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// license:BSD-3-Clause
// copyright-holders:Morten Shearman Kirkegaard, Samuel Neves, Peter Wilhelmsen
/*************************************************************************

    atarixga.cpp

    Atari XGA encryption FPGA

 **************************************************************************

    Part numbers:

        136094-0072   Moto Frenzy
        136095-0072   Space Lords
        ?             Road Riot's Revenge
        136094-0004A  Primal Rage
        ?             T-Mek

*************************************************************************/

#include "emu.h"
#include "atarixga.h"


extern const device_type ATARI_XGA = &device_creator<atari_xga_device>;

atari_xga_device::atari_xga_device(const machine_config &mconfig, const char *tag, device_t *owner, uint32_t clock)
: device_t(mconfig, ATARI_XGA, "Atari XGA", tag, owner, clock, "xga", __FILE__),
	m_mode(FPGA_RESET),
	m_address(0),
	m_ciphertext(0)
{
}



/*************************************
 *
 *  Initialization
 *
 *************************************/

void atari_xga_device::device_start()
{
	m_ram = std::make_unique<uint16_t[]>(RAM_WORDS);

	save_pointer(NAME(m_ram.get()), RAM_WORDS * sizeof(uint16_t));
	save_item(NAME(m_address));
	save_item(NAME(m_ciphertext));
}

void atari_xga_device::device_reset()
{
	memset(m_ram.get(), 0, RAM_WORDS * sizeof(uint16_t));
	m_mode = FPGA_RESET;
	m_address = 0;
	m_ciphertext = 0;
}



/*************************************
 *
 *  Definitions
 *
 *************************************/

// TODO: Add definitions for other games
// Moto Frenzy

/* key 0x10 is special, it has 15 "identical twins". */
static const uint8_t kmap[128] =
{
	0x6B,0x11,0x1B,0x19,0x4B,0x50,0x17,0x09,
	0x5D,0x69,0x43,0x33,0x0F,0x0C,0x28,0x3F,
	0x00,0x20,0x15,0x3C,0x57,0x38,0x00,0x07,
	0x49,0x25,0x61,0x2F,0x2B,0x4E,0x64,0x00,
	0x45,0x41,0x6D,0x52,0x31,0x66,0x22,0x59,
	0x00,0x70,0x6F,0x5B,0x46,0x6E,0x67,0x5A,
	0x26,0x30,0x2C,0x65,0x21,0x3D,0x58,0x00,
	0x5E,0x44,0x0D,0x40,0x6C,0x1C,0x51,0x0A,
	0x35,0x2A,0x13,0x4D,0x63,0x00,0x00,0x3A,
	0x00,0x48,0x54,0x24,0x60,0x1E,0x2E,0x01,
	0x56,0x03,0x37,0x00,0x04,0x00,0x05,0x06,
	0x00,0x55,0x1F,0x02,0x36,0x14,0x00,0x3B,
	0x5F,0x0E,0x1D,0x0B,0x27,0x2D,0x3E,0x00,
	0x00,0x5C,0x47,0x68,0x42,0x53,0x32,0x23,
	0x4A,0x62,0x4F,0x00,0x00,0x16,0x39,0x08,
	0x6A,0x34,0x10,0x29,0x12,0x1A,0x4C,0x18
};



/*************************************
 *
 *  Decryption
 *
 *************************************/

uint16_t atari_xga_device::ctz(uint16_t x)
{
	uint16_t n = 0;
	if (x == 0) return 16;
	if (!(x & 0x00FF)) n += 8, x >>= 8;
	if (!(x & 0x000F)) n += 4, x >>= 4;
	if (!(x & 0x0003)) n += 2, x >>= 2;
	if (!(x & 0x0001)) n += 1, x >>= 1;
	return n;
}

size_t atari_xga_device::popcount(uint16_t x)
{
	size_t count = 0;
	while (x != 0)
	{
		count += 1;
		x &= x - 1;
	}
	return count;
}

uint16_t atari_xga_device::parity(uint16_t x)
{
	return popcount(x) & 1;
}

uint16_t atari_xga_device::lfsr1(uint16_t x)
{
	const uint16_t bit = parity(x & 0x8016);
	return (x << 1) | bit;
}

uint16_t atari_xga_device::lfsr2(uint16_t x)
{
	uint16_t bit = parity(x & 0x002D);
	return (x >> 1) | (bit << 15);
}

