kevacoin/src/script/interpreter.cpp

// 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 "interpreter.h"

#include "core/transaction.h"
#include "crypto/ripemd160.h"
#include "crypto/sha1.h"
#include "crypto/sha2.h"
#include "eccryptoverify.h"
#include "pubkey.h"
#include "script/script.h"
#include "uint256.h"
#include "util.h"

using namespace std;

typedef vector<unsigned char> valtype;
static const valtype vchFalse(0);
static const valtype vchZero(0);
static const valtype vchTrue(1, 1);
static const CScriptNum bnZero(0);
static const CScriptNum bnOne(1);
static const CScriptNum bnFalse(0);
static const CScriptNum bnTrue(1);

bool CastToBool(const valtype& vch)
{
    for (unsigned int i = 0; i < vch.size(); i++)
    {
        if (vch[i] != 0)
        {
            // Can be negative zero
            if (i == vch.size()-1 && vch[i] == 0x80)
                return false;
            return true;
        }
    }
    return false;
}

//
// Script is a stack machine (like Forth) that evaluates a predicate
// returning a bool indicating valid or not.  There are no loops.
//
#define stacktop(i)  (stack.at(stack.size()+(i)))
#define altstacktop(i)  (altstack.at(altstack.size()+(i)))
static inline void popstack(vector<valtype>& stack)
{
    if (stack.empty())
        throw runtime_error("popstack() : stack empty");
    stack.pop_back();
}

bool static IsCompressedOrUncompressedPubKey(const valtype &vchPubKey) {
    if (vchPubKey.size() < 33)
        return error("Non-canonical public key: too short");
    if (vchPubKey[0] == 0x04) {
        if (vchPubKey.size() != 65)
            return error("Non-canonical public key: invalid length for uncompressed key");
    } else if (vchPubKey[0] == 0x02 || vchPubKey[0] == 0x03) {
        if (vchPubKey.size() != 33)
            return error("Non-canonical public key: invalid length for compressed key");
    } else {
        return error("Non-canonical public key: neither compressed nor uncompressed");
    }
    return true;
}

bool static IsDERSignature(const valtype &vchSig) {
    // See https://bitcointalk.org/index.php?topic=8392.msg127623#msg127623
    // A canonical signature exists of: <30> <total len> <02> <len R> <R> <02> <len S> <S> <hashtype>
    // Where R and S are not negative (their first byte has its highest bit not set), and not
    // excessively padded (do not start with a 0 byte, unless an otherwise negative number follows,
    // in which case a single 0 byte is necessary and even required).
    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");
    unsigned int nLenR = vchSig[3];
    if (5 + nLenR >= vchSig.size())
        return error("Non-canonical signature: S length misplaced");
    unsigned int nLenS = vchSig[5+nLenR];
    if ((unsigned long)(nLenR+nLenS+7) != vchSig.size())
        return error("Non-canonical signature: R+S length mismatch");

    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");

    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");

    return true;
}

bool static IsLowDERSignature(const valtype &vchSig) {
    if (!IsDERSignature(vchSig)) {
        return false;
    }
    unsigned int nLenR = vchSig[3];
    unsigned int nLenS = vchSig[5+nLenR];
    const unsigned char *S = &vchSig[6+nLenR];
    // If the S value is above the order of the curve divided by two, its
    // 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 true;
}

bool static IsDefinedHashtypeSignature(const valtype &vchSig) {
    if (vchSig.size() == 0) {
        return false;
    }
    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 true;
}

bool static CheckSignatureEncoding(const valtype &vchSig, unsigned int flags) {
    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 false;
    } else if ((flags & SCRIPT_VERIFY_STRICTENC) != 0 && !IsDefinedHashtypeSignature(vchSig)) {
        return false;
    }
    return true;
}

bool static CheckPubKeyEncoding(const valtype &vchSig, unsigned int flags) {
    if ((flags & SCRIPT_VERIFY_STRICTENC) != 0 && !IsCompressedOrUncompressedPubKey(vchSig)) {
        return false;
    }
    return true;
}

