BIP141: Commitment structure and deployment

Includes a fix by Suhas Daftuar and LongShao007

1 files changed, 138 insertions, 8 deletions

diff --git a/src/main.cpp b/src/main.cpp
index d1945a5094..8d91d6d625 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -293,6 +293,8 @@ struct CNodeState {
     bool fPreferHeaderAndIDs;
     //! Whether this peer will send us cmpctblocks if we request them
     bool fProvidesHeaderAndIDs;
+    //! Whether this peer can give us witnesses
+    bool fHaveWitness;
 
     CNodeState() {
         fCurrentlyConnected = false;
@@ -1119,6 +1121,11 @@ bool AcceptToMemoryPoolWorker(CTxMemPool& pool, CValidationState& state, const C
         return state.DoS(0, false, REJECT_NONSTANDARD, "premature-version2-tx");
     }
 
+    // Don't accept witness transactions before the final threshold passes
+    if (!tx.wit.IsNull() && !IsWitnessEnabled(chainActive.Tip(), Params().GetConsensus())) {
+        return state.DoS(0, false, REJECT_NONSTANDARD, "no-witness-yet", true);
+    }
+
     // Only accept nLockTime-using transactions that can be mined in the next
     // block; we don't want our mempool filled up with transactions that can't
     // be mined yet.
@@ -1459,8 +1466,17 @@ bool AcceptToMemoryPoolWorker(CTxMemPool& pool, CValidationState& state, const C
 
         // Check against previous transactions
         // This is done last to help prevent CPU exhaustion denial-of-service attacks.
-        if (!CheckInputs(tx, state, view, true, STANDARD_SCRIPT_VERIFY_FLAGS, true))
-            return false; // state filled in by CheckInputs
+        if (!CheckInputs(tx, state, view, true, STANDARD_SCRIPT_VERIFY_FLAGS, true)) {
+            // SCRIPT_VERIFY_CLEANSTACK requires SCRIPT_VERIFY_WITNESS, so we
+            // need to turn both off, and compare against just turning off CLEANSTACK
+            // to see if the failure is specifically due to witness validation.
+            if (CheckInputs(tx, state, view, true, STANDARD_SCRIPT_VERIFY_FLAGS & ~(SCRIPT_VERIFY_WITNESS | SCRIPT_VERIFY_CLEANSTACK), true) &&
+                !CheckInputs(tx, state, view, true, STANDARD_SCRIPT_VERIFY_FLAGS & ~SCRIPT_VERIFY_CLEANSTACK, true)) {
+                // Only the witness is wrong, so the transaction itself may be fine.
+                state.SetCorruptionPossible();
+            }
+            return false;
+        }
 
         // Check again against just the consensus-critical mandatory script
         // verification flags, in case of bugs in the standard flags that cause
@@ -2406,6 +2422,11 @@ bool ConnectBlock(const CBlock& block, CValidationState& state, CBlockIndex* pin
         nLockTimeFlags |= LOCKTIME_VERIFY_SEQUENCE;
     }
 
+    // Start enforcing WITNESS rules using versionbits logic.
+    if (IsWitnessEnabled(pindex->pprev, chainparams.GetConsensus())) {
+        flags |= SCRIPT_VERIFY_WITNESS;
+    }
+
     int64_t nTime2 = GetTimeMicros(); nTimeForks += nTime2 - nTime1;
     LogPrint("bench", "    - Fork checks: %.2fms [%.2fs]\n", 0.001 * (nTime2 - nTime1), nTimeForks * 0.000001);
 
@@ -3441,6 +3462,71 @@ static bool CheckIndexAgainstCheckpoint(const CBlockIndex* pindexPrev, CValidati
     return true;
 }
 
