Signature policies

Reference programs, not part of the protocol: the chain runs them like any other deployment. See Reference programs.

Three reference programs answer the policy interface with ECDSA signatures: policy-secp256k1 checks one secp256k1 key, policy-multisig-secp256k1 a threshold of several, and policy-p256 one P-256 key. Each is the rule behind policy accounts: the account names the policy and its params, here public keys, and a witness holding the right signatures authorizes a message for it. The policies keep no state, emit no events and never ask for a delay. A delay comes from a policy that consults them, such as an identity.

How they verify#

All three policies:

• take the interface's input, and succeed with an empty result on empty input, which is how they are deployed. Input that is not exactly three Bytes fields reverts with bad input;
• verify ECDSA over the signing digest, H("app/sign-v1", message), which serves directly as the 32-byte message hash: it is not hashed again;
• read public keys in SEC1 form: 02 or 03 followed by the 32-byte x coordinate (compressed, 33 bytes), or 04 followed by x and y (uncompressed, 65 bytes). The multisignature policy takes the uncompressed form only;
• read a signature as r ‖ s, 64 bytes: two 32-byte big-endian integers, each from 1 to n − 1, where n is the order of the curve;
• accept only a low S, s <= (n − 1) / 2. Whenever (r, s) is a valid signature, so is (r, n − s); requiring the low one leaves every signature a single form;
• return 01 when the signatures are valid and 00 otherwise. An invalid key, a wrong length, a zero or out-of-range scalar and a high S all return 00; nothing in the params or the witness makes a policy revert.

The single-key policies also accept a third, compact form: 05 followed by x, 33 bytes, which names the point with that x and an even y on secp256k1, and the one with the smaller of the two possible y on P-256. The point at infinity is never a key. The forms of one key are different params, so each defines a different account.

policy-secp256k1#

The result is 01 exactly when the witness is a valid low-S signature by the key. High-S signatures, invalid keys, zero or out-of-range scalars and witnesses of any other length return 00.

policy-multisig-secp256k1#

A threshold of signatures by a list of keys:

params  = u64(threshold) ‖ u64(count) ‖ key[count]
witness = entry[threshold]
entry   = u64(key_index) ‖ r ‖ s

The keys are sorted, so no key is listed twice: a 2-of-3 account always needs two different keys, and one set of keys with one threshold defines exactly one account. Different keys need not have different holders, though: a key and its negation, the point with the same x and the other y, have the secret keys d and n − d, so whoever holds one holds both. A wallet that builds a joint account from other people's keys checks that no key is another's negation. The result is 01 exactly when:

• 1 <= threshold <= count, the params hold exactly count keys with nothing after them, and every key is greater than the one before it, compared byte by byte;
• the witness holds exactly threshold entries with nothing after them;
• the indices are strictly increasing and below count, so no key signs twice;
• every key the witness names is an uncompressed key, and every listed signature is a valid low-S signature by its key over the signing digest. Keys the witness does not name are compared for their order but not checked as points.

Anything else returns 00.

policy-p256#

The result is 01 exactly when the witness is a valid low-S signature by the key, and 00 otherwise, by the same rules as policy-secp256k1.

P-256 is the curve of phone secure elements, such as the Secure Enclave and Android StrongBox, and of WebAuthn authenticators. A phone becomes a key of an identity through this policy when its secure element signs the signing digest itself, as the 32-byte hash of an ECDSA signature: the policy verifies exactly that and nothing else. A WebAuthn assertion is a different signature. In it the authenticator signs its own authenticator data together with a hash of the client data, which merely contains the challenge, so an assertion is not a valid witness even when its challenge is the signing digest. Hardware signers may also return a high S; the wallet replaces it by n − s, which is just as valid, and submits the signature as r ‖ s. The policy itself accepts only the low form.

Costs#

About 4 million C of each figure is the policy's code price, which a transaction pays only the first time it runs the policy (the cost of an invocation). A compressed key costs more because recovering its y coordinate takes a square root. The multisignature policy costs roughly one single-key verification per required signature on top of its code. P-256's field and point arithmetic is generic, where the secp256k1 policies' is specialized for their curve, but its program is built with wide arithmetic, which turns its 64-bit multiplications into single instructions, while the secp256k1 code works in 32-bit pieces. At the production network's initial 150,000,000 C per block (production profile), eight P-256 verifications fit in one block. Like every invocation, each check also reads 2 S.