I have been attempting to reverse engineer a format for a visual novel psp game ('Mirai Nikki RE: WRITE - 13 Ninme No Nikki Shoyuusha') and I have come to the understanding that the format works as a type of archive by looking at the boot.elf file of the game. From this decoded boot.elf file I learnt that music is stored in the format at3 (which is atrac3) and sound effects as fsb (fmod sound bank). There are multiple 'MIG' files which I believe are actually GIM files which is a format used by the PSP for images (these are likely the character portraits, backgrounds and ui). The string YKLZ is also repeated throughout the file and seems to act as a sort of file or directory end. The header T2FP also occurs.
After learning this I have hit a wall as I am unsure of how to progress in getting the files as I do not know how it is encrypted or where the files start and end. If anyone can tell me how I could go about reversing the decryption function in an emulator or can provide any help about the file it would be greatly appreciated.
Edit: The hex code 87 4C 74 68 also occurs multiple times throughout the file.