+bool IsWitnessEnabled(const CBlockIndex* pindexPrev, const Consensus::Params& params)
+{
+    LOCK(cs_main);
+    return (VersionBitsState(pindexPrev, params, Consensus::DEPLOYMENT_SEGWIT, versionbitscache) == THRESHOLD_ACTIVE);
+}
+
+// Compute at which vout of the block's coinbase transaction the witness
+// commitment occurs, or -1 if not found.
+static int GetWitnessCommitmentIndex(const CBlock& block)
+{
+    int commitpos = -1;
+    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) {
+            commitpos = o;
+        }
+    }
+    return commitpos;
+}
+
+void UpdateUncommittedBlockStructures(CBlock& block, const CBlockIndex* pindexPrev, const Consensus::Params& consensusParams)
+{
+    int commitpos = GetWitnessCommitmentIndex(block);
+    static const std::vector<unsigned char> nonce(32, 0x00);
+    if (commitpos != -1 && IsWitnessEnabled(pindexPrev, consensusParams) && block.vtx[0].wit.IsEmpty()) {
+        block.vtx[0].wit.vtxinwit.resize(1);
+        block.vtx[0].wit.vtxinwit[0].scriptWitness.stack.resize(1);
+        block.vtx[0].wit.vtxinwit[0].scriptWitness.stack[0] = nonce;
+    }
+}
+
+std::vector<unsigned char> GenerateCoinbaseCommitment(CBlock& block, const CBlockIndex* pindexPrev, const Consensus::Params& consensusParams)
+{
+    std::vector<unsigned char> commitment;
+    int commitpos = GetWitnessCommitmentIndex(block);
+    bool fHaveWitness = false;
+    for (size_t t = 1; t < block.vtx.size(); t++) {
+        if (!block.vtx[t].wit.IsNull()) {
+            fHaveWitness = true;
+            break;
+        }
+    }
+    std::vector<unsigned char> ret(32, 0x00);
+    if (fHaveWitness && IsWitnessEnabled(pindexPrev, consensusParams)) {
+        if (commitpos == -1) {
+            uint256 witnessroot = BlockWitnessMerkleRoot(block, NULL);
+            CHash256().Write(witnessroot.begin(), 32).Write(&ret[0], 32).Finalize(witnessroot.begin());
+            CTxOut out;
+            out.nValue = 0;
+            out.scriptPubKey.resize(38);
+            out.scriptPubKey[0] = OP_RETURN;
+            out.scriptPubKey[1] = 0x24;
+            out.scriptPubKey[2] = 0xaa;
+            out.scriptPubKey[3] = 0x21;
+            out.scriptPubKey[4] = 0xa9;
+            out.scriptPubKey[5] = 0xed;
+            memcpy(&out.scriptPubKey[6], witnessroot.begin(), 32);
+            commitment = std::vector<unsigned char>(out.scriptPubKey.begin(), out.scriptPubKey.end());
+            const_cast<std::vector<CTxOut>*>(&block.vtx[0].vout)->push_back(out);
+            block.vtx[0].UpdateHash();
+        }
+    }
+    UpdateUncommittedBlockStructures(block, pindexPrev, consensusParams);
+    return commitment;
+}
+
 bool ContextualCheckBlockHeader(const CBlockHeader& block, CValidationState& state, const Consensus::Params& consensusParams, CBlockIndex * const pindexPrev, int64_t nAdjustedTime)
 {
     // Check proof of work
@@ -3497,6 +3583,43 @@ bool ContextualCheckBlock(const CBlock& block, CValidationState& state, CBlockIn
         }
     }
 
