BIP-322: switch to tx based approach

Co-authored-by: Stepan Snigirev <stepan.snigirev@mpq.mpg.de>
Co-authored-by: Luke Dashjr <luke_github1@dashjr.org>

1 files changed, 59 insertions, 137 deletions

diff --git a/bip-0322.mediawiki b/bip-0322.mediawiki
index 95991e6..e515b56 100644
--- a/bip-0322.mediawiki
+++ b/bip-0322.mediawiki
@@ -13,119 +13,99 @@
 
 == Abstract ==
 
-A standard for interoperable generic signed messages based on the Bitcoin Script format.
-
-== Background ==
-
-* Assume two actors, a prover <code>P</code> and a verifier <code>V</code>.
-* <code>P</code> wants to prove that they own the private key <code>k</code> associated with a given address <code>A</code> (which in turn is derived from the pubkey <code>kG</code>).
-* Let <code>V</code> generate a message <code>M</code> and hand this to <code>P</code>.
-* <code>P</code> generates a signature <code>S</code> by signing the message <code>M</code> using <code>k</code>. Given <code>S</code>, <code>V</code> can prove that <code>P</code> has the private key associated with <code>A</code>.
-
-The astute reader will notice that the above is missing a critical part, namely the pubkey <code>kG</code>, without which the verifier cannot actually verify the message. The current message signing standard solves this via a cryptographic trick, wherein the signature <code>S</code> above is a special "recoverable signature" type. Given the message <code>M</code> and the signature <code>S</code>, it is then possible to recover the pubkey <code>kG</code>. The system thus derives the address for the pubkey <code>kG</code>, and if it does not match <code>A</code>, the proof is deemed invalid.
-
-While this is a neat trick, it unnecessarily restricts and complicates the message signing mechanism; for instance, it is currently not possible to sign a message for a P2SH address, because there is no pubkey to recover from the resulting signature.
+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.
 
 == Motivation ==
 
-The current message signing standard only works for P2PKH (1...) addresses. By extending it to use a Bitcoin Script based approach, it could be made more generic without causing a too big burden on implementers, who most likely have access to Bitcoin Script interpreters already.
-
-== Specification ==
+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.
 
-A new structure <code>SignatureProof</code> is added, which is a simple serializable scriptSig & witness container.
+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.
 
-=== SignatureProof container ===
+== Specification ==
 
-{|class="wikitable" style="text-align: center;"
-|-
-!Type
-!Length
-!Name
-!Comment
-|-
-|VarInt||1-8||scriptsiglen||Number of bytes in scriptSig data
-|-
-|Uint8*||[scriptsiglen]||scriptsig||ScriptSig data
-|-
-|VarInt||1-8||witlen||Number of entries in witness stack
-|-
-|Uint8[]*||[witlen]||wit||Witness stack, as [witlen] uint8* vectors, each one prepended with a varint of its size
-|}
+This BIP follows the specification of BIP-325 challenges and solutions (see Signet comparison below).
 
-In some cases, the scriptsig or wit may be empty. If both are empty, the proof is incomplete.
+Let there be two virtual transactions to_spend and to_sign.
 
-=== Result Codes ===
+The "to_spend" transaction is:
 
-A verification call will return a result code according to the table below.
+    nVersion = 0
+    nLockTime = 0
+    vin[0].prevout.hash = 0000...000
+    vin[0].prevout.n = 0xFFFFFFFF
+    vin[0].nSequence = 0
+    vin[0].scriptSig = OP_0 PUSH32[ message_hash ]
+    vin[0].scriptWitness = []
+    vout[0].nValue = 0
+    vout[0].scriptPubKey = message_challenge
 
-{|class="wikitable" style="text-align: center;"
-|-
-!Code
-!Description
-|-
-|INCOMPLETE||Empty proof.
-|-
-|INCONCLUSIVE||The given proof was consensus-valid but policy-invalid.
-|-
-|VALID||The proof was valid.
-|-
-|INVALID||The proof was invalid
-|-
-|ERROR||An error was encountered
-|}
+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.
 
