[enforser]

What

A single-file Python example (samples/python/custody_withdrawal.py) for the MPC custody API signing challenge and authorized withdrawals.

Prerequisite (stated prominently in the README and the example docstring): this API is not self-serve. Integrators must contact Gemini support, who enable the custody API for the account group, configure the MPC custody account's approved security policy with a positive API signer threshold, and register the customer's API signer keypairs (COSE_Key-encoded P-256 public keys) on the account. Every withdrawal fails closed until that enrollment is complete.

Why the verification step matters

The hard part of integrating this API is client-side: a verifier must reconstruct the canonical authorization context byte-for-byte and re-derive the challenge. Any re-serialization (different key order, whitespace, re-encoding of the embedded signing strings) produces a different digest. The example therefore embeds the verification engine in the same file and runs it on every flow:

• rebuilds the canonical context from the complete received action/policy signing strings (never re-serialized, never trusting a server-supplied digest)
• SHA-256 → unpadded base64url → GEMINI-API-SIGNATURE-V1:<challenge>
• validates the response against a locally trusted intent: account, single-leg transfer shape, asset/amount/address/memo, policy quorum, expiry window
• raises field-specific VerificationErrors for every mismatch

The verification section is deliberately stdlib-only and keyless, so integrators can audit it or lift it into their own language.

Files

File	Purpose
samples/python/custody_withdrawal.py	End-to-end flow + embedded verifier: signed POST /v1/custody/challenge → local challenge verification → ES256 (COSE P-256) assertion per approved signer → signed POST /v2/withdraw/:network/:ticker
samples/README.md	New "MPC Custody Withdrawals (Support-Enrolled Accounts Only)" section with the prerequisite, flow, and usage; endpoint table/structure updates
samples/python/requirements.txt	Adds cryptography (ES256 signing in the example only; the verification section stays stdlib-only)

Contract

Implements the /v1/custody/challenge wire contract, version "1" (message prefix GEMINI-API-SIGNATURE-V1:). The protocol constants are carried as code so drift against a future contract version fails loudly.

Validation

• End-to-end against production: XRP withdrawal with a 3-of-3 signer threshold — challenge minted, challenge verified by the embedded verifier against the live response, three ES256 assertions built, withdrawal accepted (HTTP 200)
• The assertion builder was exercised against a real registered P-256 key: the COSE encoding matches the registered public key byte-for-byte
• Note: on the wire, a transfer leg omits memo (rather than sending null) when a transfer has no memo; the verifier treats absent and null identically

[sdlc-pr-bot]

Change Control Evidence Check

Authorization — ❌ Fail

Evidence:

• Linear ticket: ⚠️ No ticket linked
• Change owner: Matthew Fors
• Type of change: Normal

Testing — ✅ Pass

Evidence:

• Tests from CI checks: ➖ No tests ran
• Security scans: ✅ Passed
• Testing summary: ✅ Found in PR description
  • Summary: XRP withdrawal functionality was tested end-to-end against production with a 3-of-3 signer threshold, resulting in successful challenge verification, three ES256 assertions, and HTTP 200 acceptance. The assertion builder was validated against a real registered P-256 key with byte-for-byte COSE encoding match to the registered public key.
• Evidence link: View run

Approval — ✅ Pass

Evidence:

• Approver: Victor Reventos Rosario
• Approval source: GitHub PR Review
• Approval timestamp: 2026-11-11 02:03 UTC

Segregation of Duties — ✅ Pass

Validated:

• PR author: Matthew Fors
• Commit author(s): enforser
• Approver: Victor Reventos Rosario
• Result: Implementer and approver are different people ✅

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Last checked: 2026-11-11 02:03 UTC

↩ Re-run Clarissa if tickets, CI, or approvals have changed.

[nostradamus-bot]

Nostradamus Risk Rating — Medium

samples/python/custody_withdrawal.py implements HMAC-SHA384 API signing and hand-rolled ES256/COSE P-256 assertions for MPC custody withdrawals — a privileged financial operation — in example code that integrators are likely to adopt verbatim; correctness of the custom COSE CBOR encoder and challenge-verification engine is the primary security concern. The change is confined entirely to samples/ with no production server-side attack surface introduced, placing it at Medium (Impact: High × Likelihood: Medium per NIST SP 800-30r1).