Leaf Version as Flags

Chris_Stewart_5 · April 12, 2024, 3:40pm

Hi Ben!

I don’t believe this can be changed anymore in v1 taproot. We serialize the entire byte representation of the leaf version into the tap leaf hash.

In other words, we commit to the full byte rather than an individual bit in the leaf version.

github.com

bitcoin/bitcoin/blob/0de63b8b46eff5cda85b4950062703324ba65a80/src/script/interpreter.cpp#L1828


      
          
              // Run the script interpreter.
              if (!EvalScript(stack, exec_script, flags, checker, sigversion, execdata, serror)) return false;
          
              // Scripts inside witness implicitly require cleanstack behaviour
              if (stack.size() != 1) return set_error(serror, SCRIPT_ERR_CLEANSTACK);
              if (!CastToBool(stack.back())) return set_error(serror, SCRIPT_ERR_EVAL_FALSE);
              return true;
          }
          
          uint256 ComputeTapleafHash(uint8_t leaf_version, Span<const unsigned char> script)
          {
              return (HashWriter{HASHER_TAPLEAF} << leaf_version << CompactSizeWriter(script.size()) << script).GetSHA256();
          }
          
          uint256 ComputeTapbranchHash(Span<const unsigned char> a, Span<const unsigned char> b)
          {
              HashWriter ss_branch{HASHER_TAPBRANCH};
              if (std::lexicographical_compare(a.begin(), a.end(), b.begin(), b.end())) {
                  ss_branch << a << b;
              } else {

Whenever we roll out witness version 2, i believe we could change semantics around this. However, when we do roll out version 2 we could totally redefine semantics for leaf versions any way. With the pace we deploy things currently, 256 leaf versions more than adequate imo.