bool static CheckMinimalPush(const valtype& data, opcodetype opcode) {
    if (data.size() == 0) {
        // Could have used OP_0.
        return opcode == OP_0;
    } else if (data.size() == 1 && data[0] >= 1 && data[0] <= 16) {
        // Could have used OP_1 .. OP_16.
        return opcode == OP_1 + (data[0] - 1);
    } else if (data.size() == 1 && data[0] == 0x81) {
        // Could have used OP_1NEGATE.
        return opcode == OP_1NEGATE;
    } else if (data.size() <= 75) {
        // Could have used a direct push (opcode indicating number of bytes pushed + those bytes).
        return opcode == data.size();
    } else if (data.size() <= 255) {
        // Could have used OP_PUSHDATA.
        return opcode == OP_PUSHDATA1;
    } else if (data.size() <= 65535) {
        // Could have used OP_PUSHDATA2.
        return opcode == OP_PUSHDATA2;
    }
    return true;
}

bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, unsigned int flags, const BaseSignatureChecker& checker)
{
    CScript::const_iterator pc = script.begin();
    CScript::const_iterator pend = script.end();
    CScript::const_iterator pbegincodehash = script.begin();
    opcodetype opcode;
    valtype vchPushValue;
    vector<bool> vfExec;
    vector<valtype> altstack;
    if (script.size() > 10000)
        return false;
    int nOpCount = 0;
    bool fRequireMinimal = (flags & SCRIPT_VERIFY_MINIMALDATA) != 0;

    try
    {
        while (pc < pend)
        {
            bool fExec = !count(vfExec.begin(), vfExec.end(), false);

            //
            // Read instruction
            //
            if (!script.GetOp(pc, opcode, vchPushValue))
                return false;
            if (vchPushValue.size() > MAX_SCRIPT_ELEMENT_SIZE)
                return false;

            // Note how OP_RESERVED does not count towards the opcode limit.
            if (opcode > OP_16 && ++nOpCount > 201)
                return false;

            if (opcode == OP_CAT ||
                opcode == OP_SUBSTR ||
                opcode == OP_LEFT ||
                opcode == OP_RIGHT ||
                opcode == OP_INVERT ||
                opcode == OP_AND ||
                opcode == OP_OR ||
                opcode == OP_XOR ||
                opcode == OP_2MUL ||
                opcode == OP_2DIV ||
                opcode == OP_MUL ||
                opcode == OP_DIV ||
                opcode == OP_MOD ||
                opcode == OP_LSHIFT ||
                opcode == OP_RSHIFT)
                return false; // Disabled opcodes.

            if (fExec && 0 <= opcode && opcode <= OP_PUSHDATA4) {
                if (fRequireMinimal && !CheckMinimalPush(vchPushValue, opcode)) {
                    return false;
                }
                stack.push_back(vchPushValue);
            } else if (fExec || (OP_IF <= opcode && opcode <= OP_ENDIF))
            switch (opcode)
            {
                //
                // Push value
                //
                case OP_1NEGATE:
                case OP_1:
                case OP_2:
                case OP_3:
                case OP_4:
                case OP_5:
                case OP_6:
                case OP_7:
                case OP_8:
                case OP_9:
                case OP_10:
                case OP_11:
                case OP_12:
                case OP_13:
                case OP_14:
                case OP_15:
                case OP_16:
                {
                    // ( -- value)
                    CScriptNum bn((int)opcode - (int)(OP_1 - 1));
                    stack.push_back(bn.getvch());
                    // The result of these opcodes should always be the minimal way to push the data
                    // they push, so no need for a CheckMinimalPush here.
                }
                break;


