script: create sane error return codes for script validation and remove logging

Attempt to codify the possible error statuses associated with script
validation. script/types.h has been created with the expectation that it will
be part of the public lib interface. The other flag enums will be moved here in
a future commit.

Logging has also been removed in order to drop the dependency on core.h. It can
be re-added to bitcoind as-needed. This makes script verification finally free
of application state and boost!

4 files changed, 304 insertions, 119 deletions

diff --git a/src/script/interpreter.cpp b/src/script/interpreter.cpp
index 5fda6248c2..cf81fe30a2 100644
--- a/src/script/interpreter.cpp
+++ b/src/script/interpreter.cpp
@@ -13,7 +13,6 @@
 #include "pubkey.h"
 #include "script/script.h"
 #include "uint256.h"
-#include "util.h"
 
 using namespace std;
 
@@ -26,6 +25,24 @@ static const CScriptNum bnOne(1);
 static const CScriptNum bnFalse(0);
 static const CScriptNum bnTrue(1);
 
+namespace {
+
+inline bool set_success(ScriptError* ret)
+{
+    if (ret)
+        *ret = SCRIPT_ERR_OK;
+    return true;
+}
+
+inline bool set_error(ScriptError* ret, const ScriptError serror)
+{
+    if (ret)
+        *ret = serror;
+    return false;
+}
+
+} // anon namespace
+
 bool CastToBool(const valtype& vch)
 {
     for (unsigned int i = 0; i < vch.size(); i++)
@@ -55,16 +72,23 @@ static inline void popstack(vector<valtype>& stack)
 }
 
 bool static IsCompressedOrUncompressedPubKey(const valtype &vchPubKey) {
-    if (vchPubKey.size() < 33)
-        return error("Non-canonical public key: too short");
+    if (vchPubKey.size() < 33) {
+        //  Non-canonical public key: too short
+        return false;
+    }
     if (vchPubKey[0] == 0x04) {
-        if (vchPubKey.size() != 65)
-            return error("Non-canonical public key: invalid length for uncompressed key");
+        if (vchPubKey.size() != 65) {
+            //  Non-canonical public key: invalid length for uncompressed key
+            return false;
+        }
     } else if (vchPubKey[0] == 0x02 || vchPubKey[0] == 0x03) {
-        if (vchPubKey.size() != 33)
-            return error("Non-canonical public key: invalid length for compressed key");
+        if (vchPubKey.size() != 33) {
+            //  Non-canonical public key: invalid length for compressed key
+            return false;
+        }
     } else {
-        return error("Non-canonical public key: neither compressed nor uncompressed");
+          //  Non-canonical public key: neither compressed nor uncompressed
+          return false;
     }
     return true;
 }
@@ -79,47 +103,74 @@ bool static IsCompressedOrUncompressedPubKey(const valtype &vchPubKey) {
  */
 bool static IsDERSignature(const valtype &vchSig) {
 
-    if (vchSig.size() < 9)
-        return error("Non-canonical signature: too short");
-    if (vchSig.size() > 73)
-        return error("Non-canonical signature: too long");
-    if (vchSig[0] != 0x30)
-        return error("Non-canonical signature: wrong type");
-    if (vchSig[1] != vchSig.size()-3)
-        return error("Non-canonical signature: wrong length marker");
+    if (vchSig.size() < 9) {
+        //  Non-canonical signature: too short
+        return false;
+    }
+    if (vchSig.size() > 73) {
+        // Non-canonical signature: too long
+        return false;
+    }
+    if (vchSig[0] != 0x30) {
+        //  Non-canonical signature: wrong type
+        return false;
+    }
+    if (vchSig[1] != vchSig.size()-3) {
+        //  Non-canonical signature: wrong length marker
+        return false;
+    }
     unsigned int nLenR = vchSig[3];
-    if (5 + nLenR >= vchSig.size())
-        return error("Non-canonical signature: S length misplaced");
+    if (5 + nLenR >= vchSig.size()) {
+        //  Non-canonical signature: S length misplaced
+        return false;
+    }
     unsigned int nLenS = vchSig[5+nLenR];
-    if ((unsigned long)(nLenR+nLenS+7) != vchSig.size())
-        return error("Non-canonical signature: R+S length mismatch");
+    if ((unsigned long)(nLenR+nLenS+7) != vchSig.size()) {
+        //  Non-canonical signature: R+S length mismatch
+        return false;
+    }
 