+    // Validation for witness commitments.
+    // * We compute the witness hash (which is the hash including witnesses) of all the block's transactions, except the
+    //   coinbase (where 0x0000....0000 is used instead).
+    // * The coinbase scriptWitness is a stack of a single 32-byte vector, containing a witness nonce (unconstrained).
+    // * We build a merkle tree with all those witness hashes as leaves (similar to the hashMerkleRoot in the block header).
+    // * There must be at least one output whose scriptPubKey is a single 36-byte push, the first 4 bytes of which are
+    //   {0xaa, 0x21, 0xa9, 0xed}, and the following 32 bytes are SHA256^2(witness root, witness nonce). In case there are
+    //   multiple, the last one is used.
+    bool fHaveWitness = false;
+    if (IsWitnessEnabled(pindexPrev, consensusParams)) {
+        int commitpos = GetWitnessCommitmentIndex(block);
+        if (commitpos != -1) {
+            bool malleated = false;
+            uint256 hashWitness = BlockWitnessMerkleRoot(block, &malleated);
+            // The malleation check is ignored; as the transaction tree itself
+            // already does not permit it, it is impossible to trigger in the
+            // witness tree.
+            if (block.vtx[0].wit.vtxinwit.size() != 1 || block.vtx[0].wit.vtxinwit[0].scriptWitness.stack.size() != 1 || block.vtx[0].wit.vtxinwit[0].scriptWitness.stack[0].size() != 32) {
+                return state.DoS(100, error("%s : invalid witness nonce size", __func__), REJECT_INVALID, "bad-witness-nonce-size", true);
+            }
+            CHash256().Write(hashWitness.begin(), 32).Write(&block.vtx[0].wit.vtxinwit[0].scriptWitness.stack[0][0], 32).Finalize(hashWitness.begin());
+            if (memcmp(hashWitness.begin(), &block.vtx[0].vout[commitpos].scriptPubKey[6], 32)) {
+                return state.DoS(100, error("%s : witness merkle commitment mismatch", __func__), REJECT_INVALID, "bad-witness-merkle-match", true);
+            }
+            fHaveWitness = true;
+        }
+    }
+
+    // No witness data is allowed in blocks that don't commit to witness data, as this would otherwise leave room for spam
+    if (!fHaveWitness) {
+        for (size_t i = 0; i < block.vtx.size(); i++) {
+            if (!block.vtx[i].wit.IsNull()) {
+                return state.DoS(100, error("%s : unexpected witness data found", __func__), REJECT_INVALID, "unexpected-witness", true);
+            }
+        }
+    }
+
     return true;
 }
 
@@ -5278,7 +5401,7 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
                     else if (!fMissingInputs2)
                     {
                         int nDos = 0;
-                        if (stateDummy.IsInvalid(nDos) && nDos > 0)
+                        if (stateDummy.IsInvalid(nDos) && nDos > 0 && (!state.CorruptionPossible() || State(fromPeer)->fHaveWitness))
                         {
                             // Punish peer that gave us an invalid orphan tx
                             Misbehaving(fromPeer, nDos);
@@ -5289,8 +5412,10 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
                         // Probably non-standard or insufficient fee/priority
                         LogPrint("mempool", "   removed orphan tx %s\n", orphanHash.ToString());
                         vEraseQueue.push_back(orphanHash);
-                        assert(recentRejects);
-                        recentRejects->insert(orphanHash);
+                        if (!stateDummy.CorruptionPossible()) {
+                            assert(recentRejects);
+                            recentRejects->insert(orphanHash);
+                        }
                     }
                     mempool.check(pcoinsTip);
                 }
@@ -5325,8 +5450,10 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
                 LogPrint("mempool", "not keeping orphan with rejected parents %s\n",tx.GetHash().ToString());
             }
         } else {
-            assert(recentRejects);
-            recentRejects->insert(tx.GetHash());
+            if (!state.CorruptionPossible()) {
+                assert(recentRejects);
+                recentRejects->insert(tx.GetHash());
+            }
 
             if (pfrom->fWhitelisted && GetBoolArg("-whitelistforcerelay", DEFAULT_WHITELISTFORCERELAY)) {
                 // Always relay transactions received from whitelisted peers, even
@@ -5355,8 +5482,11 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
             if (state.GetRejectCode() < REJECT_INTERNAL) // Never send AcceptToMemoryPool's internal codes over P2P
                 pfrom->PushMessage(NetMsgType::REJECT, strCommand, (unsigned char)state.GetRejectCode(),
                                    state.GetRejectReason().substr(0, MAX_REJECT_MESSAGE_LENGTH), inv.hash);
-            if (nDoS > 0)
+            if (nDoS > 0 && (!state.CorruptionPossible() || State(pfrom->id)->fHaveWitness)) {
+                // When a non-witness-supporting peer gives us a transaction that would
+                // be accepted if witness validation was off, we can't blame them for it.
                 Misbehaving(pfrom->GetId(), nDoS);
+            }
         }
         FlushStateToDisk(state, FLUSH_STATE_PERIODIC);
     }