fix: use correct leafVersion in control block byte for non-default version leaves #2103

1. 
   jasonandjay commented at 2:03 am on February 15, 2026: none

   Summary

   Fix the first byte of scriptPathControlBlocks[1] in the p2mr_different_version_leaves test vector. The control block incorrectly uses 0xC1 (leafVersion 192) instead of 0xFB (leafVersion 250) for a leaf explicitly declared with "leafVersion": 250.

   Problem

   The P2MR control block’s first byte encodes both the leaf version and parity bit:

   0controlBlock[0] = (leafVersion & 0xfe) | parityBit
   

   For P2MR, the parity bit is always 1. So:

   • leafVersion 192 (0xC0) → 0xC1
   • leafVersion 250 (0xFA) → 0xFB

   The test vector defines two leaves:

   0{ "id": 0, "leafVersion": 192, "script": "...OP_CHECKSIG" }
   1{ "id": 1, "leafVersion": 250, "script": "06424950333431" }
   

   But the current control block for leaf1 starts with 0xC1:

   0scriptPathControlBlocks[1] = "c1 8ad69ec7cf41c2a4001fd1f738bf1e505ce2277acdcaa63fe4765192497f47a7"
   1                               ^^
   2                               0xC1 → leafVersion = 0xC0 = 192  ✗ (should be 250)
   

   This is incorrect because a verifier would extract leafVersion = 0xC1 & 0xFE = 0xC0 = 192 from this byte, then compute tapleafHash(script, version=192) which produces a completely different hash than the correct tapleafHash(script, version=250), making it impossible to reconstruct the correct Merkle root.

   Fix

   Change the first byte from 0xC1 to 0xFB:

   0scriptPathControlBlocks[1] = "fb 8ad69ec7cf41c2a4001fd1f738bf1e505ce2277acdcaa63fe4765192497f47a7"
   1                               ^^
   2                               0xFB → leafVersion = 0xFA = 250  ✓
   

   Verification:

   0tapleafHash(script="06424950333431", version=250) = f224a923cd0021ab202ab139cc56802ddb92dcfc172b9212261a539df79a112a
   1sibling = leafHash[0] = 8ad69ec7cf41c2a4001fd1f738bf1e505ce2277acdcaa63fe4765192497f47a7
   2
   3tapBranchHash(f224a923..., 8ad69ec7...) → hashes are sorted internally
   4→ 6c2dc106ab816b73f9d07e3cd1ef2c8c1256f519748e0813e4edd2405d277bef = merkleRoot ✓
   

   Changes

   • p2mr_different_version_leaves: change scriptPathControlBlocks[1] first byte from c1 to fb
2. fix: use correct leafVersion in control block byte for non-default version leaves

   The P2MR control block's first byte encodes both the leaf version and
   parity bit: controlBlock[0] = (leafVersion & 0xfe) | parityBit.
   For the leaf with leafVersion 250 (0xFA), the correct first byte is
   0xFB (with parity bit 1), not 0xC1 (which corresponds to leafVersion
   192).
   
   Fix p2mr_different_version_leaves scriptPathControlBlocks[1] first
   byte from c1 to fb.

   c4648bae6b

Contributors
jasonandjay