Sometimes the encoding is done by block.
In this case, the decoder function could
read the size "it wants" and
append more data than asked.
The excess data will be used for the following fields.
Decoding rule :
Take 4 bytes and invert them.
Decoding function :
# This decoder append more data than asked (except when nb_of_bits_needed%32 == 0).
function void decode_invert_4_bytes (in frame frame, in uint32 nb_of_bits_needed_uint)
{
hide var int32 nb_of_bits_needed = nb_of_bits_needed_uint;
while (nb_of_bits_needed > 0)
{
hide uint8 byte1;
hide uint8 byte2;
hide uint8 byte3;
hide uint8 byte4;
# simply invert the 4 bytes read
call frame_append_data (frame, byte4);
call frame_append_data (frame, byte3);
call frame_append_data (frame, byte2);
call frame_append_data (frame, byte1);
set nb_of_bits_needed = nb_of_bits_needed - 32;
}
}
Using decoding function
struct xxx
{
uint8 field_before; # decode_invert_4_bytes is not called
decoder decode_invert_4_bytes;
uint16 field_a; # decode_invert_4_bytes will be called with nb_of_bits_needed = 16
# append 32 bits
# 16 bits are used immediately
# 16 bits remains
enum8 field_b; # decode_invert_4_bytes will NOT be called
# use 8 bits of the 16 remaining bits
# 8 bits remains
string(3) field_c; # decode_invert_4_bytes will be called with nb_of_bits_needed = 16
# append 32 bits
# which makes 40 bits available
# 24 bits are used immediately
# 16 bits remains
enum16 field_d; # decode_invert_4_bytes will NOT be called
# use 16 bits of the 16 remaining bits
# 0 bits remains
decoder nil;
uint16 field_after; # decode_invert_4_bytes is not called
}
Decoding rule :
Decrypt aes ecb
AES ECB always use blocks of 16 bytes.
Decoding function skeleton :
# This decoder append more data than asked (except when nb_of_bits_needed%(16*8) == 0).
function void decoder_aes (in frame frame, in uint32 nb_of_bits_needed_uint)
{
hide var int32 nb_of_bits_needed = nb_of_bits_needed_uint;
while (nb_of_bits_needed > 0)
{
hide uint8[16] bytes_crypted;
# Decrypt the 16 bytes
hide var uint8[16] bytes_decrypted = 0;
...
# Append the 16 bytes decrypted
call frame_append_data (frame, bytes_decrypted[0]);
call frame_append_data (frame, bytes_decrypted[1]);
...
call frame_append_data (frame, bytes_decrypted[14]);
call frame_append_data (frame, bytes_decrypted[15]);
set nb_of_bits_needed = nb_of_bits_needed - (16 * 8);
}
}
decoder_aes is built-in.
Key (16/24/32 bytes) must be specified by ascii string variable "decoder_aes_key".
As encrypted data must always be multiple of 16 bytes block, you can have 0 to 15 padding bytes in the last block.
This is your responsability to read them.
If not, this padding bytes will be wrongly used for the next fields (instead of read input data).