In 2018, I did some research into decrypting thecus nas devices and posted a poorly assembled gist.

This repository contains a more standardized version of the source code.

sudo apt install libssl-dev
gcc -o string2key string2key.c -lssl -lcrypto

python3 decrypt.py $FILENAME

This will output the proper key, which you will then use in step 3

python3 decrypt.py -t N16000 $FILENAME

Where N16000 is an example of the key returned in the output of step 2

Enjoy!

The pyDes implementation used in this script is very, very slow. If you want to decrypt faster, you can use openssl:

OPENSSL_CONF=openssl_legacy.cnf openssl des-cbc -d -in $FILENAME -out $FILENAME.decrypted.bin -iv 00000000000000000 -K $(./string2key N16000) -nopad -nosalt

The OPENSSL_CONF file provided with this cnf file enables legacy ciphers, such as DES-CBC. I highly recommend not adding this to your standard openssl.cnf file.