vgmstream/src/coding/mpeg_custom_utils_ahx.c

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2019-11-10 21:26:31 +01:00
#include "mpeg_decoder.h"
#ifdef VGM_USE_MPEG
#define MPEG_AHX_EXPECTED_FRAME_SIZE 0x414
static int ahx_decrypt_type08(uint8_t * buffer, mpeg_custom_config *config);
/* writes data to the buffer and moves offsets, transforming AHX frames as needed */
int mpeg_custom_parse_frame_ahx(VGMSTREAMCHANNEL *stream, mpeg_codec_data *data, int num_stream) {
mpeg_custom_stream *ms = data->streams[num_stream];
size_t current_data_size = 0;
size_t file_size = get_streamfile_size(stream->streamfile);
/* AHX has a 0xFFF5E0C0 header with frame size 0x414 (160kbps, 22050Hz) but they actually are much shorter */
/* read supposed frame size first (to minimize reads) */
ms->bytes_in_buffer = read_streamfile(ms->buffer, stream->offset, MPEG_AHX_EXPECTED_FRAME_SIZE, stream->streamfile);
/* find actual frame size by looking for the next frame header */
{
uint32_t current_header = get_u32be(ms->buffer);
int next_pos = 0x04;
while (next_pos <= MPEG_AHX_EXPECTED_FRAME_SIZE) {
uint32_t next_header = get_u32be(ms->buffer + next_pos);
if (current_header == next_header) {
current_data_size = next_pos;
break;
}
/* AHXs end in a 0x0c footer (0x41485845 28632943 52490000 / "AHXE(c)CRI\0\0") */
if (stream->offset + next_pos + 0x0c >= file_size) {
current_data_size = next_pos;
break;
}
next_pos++;
}
}
if (current_data_size == 0 || current_data_size > ms->buffer_size || current_data_size > MPEG_AHX_EXPECTED_FRAME_SIZE) {
VGM_LOG("MPEG AHX: incorrect data_size 0x%x\n", current_data_size);
goto fail;
}
/* 0-fill up to expected size to keep mpg123 happy */
memset(ms->buffer + current_data_size, 0, MPEG_AHX_EXPECTED_FRAME_SIZE - current_data_size);
ms->bytes_in_buffer = MPEG_AHX_EXPECTED_FRAME_SIZE;
/* decrypt if needed */
switch(data->config.encryption) {
case 0x00: break;
case 0x08: ahx_decrypt_type08(ms->buffer, &data->config); break;
default:
VGM_LOG("MPEG AHX: unknown encryption 0x%x\n", data->config.encryption);
break; /* garbled frame */
}
/* update offsets */
stream->offset += current_data_size;
if (stream->offset + 0x0c >= file_size)
stream->offset = file_size; /* skip 0x0c footer to reach EOF (shouldn't happen normally) */
return 1;
fail:
return 0;
}
/* Decrypts an AHX type 0x08 (keystring) encrypted frame. Algorithm by Thealexbarney */
static int ahx_decrypt_type08(uint8_t * buffer, mpeg_custom_config *config) {
int i, index, encrypted_bits;
uint32_t value;
uint16_t current_key;
/* encryption 0x08 modifies a few bits every frame, here we decrypt and write to data buffer */
/* derive keystring to 3 primes, using the type 0x08 method, and assign each an index of 1/2/3 (0=no key) */
/* (externally done for now, see: https://github.com/Thealexbarney/VGAudio/blob/2.0/src/VGAudio/Codecs/CriAdx/CriAdxKey.cs) */
/* read 2b from a bitstream offset to decrypt, and use it as an index to get the key.
* AHX encrypted bitstream starts at 107b (0x0d*8+3), every frame, and seem to always use index 2 */
value = get_u32be(buffer + 0x0d);
index = (value >> (32-3-2)) & 0x03;
switch(index) {
case 0: current_key = 0; break;
case 1: current_key = config->cri_key1; break;
case 2: current_key = config->cri_key2; break;
case 3: current_key = config->cri_key3; break;
default: goto fail;
}
/* AHX for DC: 16b, normal: 6b (no idea, probably some Layer II field) */
encrypted_bits = config->cri_type == 0x10 ? 16 : 6;
/* decrypt next bitstream 2b pairs, up to 16b (max key size):
* - read 2b from bitstream (from higher to lower)
* - read 2b from key (from lower to higher)
* - XOR them to decrypt */
for (i = 0; i < encrypted_bits; i+=2) {
uint32_t xor_2b = (current_key >> i) & 0x03;
value ^= ((xor_2b << (32-3-2-2)) >> i);
}
/* write output */
put_32bitBE(buffer + 0x0d, value);
return 1;
fail:
return 0;
}
#endif