feat(mobile): verify fee-payer HKDF derivation on native - #598
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feat(mobile): verify fee-payer HKDF derivation on native#598Lazyartist1 wants to merge 1 commit into
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Confirm the deterministic fee-payer (sponsor) keypair the SDK derives from the passkey is byte-identical on native and web. The wallet is fee-sponsored, so a mismatch would pay fees from the wrong account. The web derives the Ed25519 seed with WebCrypto HKDF (crypto.subtle), which has no equivalent under React Native's crypto polyfills, so this native port uses @noble/hashes HKDF-SHA-256 with the same salt/info. HKDF is a fixed standard, so the two produce the same 32 bytes and therefore the same G… address — checked against a golden address derived through the web WebCrypto path. - lib/deriveFeePayer.ts: deriveFeePayerKeypair (base64url credential id → noble HKDF → Ed25519 seed → Keypair) and deriveStoredFeePayer (from the secure-store credential id). Closes Miracle656#448
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Summary
Adds and verifies the native fee-payer (sponsor) key derivation (backlog item 20). The wallet is fee-sponsored, so the deterministic keypair the SDK derives from the passkey must be byte-identical on web and native — a mismatch would pay fees from the wrong account. This is a correctness check on the HKDF path under the native crypto polyfills.
Approach
The web derives the Ed25519 seed with WebCrypto HKDF (
crypto.subtle.deriveBits), which has no equivalent under React Native's crypto polyfills. The native port uses@noble/hashesHKDF-SHA-256 with the same salt/info. HKDF (RFC 5869) is a fixed standard, so both implementations produce the same 32 bytes → the same Ed25519 seed → the sameG…address.Changes
lib/deriveFeePayer.ts(new) — native port offrontend/wallet/lib/deriveFeePayer.ts:deriveFeePayerKeypair(credentialIdBase64url)— base64url credential id → noble HKDF-SHA-256 (saltveil:feepayer:salt:v1, infoveil:feepayer:ed25519:v1) → 32-byte Ed25519 seed →Keypair.deriveStoredFeePayer()— derives from the credential id in secure store; null when no passkey is registered.Acceptance
G…fee-payer address on web and nativeVerified: for credential id
AQIDBAUGBwgJCgsMDQ4PEA, both the web WebCrypto path and the native noble path deriveGDNLN66V4YFQQPDVNMADJSWUF2EFSLM2BHOF2LDUGVQPQUBG7H4WRFDQ. That address is pinned as the golden value in the test, so any drift in the native derivation fails CI.Testing
tsc --noEmitcleanjest lib/__tests__/deriveFeePayer.test.ts— 5/5 passing (matches the web-derived golden address, deterministic, distinct id → distinct account, derives from stored credential, null when unregistered)Closes #448