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Simplified emulator ROM decryption subroutine

This commit is contained in:
Jack Andersen 2018-06-19 08:44:34 -10:00
parent 64dd7b1643
commit 73d48c9b41
2 changed files with 76 additions and 176 deletions
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248
NESEmulator/CNESEmulator.cpp
View File

@ -523,203 +523,103 @@ static void nesEmuFdsSetup(uint8_t *src, uint8_t *dst)
}
#endif
void CNESEmulator::DecompressROM(u8* dataIn, u8* dataOut, u32 dataOutLen, u8 descrambleSeed,
u32 checkDataLen, u32 checksumMagic)
struct BitstreamState
{
for (int i=0 ; i<256 ; ++i)
u8* rPos;
int position = 0;
int tmpBuf = 0;
int decBit = 0;
BitstreamState(u8* pos) : rPos(pos) {}
void resetDecBit() { decBit = 0; }
void runDecBit()
{
descrambleSeed += dataIn[i];
dataIn[i] = descrambleSeed;
}
for (int i=0 ; i<128 ; ++i)
std::swap(dataIn[255 - i], dataIn[i]);
struct BitstreamState
{
u32 x0_remBits = 0;
u32 x4_lastByte;
u8* x8_ptr;
BitstreamState(u8* ptr) : x8_ptr(ptr) {}
} bState = {dataIn + 256};
u8* dataOutAlpha = dataOut;
while (dataOutLen != 0)
{
u32 r12 = 0;
if (bState.x0_remBits == 0)
if(position == 0)
{
bState.x4_lastByte = *bState.x8_ptr;
bState.x8_ptr += 1;
bState.x0_remBits = 8;
position = 8;
tmpBuf = *rPos++;
}
decBit <<= 1;
if(tmpBuf & 0x80)
decBit |= 1;
tmpBuf <<= 1;
position--;
}
};
u32 r11 = r12 << 1;
if (bState.x4_lastByte & 0x80)
r11 |= 1;
bState.x4_lastByte <<= 1;
bState.x0_remBits -= 1;
if (r11 == 1)
// Based on https://gist.github.com/FIX94/7593640c5cee6c37e3b23e7fcf8fe5b7
void CNESEmulator::DecryptMetroid(u8* dataIn, u8* dataOut, u32 decLen, u8 decByte,
u32 xorLen, u32 xorVal)
{
int i, j;
// simple add obfuscation
for(i = 0; i < 0x100; i++)
{
dataIn[i] += decByte;
decByte = dataIn[i];
}
// flip the first 0x100 bytes around
for (i = 0; i < 128; ++i)
std::swap(dataIn[255 - i], dataIn[i]);
// set up buffer pointers
BitstreamState bState(dataIn + 0x100);
// unscramble buffer
for(i = 0; i < decLen; i++)
{
bState.resetDecBit();
bState.runDecBit();
if(bState.decBit)
{
r12 = 0;
for (int i=0 ; i<4 ; ++i)
{
if (bState.x0_remBits == 0)
{
bState.x4_lastByte = *bState.x8_ptr;
bState.x8_ptr += 1;
bState.x0_remBits = 8;
}
r12 <<= 1;
if (bState.x4_lastByte & 0x80)
r12 |= 1;
bState.x4_lastByte <<= 1;
bState.x0_remBits -= 1;
if (bState.x0_remBits == 0)
{
bState.x4_lastByte = *bState.x8_ptr;
bState.x8_ptr += 1;
bState.x0_remBits = 8;
}
r12 <<= 1;
if (bState.x4_lastByte & 0x80)
r12 |= 1;
bState.x4_lastByte <<= 1;
bState.x0_remBits -= 1;
}
*dataOutAlpha++ = dataIn[r12 + 73];
bState.resetDecBit();
for(j = 0; j < 8; j++)
bState.runDecBit();
dataOut[i] = dataIn[bState.decBit + 0x49];
}
else
{
r12 = 0;
if (bState.x0_remBits == 0)
bState.resetDecBit();
bState.runDecBit();
if(bState.decBit)
{
bState.x4_lastByte = *bState.x8_ptr;
bState.x8_ptr += 1;
bState.x0_remBits = 8;
}
r11 = r12 << 1;
if (bState.x4_lastByte & 0x80)
r11 |= 1;
bState.x4_lastByte <<= 1;
bState.x0_remBits -= 1;
if (r11 == 1)
{
r12 = 0;