                //
                // Control
                //
                case OP_NOP:
                case OP_NOP1: case OP_NOP2: case OP_NOP3: case OP_NOP4: case OP_NOP5:
                case OP_NOP6: case OP_NOP7: case OP_NOP8: case OP_NOP9: case OP_NOP10:
                break;

                case OP_IF:
                case OP_NOTIF:
                {
                    // <expression> if [statements] [else [statements]] endif
                    bool fValue = false;
                    if (fExec)
                    {
                        if (stack.size() < 1)
                            return false;
                        valtype& vch = stacktop(-1);
                        fValue = CastToBool(vch);
                        if (opcode == OP_NOTIF)
                            fValue = !fValue;
                        popstack(stack);
                    }
                    vfExec.push_back(fValue);
                }
                break;

                case OP_ELSE:
                {
                    if (vfExec.empty())
                        return false;
                    vfExec.back() = !vfExec.back();
                }
                break;

                case OP_ENDIF:
                {
                    if (vfExec.empty())
                        return false;
                    vfExec.pop_back();
                }
                break;

                case OP_VERIFY:
                {
                    // (true -- ) or
                    // (false -- false) and return
                    if (stack.size() < 1)
                        return false;
                    bool fValue = CastToBool(stacktop(-1));
                    if (fValue)
                        popstack(stack);
                    else
                        return false;
                }
                break;

                case OP_RETURN:
                {
                    return false;
                }
                break;


                //
                // Stack ops
                //
                case OP_TOALTSTACK:
                {
                    if (stack.size() < 1)
                        return false;
                    altstack.push_back(stacktop(-1));
                    popstack(stack);
                }
                break;

                case OP_FROMALTSTACK:
                {
                    if (altstack.size() < 1)
                        return false;
                    stack.push_back(altstacktop(-1));
                    popstack(altstack);
                }
                break;

                case OP_2DROP:
                {
                    // (x1 x2 -- )
                    if (stack.size() < 2)
                        return false;
                    popstack(stack);
                    popstack(stack);
                }
                break;

                case OP_2DUP:
                {
                    // (x1 x2 -- x1 x2 x1 x2)
                    if (stack.size() < 2)
                        return false;
                    valtype vch1 = stacktop(-2);
                    valtype vch2 = stacktop(-1);
                    stack.push_back(vch1);
                    stack.push_back(vch2);
                }
                break;

                case OP_3DUP:
                {
                    // (x1 x2 x3 -- x1 x2 x3 x1 x2 x3)
                    if (stack.size() < 3)
                        return false;
                    valtype vch1 = stacktop(-3);
                    valtype vch2 = stacktop(-2);
                    valtype vch3 = stacktop(-1);
                    stack.push_back(vch1);
                    stack.push_back(vch2);
                    stack.push_back(vch3);
                }
                break;

                case OP_2OVER:
                {
                    // (x1 x2 x3 x4 -- x1 x2 x3 x4 x1 x2)
                    if (stack.size() < 4)
                        return false;
                    valtype vch1 = stacktop(-4);
                    valtype vch2 = stacktop(-3);
                    stack.push_back(vch1);
                    stack.push_back(vch2);
                }
                break;

                case OP_2ROT:
                {
                    // (x1 x2 x3 x4 x5 x6 -- x3 x4 x5 x6 x1 x2)
                    if (stack.size() < 6)
                        return false;
                    valtype vch1 = stacktop(-6);
                    valtype vch2 = stacktop(-5);
                    stack.erase(stack.end()-6, stack.end()-4);
                    stack.push_back(vch1);
                    stack.push_back(vch2);
                }
                break;

                case OP_2SWAP:
                {
                    // (x1 x2 x3 x4 -- x3 x4 x1 x2)
                    if (stack.size() < 4)
                        return false;
                    swap(stacktop(-4), stacktop(-2));
                    swap(stacktop(-3), stacktop(-1));
                }
                break;

                case OP_IFDUP:
                {
                    // (x - 0 | x x)
                    if (stack.size() < 1)
                        return false;
                    valtype vch = stacktop(-1);
                    if (CastToBool(vch))
                        stack.push_back(vch);
                }
                break;

                case OP_DEPTH:
                {
                    // -- stacksize
                    CScriptNum bn(stack.size());
                    stack.push_back(bn.getvch());
                }
                break;

