What type of encryption has been used to Zyxel VMG8924-B10A config file?

[chants]

If you can get a decrypted version of the firmware it would help answer this question. Assuming this encryption method for this model is not known, finding downloadable firmware or dumping it off the ROM chip directly are options. Usually web firmware updates are themselves encrypted leaving another task but usually doable. Disassembling the firmware in IDA ought to do and see how the file is read and decrypted. Then if there is a key in a ROM or elsewhere you must find or dump that as well. That is about all I can say. It could be a time consuming but rewarding project if you find it worth it. Some electronics repair shops could dump the ROM chips if you pay for this service. I've had them reflash a BIOS I almost bricked due to custom mods I hand patched on a laptop once so it is possible