Add constant scriptCode policy in non-segwit scripts

This disables OP_CODESEPARATOR in non-segwit scripts (even in an unexecuted branch), and makes a positive FindAndDelete result invalid. This ensures that the scriptCode serialized in SignatureHash() is always the same as the script passing to the EvalScript.

Author: jl2012

src/policy/policy.h

@@ -63,7 +63,8 @@ static constexpr unsigned int STANDARD_SCRIPT_VERIFY_FLAGS = MANDATORY_SCRIPT_VE
                                                              SCRIPT_VERIFY_LOW_S |
                                                              SCRIPT_VERIFY_WITNESS |
                                                              SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_WITNESS_PROGRAM |
-                                                             SCRIPT_VERIFY_WITNESS_PUBKEYTYPE;
+                                                             SCRIPT_VERIFY_WITNESS_PUBKEYTYPE |
+                                                             SCRIPT_VERIFY_CONST_SCRIPTCODE;
 
 /** For convenience, standard but not mandatory verify flags. */
 static constexpr unsigned int STANDARD_NOT_MANDATORY_VERIFY_FLAGS = STANDARD_SCRIPT_VERIFY_FLAGS & ~MANDATORY_SCRIPT_VERIFY_FLAGS;

src/script/interpreter.cpp

@@ -336,6 +336,10 @@ bool EvalScript(std::vector<std::vector<unsigned char> >& stack, const CScript&
                 opcode == OP_RSHIFT)
                 return set_error(serror, SCRIPT_ERR_DISABLED_OPCODE); // Disabled opcodes.
 
+            // With SCRIPT_VERIFY_CONST_SCRIPTCODE, OP_CODESEPARATOR in non-segwit script is rejected even in an unexecuted branch
+            if (opcode == OP_CODESEPARATOR && sigversion == SigVersion::BASE && (flags & SCRIPT_VERIFY_CONST_SCRIPTCODE))
+                return set_error(serror, SCRIPT_ERR_OP_CODESEPARATOR);
+
             if (fExec && 0 <= opcode && opcode <= OP_PUSHDATA4) {
                 if (fRequireMinimal && !CheckMinimalPush(vchPushValue, opcode)) {
                     return set_error(serror, SCRIPT_ERR_MINIMALDATA);
@@ -899,6 +903,9 @@ bool EvalScript(std::vector<std::vector<unsigned char> >& stack, const CScript&
 
                 case OP_CODESEPARATOR:
                 {
+                    // If SCRIPT_VERIFY_CONST_SCRIPTCODE flag is set, use of OP_CODESEPARATOR is rejected in pre-segwit
+                    // script, even in an unexecuted branch (this is checked above the opcode case statement).
+
                     // Hash starts after the code separator
                     pbegincodehash = pc;
                 }
@@ -919,7 +926,9 @@ bool EvalScript(std::vector<std::vector<unsigned char> >& stack, const CScript&
 
                     // Drop the signature in pre-segwit scripts but not segwit scripts
                     if (sigversion == SigVersion::BASE) {
-                        FindAndDelete(scriptCode, CScript(vchSig));
+                        int found = FindAndDelete(scriptCode, CScript(vchSig));
+                        if (found > 0 && (flags & SCRIPT_VERIFY_CONST_SCRIPTCODE))
+                            return set_error(serror, SCRIPT_ERR_SIG_FINDANDDELETE);
                     }
 
                     if (!CheckSignatureEncoding(vchSig, flags, serror) || !CheckPubKeyEncoding(vchPubKey, flags, sigversion, serror)) {
@@ -983,7 +992,9 @@ bool EvalScript(std::vector<std::vector<unsigned char> >& stack, const CScript&
                     {
                         valtype& vchSig = stacktop(-isig-k);
                         if (sigversion == SigVersion::BASE) {
-                            FindAndDelete(scriptCode, CScript(vchSig));
+                            int found = FindAndDelete(scriptCode, CScript(vchSig));
+                            if (found > 0 && (flags & SCRIPT_VERIFY_CONST_SCRIPTCODE))
+                                return set_error(serror, SCRIPT_ERR_SIG_FINDANDDELETE);
                         }
                     }
 

src/script/interpreter.h

@@ -111,6 +111,10 @@ enum
     // Public keys in segregated witness scripts must be compressed
     //
     SCRIPT_VERIFY_WITNESS_PUBKEYTYPE = (1U << 15),
+
+    // Making OP_CODESEPARATOR and FindAndDelete fail any non-segwit scripts
+    //
+    SCRIPT_VERIFY_CONST_SCRIPTCODE = (1U << 16),
 };
 
 bool CheckSignatureEncoding(const std::vector<unsigned char> &vchSig, unsigned int flags, ScriptError* serror);

src/script/script_error.cpp

@@ -89,6 +89,10 @@ const char* ScriptErrorString(const ScriptError serror)
             return "Witness provided for non-witness script";
         case SCRIPT_ERR_WITNESS_PUBKEYTYPE:
             return "Using non-compressed keys in segwit";
+        case SCRIPT_ERR_OP_CODESEPARATOR:
+            return "Using OP_CODESEPARATOR in non-witness script";
+        case SCRIPT_ERR_SIG_FINDANDDELETE:
+            return "Signature is found in scriptCode";
         case SCRIPT_ERR_UNKNOWN_ERROR:
         case SCRIPT_ERR_ERROR_COUNT:
         default: break;

src/script/script_error.h

@@ -64,6 +64,10 @@ typedef enum ScriptError_t
     SCRIPT_ERR_WITNESS_UNEXPECTED,
     SCRIPT_ERR_WITNESS_PUBKEYTYPE,
 
+    /* Constant scriptCode */
+    SCRIPT_ERR_OP_CODESEPARATOR,
+    SCRIPT_ERR_SIG_FINDANDDELETE,
+
     SCRIPT_ERR_ERROR_COUNT
 } ScriptError;
 

src/test/script_tests.cpp

@@ -97,6 +97,8 @@ static ScriptErrorDesc script_errors[]={
     {SCRIPT_ERR_WITNESS_MALLEATED_P2SH, "WITNESS_MALLEATED_P2SH"},
     {SCRIPT_ERR_WITNESS_UNEXPECTED, "WITNESS_UNEXPECTED"},
     {SCRIPT_ERR_WITNESS_PUBKEYTYPE, "WITNESS_PUBKEYTYPE"},
+    {SCRIPT_ERR_OP_CODESEPARATOR, "OP_CODESEPARATOR"},
+    {SCRIPT_ERR_SIG_FINDANDDELETE, "SIG_FINDANDDELETE"},
 };
 
 const char *FormatScriptError(ScriptError_t err)