                case OP_DROP:
                {
                    // (x -- )
                    if (stack.size() < 1)
                        return false;
                    popstack(stack);
                }
                break;

                case OP_DUP:
                {
                    // (x -- x x)
                    if (stack.size() < 1)
                        return false;
                    valtype vch = stacktop(-1);
                    stack.push_back(vch);
                }
                break;

                case OP_NIP:
                {
                    // (x1 x2 -- x2)
                    if (stack.size() < 2)
                        return false;
                    stack.erase(stack.end() - 2);
                }
                break;

                case OP_OVER:
                {
                    // (x1 x2 -- x1 x2 x1)
                    if (stack.size() < 2)
                        return false;
                    valtype vch = stacktop(-2);
                    stack.push_back(vch);
                }
                break;

                case OP_PICK:
                case OP_ROLL:
                {
                    // (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;
                    int n = CScriptNum(stacktop(-1), fRequireMinimal).getint();
                    popstack(stack);
                    if (n < 0 || n >= (int)stack.size())
                        return false;
                    valtype vch = stacktop(-n-1);
                    if (opcode == OP_ROLL)
                        stack.erase(stack.end()-n-1);
                    stack.push_back(vch);
                }
                break;

                case OP_ROT:
                {
                    // (x1 x2 x3 -- x2 x3 x1)
                    //  x2 x1 x3  after first swap
                    //  x2 x3 x1  after second swap
                    if (stack.size() < 3)
                        return false;
                    swap(stacktop(-3), stacktop(-2));
                    swap(stacktop(-2), stacktop(-1));
                }
                break;

                case OP_SWAP:
                {
                    // (x1 x2 -- x2 x1)
                    if (stack.size() < 2)
                        return false;
                    swap(stacktop(-2), stacktop(-1));
                }
                break;

                case OP_TUCK:
                {
                    // (x1 x2 -- x2 x1 x2)
                    if (stack.size() < 2)
                        return false;
                    valtype vch = stacktop(-1);
                    stack.insert(stack.end()-2, vch);
                }
                break;


                case OP_SIZE:
                {
                    // (in -- in size)
                    if (stack.size() < 1)
                        return false;
                    CScriptNum bn(stacktop(-1).size());
                    stack.push_back(bn.getvch());
                }
                break;


                //
                // Bitwise logic
                //
                case OP_EQUAL:
                case OP_EQUALVERIFY:
                //case OP_NOTEQUAL: // use OP_NUMNOTEQUAL
                {
                    // (x1 x2 - bool)
                    if (stack.size() < 2)
                        return false;
                    valtype& vch1 = stacktop(-2);
                    valtype& vch2 = stacktop(-1);
                    bool fEqual = (vch1 == vch2);
                    // OP_NOTEQUAL is disabled because it would be too easy to say
                    // something like n != 1 and have some wiseguy pass in 1 with extra
                    // zero bytes after it (numerically, 0x01 == 0x0001 == 0x000001)
                    //if (opcode == OP_NOTEQUAL)
                    //    fEqual = !fEqual;
                    popstack(stack);
                    popstack(stack);
                    stack.push_back(fEqual ? vchTrue : vchFalse);
                    if (opcode == OP_EQUALVERIFY)
                    {
                        if (fEqual)
                            popstack(stack);
                        else
                            return false;
                    }
                }
                break;


