It can be anything from a fixed key xor starting with the bytes 0x8a 0xec 0xb3 0xa2 to a xor with incremental pseudo number generator.
You can verify the first idea by searching those 4 bytes in the executable of the game.
I don't know why there is a 32bit size value at the beginning at the file and what are all those bytes after at the end of the file
They seem to be encrypted with AES keys. There's functions like DeALSetting$$get_OggEncryptionAesKey and AssetBundleSetting$$get_SimpleEncryptionKey, as for where to find these keys, I still don't know.