     const unsigned char *R = &vchSig[4];
-    if (R[-2] != 0x02)
-        return error("Non-canonical signature: R value type mismatch");
-    if (nLenR == 0)
-        return error("Non-canonical signature: R length is zero");
-    if (R[0] & 0x80)
-        return error("Non-canonical signature: R value negative");
-    if (nLenR > 1 && (R[0] == 0x00) && !(R[1] & 0x80))
-        return error("Non-canonical signature: R value excessively padded");
+    if (R[-2] != 0x02) {
+        //  Non-canonical signature: R value type mismatch
+        return false;
+    }
+    if (nLenR == 0) {
+        //  Non-canonical signature: R length is zero
+        return false;
+    }
+    if (R[0] & 0x80) {
+        //  Non-canonical signature: R value negative
+        return false;
+    }
+    if (nLenR > 1 && (R[0] == 0x00) && !(R[1] & 0x80)) {
+        //  Non-canonical signature: R value excessively padded
+        return false;
+    }
 
     const unsigned char *S = &vchSig[6+nLenR];
-    if (S[-2] != 0x02)
-        return error("Non-canonical signature: S value type mismatch");
-    if (nLenS == 0)
-        return error("Non-canonical signature: S length is zero");
-    if (S[0] & 0x80)
-        return error("Non-canonical signature: S value negative");
-    if (nLenS > 1 && (S[0] == 0x00) && !(S[1] & 0x80))
-        return error("Non-canonical signature: S value excessively padded");
-
+    if (S[-2] != 0x02) {
+        //  Non-canonical signature: S value type mismatch
+        return false;
+    }
+    if (nLenS == 0) {
+        //  Non-canonical signature: S length is zero
+        return false;
+    }
+    if (S[0] & 0x80) {
+        //  Non-canonical signature: S value negative
+        return false;
+    }
+    if (nLenS > 1 && (S[0] == 0x00) && !(S[1] & 0x80)) {
+        //  Non-canonical signature: S value excessively padded
+        return false;
+    }
     return true;
 }
 
-bool static IsLowDERSignature(const valtype &vchSig) {
+bool static IsLowDERSignature(const valtype &vchSig, ScriptError* serror) {
     if (!IsDERSignature(vchSig)) {
-        return false;
+        return set_error(serror, SCRIPT_ERR_SIG_DER);
     }
     unsigned int nLenR = vchSig[3];
     unsigned int nLenS = vchSig[5+nLenR];
@@ -128,7 +179,7 @@ bool static IsLowDERSignature(const valtype &vchSig) {
     // complement modulo the order could have been used instead, which is
     // one byte shorter when encoded correctly.
     if (!eccrypto::CheckSignatureElement(S, nLenS, true))
-        return error("Non-canonical signature: S value is unnecessarily high");
+        return set_error(serror, SCRIPT_ERR_SIG_HIGH_S);
 
     return true;
 }
@@ -139,18 +190,19 @@ bool static IsDefinedHashtypeSignature(const valtype &vchSig) {
     }
     unsigned char nHashType = vchSig[vchSig.size() - 1] & (~(SIGHASH_ANYONECANPAY));
     if (nHashType < SIGHASH_ALL || nHashType > SIGHASH_SINGLE)
-        return error("Non-canonical signature: unknown hashtype byte");
+        return false;
 
     return true;
 }
 
-bool static CheckSignatureEncoding(const valtype &vchSig, unsigned int flags) {
+bool static CheckSignatureEncoding(const valtype &vchSig, unsigned int flags, ScriptError* serror) {
     if ((flags & (SCRIPT_VERIFY_DERSIG | SCRIPT_VERIFY_LOW_S | SCRIPT_VERIFY_STRICTENC)) != 0 && !IsDERSignature(vchSig)) {
-        return false;
-    } else if ((flags & SCRIPT_VERIFY_LOW_S) != 0 && !IsLowDERSignature(vchSig)) {
+        return set_error(serror, SCRIPT_ERR_SIG_DER);
+    } else if ((flags & SCRIPT_VERIFY_LOW_S) != 0 && !IsLowDERSignature(vchSig, serror)) {
+        // serror is set
         return false;
     } else if ((flags & SCRIPT_VERIFY_STRICTENC) != 0 && !IsDefinedHashtypeSignature(vchSig)) {
-        return false;
+        return set_error(serror, SCRIPT_ERR_SIG_HASHTYPE);
     }
     return true;
 }
@@ -185,7 +237,7 @@ bool static CheckMinimalPush(const valtype& data, opcodetype opcode) {
     return true;
 }
 
-bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, unsigned int flags, const BaseSignatureChecker& checker)
+bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, unsigned int flags, const BaseSignatureChecker& checker, ScriptError* serror)
 {
     CScript::const_iterator pc = script.begin();
     CScript::const_iterator pend = script.end();
@@ -194,8 +246,9 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
     valtype vchPushValue;
     vector<bool> vfExec;
     vector<valtype> altstack;
+    set_error(serror, SCRIPT_ERR_UNKNOWN_ERROR);
     if (script.size() > 10000)
-        return false;
+        return set_error(serror, SCRIPT_ERR_SCRIPT_SIZE);
     int nOpCount = 0;
     bool fRequireMinimal = (flags & SCRIPT_VERIFY_MINIMALDATA) != 0;
 
@@ -209,13 +262,13 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
             // Read instruction
             //
             if (!script.GetOp(pc, opcode, vchPushValue))
-                return false;
+                return set_error(serror, SCRIPT_ERR_BAD_OPCODE);
             if (vchPushValue.size() > MAX_SCRIPT_ELEMENT_SIZE)
-                return false;
+                return set_error(serror, SCRIPT_ERR_PUSH_SIZE);
 
             // Note how OP_RESERVED does not count towards the opcode limit.
             if (opcode > OP_16 && ++nOpCount > 201)
-                return false;
+                return set_error(serror, SCRIPT_ERR_OP_COUNT);
 
             if (opcode == OP_CAT ||
                 opcode == OP_SUBSTR ||
@@ -232,11 +285,11 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                 opcode == OP_MOD ||
                 opcode == OP_LSHIFT ||
                 opcode == OP_RSHIFT)
-                return false; // Disabled opcodes.
+                return set_error(serror, SCRIPT_ERR_DISABLED_OPCODE); // Disabled opcodes.
 
             if (fExec && 0 <= opcode && opcode <= OP_PUSHDATA4) {
                 if (fRequireMinimal && !CheckMinimalPush(vchPushValue, opcode)) {
-                    return false;
+                    return set_error(serror, SCRIPT_ERR_MINIMALDATA);
                 }
                 stack.push_back(vchPushValue);
             } else if (fExec || (OP_IF <= opcode && opcode <= OP_ENDIF))
@@ -288,7 +341,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                     if (fExec)
                     {
                         if (stack.size() < 1)
-                            return false;
+                            return set_error(serror, SCRIPT_ERR_UNBALANCED_CONDITIONAL);
                         valtype& vch = stacktop(-1);
                         fValue = CastToBool(vch);
                         if (opcode == OP_NOTIF)
@@ -302,7 +355,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                 case OP_ELSE:
                 {
                     if (vfExec.empty())
-                        return false;
+                        return set_error(serror, SCRIPT_ERR_UNBALANCED_CONDITIONAL);
                     vfExec.back() = !vfExec.back();
                 }
                 break;
@@ -310,7 +363,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                 case OP_ENDIF:
                 {
                     if (vfExec.empty())
-                        return false;
+                        return set_error(serror, SCRIPT_ERR_UNBALANCED_CONDITIONAL);
                     vfExec.pop_back();
                 }
                 break;
@@ -320,18 +373,18 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                     // (true -- ) or
                     // (false -- false) and return
                     if (stack.size() < 1)
-                        return false;
+                        return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                     bool fValue = CastToBool(stacktop(-1));
                     if (fValue)
                         popstack(stack);
                     else
-                        return false;
+                        return set_error(serror, SCRIPT_ERR_VERIFY);
                 }
                 break;
 
                 case OP_RETURN:
                 {
-                    return false;
+                    return set_error(serror, SCRIPT_ERR_OP_RETURN);
                 }
                 break;
 
