BIP112: Implement CHECKSEQUENCEVERIFY

- Replace NOP3 with CHECKSEQUENCEVERIFY (BIP112)
  <nSequence> CHECKSEQUENCEVERIFY -> <nSequence>
- Fails if txin.nSequence < nSequence, allowing funds of a txout to be locked for a number of blocks or a duration of time after its inclusion in a block.
- Pull most of CheckLockTime() out into VerifyLockTime(), a local function that will be reused for CheckSequence()
- Add bitwise AND operator to CScriptNum
- Enable CHECKSEQUENCEVERIFY as a standard script verify flag
- Transactions that fail CSV verification will be rejected from the mempool, making it easy to test the feature. However blocks containing "invalid" CSV-using transactions will still be accepted; this is *not* the soft-fork required to actually enable CSV for production use.

1 files changed, 82 insertions, 7 deletions

diff --git a/src/script/interpreter.cpp b/src/script/interpreter.cpp
index 901f901f01..4e87006f57 100644
--- a/src/script/interpreter.cpp
+++ b/src/script/interpreter.cpp
@@ -373,7 +373,44 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                     break;
                 }
 
-                case OP_NOP1: case OP_NOP3: case OP_NOP4: case OP_NOP5:
+                case OP_CHECKSEQUENCEVERIFY:
+                {
+                    if (!(flags & SCRIPT_VERIFY_CHECKSEQUENCEVERIFY)) {
+                        // not enabled; treat as a NOP3
+                        if (flags & SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_NOPS) {
+                            return set_error(serror, SCRIPT_ERR_DISCOURAGE_UPGRADABLE_NOPS);
+                        }
+                        break;
+                    }
+
+                    if (stack.size() < 1)
+                        return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
+
+                    // nSequence, like nLockTime, is a 32-bit unsigned integer
+                    // field. See the comment in CHECKLOCKTIMEVERIFY regarding
+                    // 5-byte numeric operands.
+                    const CScriptNum nSequence(stacktop(-1), fRequireMinimal, 5);
+
+                    // In the rare event that the argument may be < 0 due to
+                    // some arithmetic being done first, you can always use
+                    // 0 MAX CHECKSEQUENCEVERIFY.
+                    if (nSequence < 0)
+                        return set_error(serror, SCRIPT_ERR_NEGATIVE_LOCKTIME);
+
+                    // To provide for future soft-fork extensibility, if the
+                    // operand has the disabled lock-time flag set,
+                    // CHECKSEQUENCEVERIFY behaves as a NOP.
+                    if ((nSequence & CTxIn::SEQUENCE_LOCKTIME_DISABLE_FLAG) != 0)
+                        break;
+
+                    // Compare the specified sequence number with the input.
+                    if (!checker.CheckSequence(nSequence))
+                        return set_error(serror, SCRIPT_ERR_UNSATISFIED_LOCKTIME);
+
+                    break;
+                }
+
+                case OP_NOP1: case OP_NOP4: case OP_NOP5:
                 case OP_NOP6: case OP_NOP7: case OP_NOP8: case OP_NOP9: case OP_NOP10:
                 {
                     if (flags & SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_NOPS)
@@ -1120,27 +1157,33 @@ bool TransactionSignatureChecker::CheckSig(const vector<unsigned char>& vchSigIn
     return true;
 }
 
-bool TransactionSignatureChecker::CheckLockTime(const CScriptNum& nLockTime) const
+static bool VerifyLockTime(int64_t txToLockTime, int64_t nThreshold, const CScriptNum& nLockTime)
 {
     // There are two kinds of nLockTime: lock-by-blockheight
     // and lock-by-blocktime, distinguished by whether
-    // nLockTime < LOCKTIME_THRESHOLD.
+    // nLockTime < nThreshold (either LOCKTIME_THRESHOLD or
+    // CTxIn::SEQUENCE_LOCKTIME_TYPE_FLAG).
     //
     // We want to compare apples to apples, so fail the script
     // unless the type of nLockTime being tested is the same as
     // the nLockTime in the transaction.
     if (!(
-        (txTo->nLockTime <  LOCKTIME_THRESHOLD && nLockTime <  LOCKTIME_THRESHOLD) ||
-        (txTo->nLockTime >= LOCKTIME_THRESHOLD && nLockTime >= LOCKTIME_THRESHOLD)
+        (txToLockTime <  nThreshold && nLockTime <  nThreshold) ||
+        (txToLockTime >= nThreshold && nLockTime >= nThreshold)
     ))
         return false;
 
     // Now that we know we're comparing apples-to-apples, the
     // comparison is a simple numeric one.
-    if (nLockTime > (int64_t)txTo->nLockTime)
+    if (nLockTime > txToLockTime)
         return false;
 
-    // Finally the nLockTime feature can be disabled and thus
+    return true;
+}
+
+bool TransactionSignatureChecker::CheckLockTime(const CScriptNum& nLockTime) const
+{
+    // The nLockTime feature can be disabled and thus
     // CHECKLOCKTIMEVERIFY bypassed if every txin has been
     // finalized by setting nSequence to maxint. The
     // transaction would be allowed into the blockchain, making
@@ -1153,6 +1196,38 @@ bool TransactionSignatureChecker::CheckLockTime(const CScriptNum& nLockTime) con
     if (CTxIn::SEQUENCE_FINAL == txTo->vin[nIn].nSequence)
         return false;
 
+    if (!::VerifyLockTime((int64_t)txTo->nLockTime, LOCKTIME_THRESHOLD, nLockTime))
+        return false;
+
+    return true;
+}
+
+bool TransactionSignatureChecker::CheckSequence(const CScriptNum& nSequence) const
+{
+    // Relative lock times are supported by comparing the passed
+    // in operand to the sequence number of the input.
+    const int64_t txToSequence = (int64_t)txTo->vin[nIn].nSequence;
+
+    // Fail if the transaction's version number is not set high
+    // enough to trigger BIP 68 rules.
+    if (static_cast<uint32_t>(txTo->nVersion) < 2)
+        return false;
+
+    // Sequence numbers with their most significant bit set are not
+    // consensus constrained. Testing that the transaction's sequence
+    // number do not have this bit set prevents using this property
+    // to get around a CHECKSEQUENCEVERIFY check.
+    if (txToSequence & CTxIn::SEQUENCE_LOCKTIME_DISABLE_FLAG)
+        return false;
+
+    // Mask off any bits that do not have consensus-enforced meaning
+    // before doing the integer comparisons of ::VerifyLockTime.
+    const uint32_t nLockTimeMask = CTxIn::SEQUENCE_LOCKTIME_TYPE_FLAG
+                                 | CTxIn::SEQUENCE_LOCKTIME_MASK;
+
+    if (!::VerifyLockTime(txToSequence & nLockTimeMask, CTxIn::SEQUENCE_LOCKTIME_TYPE_FLAG, nSequence & nLockTimeMask))
+        return false;
+
     return true;
 }