Just like every other COD fastfile. It's just compressed in a custom format and encrypted
- - - Updated - - -
Edit:
Did some reversing for you: ( Latest version for MP: 1.13 )
Code:#define FF_SIGNATURE "\x53\x31\x66\x66\x75\x31\x30\x30\x2E\x07" // "S1ffu100" FS_OpenFile() - 0x1404B7890 struct FFDef{ HANDLE ff_handle; // 0x0000 INT64 ff_posInMemory; // 0x0008 - The relative index position in memory char* ff_signature; // 0x0010 char ff_name[64]; // 0x0018 - This seems to be encrypted. It's xor'd with a simple key. }