@@ -342,7 +395,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                 case OP_TOALTSTACK:
                 {
                     if (stack.size() < 1)
-                        return false;
+                        return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                     altstack.push_back(stacktop(-1));
                     popstack(stack);
                 }
@@ -351,7 +404,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                 case OP_FROMALTSTACK:
                 {
                     if (altstack.size() < 1)
-                        return false;
+                        return set_error(serror, SCRIPT_ERR_INVALID_ALTSTACK_OPERATION);
                     stack.push_back(altstacktop(-1));
                     popstack(altstack);
                 }
@@ -361,7 +414,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                 {
                     // (x1 x2 -- )
                     if (stack.size() < 2)
-                        return false;
+                        return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                     popstack(stack);
                     popstack(stack);
                 }
@@ -371,7 +424,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                 {
                     // (x1 x2 -- x1 x2 x1 x2)
                     if (stack.size() < 2)
-                        return false;
+                        return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                     valtype vch1 = stacktop(-2);
                     valtype vch2 = stacktop(-1);
                     stack.push_back(vch1);
@@ -383,7 +436,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                 {
                     // (x1 x2 x3 -- x1 x2 x3 x1 x2 x3)
                     if (stack.size() < 3)
-                        return false;
+                        return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                     valtype vch1 = stacktop(-3);
                     valtype vch2 = stacktop(-2);
                     valtype vch3 = stacktop(-1);
@@ -397,7 +450,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                 {
                     // (x1 x2 x3 x4 -- x1 x2 x3 x4 x1 x2)
                     if (stack.size() < 4)
-                        return false;
+                        return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                     valtype vch1 = stacktop(-4);
                     valtype vch2 = stacktop(-3);
                     stack.push_back(vch1);
@@ -409,7 +462,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                 {
                     // (x1 x2 x3 x4 x5 x6 -- x3 x4 x5 x6 x1 x2)
                     if (stack.size() < 6)
-                        return false;
+                        return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                     valtype vch1 = stacktop(-6);
                     valtype vch2 = stacktop(-5);
                     stack.erase(stack.end()-6, stack.end()-4);
@@ -422,7 +475,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                 {
                     // (x1 x2 x3 x4 -- x3 x4 x1 x2)
                     if (stack.size() < 4)
-                        return false;
+                        return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                     swap(stacktop(-4), stacktop(-2));
                     swap(stacktop(-3), stacktop(-1));
                 }
@@ -432,7 +485,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                 {
                     // (x - 0 | x x)
                     if (stack.size() < 1)
-                        return false;
+                        return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                     valtype vch = stacktop(-1);
                     if (CastToBool(vch))
                         stack.push_back(vch);
@@ -451,7 +504,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                 {
                     // (x -- )
                     if (stack.size() < 1)
-                        return false;
+                        return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                     popstack(stack);
                 }
                 break;
@@ -460,7 +513,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                 {
                     // (x -- x x)
                     if (stack.size() < 1)
-                        return false;
+                        return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                     valtype vch = stacktop(-1);
                     stack.push_back(vch);
                 }
@@ -470,7 +523,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                 {
                     // (x1 x2 -- x2)
                     if (stack.size() < 2)
-                        return false;
+                        return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                     stack.erase(stack.end() - 2);
                 }
                 break;
@@ -479,7 +532,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                 {
                     // (x1 x2 -- x1 x2 x1)
                     if (stack.size() < 2)
-                        return false;
+                        return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                     valtype vch = stacktop(-2);
                     stack.push_back(vch);
                 }
@@ -491,11 +544,11 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                     // (xn ... x2 x1 x0 n - xn ... x2 x1 x0 xn)
                     // (xn ... x2 x1 x0 n - ... x2 x1 x0 xn)
                     if (stack.size() < 2)
-                        return false;
+                        return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                     int n = CScriptNum(stacktop(-1), fRequireMinimal).getint();
                     popstack(stack);
                     if (n < 0 || n >= (int)stack.size())
-                        return false;
+                        return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                     valtype vch = stacktop(-n-1);
                     if (opcode == OP_ROLL)
                         stack.erase(stack.end()-n-1);
@@ -509,7 +562,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                     //  x2 x1 x3  after first swap
                     //  x2 x3 x1  after second swap
                     if (stack.size() < 3)
-                        return false;
+                        return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                     swap(stacktop(-3), stacktop(-2));
                     swap(stacktop(-2), stacktop(-1));
                 }
@@ -519,7 +572,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                 {
                     // (x1 x2 -- x2 x1)
                     if (stack.size() < 2)
-                        return false;
+                        return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                     swap(stacktop(-2), stacktop(-1));
                 }
                 break;
@@ -528,7 +581,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                 {
                     // (x1 x2 -- x2 x1 x2)
                     if (stack.size() < 2)
-                        return false;
+                        return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                     valtype vch = stacktop(-1);
                     stack.insert(stack.end()-2, vch);
                 }
@@ -539,7 +592,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                 {
                     // (in -- in size)
                     if (stack.size() < 1)
-                        return false;
+                        return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                     CScriptNum bn(stacktop(-1).size());
                     stack.push_back(bn.getvch());
                 }
@@ -555,7 +608,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                 {
                     // (x1 x2 - bool)
                     if (stack.size() < 2)
-                        return false;
+                        return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                     valtype& vch1 = stacktop(-2);
                     valtype& vch2 = stacktop(-1);
                     bool fEqual = (vch1 == vch2);
@@ -572,7 +625,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                         if (fEqual)
                             popstack(stack);
                         else
-                            return false;
+                            return set_error(serror, SCRIPT_ERR_EQUALVERIFY);
                     }
                 }
                 break;
@@ -590,7 +643,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                 {
                     // (in -- out)
                     if (stack.size() < 1)
-                        return false;
+                        return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                     CScriptNum bn(stacktop(-1), fRequireMinimal);
                     switch (opcode)
                     {
@@ -623,7 +676,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                 {
                     // (x1 x2 -- out)
                     if (stack.size() < 2)
-                        return false;
+                        return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                     CScriptNum bn1(stacktop(-2), fRequireMinimal);
                     CScriptNum bn2(stacktop(-1), fRequireMinimal);
                     CScriptNum bn(0);
@@ -659,7 +712,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                         if (CastToBool(stacktop(-1)))
                             popstack(stack);
                         else
-                            return false;
+                            return set_error(serror, SCRIPT_ERR_NUMEQUALVERIFY);
                     }
                 }
                 break;
@@ -668,7 +721,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                 {
                     // (x min max -- out)
                     if (stack.size() < 3)
-                        return false;
+                        return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                     CScriptNum bn1(stacktop(-3), fRequireMinimal);
                     CScriptNum bn2(stacktop(-2), fRequireMinimal);
                     CScriptNum bn3(stacktop(-1), fRequireMinimal);
@@ -692,7 +745,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                 {
                     // (in -- hash)
                     if (stack.size() < 1)
-                        return false;
+                        return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                     valtype& vch = stacktop(-1);
                     valtype vchHash((opcode == OP_RIPEMD160 || opcode == OP_SHA1 || opcode == OP_HASH160) ? 20 : 32);
                     if (opcode == OP_RIPEMD160)
@@ -722,7 +775,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                 {
                     // (sig pubkey -- bool)
                     if (stack.size() < 2)
-                        return false;
+                        return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
 