-== Signing and Verifying ==
+The "to_sign" transaction is:
 
-If the challenge consists of an address is in the P2PKH (legacy) format, sign using the legacy format (further information below). Otherwise continue as stated below.
+    nVersion = 0 or as appropriate (e.g. 2, for time locks)
+    nLockTime = 0 or as appropriate (for time locks)
+    vin[0].prevout.hash = to_spend.txid
+    vin[0].prevout.n = 0
+    vin[0].nSequence = 0 or as appropriate (for time locks)
+    vin[0].scriptWitness = message_signature
+    vout[0].nValue = 0
+    vout[0].scriptPubKey = OP_RETURN
 
-For both cases, generate a sighash based on the given scriptPubKey and message as follows:
+When a proof of funds is being created, additional inputs should be included for virtually spending transaction outputs of desired value.
 
-# Define the message pre-image as the sequence "Bitcoin Signed Message:\n" concatenated with the message, encoded in UTF-8 using Normalization Form Compatibility Decomposition (NFKD)
-# Let sighash = sha256(sha256(scriptPubKey || pre-image))
+* All signatures must use the SIGHASH_ALL flag.
+* 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.
 
-A private key may be used directly to sign a message. In this case, its P2WPKH bech32 address shall be derived, and used as the input.
+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.
 
-=== Signing ===
+A validator must verify it is valid and meets the description of virtual transactions as specified above. See "Validation" below.
 
-The signature is generated as follows:
+=== Validation ===
 
-# Derive the private key privkey for the scriptPubKey; FAIL if not VALID
-# Generate and return a signature sig with privkey=privkey, sighash=sighash
-
-=== Verifying ===
+To validate a simple proof, the following steps must be taken:
 
-Verify a proof, given a standard flags value, a script sig, an optional witness, and a derived sighash as described above.
+# construct the to_spend and to_sign transactions, based on the specification above
+# check the signature using consensus rules, then upgradable rules
 
-While omitted below, ERROR is returned if an unforeseen error occurs at any point in the process. A concrete example of this is if a legacy proof is given as input to a non-legacy address; the deserialization of the proof will fail in this case, and this should result in an ERROR result.
+To validate a proof of funds, the following steps must be taken:
 
-# Verify Script with flags=consensus flags (currently P2SH, DERSIG, NULLDUMMY, CLTV, CSV, WITNESS), scriptSig=script sig, scriptPubKey=scriptPubKey, witness=witness, and sighash=sighash
-# Return INVALID if verification fails
-# Verify Script with flags=standard flags (above plus STRICTENC, MINIMALDATA, etc.), scriptSig=script sig, scriptPubKey=scriptPubKey, witness=witness, and sighash=sighash
-# Return VALID if verification succeeds, otherwise return INCONCLUSIVE
+# 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
 
 == Legacy format ==
 
-The legacy format is restricted to the legacy P2PKH address format.
+New proofs should use the new format for all invoice address formats, including P2PKH.
 
-Any other input (i.e. non-P2PKH address format) must be signed using the new format described above.
+The legacy format MAY be used, but must be restricted to the legacy P2PKH invoice address format.
 
 === Signing ===
 
-Given the P2PKH 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> and the message <code>m</code>, and the pubkey-hash function <code>pkh(P) = ripemd160(sha256(P))</code>:
 
 # let <code>p</code> be the pubkey-hash <code>pkh(P)</code> for the pubkey <code>P</code>, contained in <code>a</code>
 # let <code>x</code> be the private key associated with <code>P</code> so that <code>pkh(xG) = p</code>
-# let <code>digest</code> be <code>SHA56d("Bitcoin Signed Message:\n"||m)</code>
+# let <code>digest</code> be <code>SHA56d(0x18||"Bitcoin Signed Message:\n"||compactint(len(m))||m)</code>
 # create a compact signature <code>sig</code> (aka "recoverable ECDSA signature") using <code>x</code> on <code>digest</code>
 
 The resulting proof is <code>sig</code>, serialized using the base64 encoding.
 
 === Verifying ===
 
-Given the P2PKH 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>:
+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>:
 
 # let <code>p</code> be the pubkey-hash <code>pkh(P)</code> for the pubkey <code>P</code>, contained in <code>a</code>
-# let <code>digest</code> be <code>SHA56d("Bitcoin Signed Message:\n"||m)</code>
+# let <code>digest</code> be <code>SHA56d(0x18||"Bitcoin Signed Message:\n"||compactint(len(m))||m)</code>
 # attempt pubkey recovery for <code>digest</code> using the signature <code>sig</code> and store the resulting pubkey into <code>Q</code>
 ## fail verification if pubkey recovery above fails
 # let <code>q</code> be the pubkey-hash <code>pkh(Q)</code> for the pubkey <code>Q</code>
@@ -137,7 +117,7 @@ This specification is backwards compatible with the legacy signmessage/verifymes
 
 == Reference implementation ==
 
-# Pull request to Bitcoin Core: https://github.com/bitcoin/bitcoin/pull/16440
+TODO
 
 == Acknowledgements ==
 
@@ -153,9 +133,9 @@ This document is licensed under the Creative Commons CC0 1.0 Universal license.
 
