// 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(address_space &space, int offset, int data, int param1, int param2) { static int lastpc, lastoffset, lastencword, lastdecword; int thispc = space.device().state().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; }