for (int i=0 ; i<3 ; ++i)
{
if (bState.x0_remBits == 0)
{
bState.x4_lastByte = *bState.x8_ptr;
bState.x8_ptr += 1;
bState.x0_remBits = 8;
}
r12 <<= 1;
if (bState.x4_lastByte & 0x80)
r12 |= 1;
bState.x4_lastByte <<= 1;
bState.x0_remBits -= 1;
if (bState.x0_remBits == 0)
{
bState.x4_lastByte = *bState.x8_ptr;
bState.x8_ptr += 1;
bState.x0_remBits = 8;
}
r12 <<= 1;
if (bState.x4_lastByte & 0x80)
r12 |= 1;
bState.x4_lastByte <<= 1;
bState.x0_remBits -= 1;
}
*dataOutAlpha++ = dataIn[r12 + 9];
bState.resetDecBit();
for(j = 0; j < 6; j++)
bState.runDecBit();
dataOut[i] = dataIn[bState.decBit + 9];
}
else
{
r12 = 0;
if (bState.x0_remBits == 0)
bState.resetDecBit();
bState.runDecBit();
if(bState.decBit)
{
bState.x4_lastByte = *bState.x8_ptr;
bState.x8_ptr += 1;
bState.x0_remBits = 8;
}
r11 = r12 << 1;
if (bState.x4_lastByte & 0x80)
r11 |= 1;
bState.x4_lastByte <<= 1;
bState.x0_remBits -= 1;
if (r11 == 1)
{
r12 = 0;
for (int i=0 ; i<3 ; ++i)
{
if (bState.x0_remBits == 0)
{
bState.x4_lastByte = *bState.x8_ptr;
bState.x8_ptr += 1;
bState.x0_remBits = 8;
}
r12 <<= 1;
if (bState.x4_lastByte & 0x80)
r12 |= 1;
bState.x4_lastByte <<= 1;
bState.x0_remBits -= 1;
}
*dataOutAlpha++ = dataIn[r12 + 1];
bState.resetDecBit();
for(j = 0; j < 3; j++)
bState.runDecBit();
dataOut[i] = dataIn[bState.decBit + 1];
}
else
{
*dataOutAlpha++ = dataIn[0];
}
dataOut[i] = dataIn[bState.decBit];
}
}
--dataOutLen;
}
u32 tmpWord = 0;
for (int i=0 ; i<checkDataLen ; ++i)
// do checksum fixups
unsigned int xorTmpVal = 0;
for(i = 0; i < xorLen; i++)
{
tmpWord ^= dataOut[i];
for (int j=0 ; j<8 ; ++j)
xorTmpVal ^= dataOut[i];
for(j = 0; j < 8; j++)
{
if (tmpWord & 0x1)
tmpWord = (tmpWord >> 1) ^ checksumMagic;
if(xorTmpVal & 1)
{
xorTmpVal >>= 1;
xorTmpVal ^= xorVal;
}
else
tmpWord = (tmpWord >> 1);
xorTmpVal >>= 1;
}
}
dataOut[checkDataLen - 1] = (tmpWord >> 8) & 0xff;
dataOut[checkDataLen - 2] = tmpWord & 0xff;
// write in calculated checksum
dataOut[xorLen - 1] = u8((xorTmpVal >> 8) & 0xFF);
dataOut[xorLen - 2] = u8(xorTmpVal & 0xFF);
}
void CNESEmulator::ProcessUserInput(const CFinalInput& input, int)
@ -871,7 +771,7 @@ void CNESEmulator::Update()
m_dvdReq.reset();
emuNesROMsize = 0x20000;
emuNesROM = (uint8_t*)malloc(emuNesROMsize);
DecompressROM(m_nesEmuPBuf.get(), emuNesROM);
DecryptMetroid(m_nesEmuPBuf.get(), emuNesROM);
m_nesEmuPBuf.reset();
InitializeEmulator();
}

4
NESEmulator/CNESEmulator.hpp
View File

@ -70,8 +70,8 @@ private:
EPasswordEntryState x34_passwordEntryState = EPasswordEntryState::NotPasswordScreen;
bool x38_passwordPending = false;
u8 x39_passwordToNES[18];
static void DecompressROM(u8* dataIn, u8* dataOut, u32 dataOutLen = 0x20000, u8 descrambleSeed = 0xe9,
u32 checkDataLen = 0x1FFFC, u32 checksumMagic = 0xA663);
static void DecryptMetroid(u8* dataIn, u8* dataOut, u32 decLen = 0x20000, u8 decByte = 0xe9,
u32 xorLen = 0x1FFFC, u32 xorVal = 0xA663);
void InitializeEmulator();
void DeinitializeEmulator();
void NesEmuMainLoop(bool forceDraw = false);