// license:BSD-3-Clause
// copyright-holders:Nicola Salmoria, David Widel
/***************************************************************************
NEC MC-8123 encryption emulation
Decryption tables provided by David Widel
Decryption algorithm by Nicola Salmoria
The NEC MC-8123 is a Z80 with built-in encryption.
It contains 0x2000 bytes of battery-backed RAM, when the battery dies the
game stops working. It's possible that the CPU would work as a normal Z80
in that case, this hasn't been verified.
When the CPU reads a byte from memory, it uses 12 bits of the address and
the information of whether the byte is an opcode or not to select a byte in
the internal RAM. The byte controls how the decryption works.
A special value in the internal RAM instructs the CPU to not apply the decryption
algorithm at all. This is necessary for the CPU to be able to use RAM areas
normally.
In theory, the size of the key would make the encryption quite secure.
In practice, the contents of the internal RAM were generated using a linear
congruential generator, so the whole key can be generated starting from a single
24-bit seed.
The LCG is the same that was used to generate the FD1094 keys.
Note that the algorithm skips the special value that would instruct the CPU to
not decrypt at that address.
void generate_key(uint8_t (&key)[0x2000], uint32_t seed, unsigned upper_bound)
{
unsigned i;
for (i = 0; i < upper_bound; ++i)
{
uint8_t byteval;
do
{
seed *= 0x29;
seed += seed << 16;
byteval = (~seed >> 16) & 0xff;
}
while (byteval == 0xff);
key[i] = byteval;
}
for ( ; i < 0x2000; ++i)
key[i] = 0xff;
}
The known games that use this CPU are:
CPU # Game Notes Seed Upper Limit
-------- ------------------------- ------------------ ------ -----
317-5014 DakkoChan Jansoh NEC MC-8123B 206850 1C00
317-0029 Block Gal NEC MC-8123B 651 091755 1800?
317-0030 Perfect Billiard 9451EC 1C00
317-0042 Opa Opa B31FD0 1C00
317-0043 Wonder Boy Monster Land 640708 1C00
317-0054 Shinobi (sound CPU) NEC MC-8123B 652 088992 1800
317-0057 Fantasy Zone 2 ADFACE 1800
317-0064 Ufo Senshi Yohko Chan 880603 1C00
317-0066 Altered Beast (sound CPU) NEC MC-8123B 704 ED8600 1800
317-5002 Gigas NEC MC-8123 638 234567 1C00
317-5012 Ganbare Chinsan Ooshoubu NEC MC-8123A 804B54 1C00
317-5??? Ninja Kid II (sound CPU) NEC MC-8123A 646 27998D 1800
317-???? Center Court (sound CPU) NEC MC-8123B 703 640506 1800
312-???? Omega 861226 1C00?