                     valtype& vchSig    = stacktop(-2);
                     valtype& vchPubKey = stacktop(-1);
@@ -733,10 +786,10 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                     // Drop the signature, since there's no way for a signature to sign itself
                     scriptCode.FindAndDelete(CScript(vchSig));
 
-                    if (!CheckSignatureEncoding(vchSig, flags)) {
+                    if (!CheckSignatureEncoding(vchSig, flags, serror)) {
+                        //serror is set
                         return false;
                     }
-
                     bool fSuccess = CheckPubKeyEncoding(vchPubKey, flags) && checker.CheckSig(vchSig, vchPubKey, scriptCode);
 
                     popstack(stack);
@@ -747,7 +800,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                         if (fSuccess)
                             popstack(stack);
                         else
-                            return false;
+                            return set_error(serror, SCRIPT_ERR_CHECKSIGVERIFY);
                     }
                 }
                 break;
@@ -759,26 +812,26 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
 
                     int i = 1;
                     if ((int)stack.size() < i)
-                        return false;
+                        return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
 
                     int nKeysCount = CScriptNum(stacktop(-i), fRequireMinimal).getint();
                     if (nKeysCount < 0 || nKeysCount > 20)
-                        return false;
+                        return set_error(serror, SCRIPT_ERR_PUBKEY_COUNT);
                     nOpCount += nKeysCount;
                     if (nOpCount > 201)
-                        return false;
+                        return set_error(serror, SCRIPT_ERR_OP_COUNT);
                     int ikey = ++i;
                     i += nKeysCount;
                     if ((int)stack.size() < i)
-                        return false;
+                        return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
 
                     int nSigsCount = CScriptNum(stacktop(-i), fRequireMinimal).getint();
                     if (nSigsCount < 0 || nSigsCount > nKeysCount)
-                        return false;
+                        return set_error(serror, SCRIPT_ERR_SIG_COUNT);
                     int isig = ++i;
                     i += nSigsCount;
                     if ((int)stack.size() < i)
-                        return false;
+                        return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
 
