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

1 files changed, 8 insertions, 4 deletions

diff --git a/bip-0342.mediawiki b/bip-0342.mediawiki
index 87e07ae..bbefcaa 100644
--- a/bip-0342.mediawiki
+++ b/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.