***************************************************************************/
#include "emu.h"
#include "mc8123.h"
DEFINE_DEVICE_TYPE(MC8123, mc8123_device, "mc8123", "MC-8123")
//-------------------------------------------------
// mc8123_device - constructor
//-------------------------------------------------
mc8123_device::mc8123_device(const machine_config &mconfig, const char *tag, device_t *owner, u32 clock)
: z80_device(mconfig, MC8123, tag, owner, clock)
, m_key(*this, "key", 0x2000)
{
}
namespace {
template <typename T> constexpr u8 BITS(T b) { return u8(1) << b; }
template <typename T, typename... U> constexpr u8 BITS(T b, U... c) { return (u8(1) << b) | BITS(c...); }
} // anonymous namespace
u8 mc8123_device::decrypt_type0(u8 val, u8 param, unsigned swap)
{
if (swap == 0) val = bitswap<8>(val,7,5,3,1,2,0,6,4);
if (swap == 1) val = bitswap<8>(val,5,3,7,2,1,0,4,6);
if (swap == 2) val = bitswap<8>(val,0,3,4,6,7,1,5,2);
if (swap == 3) val = bitswap<8>(val,0,7,3,2,6,4,1,5);
if (BIT(param,3) && BIT(val,7))
val ^= BITS(5,3,0);
if (BIT(param,2) && BIT(val,6))
val ^= BITS(7,2,1);
if (BIT(val,6)) val ^= BITS(7);
if (BIT(param,1) && BIT(val,7))
val ^= BITS(6);
if (BIT(val,2)) val ^= BITS(5,0);
val ^= BITS(4,3,1);
if (BIT(param,2)) val ^= BITS(5,2,0);
if (BIT(param,1)) val ^= BITS(7,6);
if (BIT(param,0)) val ^= BITS(5,0);
if (BIT(param,0)) val = bitswap<8>(val,7,6,5,1,4,3,2,0);
return val;
}
u8 mc8123_device::decrypt_type1a(u8 val, u8 param, unsigned swap)
{
if (swap == 0) val = bitswap<8>(val,4,2,6,5,3,7,1,0);
if (swap == 1) val = bitswap<8>(val,6,0,5,4,3,2,1,7);
if (swap == 2) val = bitswap<8>(val,2,3,6,1,4,0,7,5);
if (swap == 3) val = bitswap<8>(val,6,5,1,3,2,7,0,4);
if (BIT(param,2)) val = bitswap<8>(val,7,6,1,5,3,2,4,0);
if (BIT(val,1)) val ^= BITS(0);
if (BIT(val,6)) val ^= BITS(3);
if (BIT(val,7)) val ^= BITS(6,3);
if (BIT(val,2)) val ^= BITS(6,3,1);
if (BIT(val,4)) val ^= BITS(7,6,2);
if (BIT(val,7) ^ BIT(val,2))
val ^= BITS(4);
val ^= BITS(6,3,1,0);
if (BIT(param,3)) val ^= BITS(7,2);
if (BIT(param,1)) val ^= BITS(6,3);
if (BIT(param,0)) val = bitswap<8>(val,7,6,1,4,3,2,5,0);
return val;
}
u8 mc8123_device::decrypt_type1b(u8 val, u8 param, unsigned swap)
{
if (swap == 0) val = bitswap<8>(val,1,0,3,2,5,6,4,7);
if (swap == 1) val = bitswap<8>(val,2,0,5,1,7,4,6,3);
if (swap == 2) val = bitswap<8>(val,6,4,7,2,0,5,1,3);
if (swap == 3) val = bitswap<8>(val,7,1,3,6,0,2,5,4);
if (BIT(val,2) && BIT(val,0))
val ^= BITS(7,4);
if (BIT(val,7)) val ^= BITS(2);
if (BIT(val,5)) val ^= BITS(7,2);
if (BIT(val,1)) val ^= BITS(5);
if (BIT(val,6)) val ^= BITS(1);
if (BIT(val,4)) val ^= BITS(6,5);
if (BIT(val,0)) val ^= BITS(6,2,1);
if (BIT(val,3)) val ^= BITS(7,6,2,1,0);