                //
                // Numeric
                //
                case OP_1ADD:
                case OP_1SUB:
                case OP_NEGATE:
                case OP_ABS:
                case OP_NOT:
                case OP_0NOTEQUAL:
                {
                    // (in -- out)
                    if (stack.size() < 1)
                        return false;
                    CScriptNum bn(stacktop(-1), fRequireMinimal);
                    switch (opcode)
                    {
                    case OP_1ADD:       bn += bnOne; break;
                    case OP_1SUB:       bn -= bnOne; break;
                    case OP_NEGATE:     bn = -bn; break;
                    case OP_ABS:        if (bn < bnZero) bn = -bn; break;
                    case OP_NOT:        bn = (bn == bnZero); break;
                    case OP_0NOTEQUAL:  bn = (bn != bnZero); break;
                    default:            assert(!"invalid opcode"); break;
                    }
                    popstack(stack);
                    stack.push_back(bn.getvch());
                }
                break;

                case OP_ADD:
                case OP_SUB:
                case OP_BOOLAND:
                case OP_BOOLOR:
                case OP_NUMEQUAL:
                case OP_NUMEQUALVERIFY:
                case OP_NUMNOTEQUAL:
                case OP_LESSTHAN:
                case OP_GREATERTHAN:
                case OP_LESSTHANOREQUAL:
                case OP_GREATERTHANOREQUAL:
                case OP_MIN:
                case OP_MAX:
                {
                    // (x1 x2 -- out)
                    if (stack.size() < 2)
                        return false;
                    CScriptNum bn1(stacktop(-2), fRequireMinimal);
                    CScriptNum bn2(stacktop(-1), fRequireMinimal);
                    CScriptNum bn(0);
                    switch (opcode)
                    {
                    case OP_ADD:
                        bn = bn1 + bn2;
                        break;

                    case OP_SUB:
                        bn = bn1 - bn2;
                        break;

                    case OP_BOOLAND:             bn = (bn1 != bnZero && bn2 != bnZero); break;
                    case OP_BOOLOR:              bn = (bn1 != bnZero || bn2 != bnZero); break;
                    case OP_NUMEQUAL:            bn = (bn1 == bn2); break;
                    case OP_NUMEQUALVERIFY:      bn = (bn1 == bn2); break;
                    case OP_NUMNOTEQUAL:         bn = (bn1 != bn2); break;
                    case OP_LESSTHAN:            bn = (bn1 < bn2); break;
                    case OP_GREATERTHAN:         bn = (bn1 > bn2); break;
                    case OP_LESSTHANOREQUAL:     bn = (bn1 <= bn2); break;
                    case OP_GREATERTHANOREQUAL:  bn = (bn1 >= bn2); break;
                    case OP_MIN:                 bn = (bn1 < bn2 ? bn1 : bn2); break;
                    case OP_MAX:                 bn = (bn1 > bn2 ? bn1 : bn2); break;
                    default:                     assert(!"invalid opcode"); break;
                    }
                    popstack(stack);
                    popstack(stack);
                    stack.push_back(bn.getvch());

                    if (opcode == OP_NUMEQUALVERIFY)
                    {
                        if (CastToBool(stacktop(-1)))
                            popstack(stack);
                        else
                            return false;
                    }
                }
                break;

                case OP_WITHIN:
                {
                    // (x min max -- out)
                    if (stack.size() < 3)
                        return false;
                    CScriptNum bn1(stacktop(-3), fRequireMinimal);
                    CScriptNum bn2(stacktop(-2), fRequireMinimal);
                    CScriptNum bn3(stacktop(-1), fRequireMinimal);
                    bool fValue = (bn2 <= bn1 && bn1 < bn3);
                    popstack(stack);
                    popstack(stack);
                    popstack(stack);
                    stack.push_back(fValue ? vchTrue : vchFalse);
                }
                break;


                //
                // Crypto
                //
                case OP_RIPEMD160:
                case OP_SHA1:
                case OP_SHA256:
                case OP_HASH160:
                case OP_HASH256:
                {
                    // (in -- hash)
                    if (stack.size() < 1)
                        return false;
                    valtype& vch = stacktop(-1);
                    valtype vchHash((opcode == OP_RIPEMD160 || opcode == OP_SHA1 || opcode == OP_HASH160) ? 20 : 32);
                    if (opcode == OP_RIPEMD160)
                        CRIPEMD160().Write(begin_ptr(vch), vch.size()).Finalize(begin_ptr(vchHash));
                    else if (opcode == OP_SHA1)
                        CSHA1().Write(begin_ptr(vch), vch.size()).Finalize(begin_ptr(vchHash));
                    else if (opcode == OP_SHA256)
                        CSHA256().Write(begin_ptr(vch), vch.size()).Finalize(begin_ptr(vchHash));
                    else if (opcode == OP_HASH160)
                        CHash160().Write(begin_ptr(vch), vch.size()).Finalize(begin_ptr(vchHash));
                    else if (opcode == OP_HASH256)
                        CHash256().Write(begin_ptr(vch), vch.size()).Finalize(begin_ptr(vchHash));
                    popstack(stack);
                    stack.push_back(vchHash);
                }
                break;                                   