                     // Subset of script starting at the most recent codeseparator
                     CScript scriptCode(pbegincodehash, pend);
@@ -796,7 +849,8 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                         valtype& vchSig    = stacktop(-isig);
                         valtype& vchPubKey = stacktop(-ikey);
 
-                        if (!CheckSignatureEncoding(vchSig, flags)) {
+                        if (!CheckSignatureEncoding(vchSig, flags, serror)) {
+                            // serror is set
                             return false;
                         }
 
@@ -827,9 +881,9 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                     // so optionally verify it is exactly equal to zero prior
                     // to removing it from the stack.
                     if (stack.size() < 1)
-                        return false;
+                        return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                     if ((flags & SCRIPT_VERIFY_NULLDUMMY) && stacktop(-1).size())
-                        return error("CHECKMULTISIG dummy argument not null");
+                        return set_error(serror, SCRIPT_ERR_SIG_NULLDUMMY);
                     popstack(stack);
 
                     stack.push_back(fSuccess ? vchTrue : vchFalse);
@@ -839,29 +893,29 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                         if (fSuccess)
                             popstack(stack);
                         else
-                            return false;
+                            return set_error(serror, SCRIPT_ERR_CHECKMULTISIGVERIFY);
                     }
                 }
                 break;
 
                 default:
-                    return false;
+                    return set_error(serror, SCRIPT_ERR_BAD_OPCODE);
             }
 
             // Size limits
             if (stack.size() + altstack.size() > 1000)
-                return false;
+                return set_error(serror, SCRIPT_ERR_STACK_SIZE);
         }
     }
     catch (...)
     {
-        return false;
+        return set_error(serror, SCRIPT_ERR_UNKNOWN_ERROR);
     }
 
     if (!vfExec.empty())
-        return false;
+        return set_error(serror, SCRIPT_ERR_UNBALANCED_CONDITIONAL);
 
-    return true;
+    return set_success(serror);
 }
 
 namespace {
@@ -966,14 +1020,14 @@ public:
 uint256 SignatureHash(const CScript& scriptCode, const CTransaction& txTo, unsigned int nIn, int nHashType)
 {
     if (nIn >= txTo.vin.size()) {
-        LogPrintf("ERROR: SignatureHash() : nIn=%d out of range\n", nIn);
+        //  nIn out of range
         return 1;
     }
 
     // Check for invalid use of SIGHASH_SINGLE
     if ((nHashType & 0x1f) == SIGHASH_SINGLE) {
         if (nIn >= txTo.vout.size()) {
-            LogPrintf("ERROR: SignatureHash() : nOut=%d out of range\n", nIn);
+            //  nOut out of range
             return 1;
         }
     }
@@ -1013,30 +1067,35 @@ bool SignatureChecker::CheckSig(const vector<unsigned char>& vchSigIn, const vec
     return true;
 }
 
-bool VerifyScript(const CScript& scriptSig, const CScript& scriptPubKey, unsigned int flags, const BaseSignatureChecker& checker)
+bool VerifyScript(const CScript& scriptSig, const CScript& scriptPubKey, unsigned int flags, const BaseSignatureChecker& checker, ScriptError* serror)
 {
+    set_error(serror, SCRIPT_ERR_UNKNOWN_ERROR);
+
     if ((flags & SCRIPT_VERIFY_SIGPUSHONLY) != 0 && !scriptSig.IsPushOnly()) {
-        return false;
+        return set_error(serror, SCRIPT_ERR_SIG_PUSHONLY);
     }
 
     vector<vector<unsigned char> > stack, stackCopy;
-    if (!EvalScript(stack, scriptSig, flags, checker))
+    if (!EvalScript(stack, scriptSig, flags, checker, serror))
+        // serror is set
         return false;
     if (flags & SCRIPT_VERIFY_P2SH)
         stackCopy = stack;
-    if (!EvalScript(stack, scriptPubKey, flags, checker))
+    if (!EvalScript(stack, scriptPubKey, flags, checker, serror))
+        // serror is set
         return false;
     if (stack.empty())
-        return false;
+        return set_error(serror, SCRIPT_ERR_EVAL_FALSE);
 
     if (CastToBool(stack.back()) == false)
-        return false;
+        return set_error(serror, SCRIPT_ERR_EVAL_FALSE);
 
