Merge #18780: validation: add const for minimum witness commitment size

692f8307fc1449299b90182e7d79efb81a55d7ab test: add test for witness commitment index (fanquake)
06442549f8b725f46c1c727e9eb6fde6b843503c validation: Add minimum witness commitment size constant (fanquake)

Pull request description:

  https://github.com/bitcoin/bitcoin/commit/16101de5f33be494019a3f81755e204d00c22347: Per [BIP 141](https://github.com/bitcoin/bips/blob/master/bip-0141.mediawiki#Commitment_structure), the witness commitment structure is at least 38 bytes,
  OP_RETURN (0x6a) + 36 (0x24) + 4 byte header (0xaa21a9ed) + 32 byte
  SHA256 hash. It can be longer, however any additional data has no
  consensus meaning.

  https://github.com/bitcoin/bitcoin/commit/54f8c48d6ac973024df35c4db038791b7958a51d: As per BIP 141, if there is more than 1 pubkey that matches the witness
  commitment structure, the one with the highest output index should be
  chosen. This adds a sanity check that we are doing that, which will fail
  if anyone tries to "optimize" GetWitnessCommitmentIndex() by returning
  early.

ACKs for top commit:
  MarcoFalke:
    ACK 692f8307fc1449299b90182e7d79efb81a55d7ab 🌵
  jonatack:
    Code review ACK 692f830
  ajtowns:
    ACK 692f8307fc1449299b90182e7d79efb81a55d7ab
  jnewbery:
    utACK 692f8307fc1449299b90182e7d79efb81a55d7ab
  laanwj:
    ACK 692f8307fc1449299b90182e7d79efb81a55d7ab

Tree-SHA512: 7af3fe4b8a52fea2cdd0aec95f7bb935351a77b73d934bc88d6625a3503311b2a062cba5190b2228f97caa76840db3889032d910fc8e318ca8e7810a8afbafa0

1 files changed, 9 insertions, 2 deletions

diff --git a/src/validation.cpp b/src/validation.cpp
index d18803b342..259397051c 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -3386,7 +3386,14 @@ int GetWitnessCommitmentIndex(const CBlock& block)
     int commitpos = -1;
     if (!block.vtx.empty()) {
         for (size_t o = 0; o < block.vtx[0]->vout.size(); o++) {
-            if (block.vtx[0]->vout[o].scriptPubKey.size() >= 38 && block.vtx[0]->vout[o].scriptPubKey[0] == OP_RETURN && block.vtx[0]->vout[o].scriptPubKey[1] == 0x24 && block.vtx[0]->vout[o].scriptPubKey[2] == 0xaa && block.vtx[0]->vout[o].scriptPubKey[3] == 0x21 && block.vtx[0]->vout[o].scriptPubKey[4] == 0xa9 && block.vtx[0]->vout[o].scriptPubKey[5] == 0xed) {
+            const CTxOut& vout = block.vtx[0]->vout[o];
+            if (vout.scriptPubKey.size() >= MINIMUM_WITNESS_COMMITMENT &&
+                vout.scriptPubKey[0] == OP_RETURN &&
+                vout.scriptPubKey[1] == 0x24 &&
+                vout.scriptPubKey[2] == 0xaa &&
+                vout.scriptPubKey[3] == 0x21 &&
+                vout.scriptPubKey[4] == 0xa9 &&
+                vout.scriptPubKey[5] == 0xed) {
                 commitpos = o;
             }
         }
@@ -3417,7 +3424,7 @@ std::vector<unsigned char> GenerateCoinbaseCommitment(CBlock& block, const CBloc
             CHash256().Write(witnessroot.begin(), 32).Write(ret.data(), 32).Finalize(witnessroot.begin());
             CTxOut out;
             out.nValue = 0;
-            out.scriptPubKey.resize(38);
+            out.scriptPubKey.resize(MINIMUM_WITNESS_COMMITMENT);
             out.scriptPubKey[0] = OP_RETURN;
             out.scriptPubKey[1] = 0x24;
             out.scriptPubKey[2] = 0xaa;