                case OP_CODESEPARATOR:
                {
                    // Hash starts after the code separator
                    pbegincodehash = pc;
                }
                break;

                case OP_CHECKSIG:
                case OP_CHECKSIGVERIFY:
                {
                    // (sig pubkey -- bool)
                    if (stack.size() < 2)
                        return false;

                    valtype& vchSig    = stacktop(-2);
                    valtype& vchPubKey = stacktop(-1);

                    // Subset of script starting at the most recent codeseparator
                    CScript scriptCode(pbegincodehash, pend);

                    // Drop the signature, since there's no way for a signature to sign itself
                    scriptCode.FindAndDelete(CScript(vchSig));

                    if (!CheckSignatureEncoding(vchSig, flags)) {
                        return false;
                    }

                    bool fSuccess = CheckPubKeyEncoding(vchPubKey, flags) && checker.CheckSig(vchSig, vchPubKey, scriptCode);

                    popstack(stack);
                    popstack(stack);
                    stack.push_back(fSuccess ? vchTrue : vchFalse);
                    if (opcode == OP_CHECKSIGVERIFY)
                    {
                        if (fSuccess)
                            popstack(stack);
                        else
                            return false;
                    }
                }
                break;

                case OP_CHECKMULTISIG:
                case OP_CHECKMULTISIGVERIFY:
                {
                    // ([sig ...] num_of_signatures [pubkey ...] num_of_pubkeys -- bool)

                    int i = 1;
                    if ((int)stack.size() < i)
                        return false;

                    int nKeysCount = CScriptNum(stacktop(-i), fRequireMinimal).getint();
                    if (nKeysCount < 0 || nKeysCount > 20)
                        return false;
                    nOpCount += nKeysCount;
                    if (nOpCount > 201)
                        return false;
                    int ikey = ++i;
                    i += nKeysCount;
                    if ((int)stack.size() < i)
                        return false;

                    int nSigsCount = CScriptNum(stacktop(-i), fRequireMinimal).getint();
                    if (nSigsCount < 0 || nSigsCount > nKeysCount)
                        return false;
                    int isig = ++i;
                    i += nSigsCount;
                    if ((int)stack.size() < i)
                        return false;

                    // Subset of script starting at the most recent codeseparator
                    CScript scriptCode(pbegincodehash, pend);

                    // Drop the signatures, since there's no way for a signature to sign itself
                    for (int k = 0; k < nSigsCount; k++)
                    {
                        valtype& vchSig = stacktop(-isig-k);
                        scriptCode.FindAndDelete(CScript(vchSig));
                    }

                    bool fSuccess = true;
                    while (fSuccess && nSigsCount > 0)
                    {
                        valtype& vchSig    = stacktop(-isig);
                        valtype& vchPubKey = stacktop(-ikey);

                        if (!CheckSignatureEncoding(vchSig, flags)) {
                            return false;
                        }

                        // Check signature
                        bool fOk = CheckPubKeyEncoding(vchPubKey, flags) && checker.CheckSig(vchSig, vchPubKey, scriptCode);

                        if (fOk) {
                            isig++;
                            nSigsCount--;
                        }
                        ikey++;
                        nKeysCount--;

                        // If there are more signatures left than keys left,
                        // then too many signatures have failed
                        if (nSigsCount > nKeysCount)
                            fSuccess = false;
                    }