uint16_t atari_xga_device::powers2(uint8_t k, uint16_t x)
{
	static const uint16_t L[16] = {
			0x5E85,0xBD0B,0x2493,0x17A3,
			0x2F47,0x0005,0x000B,0x0017,
			0x002F,0x005E,0x00BD,0x017A,
			0x02F4,0x05E8,0x0BD0,0x17A1
		};

	uint16_t t = (x == 16) ? (L[4] ^ L[5]) : L[x];

	for (size_t i = 0; i < k; ++i)
		t = lfsr1(t);

	return t;
}

uint16_t atari_xga_device::decipher(uint8_t k, uint16_t c)
{
	uint16_t p = 0;

	/* Only 128 keys internally, if high bit set,
	then find the 7-bit "twin" by xor 0xA8. */
	if (k & 0x80)
		k ^= 0xA8;

	k = kmap[k];

	if ((c & (c - 1)) == 0)
		return powers2(k, ctz(c));

	for (uint16_t bit = 0; bit < 5; ++bit)
	{
		if ((c >> bit) & 1)
		{
			p ^= powers2(k, bit);
		}
	}

	for (uint16_t bit = 5; bit < 16; ++bit)
	{
		if ((c >> bit) & 1)
		{
			p ^= powers2(k, bit + 1);
		}
	}

	uint16_t x = 0x8010;
	for (uint16_t i = 0; i < k + 3; ++i)
	{
		if (x == c)
		{
			return (p == 1) ? 0 : lfsr2(p);
		}
		x = lfsr2(x);
	}

	return p;
}



/*************************************
 *
 *  Write/Read access
 *
 *************************************/

WRITE32_MEMBER(atari_xga_device::write)
{
	switch (m_mode)
	{
	case FPGA_RESET:
		return;

	case FPGA_SETKEY:
		/* Write table to FPGA SRAM. */
		if (ACCESSING_BITS_16_31)
			m_ram[offset << 1] = uint16_t (data >> 16);
		if (ACCESSING_BITS_0_15)
			m_ram[(offset << 1) + 1] = uint16_t(data & 0xFFFF);
		break;

	case FPGA_DECIPHER:
		/* Send Ciphertext to FPGA for decryption. */
		if (ACCESSING_BITS_16_31)
		{
			m_address = offset << 2;
			m_ciphertext = uint16_t(data >> 16);
		}
		if (ACCESSING_BITS_0_15)
		{
			m_address = (offset << 2) + 2;
			m_ciphertext = uint16_t(data & 0xFFFF);
		}
		break;
	}
}

READ32_MEMBER(atari_xga_device::read)
{
	switch (offset << 2)
	{
	case 0x0FC0:
		m_mode = FPGA_RESET;
		break;
	case 0x0010:
		m_mode = FPGA_SETKEY;
		break;
	case 0x0020:
		m_mode = FPGA_DECIPHER;
		break;
	}

	if (m_mode == FPGA_RESET)
		return 0;

	uint32_t plaintext = 0;
	if (m_mode == FPGA_DECIPHER)
	{
		uint16_t address = (offset << 2) - 0x400;

		if (ACCESSING_BITS_0_15)
			address += 2;

		/* Reply with decrypted plaintext */
		if (address == m_address)
		{
			uint16_t key_offset, key_byte;

			/* Algorithm to select key byte based on offset. */
			key_offset = ((((address >>  4) & 1) ^ 1) << 0)
						^ ((((address >>  2) & 1) ^ 0) << 1)
						^ ((((address >>  8) & 1) ^ 0) << 2)
						^ ((((address >>  3) & 1) ^ 1) << 3)
						^ ((((address >>  1) & 1) ^ 1) << 4)
						^ ((((address >>  6) & 1) ^ 0) << 5)
						^ ((((address >>  7) & 1) ^ 0) << 6)
						^ ((((address >>  5) & 1) ^ 1) << 7)
						^ ((((address >>  9) & 1) ^ 0) << 8)
						^ ((((address >> 10) & 1) ^ 0) << 9);
			key_byte = m_ram[key_offset];

			/* And now for the full magic. */
			plaintext = decipher(key_byte, m_ciphertext);

			if (ACCESSING_BITS_16_31)
				plaintext <<= 16;
		}
	}

	return plaintext;
}