== QNAP Firmware decryptor ==

This small tool can encrypt and decrypt official firmware images for QNAP NAS,
provided you know the key. It is a free software implementation of the same
algorithm used in the official "PC1" tool present in QNAP NAS firmwares (at
least the TS-x31 series).

Official firmware images downloadable from the QNAP website are partially
encrypted using what appears to be a custom symmetric cipher.

Metadata is appended to the bottom of the encrypted image to indicate the
length of the encrypted part as well as details about the product model the
firmware image targets and version information.

The key is a sequence of ASCII characters. Key length must be even, otherwise
the last byte is ignored. Extended ASCII (> 127) characters should not be used
as they are sign-extended when - I believe - they shouldn't.

The key used for official firmwares is not disclosed here. If you own a QNAP
NAS, you can easily extract it yourself is you have shell access via SSH or the
serial console.