TL;DR
This guide shows you how to recover an s2k mode 0 session key when all you have is the original passphrase. This often happens with older encryption (like PGP) where a weak key derivation function was used. It’s important to understand this method isn’t secure and should be replaced with stronger encryption.
Recovering the Session Key
s2k mode 0 uses a simple hashing process to derive a session key from your passphrase. Because it’s weak, we can often crack it using brute-force or dictionary attacks if the passphrase isn’t strong enough. This guide focuses on using OpenSSL for recovery.
Understand the Basics: s2k mode 0 takes your passphrase and hashes it repeatedly. The number of repetitions (the ‘count’) is crucial. If you know this count, recovery is much easier. If not, you’ll need to try different counts.
The hash function used is typically MD5 or SHA1.
Longer passphrases are harder to crack.
Install OpenSSL: If you don’t have it already, install OpenSSL. On most Linux systems:
sudo apt-get update && sudo apt-get install openssl
On macOS (using Homebrew):
brew install openssl
Hash the Passphrase: Use OpenSSL to generate hashes of your passphrase. We’ll start with MD5, as it’s a common default.
openssl md5 -pass pass:”your_passphrase”
Replace “your_passphrase” with the actual passphrase. Note the output hash – you’ll need this later if it matches.
Repeat Hashing (if count is known): If you know the ‘count’, repeat the hashing process that many times. For example, if the count is 10:
for i in {1..10}; do openssl md5 -pass pass:”your_passphrase” | awk ‘{print $4}’; done
This script loops ten times, hashing the passphrase and printing only the hash value each time. You can adapt this for SHA1 or other algorithms.
Try Different Counts (if count is unknown): If you don’t know the count, try a range of values. Start with small numbers (e.g., 1 to 100) and increase if necessary. This can take a long time!
for count in {1..100}; do openssl md5 -pass pass:”your_passphrase” | awk ‘{print $4}’; done
Compare Hashes: Compare the generated hashes with any known session key hash you have. If they match, you’ve found your session key.
If you are trying to decrypt a file, the decryption software may give an error message indicating the correct hash format or algorithm.
Using SHA1 (if MD5 fails): If MD5 doesn’t work, try SHA1:
openssl sha1 -pass pass:”your_passphrase”
Repeat steps 4 and 5 with SHA1.
Automated Cracking (advanced): For more complex scenarios, consider using tools like John the Ripper or Hashcat. These tools can perform dictionary attacks and brute-force cracking efficiently.
These tools require some technical knowledge to set up and use effectively.
Important Security Note
s2k mode 0 is considered very insecure. If you’re using it, upgrade to a more modern encryption method immediately (e.g., AES with a strong key derivation function like PBKDF2). This guide is for recovery purposes only and should not be used as a long-term security solution.
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