1 files changed, 27 insertions, 10 deletions

diff --git a/src/script/interpreter.cpp b/src/script/interpreter.cpp
index e979f61f6e..8a06f3d118 100644
--- a/src/script/interpreter.cpp
+++ b/src/script/interpreter.cpp
@@ -189,6 +189,11 @@ bool static IsDefinedHashtypeSignature(const valtype &vchSig) {
 }
 
 bool static CheckSignatureEncoding(const valtype &vchSig, unsigned int flags, ScriptError* serror) {
+    // Empty signature. Not strictly DER encoded, but allowed to provide a
+    // compact way to provide an invalid signature for use with CHECK(MULTI)SIG
+    if (vchSig.size() == 0) {
+        return true;
+    }
     if ((flags & (SCRIPT_VERIFY_DERSIG | SCRIPT_VERIFY_LOW_S | SCRIPT_VERIFY_STRICTENC)) != 0 && !IsDERSignature(vchSig)) {
         return set_error(serror, SCRIPT_ERR_SIG_DER);
     } else if ((flags & SCRIPT_VERIFY_LOW_S) != 0 && !IsLowDERSignature(vchSig, serror)) {
@@ -1098,7 +1103,6 @@ bool VerifyScript(const CScript& scriptSig, const CScript& scriptPubKey, unsigne
         return false;
     if (stack.empty())
         return set_error(serror, SCRIPT_ERR_EVAL_FALSE);
-
     if (CastToBool(stack.back()) == false)
         return set_error(serror, SCRIPT_ERR_EVAL_FALSE);
 
@@ -1109,24 +1113,37 @@ bool VerifyScript(const CScript& scriptSig, const CScript& scriptPubKey, unsigne
         if (!scriptSig.IsPushOnly())
             return set_error(serror, SCRIPT_ERR_SIG_PUSHONLY);
 
-        // stackCopy cannot be empty here, because if it was the
+        // Restore stack.
+        swap(stack, stackCopy);
+
+        // stack cannot be empty here, because if it was the
         // P2SH  HASH <> EQUAL  scriptPubKey would be evaluated with
         // an empty stack and the EvalScript above would return false.
-        assert(!stackCopy.empty());
+        assert(!stack.empty());
 
-        const valtype& pubKeySerialized = stackCopy.back();
+        const valtype& pubKeySerialized = stack.back();
         CScript pubKey2(pubKeySerialized.begin(), pubKeySerialized.end());
-        popstack(stackCopy);
+        popstack(stack);
 
-        if (!EvalScript(stackCopy, pubKey2, flags, checker, serror))
+        if (!EvalScript(stack, pubKey2, flags, checker, serror))
             // serror is set
             return false;
-        if (stackCopy.empty())
+        if (stack.empty())
             return set_error(serror, SCRIPT_ERR_EVAL_FALSE);
-        if (!CastToBool(stackCopy.back()))
+        if (!CastToBool(stack.back()))
             return set_error(serror, SCRIPT_ERR_EVAL_FALSE);
-        else
-            return set_success(serror);
+    }
+
+    // The CLEANSTACK check is only performed after potential P2SH evaluation,
+    // as the non-P2SH evaluation of a P2SH script will obviously not result in
+    // a clean stack (the P2SH inputs remain).
+    if ((flags & SCRIPT_VERIFY_CLEANSTACK) != 0) {
+        // Disallow CLEANSTACK without P2SH, as otherwise a switch CLEANSTACK->P2SH+CLEANSTACK
+        // would be possible, which is not a softfork (and P2SH should be one).
+        assert((flags & SCRIPT_VERIFY_P2SH) != 0);
+        if (stack.size() != 1) {
+            return set_error(serror, SCRIPT_ERR_CLEANSTACK);
+        }
     }
 
     return set_success(serror);