     // Additional validation for spend-to-script-hash transactions:
     if ((flags & SCRIPT_VERIFY_P2SH) && scriptPubKey.IsPayToScriptHash())
     {
-        if (!scriptSig.IsPushOnly()) // scriptSig must be literals-only
-            return false;            // or validation fails
+        // scriptSig must be literals-only or validation fails
+        if (!scriptSig.IsPushOnly())
+            return set_error(serror, SCRIPT_ERR_SIG_PUSHONLY);
 
         // stackCopy cannot be empty here, because if it was the
         // P2SH  HASH <> EQUAL  scriptPubKey would be evaluated with
@@ -1047,12 +1106,16 @@ bool VerifyScript(const CScript& scriptSig, const CScript& scriptPubKey, unsigne
         CScript pubKey2(pubKeySerialized.begin(), pubKeySerialized.end());
         popstack(stackCopy);
 
-        if (!EvalScript(stackCopy, pubKey2, flags, checker))
+        if (!EvalScript(stackCopy, pubKey2, flags, checker, serror))
+            // serror is set
             return false;
         if (stackCopy.empty())
-            return false;
-        return CastToBool(stackCopy.back());
+            return set_error(serror, SCRIPT_ERR_EVAL_FALSE);
+        if (!CastToBool(stackCopy.back()))
+            return set_error(serror, SCRIPT_ERR_EVAL_FALSE);
+        else
+            return set_success(serror);
     }
 
-    return true;
+    return set_success(serror);
 }
diff --git a/src/script/interpreter.h b/src/script/interpreter.h
index ed899fc411..14cccc558f 100644
--- a/src/script/interpreter.h
+++ b/src/script/interpreter.h
@@ -6,6 +6,8 @@
 #ifndef BITCOIN_SCRIPT_INTERPRETER_H
 #define BITCOIN_SCRIPT_INTERPRETER_H
 
+#include "script_error.h"
+
 #include <vector>
 #include <stdint.h>
 #include <string>
@@ -85,7 +87,7 @@ public:
     bool CheckSig(const std::vector<unsigned char>& scriptSig, const std::vector<unsigned char>& vchPubKey, const CScript& scriptCode) const;
 };
 
-bool EvalScript(std::vector<std::vector<unsigned char> >& stack, const CScript& script, unsigned int flags, const BaseSignatureChecker& checker);
-bool VerifyScript(const CScript& scriptSig, const CScript& scriptPubKey, unsigned int flags, const BaseSignatureChecker& checker);
+bool EvalScript(std::vector<std::vector<unsigned char> >& stack, const CScript& script, unsigned int flags, const BaseSignatureChecker& checker, ScriptError* error = NULL);
+bool VerifyScript(const CScript& scriptSig, const CScript& scriptPubKey, unsigned int flags, const BaseSignatureChecker& checker, ScriptError* error = NULL);
 