val ^= BITS(6,4,0);
if (BIT(param,3)) val ^= BITS(4,1);
if (BIT(param,2)) val ^= BITS(7,6,3,0);
if (BIT(param,1)) val ^= BITS(4,3);
if (BIT(param,0)) val ^= BITS(6,2,1,0);
return val;
}
u8 mc8123_device::decrypt_type2a(u8 val, u8 param, unsigned swap)
{
if (swap == 0) val = bitswap<8>(val,0,1,4,3,5,6,2,7);
if (swap == 1) val = bitswap<8>(val,6,3,0,5,7,4,1,2);
if (swap == 2) val = bitswap<8>(val,1,6,4,5,0,3,7,2);
if (swap == 3) val = bitswap<8>(val,4,6,7,5,2,3,1,0);
if (BIT(val,3) || (BIT(param,1) && BIT(val,2)))
val = bitswap<8>(val,6,0,7,4,3,2,1,5);
if (BIT(val,5)) val ^= BITS(7);
if (BIT(val,6)) val ^= BITS(5);
if (BIT(val,0)) val ^= BITS(6);
if (BIT(val,4)) val ^= BITS(3,0);
if (BIT(val,1)) val ^= BITS(2);
val ^= BITS(7,6,5,4,1);
if (BIT(param,2)) val ^= BITS(4,3,2,1,0);
if (BIT(param,3))
{
if (BIT(param,0))
val = bitswap<8>(val,7,6,5,3,4,1,2,0);
else
val = bitswap<8>(val,7,6,5,1,2,4,3,0);
}
else if (BIT(param,0))
{
val = bitswap<8>(val,7,6,5,2,1,3,4,0);
}
return val;
}
u8 mc8123_device::decrypt_type2b(u8 val, u8 param, unsigned swap)
{
// only 0x20 possible encryptions for this method - all others have 0x40
// this happens because BIT(param,2) cancels the other three
if (swap == 0) val = bitswap<8>(val,1,3,4,6,5,7,0,2);
if (swap == 1) val = bitswap<8>(val,0,1,5,4,7,3,2,6);
if (swap == 2) val = bitswap<8>(val,3,5,4,1,6,2,0,7);
if (swap == 3) val = bitswap<8>(val,5,2,3,0,4,7,6,1);
if (BIT(val,7) && BIT(val,3))
val ^= BITS(6,4,0);
if (BIT(val,7)) val ^= BITS(2);
if (BIT(val,5)) val ^= BITS(7,3);
if (BIT(val,1)) val ^= BITS(5);
if (BIT(val,4)) val ^= BITS(7,5,3,1);
if (BIT(val,7) && BIT(val,5))
val ^= BITS(4,0);
if (BIT(val,5) && BIT(val,1))
val ^= BITS(4,0);
if (BIT(val,6)) val ^= BITS(7,5);
if (BIT(val,3)) val ^= BITS(7,6,5,1);
if (BIT(val,2)) val ^= BITS(3,1);
val ^= BITS(7,3,2,1);
if (BIT(param,3)) val ^= BITS(6,3,1);
if (BIT(param,2)) val ^= BITS(7,6,5,3,2,1); // same as the other three combined
if (BIT(param,1)) val ^= BITS(7);
if (BIT(param,0)) val ^= BITS(5,2);
return val;
}
u8 mc8123_device::decrypt_type3a(u8 val, u8 param, unsigned swap)
{
if (swap == 0) val = bitswap<8>(val,5,3,1,7,0,2,6,4);
if (swap == 1) val = bitswap<8>(val,3,1,2,5,4,7,0,6);
if (swap == 2) val = bitswap<8>(val,5,6,1,2,7,0,4,3);
if (swap == 3) val = bitswap<8>(val,5,6,7,0,4,2,1,3);
if (BIT(val,2)) val ^= BITS(7,5,4);
if (BIT(val,3)) val ^= BITS(0);
if (BIT(param,0)) val = bitswap<8>(val,7,2,5,4,3,1,0,6);
if (BIT(val,1)) val ^= BITS(6,0);
if (BIT(val,3)) val ^= BITS(4,2,1);
if (BIT(param,3)) val ^= BITS(4,3);
if (BIT(val,3)) val = bitswap<8>(val,5,6,7,4,3,2,1,0);
if (BIT(val,5)) val ^= BITS(2,1);
val ^= BITS(6,5,4,3);
if (BIT(param,2)) val ^= BITS(7);
if (BIT(param,1)) val ^= BITS(4);
