Help identify crypto

[The Old Pirate]

Yes, I tried these too, unfortunately to no avail. Even by looking over the assembly code of the algo I couldn't locate any specific constants (except for 0x8000, 0xFFFD, 0xFFFE, 0xFFFF and 0x10000).

Some additional info: the resulting plaintext message is 256 bytes long as well. From what I see, during the decryption process the encrypted block undergoes many tranformations (at some point even doubles it's size).

Any guess?