# Capability Spec Delta: crypto-curve (crypto-completion) **Package**: `@suaveplan/crypto-curve` **Status**: PROPOSED > Every requirement below carries a `**Verified by:**` line at authoring time. ## ADDED Requirements ### Requirement: secp256k1 ECDSA signing with recovery byte The `secp256k1` primitive SHALL expose `signWithRecovery(digest, privateKey)` returning a 65-byte signature (64-byte compact signature followed by a single recovery byte). The recovery byte SHALL be the byte returned by `secp256k1.sign(digest, privateKey).recovery` from the `@noble/curves` library; valid values are `0` or `1`. #### Scenario: produces 65-byte signature - **WHEN** `signWithRecovery(digest, privateKey)` is called for a valid digest and private key - **THEN** the result is a `Uint8Array` of length 65 AND `result[64]` is `0` or `1` #### Scenario: signWithRecovery output verifies via standard verify - **WHEN** the first 64 bytes of `signWithRecovery(digest, privateKey)` are passed to the existing `verify(digest, sig64, publicKey)` - **THEN** the signature verifies successfully **Verified by:** `packages/crypto/crypto-curve/src/primitives/secp256k1/secp256k1.test.ts::"signWithRecovery returns 65-byte signature with recovery byte 0 or 1"` AND `packages/crypto/crypto-curve/src/primitives/secp256k1/secp256k1.test.ts::"signWithRecovery output verifies via standard verify on first 64 bytes"` --- ### Requirement: secp256k1 public-key recovery The `secp256k1` primitive SHALL expose `recoverPublicKey(signatureWithRecovery, digest)` returning the 33-byte compressed public key recovered from a 65-byte signature-with-recovery and the original digest. Inputs may be supplied as `Uint8Array` or hex strings. #### Scenario: recovers the signing public key - **WHEN** `signWithRecovery(digest, privateKey)` produces `sig` and `recoverPublicKey(sig, digest)` is called - **THEN** the recovered public key equals `getPublicKey(privateKey)` byte-for-byte #### Scenario: missing recovery byte rejected - **WHEN** a 64-byte signature (without recovery byte) is passed to `recoverPublicKey` - **THEN** it throws `SuaveplanError` with code `CRYPTO_CURVE.RECOVERY_BYTE_MISSING` and `context.signatureLength === 64` #### Scenario: invalid recovery byte rejected - **WHEN** a 65-byte signature is passed but `signatureWithRecovery[64]` is `2`, `3`, or any value > 3 - **THEN** it throws `SuaveplanError` with code `CRYPTO_CURVE.RECOVERY_BYTE_INVALID` and `context.recoveryByte` reflects the offending value **Verified by:** `packages/crypto/crypto-curve/src/primitives/secp256k1/secp256k1.test.ts::"recoverPublicKey returns the same key as getPublicKey for the signer"` AND `packages/crypto/crypto-curve/src/primitives/secp256k1/secp256k1.spec-conformance.test.ts::"throws RECOVERY_BYTE_MISSING for 64-byte signature"` AND `packages/crypto/crypto-curve/src/primitives/secp256k1/secp256k1.spec-conformance.test.ts::"throws RECOVERY_BYTE_INVALID when recovery byte is out of range"` --- ### Requirement: secp256k1 verify-with-recovery The `secp256k1` primitive SHALL expose `verifyWithRecovery(signatureWithRecovery, digest, expectedPublicKey)` returning `true` only when the public key recovered from the signature equals `expectedPublicKey` (compared in constant time on the byte buffer). When the recovered key differs, the function SHALL return `false` (it does NOT throw on mismatch — mismatch is a routine path). #### Scenario: returns true on matching key - **WHEN** `verifyWithRecovery(sig, digest, expectedPublicKey)` is called with the public key that produced `sig` - **THEN** the result is `true` #### Scenario: returns false on non-matching key - **WHEN** `verifyWithRecovery(sig, digest, otherPublicKey)` is called with a public key that did not produce `sig` - **THEN** the result is `false` (no exception is thrown) #### Scenario: comparison is constant-time - **WHEN** the recovered public key and `expectedPublicKey` differ in their first byte vs differ only in their last byte - **THEN** the timing distribution observed across many invocations does not reveal the position of the difference (verified via a statistical timing test) **Verified by:** `packages/crypto/crypto-curve/src/primitives/secp256k1/secp256k1.test.ts::"verifyWithRecovery returns true on matching public key"` AND `packages/crypto/crypto-curve/src/primitives/secp256k1/secp256k1.test.ts::"verifyWithRecovery returns false on non-matching public key"` AND `packages/crypto/crypto-curve/src/primitives/secp256k1/secp256k1.spec-conformance.test.ts::"verifyWithRecovery comparison is constant-time"`