 == Consensus and standard flags ==
 
-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 policy flags (which result in a transaction being accepted only if it is contained within an actual block, and rejected otherwise, if violated). The policy flags are a super-set of the consensus flags.
+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.
 
-BIP322 specifies that a proof that validates for both rulesets is valid, a proof that validates for consensus rules, but not for policy rules, is "inconclusive", and a proof that does not validate for consensus rules is "invalid" (regardless of policy rule validation).
+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.
 
@@ -168,7 +148,7 @@ The ruleset sometimes changes. This BIP does not intend to be complete, nor does
 * CHECKSEQUENCEVERIFY: enable CHECKSEQUENCEVERIFY ([https://github.com/bitcoin/bips/blob/master/bip-0112.mediawiki BIP112])
 * WITNESS: enable WITNESS ([https://github.com/bitcoin/bips/blob/master/bip-0141.mediawiki BIP141])
 
-=== Policy rules ===
+=== Upgradable rules ===
 
 All of the above, plus (subject to change):
 
@@ -185,62 +165,4 @@ All of the above, plus (subject to change):
 
 == Test vectors ==
 
-(TODO: update test vectors, which are based on previous iteration where signature proofs contained additional data)
-
-== Native segwit test vector ==
-
-<pre>
-address      = bcrt1qe7nte4zk4ayly5tc53dtdjupgkz0lr8azx3rzz
-scriptpubkey = 0014cfa6bcd456af49f25178a45ab6cb814584ff8cfd
-message      = hello
-preimage     = 0014cfa6bcd456af49f25178a45ab6cb814584ff8cfd426974636f696e205369
-               676e6564204d6573736167653a0a68656c6c6f
-               (scriptpubkey || "Bitcoin Signed Message:\nhello")
-sighash      = 790eef86c204f0bff969ff822121317aa34eff0215dbd30ccf031e7b2f3f0cc1
-               (sha256d(preimage), displayed in big-endian)
-</pre>
-
-The proof becomes:
-
-<pre>
-HEX:    01000000010002473044022075b4fb40421d55c55462879cb352a85eeb3af2138d3f0290
-        2c9143f12870f5f70220119c2995c1661138142f3899c1fd6d1af7e790e0e081be72db9c
-        e7bf5b5b932901210290beccd02b73eca57467b2b6f1e47161a9b76a5e67586e7c1dee9e
-        a6e2dcd869
-
-Base64: AQAAAAEAAkcwRAIgdbT7QEIdVcVUYoecs1KoXus68hONPwKQLJFD8Shw9fcCIBGcKZXBZhE4
-        FC84mcH9bRr355Dg4IG+ctuc579bW5MpASECkL7M0Ctz7KV0Z7K28eRxYam3al5nWG58He6e
-        puLc2Gk=
-</pre>
-
-Split into components:
-
-{|class="wikitable" style="text-align: center;"
-|-
-!Type
-!Length
-!Name
-!Value
-!Comment
-|-
-|Uint32||4||flags||<code>01000000</code>||proof format version
-|-
-|Uint8||1||entries||<code>01</code>||1 entry
-|-
-|VarInt||1-8||scriptsiglen||<code>00</code>||0 byte scriptsig
-|-
-|VarInt||1-8||wit entries||<code>02</code>||2 witness stack entries
-|-
-|VarInt||1-8||entry1len||<code>47</code>||71 byte entry
-|-
-|Uint8[71]||71||entry1||<code>3044022075b4fb40421d55c55462879cb352a85eeb3af213
-8d3f02902c9143f12870f5f70220119c2995c1661138142f
-3899c1fd6d1af7e790e0e081be72db9ce7bf5b5b932901</code>||Witness stack item 1
-|-
-|VarInt||1-8||entry2len||<code>21</code>||33 byte entry
-|-
-|Uint8[33]||33||entry2||<code>0290beccd02b73eca57467b2b6f1e47161a9b76a5e67586e
-7c1dee9ea6e2dcd869</code>||Witness stack item 2
-|}
-
-The above test vector is for a bech32 P2WPKH (native segwit) address. (Once BIP solidifies, will add test vector for other types.)
+TODO