Add policy: null signature for failed CHECK(MULTI)SIG

src/policy/policy.h

@@ -43,6 +43,7 @@ static const unsigned int STANDARD_SCRIPT_VERIFY_FLAGS = MANDATORY_SCRIPT_VERIFY
                                                          SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_NOPS |
                                                          SCRIPT_VERIFY_CLEANSTACK |
                                                          SCRIPT_VERIFY_MINIMALIF |
+                                                         SCRIPT_VERIFY_NULLFAIL |
                                                          SCRIPT_VERIFY_CHECKLOCKTIMEVERIFY |
                                                          SCRIPT_VERIFY_CHECKSEQUENCEVERIFY |
                                                          SCRIPT_VERIFY_LOW_S |

src/script/interpreter.cpp

@@ -885,6 +885,9 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                     }
                     bool fSuccess = checker.CheckSig(vchSig, vchPubKey, scriptCode, sigversion);
 
+                    if (!fSuccess && (flags & SCRIPT_VERIFY_NULLFAIL) && vchSig.size())
+                        return set_error(serror, SCRIPT_ERR_SIG_NULLFAIL);
+
                     popstack(stack);
                     popstack(stack);
                     stack.push_back(fSuccess ? vchTrue : vchFalse);
@@ -914,6 +917,9 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                     if (nOpCount > MAX_OPS_PER_SCRIPT)
                         return set_error(serror, SCRIPT_ERR_OP_COUNT);
                     int ikey = ++i;
+                    // ikey2 is the position of last non-signature item in the stack. Top stack item = 1.
+                    // With SCRIPT_VERIFY_NULLFAIL, this is used for cleanup if operation fails.
+                    int ikey2 = nKeysCount + 2;
                     i += nKeysCount;
                     if ((int)stack.size() < i)
                         return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
@@ -970,8 +976,14 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                     }
 
                     // Clean up stack of actual arguments
-                    while (i-- > 1)
+                    while (i-- > 1) {
+                        // If the operation failed, we require that all signatures must be empty vector
+                        if (!fSuccess && (flags & SCRIPT_VERIFY_NULLFAIL) && !ikey2 && stacktop(-1).size())
+                            return set_error(serror, SCRIPT_ERR_SIG_NULLFAIL);
+                        if (ikey2 > 0)
+                            ikey2--;
                         popstack(stack);
+                    }
 
                     // A bug causes CHECKMULTISIG to consume one extra argument
                     // whose contents were not checked in any way.

src/script/interpreter.h

@@ -98,6 +98,10 @@ enum
     // Segwit script only: Require the argument of OP_IF/NOTIF to be exactly 0x01 or empty vector
     //
     SCRIPT_VERIFY_MINIMALIF = (1U << 13),
+
+    // Signature(s) must be empty vector if an CHECK(MULTI)SIG operation failed
+    //
+    SCRIPT_VERIFY_NULLFAIL = (1U << 14),
 };
 
 bool CheckSignatureEncoding(const std::vector<unsigned char> &vchSig, unsigned int flags, ScriptError* serror);

src/script/script_error.cpp

@@ -65,6 +65,8 @@ const char* ScriptErrorString(const ScriptError serror)
             return "Dummy CHECKMULTISIG argument must be zero";
         case SCRIPT_ERR_MINIMALIF:
             return "OP_IF/NOTIF argument must be minimal";
+        case SCRIPT_ERR_SIG_NULLFAIL:
+            return "Signature must be zero for failed CHECK(MULTI)SIG operation";
         case SCRIPT_ERR_DISCOURAGE_UPGRADABLE_NOPS:
             return "NOPx reserved for soft-fork upgrades";
         case SCRIPT_ERR_DISCOURAGE_UPGRADABLE_WITNESS_PROGRAM:

src/script/script_error.h

@@ -49,6 +49,7 @@ typedef enum ScriptError_t
     SCRIPT_ERR_PUBKEYTYPE,
     SCRIPT_ERR_CLEANSTACK,
     SCRIPT_ERR_MINIMALIF,
+    SCRIPT_ERR_SIG_NULLFAIL,
 
