Expose spend-auth signing and verification accessors - #543
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Add three public methods needed for non-transaction signatures (such as UFVK binding signatures in zcash_keys): - SpendAuthorizingKey::sign signs a message with the unrandomized key, delegating to the inner redpallas::SigningKey. - SpendValidatingKey::to_verification_key returns the unrandomized redpallas::VerificationKey, enabling verification of non-spend signatures. - FullViewingKey::ak exposes the spend validating key, which was previously private with no public accessor. Co-Authored-By: Claude <noreply@anthropic.com>
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Motivation
Downstream consumers (notably
zcash_keysin librustzcash) need to sign and verify non-transaction messages with spend-authorizing keys. For example, the proposed UFVK binding signature inzcash_keyssigns a UFVK's canonical encoding with the account'sUnifiedSpendingKeyto detect tampering of the stored UFVK in a wallet database (audit finding COR-1649).Currently, the signing and verification primitives are only accessible through the
randomizepath, which applies a per-spend randomizer. For non-transaction signatures, no randomizer is needed, but there is no public way to sign with the unrandomized key or to obtain the unrandomized verification key.Changes
SpendAuthorizingKey::signsigns a message with the unrandomized spend-authorizing key, delegating to the innerredpallas::SigningKey.SpendValidatingKey::to_verification_keyreturns the unrandomizedredpallas::VerificationKey, enabling verification of non-spend signatures.FullViewingKey::akexposes the spend validating key, which was previously private with no public accessor. (Sapling'sFullViewingKey.vk.akis already public; this aligns the two crates.)All three are pure delegation methods with no new logic. The existing
randomizepath is unchanged.COR-1649
Co-Authored-By: Claude noreply@anthropic.com