// license:BSD-3-Clause
// copyright-holders:Manuel Abadia
/*
Gaelco video RAM encryption
Thanks to GAELCO SA for information on the algorithm.
*/
#include "emu.h"
#include "includes/gaelcrpt.h"
static int decrypt(int const param1, int const param2, int const enc_prev_word, int const dec_prev_word, int const enc_word)
{
int const swap = (BIT(dec_prev_word, 8) << 1) | BIT(dec_prev_word, 7);
int const type = (BIT(dec_prev_word,12) << 1) | BIT(dec_prev_word, 2);
int res=0;
int k=0;
switch (swap)
{
case 0: res = bitswap<16>(enc_word, 1, 2, 0,14,12,15, 4, 8,13, 7, 3, 6,11, 5,10, 9); break;
case 1: res = bitswap<16>(enc_word, 14,10, 4,15, 1, 6,12,11, 8, 0, 9,13, 7, 3, 5, 2); break;
case 2: res = bitswap<16>(enc_word, 2,13,15, 1,12, 8,14, 4, 6, 0, 9, 5,10, 7, 3,11); break;
case 3: res = bitswap<16>(enc_word, 3, 8, 1,13,14, 4,15, 0,10, 2, 7,12, 6,11, 9, 5); break;
}
res ^= param2;
switch (type)
{
case 0:
k = (0 << 0) |
(1 << 1) |
(0 << 2) |
(1 << 3) |
(1 << 4) |
(1 << 5);
break;
case 1:
k = (BIT(dec_prev_word, 0) << 0) |
(BIT(dec_prev_word, 1) << 1) |
(BIT(dec_prev_word, 1) << 2) |
(BIT(enc_prev_word, 3) << 3) |
(BIT(enc_prev_word, 8) << 4) |
(BIT(enc_prev_word,15) << 5);
break;
case 2:
k = (BIT(enc_prev_word, 5) << 0) |
(BIT(dec_prev_word, 5) << 1) |
(BIT(enc_prev_word, 7) << 2) |
(BIT(enc_prev_word, 3) << 3) |
(BIT(enc_prev_word,13) << 4) |
(BIT(enc_prev_word,14) << 5);
break;
case 3:
k = (BIT(enc_prev_word, 0) << 0) |
(BIT(enc_prev_word, 9) << 1) |
(BIT(enc_prev_word, 6) << 2) |
(BIT(dec_prev_word, 4) << 3) |
(BIT(enc_prev_word, 2) << 4) |
(BIT(dec_prev_word,11) << 5);
break;
}
k ^= param1;
res = (res & 0xffc0) | ((res + k) & 0x003f);
res ^= param1;
switch (type)
{
case 0:
k = (BIT(enc_word, 9) << 0) |
(BIT(res,2) << 1) |
(BIT(enc_word, 5) << 2) |
(BIT(res,5) << 3) |
(BIT(res,4) << 4);
break;
case 1:
k = (BIT(dec_prev_word, 2) << 0) | // always 1
(BIT(enc_prev_word, 4) << 1) |
(BIT(dec_prev_word,14) << 2) |
(BIT(res, 1) << 3) |
(BIT(dec_prev_word,12) << 4); // always 0
break;
case 2:
k = (BIT(enc_prev_word, 6) << 0) |
(BIT(dec_prev_word, 6) << 1) |
(BIT(dec_prev_word,15) << 2) |
(BIT(res,0) << 3) |
(BIT(dec_prev_word, 7) << 4);
break;
case 3:
k = (BIT(dec_prev_word, 2) << 0) | // always 1
(BIT(dec_prev_word, 9) << 1) |
(BIT(enc_prev_word, 5) << 2) |
(BIT(dec_prev_word, 1) << 3) |
(BIT(enc_prev_word,10) << 4);
break;
}
k ^= param1;
res = (res & 0x003f) |
((res + (k << 6)) & 0x07c0) |
((res + (k << 11)) & 0xf800);
res ^= (param1 << 6) | (param1 << 11);
return bitswap<16>(res, 2,6,0,11,14,12,7,10,5,4,8,3,9,1,13,15);
}
uint16_t gaelco_decrypt(cpu_device &cpu, int offset, int data, int param1, int param2)
{
static int lastpc, lastoffset, lastencword, lastdecword;
int thispc = cpu.pc();
// int savedata = data;
/* check if 2nd half of 32 bit */
if(lastpc == thispc && offset == lastoffset + 1)
{
lastpc = 0;
data = decrypt(param1, param2, lastencword, lastdecword, data);
}
else
{
/* code as 1st word */
lastpc = thispc;
lastoffset = offset;
lastencword = data;
/* high word returned */
data = decrypt(param1, param2, 0, 0, data);
lastdecword = data;
// logerror("%s : data1 = %4x > %4x @ %8x\n",machine().describe_context(),savedata,data,lastoffset);
}
return data;
}