     /* softfork safeness */
     SCRIPT_ERR_DISCOURAGE_UPGRADABLE_NOPS,

src/test/data/script_tests.json

@@ -1491,6 +1491,27 @@
     "OK",
     "BIP66 example 4, with DERSIG"
 ],
+[
+    "0x09 0x300602010102010101",
+    "0x21 0x038282263212c609d9ea2a6e3e172de238d8c39cabd5ac1ca10646e23fd5f51508 CHECKSIG NOT",
+    "DERSIG",
+    "OK",
+    "BIP66 example 4, with DERSIG, non-null DER-compliant signature"
+],
+[
+    "0",
+    "0x21 0x038282263212c609d9ea2a6e3e172de238d8c39cabd5ac1ca10646e23fd5f51508 CHECKSIG NOT",
+    "DERSIG,NULLFAIL",
+    "OK",
+    "BIP66 example 4, with DERSIG and NULLFAIL"
+],
+[
+    "0x09 0x300602010102010101",
+    "0x21 0x038282263212c609d9ea2a6e3e172de238d8c39cabd5ac1ca10646e23fd5f51508 CHECKSIG NOT",
+    "DERSIG,NULLFAIL",
+    "NULLFAIL",
+    "BIP66 example 4, with DERSIG and NULLFAIL, non-null DER-compliant signature"
+],
 [
     "1",
     "0x21 0x038282263212c609d9ea2a6e3e172de238d8c39cabd5ac1ca10646e23fd5f51508 CHECKSIG",
@@ -2208,5 +2229,15 @@
 [["645168", 0.00000001], "0x22 0x0020f913eacf2e38a5d6fc3a8311d72ae704cb83866350a984dd3e5eb76d2a8c28e8", "HASH160 0x14 0xdbb7d1c0a56b7a9c423300c8cca6e6e065baf1dc EQUAL", "P2SH,WITNESS", "UNBALANCED_CONDITIONAL"],
 [["645168", 0.00000001], "0x22 0x0020f913eacf2e38a5d6fc3a8311d72ae704cb83866350a984dd3e5eb76d2a8c28e8", "HASH160 0x14 0xdbb7d1c0a56b7a9c423300c8cca6e6e065baf1dc EQUAL", "P2SH,WITNESS,MINIMALIF", "UNBALANCED_CONDITIONAL"],
 
- ["The End"]
+["NULLFAIL should cover all signatures and signatures only"],
+["0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0", "0x01 0x14 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0x01 0x14 CHECKMULTISIG NOT", "DERSIG", "OK", "BIP66 and NULLFAIL-compliant"],
+["0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0", "0x01 0x14 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0x01 0x14 CHECKMULTISIG NOT", "DERSIG,NULLFAIL", "OK", "BIP66 and NULLFAIL-compliant"],
+["1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0", "0x01 0x14 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0x01 0x14 CHECKMULTISIG NOT", "DERSIG,NULLFAIL", "OK", "BIP66 and NULLFAIL-compliant, not NULLDUMMY-compliant"],
+["1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0", "0x01 0x14 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0x01 0x14 CHECKMULTISIG NOT", "DERSIG,NULLFAIL,NULLDUMMY", "SIG_NULLDUMMY", "BIP66 and NULLFAIL-compliant, not NULLDUMMY-compliant"],
+["0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0x09 0x300602010102010101", "0x01 0x14 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0x01 0x14 CHECKMULTISIG NOT", "DERSIG", "OK", "BIP66-compliant but not NULLFAIL-compliant"],
+["0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0x09 0x300602010102010101", "0x01 0x14 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0x01 0x14 CHECKMULTISIG NOT", "DERSIG,NULLFAIL", "NULLFAIL", "BIP66-compliant but not NULLFAIL-compliant"],
+["0 0x09 0x300602010102010101 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0", "0x01 0x14 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0x01 0x14 CHECKMULTISIG NOT", "DERSIG", "OK", "BIP66-compliant but not NULLFAIL-compliant"],
+["0 0x09 0x300602010102010101 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0", "0x01 0x14 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0x01 0x14 CHECKMULTISIG NOT", "DERSIG,NULLFAIL", "NULLFAIL", "BIP66-compliant but not NULLFAIL-compliant"],
+
+["The End"]
 ]

src/test/script_tests.cpp

@@ -90,6 +90,7 @@ static ScriptErrorDesc script_errors[]={
     {SCRIPT_ERR_PUBKEYTYPE, "PUBKEYTYPE"},
     {SCRIPT_ERR_CLEANSTACK, "CLEANSTACK"},
     {SCRIPT_ERR_MINIMALIF, "MINIMALIF"},
+    {SCRIPT_ERR_SIG_NULLFAIL, "NULLFAIL"},
     {SCRIPT_ERR_DISCOURAGE_UPGRADABLE_NOPS, "DISCOURAGE_UPGRADABLE_NOPS"},
     {SCRIPT_ERR_DISCOURAGE_UPGRADABLE_WITNESS_PROGRAM, "DISCOURAGE_UPGRADABLE_WITNESS_PROGRAM"},
     {SCRIPT_ERR_WITNESS_PROGRAM_WRONG_LENGTH, "WITNESS_PROGRAM_WRONG_LENGTH"},

src/test/transaction_tests.cpp

@@ -51,6 +51,7 @@ static std::map<string, unsigned int> mapFlagNames = boost::assign::map_list_of
     (string("DISCOURAGE_UPGRADABLE_NOPS"), (unsigned int)SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_NOPS)
     (string("CLEANSTACK"), (unsigned int)SCRIPT_VERIFY_CLEANSTACK)
     (string("MINIMALIF"), (unsigned int)SCRIPT_VERIFY_MINIMALIF)
+    (string("NULLFAIL"), (unsigned int)SCRIPT_VERIFY_NULLFAIL)
     (string("CHECKLOCKTIMEVERIFY"), (unsigned int)SCRIPT_VERIFY_CHECKLOCKTIMEVERIFY)
     (string("CHECKSEQUENCEVERIFY"), (unsigned int)SCRIPT_VERIFY_CHECKSEQUENCEVERIFY)
     (string("WITNESS"), (unsigned int)SCRIPT_VERIFY_WITNESS)