Brute Forcing Crypt Files

TL;DR

Yes, old-fashioned crypt files can be brute forced, especially if they use weak encryption or short keys. The process involves trying every possible key combination until the correct one is found. Modern computers and specialized tools make this feasible for simpler ciphers.

Understanding Crypt Files

Before we start, let’s clarify what a ‘crypt file’ usually means in this context. We’re talking about files encrypted using older methods like:

• Symmetric-key encryption: The same key is used for both encrypting and decrypting the file (e.g., DES, RC4).
• Simple ciphers: Caesar ciphers, substitution ciphers, or transposition ciphers.

These are less secure than modern encryption algorithms like AES.

Step-by-step Guide to Brute Forcing

1. Identify the Encryption Method: This is crucial. Knowing what cipher was used determines which tools and techniques you’ll need.
   • Examine file headers (using a hex editor) for clues.
   • Look for any accompanying documentation or software that might indicate the encryption type.
   • If unsure, try common cipher identification tools (see ‘Tools’ section below).
2. Determine Key Length and Character Set:
   • For symmetric-key ciphers, find out the key length (e.g., 56 bits for DES, 128 bits for RC4).
   • Identify the character set used in the key (e.g., lowercase letters only, uppercase and lowercase letters, numbers, symbols). A larger character set significantly increases brute-force time.
3. Choose a Brute-Force Tool: Several tools can automate the process.
   • John the Ripper: A versatile password cracker that supports many ciphers and hash types.
   • Hashcat: Another powerful tool, known for its speed and GPU support.
   • CyberChef: A web-based tool useful for simpler ciphers and manual analysis.
4. Run the Brute-Force Attack: The exact command will depend on the chosen tool and encryption method.

   Example using John the Ripper (DES):

   john --wordlist=/usr/share/wordlists/rockyou.txt encrypted_file.crypt

   This command tells John to try cracking encrypted_file.crypt using the words in the rockyou.txt wordlist.

5. Monitor Progress and Adjust: Brute-forcing can take a long time, from minutes to years depending on key length and complexity.
   • Check the tool’s output for progress updates.
   • If no results are found after a reasonable amount of time, consider:
     • Trying a different wordlist.
     • Increasing the key space (if you suspect a longer key).
     • Using rule-based attacks to generate variations of known passwords.

Tools

• John the Ripper: https://www.openwall.com/john/
• Hashcat: https://hashcat.net/
• CyberChef: https://gchq.github.io/CyberChef/

Important Considerations

• Legal Implications: Brute-forcing a file without permission is illegal in many jurisdictions. Only attempt this on files you own or have explicit authorization to access.
• Resource Intensive: Brute-force attacks require significant computing power and time. A powerful GPU can dramatically speed up the process.
• Key Space: The number of possible key combinations grows exponentially with key length. Even a relatively short key (e.g., 8 characters) can take years to crack using brute force.