 #endif // BITCOIN_SCRIPT_INTERPRETER_H
diff --git a/src/script/script_error.cpp b/src/script/script_error.cpp
new file mode 100644
index 0000000000..4a3df268ec
--- /dev/null
+++ b/src/script/script_error.cpp
@@ -0,0 +1,67 @@
+// Copyright (c) 2009-2010 Satoshi Nakamoto
+// Copyright (c) 2009-2014 The Bitcoin developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include "script_error.h"
+
+const char* ScriptErrorString(const ScriptError serror)
+{
+    switch (serror)
+    {
+        case SCRIPT_ERR_OK:
+            return "No error";
+        case SCRIPT_ERR_EVAL_FALSE:
+            return "Script evaluated without error but finished with a false/empty top stack element";
+        case SCRIPT_ERR_VERIFY:
+            return "Script failed an OP_VERIFY operation";
+        case SCRIPT_ERR_EQUALVERIFY:
+            return "Script failed an OP_EQUALVERIFY operation";
+        case SCRIPT_ERR_CHECKMULTISIGVERIFY:
+            return "Script failed an OP_CHECKMULTISIGVERIFY operation";
+        case SCRIPT_ERR_CHECKSIGVERIFY:
+            return "Script failed an OP_CHECKSIGVERIFY operation";
+        case SCRIPT_ERR_NUMEQUALVERIFY:
+            return "Script failed an OP_NUMEQUALVERIFY operation";
+        case SCRIPT_ERR_SCRIPT_SIZE:
+            return "Script is too big";
+        case SCRIPT_ERR_PUSH_SIZE:
+            return "Push value size limit exceeded";
+        case SCRIPT_ERR_OP_COUNT:
+            return "Operation limit exceeded";
+        case SCRIPT_ERR_STACK_SIZE:
+            return "Stack size limit exceeded";
+        case SCRIPT_ERR_SIG_COUNT:
+            return "Signature count negative or greater than pubkey count";
+        case SCRIPT_ERR_PUBKEY_COUNT:
+            return "Pubkey count negative or limit exceeded";
+        case SCRIPT_ERR_BAD_OPCODE:
+            return "Opcode missing or not understood";
+        case SCRIPT_ERR_DISABLED_OPCODE:
+            return "Attempted to use a disabled opcode";
+        case SCRIPT_ERR_INVALID_STACK_OPERATION:
+            return "Operation not valid with the current stack size";
+        case SCRIPT_ERR_INVALID_ALTSTACK_OPERATION:
+            return "Operation not valid with the current altstack size";
+        case SCRIPT_ERR_OP_RETURN:
+            return "OP_RETURN was encountered";
+        case SCRIPT_ERR_UNBALANCED_CONDITIONAL:
+            return "Invalid OP_IF construction";
+        case SCRIPT_ERR_SIG_HASHTYPE:
+            return "Signature hash type missing or not understood";
+        case SCRIPT_ERR_SIG_DER:
+            return "Non-canonical DER signature";
+        case SCRIPT_ERR_MINIMALDATA:
+            return "Data push larger than necessary";
+        case SCRIPT_ERR_SIG_PUSHONLY:
+            return "Only non-push operators allowed in signatures";
+        case SCRIPT_ERR_SIG_HIGH_S:
+            return "Non-canonical signature: S value is unnecessarily high";
+        case SCRIPT_ERR_SIG_NULLDUMMY:
+            return "Dummy CHECKMULTISIG argument must be zero";
+        case SCRIPT_ERR_UNKNOWN_ERROR:
+        case SCRIPT_ERR_ERROR_COUNT:
+        default: break;
+    }
+    return "unknown error";
+}
diff --git a/src/script/script_error.h b/src/script/script_error.h
new file mode 100644
index 0000000000..ae6626b257
--- /dev/null
+++ b/src/script/script_error.h
@@ -0,0 +1,53 @@
+// Copyright (c) 2009-2010 Satoshi Nakamoto
+// Copyright (c) 2009-2014 The Bitcoin developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_SCRIPT_ERROR_H
+#define BITCOIN_SCRIPT_ERROR_H
+
+typedef enum ScriptError_t
+{
+    SCRIPT_ERR_OK = 0,
+    SCRIPT_ERR_UNKNOWN_ERROR,
+    SCRIPT_ERR_EVAL_FALSE,
+    SCRIPT_ERR_OP_RETURN,
+
+    /* Max sizes */
+    SCRIPT_ERR_SCRIPT_SIZE,
+    SCRIPT_ERR_PUSH_SIZE,
+    SCRIPT_ERR_OP_COUNT,
+    SCRIPT_ERR_STACK_SIZE,
+    SCRIPT_ERR_SIG_COUNT,
+    SCRIPT_ERR_PUBKEY_COUNT,
+
+    /* Failed verify operations */
+    SCRIPT_ERR_VERIFY,
+    SCRIPT_ERR_EQUALVERIFY,
+    SCRIPT_ERR_CHECKMULTISIGVERIFY,
+    SCRIPT_ERR_CHECKSIGVERIFY,
+    SCRIPT_ERR_NUMEQUALVERIFY,
+
+    /* Logical/Format/Canonical errors */
+    SCRIPT_ERR_BAD_OPCODE,
+    SCRIPT_ERR_DISABLED_OPCODE,
+    SCRIPT_ERR_INVALID_STACK_OPERATION,
+    SCRIPT_ERR_INVALID_ALTSTACK_OPERATION,
+    SCRIPT_ERR_UNBALANCED_CONDITIONAL,
+
+    /* BIP62 */
+    SCRIPT_ERR_SIG_HASHTYPE,
+    SCRIPT_ERR_SIG_DER,
+    SCRIPT_ERR_MINIMALDATA,
+    SCRIPT_ERR_SIG_PUSHONLY,
+    SCRIPT_ERR_SIG_HIGH_S,
+    SCRIPT_ERR_SIG_NULLDUMMY,
+
+    SCRIPT_ERR_ERROR_COUNT
+} ScriptError;
+
+#define SCRIPT_ERR_LAST SCRIPT_ERR_ERROR_COUNT
+
+const char* ScriptErrorString(const ScriptError error);
+
+#endif // BITCOIN_SCRIPT_ERROR_H