BIP341/342: Clarify SigHash extensions

* Pull the definition of the extension in BIP342 to its own section
* Add a section to BIP341 on validating script path signatures
* Clarify that SigMsg does not produce the message being signed, but
a common portion of it

bip-0341.mediawiki

@@ -88,13 +88,13 @@ We first define a reusable common signature message calculation function, follow
 
 ==== Common signature message ====
 
-The function ''SigMsg(hash_type, ext_flag)'' computes the message being signed as a byte array. It is implicitly also a function of the spending transaction and the outputs it spends, but these are not listed to keep notation simple.
+The function ''SigMsg(hash_type, ext_flag)'' computes the common portion of the message being signed as a byte array. It is implicitly also a function of the spending transaction and the outputs it spends, but these are not listed to keep notation simple.
 
 The parameter ''hash_type'' is an 8-bit unsigned value. The <code>SIGHASH</code> encodings from the legacy script system are reused, including <code>SIGHASH_ALL</code>, <code>SIGHASH_NONE</code>, <code>SIGHASH_SINGLE</code>, and <code>SIGHASH_ANYONECANPAY</code>. We define a new ''hashtype'' <code>SIGHASH_DEFAULT</code> (value ''0x00'') which results in signing over the whole transaction just as for <code>SIGHASH_ALL</code>. The following restrictions apply, which cause validation failure if violated:
 * Using any undefined ''hash_type'' (not ''0x00'', ''0x01'', ''0x02'', ''0x03'', ''0x81'', ''0x82'', or ''0x83''<ref>'''Why reject unknown ''hash_type'' values?''' By doing so, it is easier to reason about the worst case amount of signature hashing an implementation with adequate caching must perform.</ref>).
 * Using <code>SIGHASH_SINGLE</code> without a "corresponding output" (an output with the same index as the input being verified).
 
-The parameter ''ext_flag'' is an integer in range 0-127, and is used for indicating (in the message) that extensions are added at the end of the message<ref>'''What extensions use the ''ext_flag'' mechanism?''' [[bip-0342.mediawiki|BIP342]] reuses the same common signature message algorithm, but adds BIP342-specific data at the end, which is indicated using ''ext_flag = 1''.</ref>.
+The parameter ''ext_flag'' is an integer in range 0-127, and is used for indicating (in the message) that extensions are appended to the output of ''SigMsg()''<ref>'''What extensions use the ''ext_flag'' mechanism?''' [[bip-0342.mediawiki#common-signature-message-extension|BIP342]] reuses the same common signature message algorithm, but adds BIP342-specific data at the end, which is indicated using ''ext_flag = 1''.</ref>.
 
 If the parameters take acceptable values, the message is the concatenation of the following data, in order (with byte size of each item listed in parentheses). Numerical values in 2, 4, or 8-byte are encoded in little-endian.
 

bip-0342.mediawiki

@@ -104,13 +104,17 @@ The following rules apply to <code>OP_CHECKSIG</code>, <code>OP_CHECKSIGVERIFY</
 *** For <code>OP_CHECKSIG</code>, a 1-byte value <code>0x01</code> is pushed onto the stack.
 *** For <code>OP_CHECKSIGADD</code>, a <code>CScriptNum</code> with value of <code>n + 1</code> is pushed onto the stack.
 
+===Common Signature Message Extension===
+
+We define the tapscript message extension ''ext'' to [[bip-0341.mediawiki#common-signature-message|BIP341 Common Signature Message]], indicated by ''ext_flag = 1'':
+* ''tapleaf_hash'' (32): the tapleaf hash as defined in [[bip-0341.mediawiki#design|BIP341]]
+* ''key_version'' (1): a constant value ''0x00'' representing the current version of public keys in the tapscript signature opcode execution.
+* ''codesep_pos'' (4): the opcode position of the last executed <code>OP_CODESEPARATOR</code> before the currently executed signature opcode, with the value in little endian (or ''0xffffffff'' if none executed). The first opcode in a script has a position of 0. A multi-byte push opcode is counted as one opcode, regardless of the size of data being pushed. Opcodes in parsed but unexecuted branches count towards this value as well.
+
 ===Signature validation===
 
 To validate a signature ''sig'' with public key ''p'':
-* Compute the tapscript message extension ''ext'', consisting of the concatenation of:
-** ''tapleaf_hash'' (32): the tapleaf hash as defined in [[bip-0341.mediawiki#design|BIP341]]
-** ''key_version'' (1): a constant value ''0x00'' representing the current version of public keys in the tapscript signature opcode execution.
-** ''codesep_pos'' (4): the opcode position of the last executed <code>OP_CODESEPARATOR</code> before the currently executed signature opcode, with the value in little endian (or ''0xffffffff'' if none executed). The first opcode in a script has a position of 0. A multi-byte push opcode is counted as one opcode, regardless of the size of data being pushed. Opcodes in parsed but unexecuted branches count towards this value as well.
+* Compute the tapscript message extension ''ext'' described above.
 * If the ''sig'' is 64 bytes long, return ''Verify(p, hash<sub>TapSighash</sub>(0x00 || SigMsg(0x00, 1) || ext), sig)'', where ''Verify'' is defined in [[bip-0340.mediawiki#design|BIP340]].
 * If the ''sig'' is 65 bytes long, return ''sig[64] &ne; 0x00 and Verify(p, hash<sub>TapSighash</sub>(0x00 || SigMsg(sig[64], 1) || ext), sig[0:64])''.
 * Otherwise, fail.