                    // Clean up stack of actual arguments
                    while (i-- > 1)
                        popstack(stack);

                    // A bug causes CHECKMULTISIG to consume one extra argument
                    // whose contents were not checked in any way.
                    //
                    // Unfortunately this is a potential source of mutability,
                    // so optionally verify it is exactly equal to zero prior
                    // to removing it from the stack.
                    if (stack.size() < 1)
                        return false;
                    if ((flags & SCRIPT_VERIFY_NULLDUMMY) && stacktop(-1).size())
                        return error("CHECKMULTISIG dummy argument not null");
                    popstack(stack);

                    stack.push_back(fSuccess ? vchTrue : vchFalse);

                    if (opcode == OP_CHECKMULTISIGVERIFY)
                    {
                        if (fSuccess)
                            popstack(stack);
                        else
                            return false;
                    }
                }
                break;

                default:
                    return false;
            }

            // Size limits
            if (stack.size() + altstack.size() > 1000)
                return false;
        }
    }
    catch (...)
    {
        return false;
    }

    if (!vfExec.empty())
        return false;

    return true;
}

namespace {

/** Wrapper that serializes like CTransaction, but with the modifications
 *  required for the signature hash done in-place
 */
class CTransactionSignatureSerializer {
private:
    const CTransaction &txTo;  // reference to the spending transaction (the one being serialized)
    const CScript &scriptCode; // output script being consumed
    const unsigned int nIn;    // input index of txTo being signed
    const bool fAnyoneCanPay;  // whether the hashtype has the SIGHASH_ANYONECANPAY flag set
    const bool fHashSingle;    // whether the hashtype is SIGHASH_SINGLE
    const bool fHashNone;      // whether the hashtype is SIGHASH_NONE

public:
    CTransactionSignatureSerializer(const CTransaction &txToIn, const CScript &scriptCodeIn, unsigned int nInIn, int nHashTypeIn) :
        txTo(txToIn), scriptCode(scriptCodeIn), nIn(nInIn),
        fAnyoneCanPay(!!(nHashTypeIn & SIGHASH_ANYONECANPAY)),
        fHashSingle((nHashTypeIn & 0x1f) == SIGHASH_SINGLE),
        fHashNone((nHashTypeIn & 0x1f) == SIGHASH_NONE) {}

    /** Serialize the passed scriptCode, skipping OP_CODESEPARATORs */
    template<typename S>
    void SerializeScriptCode(S &s, int nType, int nVersion) const {
        CScript::const_iterator it = scriptCode.begin();
        CScript::const_iterator itBegin = it;
        opcodetype opcode;
        unsigned int nCodeSeparators = 0;
        while (scriptCode.GetOp(it, opcode)) {
            if (opcode == OP_CODESEPARATOR)
                nCodeSeparators++;
        }
        ::WriteCompactSize(s, scriptCode.size() - nCodeSeparators);
        it = itBegin;
        while (scriptCode.GetOp(it, opcode)) {
            if (opcode == OP_CODESEPARATOR) {
                s.write((char*)&itBegin[0], it-itBegin-1);
                itBegin = it;
            }
        }
        if (itBegin != scriptCode.end())
            s.write((char*)&itBegin[0], it-itBegin);
    }

    /** Serialize an input of txTo */
    template<typename S>
    void SerializeInput(S &s, unsigned int nInput, int nType, int nVersion) const {
        // In case of SIGHASH_ANYONECANPAY, only the input being signed is serialized
        if (fAnyoneCanPay)
            nInput = nIn;
        // Serialize the prevout
        ::Serialize(s, txTo.vin[nInput].prevout, nType, nVersion);
        // Serialize the script
        if (nInput != nIn)
            // Blank out other inputs' signatures
            ::Serialize(s, CScript(), nType, nVersion);
        else
            SerializeScriptCode(s, nType, nVersion);
        // Serialize the nSequence
        if (nInput != nIn && (fHashSingle || fHashNone))
            // let the others update at will
            ::Serialize(s, (int)0, nType, nVersion);
        else
            ::Serialize(s, txTo.vin[nInput].nSequence, nType, nVersion);
    }