if (BIT(param,0)) val ^= BITS(0);
return val;
}
u8 mc8123_device::decrypt_type3b(u8 val, u8 param, unsigned swap)
{
if (swap == 0) val = bitswap<8>(val,3,7,5,4,0,6,2,1);
if (swap == 1) val = bitswap<8>(val,7,5,4,6,1,2,0,3);
if (swap == 2) val = bitswap<8>(val,7,4,3,0,5,1,6,2);
if (swap == 3) val = bitswap<8>(val,2,6,4,1,3,7,0,5);
if (BIT(val,2)) val ^= BITS(7);
if (BIT(val,7)) val = bitswap<8>(val,7,6,3,4,5,2,1,0);
if (BIT(param,3)) val ^= BITS(7);
if (BIT(val,4)) val ^= BITS(6);
if (BIT(val,1)) val ^= BITS(6,4,2);
if (BIT(val,7) && BIT(val,6))
val ^= BITS(1);
if (BIT(val,7)) val ^= BITS(1);
if (BIT(param,3)) val ^= BITS(7);
if (BIT(param,2)) val ^= BITS(0);
if (BIT(param,3)) val = bitswap<8>(val,4,6,3,2,5,0,1,7);
if (BIT(val,4)) val ^= BITS(1);
if (BIT(val,5)) val ^= BITS(4);
if (BIT(val,7)) val ^= BITS(2);
val ^= BITS(5,3,2);
if (BIT(param,1)) val ^= BITS(7);
if (BIT(param,0)) val ^= BITS(3);
return val;
}
u8 mc8123_device::decrypt_internal(u8 val, u8 key, bool opcode)
{
unsigned type = 0;
unsigned swap = 0;
u8 param = 0;
key ^= 0xff;
// no encryption
if (key == 0x00)
return val;
type ^= BIT(key,0) << 0;
type ^= BIT(key,2) << 0;
type ^= BIT(key,0) << 1;
type ^= BIT(key,1) << 1;
type ^= BIT(key,2) << 1;
type ^= BIT(key,4) << 1;
type ^= BIT(key,4) << 2;
type ^= BIT(key,5) << 2;
swap ^= BIT(key,0) << 0;
swap ^= BIT(key,1) << 0;
swap ^= BIT(key,2) << 1;
swap ^= BIT(key,3) << 1;
param ^= BIT(key,0) << 0;
param ^= BIT(key,0) << 1;
param ^= BIT(key,2) << 1;
param ^= BIT(key,3) << 1;
param ^= BIT(key,0) << 2;
param ^= BIT(key,1) << 2;
param ^= BIT(key,6) << 2;
param ^= BIT(key,1) << 3;
param ^= BIT(key,6) << 3;
param ^= BIT(key,7) << 3;
if (!opcode)
{
param ^= BITS(0);
type ^= BITS(0);
}
switch (type)
{
default:
case 0: return decrypt_type0(val, param, swap);
case 1: return decrypt_type0(val, param, swap);
case 2: return decrypt_type1a(val, param, swap);
case 3: return decrypt_type1b(val, param, swap);
case 4: return decrypt_type2a(val, param, swap);
case 5: return decrypt_type2b(val, param, swap);
case 6: return decrypt_type3a(val, param, swap);
case 7: return decrypt_type3b(val, param, swap);
}
}
u8 mc8123_device::decrypt(offs_t addr, u8 val, bool opcode)
{
// pick the translation table from bits fd57 of the address
offs_t const tbl_num = bitswap<12>(addr,15,14,13,12,11,10,8,6,4,2,1,0);
return decrypt_internal(val, m_key[tbl_num | (opcode ? 0x0000 : 0x1000)], opcode);
}
void mc8123_device::decode(u8 *rom, u8 *opcodes, unsigned length)
{
for (unsigned i = 0; i < length; i++)
{
unsigned const adr = (i >= 0xc000) ? ((i & 0x3fff) | 0x8000) : i;
u8 const src = rom[i];
// decode the opcodes
opcodes[i] = decrypt(adr, src, true);
// decode the data
rom[i] = decrypt(adr, src, false);
}
}