A standard for interoperable signed messages based on the Bitcoin Script format, either for proving fund availability, or committing to a message as the intended recipient of funds sent to the invoice address.
The current message signing standard only works for P2PKH (1...) invoice addresses. We propose to extend and generalize the standard by using a Bitcoin Script based approach. This ensures that any coins, no matter what script they are controlled by, can in-principle be signed for. For easy interoperability with existing signing hardware, we also define a signature message format which resembles a Bitcoin transaction (except that it contains an invalid input, so it cannot be spent on any real network).
Additionally, the current message signature format uses ECDSA signatures which do not commit to the public key, meaning that they do not actually prove knowledge of any secret keys. (Indeed, valid signatures can be tweaked by 3rd parties to become valid signatures on certain related keys.)
Ultimately no message signing protocol can actually prove control of funds, both because a signature is obsolete as soon as it is created, and because the possessor of a secret key may be willing to sign messages on others' behalf even if it would not sign actual transactions. No signmessage protocol can fix these limitations.
This BIP specifies three formats for signing messages: ''legacy'', ''simple'' and ''full''. Additionally, a variant of the ''full'' format can be used to demonstrate control over a set of UTXOs.
New proofs should use the new format for all invoice address formats, including P2PKH.
The legacy format MAY be used, but must be restricted to the legacy P2PKH invoice address format.
=== Simple ===
A ''simple'' signature consists of a witness stack, consensus encoded as a vector of vectors of bytes, and base64-encoded. Validators should construct <code>to_spend</code> and <code>to_sign</code> as defined below, with default values for all fields except that
* <code>message_hash</code> is a BIP340-tagged hash of the message, as specified below
* <code>message_challenge</code> in <code>to_spend</code> is set to the scriptPubKey being signed with
* <code>message_signature</code> in <code>to_sign</code> is set to the provided simple signature.
and then proceed as they would for a full signature.
=== Full ===
Full signatures follow an analogous specification to the BIP-325 challenges and solutions used by Signet.
Let there be two virtual transactions <code>to_spend</code> and <code>to_sign</code>.
where message_hash is a BIP340-tagged hash of the message, i.e. sha256_tag(m), where tag = "BIP0322-signed-message", and message_challenge is the to be proven (public) key script.
For proving funds, message_challenge shall be simply OP_TRUE.
* The proof is considered valid, inconclusive, or invalid based on whether the to_sign transaction is a valid spend of the to_spend transaction or not, according to the rules specified in the "Consensus and standard flags" section below.
* Proofs of funds may be encumbered with the in_future flag, according to the rules specified in the "Locktime and Sequence" section below, in which case we refer to the result in text form as "valid_in_future", "inconclusive_in_future", etc.
Proofs of funds are the base64-encoding of the to_spend and to_sign transactions concatenated in standard network serialisation, and proofs without additional inputs or time locks (simple proofs) are the base64-encoding of the to_sign script witness.
A signer may construct a proof of funds, demonstrating control of a set of UTXOs, by constructing a full signature as above, with the following modifications.
* <code>message_challenge</code> is unused and shall be set to <code>OP_TRUE</code>
* Similarly, <code>message_signature</code> is then empty.
* All outputs that the signer wishes to demonstrate control of are included as additional inputs of <code>to_sign</code>, and their witness and scriptSig data should be set as though these outputs were actually being spent.
Unlike an ordinary signature, validators of a proof of funds need access to the current UTXO set, to learn that the claimed inputs exist on the blockchain, and to learn their scriptPubKeys.
# deserialize the to_spend and to_sign transactions from the proof, and fail if the proof contains extraneous bytes
# verify that the to_sign transaction uses all inputs covered by the proof of funds, exactly once
# reconstruct the to_spend' and to_sign' transactions, based on the specification above, copying the version, lock time, and sequence values
# verify that to_spend = to_spend', that to_sign has at least 1 input, has exactly 1 output, and that to_sign.vin[0] = to_sign'.vin[0]
# set the "in_future" flag if the transaction's lock time is in the future according to consensus rules
# establish a "coins map", a mapping of outpoints (hash, vout) to coins (scriptPubKey, amount), initialized to coins_map(to_spend.txid, 0) = (to_spend.vout[0], 0)
# for each proof of fund input, set the corresponding values in the coins map; abort if the input cannot be found
# check the signature of each input using consensus rules, then upgradable rules
Given the P2PKH invoice address <code>a</code> and the message <code>m</code>, and the pubkey-hash function <code>pkh(P) = ripemd160(sha256(P))</code>:
Given the P2PKH invoice address <code>a</code>, the message <code>m</code>, the compact signature <code>sig</code>, and the pubkey-hash function <code>pkh(P) = ripemd160(sha256(P))</code>:
Each flag is associated with some type of enforced rule (most often a soft fork). There are two sets of flags: consensus flags (which result in a block being rejected, if violated), and upgradable flags (which are typically policy-rejected by nodes specifically for the purpose of future network upgrades). The upgradable flags are a super-set of the consensus flags.
This BIP specifies that a proof that validates for both rulesets is valid, a proof that validates for consensus rules, but not for upgradable rules, is "inconclusive", and a proof that does not validate for consensus rules is "invalid" (regardless of upgradable rule validation).
The ruleset sometimes changes. This BIP does not intend to be complete, nor does it indicate enforcement of rules, it simply lists the rules as they stand at the point of writing.
