Nemucod Decrypt

A Ruby tool to decrypt Nemucod ransomware.

Development Status

• Key derivation and file decryption
• Passing parameters in from a command line
• Packaged up as a gem

Usage

Install the uncrypt_nemucod gem following your preference. Also needed is the cli dependency gem, slop.

Assuming a file fruit.pdf.crypted in the local directory, and a non-crypted version of the same file, fruit.pdf, first derive the decrypt key:

$ uncrypt_nemucod --derive-key -k decrypt.key fruit.pdf.crypted fruit.pdf
Recovering key...

Key file is 1024 bytes long and contains 20 NUL bytes.

Now, decrypt all .crypted files in the current directory:

dutero-basilius ~/Desktop> uncrypt_nemucod --decrypt -k decrypt.key *.crypted
Decrypting example_file.pdf.crypted...OK
Decrypting example_file.jpg.crypted...OK
Decrypting example_file.txt.crypted...OK
Decrypting example_file.gif.crypted...OK
Decrypting example_file.doc.crypted...OK
Decrypting example_file.xls.crypted...OK
Decrypting example_file.wav.crypted...OK
Decrypting example_file.mp3.crypted...OK
Decrypting example_file.m4a.crypted...OK
Decrypting example_file.ppt.crypted...OK
Decrypting example_file.mid.crypted...OK
Decrypting example_file.exe.crypted...OK
Decrypting example_file.png.crypted...OK

Finding an unencrypted file

Assuming your hard drive is encrypted, how do you go about finding an unencrypted version of one of your files? How do you know it's the right one?

Start here, and put your detective hat on:

• Stock files, such as sample pictures or documents that came with the operating system: If one of them has been encrypted, find the same file on another computer or on the internet.
• Files recently downloaded from the internet: look through your downloads directory for images or recently downloaded from the internet. Find the same file on the internet again.
• Backups: Check your backups...you do have some backups, right? Ok. Did you ever copy a file to a thumb drive? Or send one over email? Unfortuately printing a file probably won't help.

How do you know you've got the right file?

• Check the size, in bytes. Both the crypted and uncrypted file should match exactly.

Background

A ransomware dubbed Nemucod or DECRYPT.txt rapidly encrypts files using a weak XOR encryption. Without the key, it is still difficult to recover the ransomed data. However, the XOR encryption key is easily derived by comparing a known good file to its encrypted counterpart. I read about the encryption technique and the possibility of deriving a key and decrypting files manually sounded like a great learning experience, so here it is.

The Nemucod ransomware is easy to identify by a signature text file it leaves on the Windows desktop of a victim computer. I've redacted some parts of this sample:

ATTENTION!

All your documents, photos, databases and other important personal files were encrypted using strong RSA-1024 algorithm with a unique key. To restore your files you have to pay BTC (bitcoins). Please follow this manual:

1. Create Bitcoin wallet here: https://blockchain.info/wallet/new
2. Buy BTC with cash, using search here: https://localbitcoins.com/buy_bitcoins
3. Send BTC to this Bitcoin address:
4. Open one of the following links in your browser to download decryptor:
5. Run decryptor to restore your files.

PLEASE REMEMBER:

• If you do not pay in 3 days YOU LOOSE ALL YOUR FILES.
• Nobody can help you except us.
• It's useless to reinstall Windows, update antivirus software, etc.
• Your files can be decrypted only after you make payment.
• You can find this manual on your desktop (DECRYPT.txt).

A family member recently became a victim of this scam and asked me for help, but the decryptor available didn't look like it'd run on a mac or linux. I'm always interested in diving into some malware and bit math, so I built this ruby tool to derive the key and decrypt ransomed files.

Send me a note if it proves to be of any use to you.

Sources:

• https://www.bleepingcomputer.com/news/security/decryptor-released-for-the-nemucod-trojans-crypted-ransomware/