• Handle signature verification failures gracefully with clear errors
• Log all signing and verification events for audit

Fetch the specification for exact signature formats, supported algorithms, attestation requirements, and verification procedures before implementing.