diff options
Diffstat (limited to 'src/mame/machine/gaelcrpt.cpp')
-rw-r--r-- | src/mame/machine/gaelcrpt.cpp | 154 |
1 files changed, 154 insertions, 0 deletions
diff --git a/src/mame/machine/gaelcrpt.cpp b/src/mame/machine/gaelcrpt.cpp new file mode 100644 index 00000000000..4f2b0f57a1e --- /dev/null +++ b/src/mame/machine/gaelcrpt.cpp @@ -0,0 +1,154 @@ +// 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 = BITSWAP16(enc_word, 1, 2, 0,14,12,15, 4, 8,13, 7, 3, 6,11, 5,10, 9); break; + case 1: res = BITSWAP16(enc_word, 14,10, 4,15, 1, 6,12,11, 8, 0, 9,13, 7, 3, 5, 2); break; + case 2: res = BITSWAP16(enc_word, 2,13,15, 1,12, 8,14, 4, 6, 0, 9, 5,10, 7, 3,11); break; + case 3: res = BITSWAP16(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 BITSWAP16(res, 2,6,0,11,14,12,7,10,5,4,8,3,9,1,13,15); +} + + + +UINT16 gaelco_decrypt(address_space &space, int offset, int data, int param1, int param2) +{ + static int lastpc, lastoffset, lastencword, lastdecword; + + int thispc = space.device().safe_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",space.machine().describe_context(),savedata,data,lastoffset); + } + return data; +} |