    /** Serialize an output of txTo */
    template<typename S>
    void SerializeOutput(S &s, unsigned int nOutput, int nType, int nVersion) const {
        if (fHashSingle && nOutput != nIn)
            // Do not lock-in the txout payee at other indices as txin
            ::Serialize(s, CTxOut(), nType, nVersion);
        else
            ::Serialize(s, txTo.vout[nOutput], nType, nVersion);
    }

    /** Serialize txTo */
    template<typename S>
    void Serialize(S &s, int nType, int nVersion) const {
        // Serialize nVersion
        ::Serialize(s, txTo.nVersion, nType, nVersion);
        // Serialize vin
        unsigned int nInputs = fAnyoneCanPay ? 1 : txTo.vin.size();
        ::WriteCompactSize(s, nInputs);
        for (unsigned int nInput = 0; nInput < nInputs; nInput++)
             SerializeInput(s, nInput, nType, nVersion);
        // Serialize vout
        unsigned int nOutputs = fHashNone ? 0 : (fHashSingle ? nIn+1 : txTo.vout.size());
        ::WriteCompactSize(s, nOutputs);
        for (unsigned int nOutput = 0; nOutput < nOutputs; nOutput++)
             SerializeOutput(s, nOutput, nType, nVersion);
        // Serialie nLockTime
        ::Serialize(s, txTo.nLockTime, nType, nVersion);
    }
};

} // anon namespace

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);
        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);
            return 1;
        }
    }

    // Wrapper to serialize only the necessary parts of the transaction being signed
    CTransactionSignatureSerializer txTmp(txTo, scriptCode, nIn, nHashType);

    // Serialize and hash
    CHashWriter ss(SER_GETHASH, 0);
    ss << txTmp << nHashType;
    return ss.GetHash();
}

bool SignatureChecker::VerifySignature(const std::vector<unsigned char>& vchSig, const CPubKey& pubkey, const uint256& sighash) const
{
    return pubkey.Verify(sighash, vchSig);
}

bool SignatureChecker::CheckSig(const vector<unsigned char>& vchSigIn, const vector<unsigned char>& vchPubKey, const CScript& scriptCode) const
{
    CPubKey pubkey(vchPubKey);
    if (!pubkey.IsValid())
        return false;

    // Hash type is one byte tacked on to the end of the signature
    vector<unsigned char> vchSig(vchSigIn);
    if (vchSig.empty())
        return false;
    int nHashType = vchSig.back();
    vchSig.pop_back();

    uint256 sighash = SignatureHash(scriptCode, txTo, nIn, nHashType);

    if (!VerifySignature(vchSig, pubkey, sighash))
        return false;

    return true;
}

bool VerifyScript(const CScript& scriptSig, const CScript& scriptPubKey, unsigned int flags, const BaseSignatureChecker& checker)
{
    if ((flags & SCRIPT_VERIFY_SIGPUSHONLY) != 0 && !scriptSig.IsPushOnly()) {
        return false;
    }

    vector<vector<unsigned char> > stack, stackCopy;
    if (!EvalScript(stack, scriptSig, flags, checker))
        return false;
    if (flags & SCRIPT_VERIFY_P2SH)
        stackCopy = stack;
    if (!EvalScript(stack, scriptPubKey, flags, checker))
        return false;
    if (stack.empty())
        return false;

    if (CastToBool(stack.back()) == false)
        return 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

        // stackCopy 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());

        const valtype& pubKeySerialized = stackCopy.back();
        CScript pubKey2(pubKeySerialized.begin(), pubKeySerialized.end());
        popstack(stackCopy);

        if (!EvalScript(stackCopy, pubKey2, flags, checker))
            return false;
        if (stackCopy.empty())
            return false;
        return CastToBool(stackCopy.back());
    }

    return true;
}