Thecus NAS Firmware Decryption


The password I’ve seen consistently used across a few different firmware images is N16000. The cipher is DES-CBC using the MIT Kerberos DES_to_string_key function for key derivation.


Back in 2018 I did some research into Thecus NAS firmware. I found that the firmware was encrypted, and I figured out how to decrypt it using parts of the filename. The firmware for Thecus NAS are encrypted with DES-CBC using the MIT Kerberos DES_string_to_key function. I wrote up some basics scripts to complete this task and posted them here.

Flash forward to earlier this week, its been six years since I looked at this research and someone online reached out to me to ask a few questions about my scripts. For whatever reason they no longer were working. Thanks to pyro_phoenix for reaching out, and also for figuring out how to enable openssl cli to decrypt the firmware.

It turns out DES-CBC is deprecated and the openssl cli will no longer allow the user to use DEC-CBC or DES-ECB by default. We can work around this by using a custom openssl.cnf file that enables legacy ciphers. Thanks again to pyro_phoenix for pointing out how to make the openssl cli command work for DES-CBC. This is an example of the openssl cli output when trying to use DES-CBC as the cipher:

hex string is too long, ignoring excess
Error setting cipher DES-CBC
4097B607E1750000:error:0308010C:digital envelope routines:inner_evp_generic_fetch:unsupported:../crypto/evp/evp_fetch.c:386:Global default library context, Algorithm (DES-CBC : 8), Properties ()

However, I did end up rewriting my origin bash scripts into a python program. The python program iterates through a list of all possible model names (I used the thecus website to build a collection of model names) and attempts to use each as the key to the encrypted firmware blob.

New Scripts

My new python-based scripts are available on github for those who are interested. The new script still uses the string2key program I wrote, which is included in the repository. This program utilizes the DES_string_to_key function from openssl to derive a key from a string. I looked at several difficult python re-implementations of this function and none of the python-based solutions worked properly. Thus I decided to resort to shelling out to this small c program to handle key derivation.

The script is able to decrypt the firmware file itself using the pyDes python-only implementation of the DES cryptographic functions. The encrypt/decrypt functions are very very slow, as the author of the library notes: 10Kb/s. The average Thecus firmware file is about 180 megabytes in size, which means it takes hours to decrypt using the pyDes library. However, with using openssl cli, the time is reduced to less than a minute.

The example openssl_legacy.cnf file provided in the repo can be used to enable legacy ciphers such as DES-CBC. You can pass this config file path using the OPENSSL_CONF environment variable. Even the original gist scripts should work if you set this environment variable to the provided openssl_legacy.cnf file.


sudo apt install libssl-dev
gcc -o string2key string2key.c -lssl -lcrypto
OPENSSL_CONF=openssl_legacy.cnf openssl des-cbc -d -in Thecus_x86_64_FW.2.06.03.cdv_build9857_N2800_N4510U_N4800_N5550_N7510.rom -out Thecus_x86_64_FW.2.06.03.cdv_build9857_N2800_N4510U_N4800_N5550_N7510.rom.decrypted.bin -iv 00000000000000000 -K $(./string2key N16000) -nopad -nosalt


I haven’t looked at or thought about this research in 6 years and it was fun to revisit. I can’t really remember how I figured all of this out originally, but it was fun to formalize the research into a git repository instead of gists.

Lastly, I want to credit pyro_phoenix for reaching out and putting this back on my radar!

If you find anything cool in the firmware images, give me a shout out!