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 approach minimizes the burden for implementers as message signing can be expected to be part of a library or project that includes Bitcoin Script interpreters already.
Additionally, the current message signing only proves that the message has been committed to by the recipient of a given invoice address.
It does not prove anything about the invoice address itself, nor that the signer has access to the private keys used to implement this invoice.
More importantly, it does not prove ownership nor access to any funds, even if the same private key would be a valid signer for spending them - and this is a commonly desired use case.
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.
# 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.
