1  // Copyright (c) 2019-2021 The Bitcoin Core developers
  2  // Distributed under the MIT software license, see the accompanying
  3  // file COPYING or http://www.opensource.org/licenses/mit-license.php.
  4  
  5  #include <signet.h>
  6  
  7  #include <array>
  8  #include <cstdint>
  9  #include <vector>
 10  
 11  #include <common/system.h>
 12  #include <consensus/merkle.h>
 13  #include <consensus/params.h>
 14  #include <consensus/validation.h>
 15  #include <core_io.h>
 16  #include <hash.h>
 17  #include <logging.h>
 18  #include <primitives/block.h>
 19  #include <primitives/transaction.h>
 20  #include <script/interpreter.h>
 21  #include <span.h>
 22  #include <streams.h>
 23  #include <uint256.h>
 24  #include <util/strencodings.h>
 25  
 26  static constexpr uint8_t SIGNET_HEADER[4] = {0xec, 0xc7, 0xda, 0xa2};
 27  
 28  static constexpr unsigned int BLOCK_SCRIPT_VERIFY_FLAGS = SCRIPT_VERIFY_P2SH | SCRIPT_VERIFY_WITNESS | SCRIPT_VERIFY_DERSIG | SCRIPT_VERIFY_NULLDUMMY;
 29  
 30  static bool FetchAndClearCommitmentSection(const Span<const uint8_t> header, CScript& witness_commitment, std::vector<uint8_t>& result)
 31  {
 32      CScript replacement;
 33      bool found_header = false;
 34      result.clear();
 35  
 36      opcodetype opcode;
 37      CScript::const_iterator pc = witness_commitment.begin();
 38      std::vector<uint8_t> pushdata;
 39      while (witness_commitment.GetOp(pc, opcode, pushdata)) {
 40          if (pushdata.size() > 0) {
 41              if (!found_header && pushdata.size() > (size_t)header.size() && Span{pushdata}.first(header.size()) == header) {
 42                  // pushdata only counts if it has the header _and_ some data
 43                  result.insert(result.end(), pushdata.begin() + header.size(), pushdata.end());
 44                  pushdata.erase(pushdata.begin() + header.size(), pushdata.end());
 45                  found_header = true;
 46              }
 47              replacement << pushdata;
 48          } else {
 49              replacement << opcode;
 50          }
 51      }
 52  
 53      if (found_header) witness_commitment = replacement;
 54      return found_header;
 55  }
 56  
 57  static uint256 ComputeModifiedMerkleRoot(const CMutableTransaction& cb, const CBlock& block)
 58  {
 59      std::vector<uint256> leaves;
 60      leaves.resize(block.vtx.size());
 61      leaves[0] = cb.GetHash();
 62      for (size_t s = 1; s < block.vtx.size(); ++s) {
 63          leaves[s] = block.vtx[s]->GetHash();
 64      }
 65      return ComputeMerkleRoot(std::move(leaves));
 66  }
 67  
 68  std::optional<SignetTxs> SignetTxs::Create(const CBlock& block, const CScript& challenge)
 69  {
 70      CMutableTransaction tx_to_spend;
 71      tx_to_spend.nVersion = 0;
 72      tx_to_spend.nLockTime = 0;
 73      tx_to_spend.vin.emplace_back(COutPoint(), CScript(OP_0), 0);
 74      tx_to_spend.vout.emplace_back(0, challenge);
 75  
 76      CMutableTransaction tx_spending;
 77      tx_spending.nVersion = 0;
 78      tx_spending.nLockTime = 0;
 79      tx_spending.vin.emplace_back(COutPoint(), CScript(), 0);
 80      tx_spending.vout.emplace_back(0, CScript(OP_RETURN));
 81  
 82      // can't fill any other fields before extracting signet
 83      // responses from block coinbase tx
 84  
 85      // find and delete signet signature
 86      if (block.vtx.empty()) return std::nullopt; // no coinbase tx in block; invalid
 87      CMutableTransaction modified_cb(*block.vtx.at(0));
 88  
 89      const int cidx = GetWitnessCommitmentIndex(block);
 90      if (cidx == NO_WITNESS_COMMITMENT) {
 91          return std::nullopt; // require a witness commitment
 92      }
 93  
 94      CScript& witness_commitment = modified_cb.vout.at(cidx).scriptPubKey;
 95  
 96      std::vector<uint8_t> signet_solution;
 97      if (!FetchAndClearCommitmentSection(SIGNET_HEADER, witness_commitment, signet_solution)) {
 98          // no signet solution -- allow this to support OP_TRUE as trivial block challenge
 99      } else {
100          try {
101              SpanReader v{signet_solution};
102              v >> tx_spending.vin[0].scriptSig;
103              v >> tx_spending.vin[0].scriptWitness.stack;
104              if (!v.empty()) return std::nullopt; // extraneous data encountered
105          } catch (const std::exception&) {
106              return std::nullopt; // parsing error
107          }
108      }
109      uint256 signet_merkle = ComputeModifiedMerkleRoot(modified_cb, block);
110  
111      std::vector<uint8_t> block_data;
112      VectorWriter writer{block_data, 0};
113      writer << block.nVersion;
114      writer << block.hashPrevBlock;
115      writer << signet_merkle;
116      writer << block.nTime;
117      tx_to_spend.vin[0].scriptSig << block_data;
118      tx_spending.vin[0].prevout = COutPoint(tx_to_spend.GetHash(), 0);
119  
120      return SignetTxs{tx_to_spend, tx_spending};
121  }
122  
123  // Signet block solution checker
124  bool CheckSignetBlockSolution(const CBlock& block, const Consensus::Params& consensusParams)
125  {
126      if (block.GetHash() == consensusParams.hashGenesisBlock) {
127          // genesis block solution is always valid
128          return true;
129      }
130  
131      const CScript challenge(consensusParams.signet_challenge.begin(), consensusParams.signet_challenge.end());
132      const std::optional<SignetTxs> signet_txs = SignetTxs::Create(block, challenge);
133  
134      if (!signet_txs) {
135          LogPrint(BCLog::VALIDATION, "CheckSignetBlockSolution: Errors in block (block solution parse failure)\n");
136          return false;
137      }
138  
139      const CScript& scriptSig = signet_txs->m_to_sign.vin[0].scriptSig;
140      const CScriptWitness& witness = signet_txs->m_to_sign.vin[0].scriptWitness;
141  
142      PrecomputedTransactionData txdata;
143      txdata.Init(signet_txs->m_to_sign, {signet_txs->m_to_spend.vout[0]});
144      TransactionSignatureChecker sigcheck(&signet_txs->m_to_sign, /* nInIn= */ 0, /* amountIn= */ signet_txs->m_to_spend.vout[0].nValue, txdata, MissingDataBehavior::ASSERT_FAIL);
145  
146      if (!VerifyScript(scriptSig, signet_txs->m_to_spend.vout[0].scriptPubKey, &witness, BLOCK_SCRIPT_VERIFY_FLAGS, sigcheck)) {
147          LogPrint(BCLog::VALIDATION, "CheckSignetBlockSolution: Errors in block (block solution invalid)\n");
148          return false;
149      }
150      return true;
151  }