Brute-Force Password Hashes: Algorithm Unknown?

TL;DR

It’s very difficult to brute-force a password hash without knowing the algorithm used, but not impossible. You can try ‘hashcat’ with multiple modes and wordlists, or use rainbow tables (though these are less effective now). Success depends on the password complexity, length, and available computing power.

Understanding the Problem

Password hashes aren’t the passwords themselves; they’re one-way transformations. Brute-forcing means trying many possible passwords until you find one that produces a hash matching the stored value. Knowing the algorithm (like MD5, SHA256, bcrypt) is crucial because it tells you what transformation to reverse.

Step 1: Identify Potential Algorithms

1. Context Clues: Where did you get the hash? The application or system might hint at the algorithm used. Check documentation or configuration files.
2. Hash Length: Different algorithms produce hashes of different lengths. This can narrow down possibilities.
   • MD5: 32 characters (16 bytes)
   • SHA-1: 40 characters (20 bytes)
   • SHA-256: 64 characters (32 bytes)
   • bcrypt: Typically 60+ characters
3. Format Indicators: Some hashes include prefixes or suffixes indicating the algorithm. For example, bcrypt hashes often start with ‘$2a$’ or similar.

Step 2: Using Hashcat

Hashcat is a powerful password cracking tool. It supports many algorithms and attack modes.

1. Install Hashcat: Download from https://hashcat.net/ and follow the installation instructions for your operating system.
2. Basic Command Structure:

   hashcat -m [mode] hashfile wordlist

   Where:

   • -m [mode] specifies the algorithm mode (e.g., 0 for MD5, 1400 for bcrypt). Use 9600 for ‘Unknown’ or ‘Generic’.
   • hashfile is the file containing the hash you want to crack.
   • wordlist is a text file with potential passwords.
3. Trying Multiple Modes: If unsure, try several common modes.

   hashcat -m 0 hashfile wordlist && hashcat -m 1400 hashfile wordlist && hashcat -m 9600 hashfile wordlist

4. Wordlists: Use comprehensive wordlists like RockYou.txt (often requires downloading separately). Consider using rule-based attacks to generate variations.

   hashcat -m 9600 hashfile rockyou.txt -r rules/best64.rule

Step 3: Rainbow Tables (Less Effective Now)

Rainbow tables pre-calculate hashes for common passwords. They’re faster than brute-forcing but require significant storage space and are less effective against salted hashes.

1. Generate or Download: You can generate rainbow tables using tools like Ophcrack, or download pre-made ones (be cautious about the source).
2. Use with Cracking Tools: Tools like John the Ripper can use rainbow tables to crack hashes.

   john --format=md5 hashfile rainbow_table.rtf

Step 4: Considerations and Limitations

• Salted Hashes: Salts are random values added to passwords before hashing, making rainbow tables ineffective and significantly increasing brute-force time.
• Password Complexity: Longer, more complex passwords with mixed characters (uppercase, lowercase, numbers, symbols) take exponentially longer to crack.
• Computing Power: Brute-forcing is computationally intensive. GPUs are much faster than CPUs for this task. Consider using cloud cracking services if you lack sufficient hardware.
• Time and Resources: Cracking a strong password can take days, weeks, or even years with limited resources.

Step 5: Ethical Considerations

Only attempt to crack passwords for systems you own or have explicit permission to test. Unauthorized access is illegal.