Kerberos 5, etype 18, Pre-Auth Hash

What is Kerberos 5, etype 18, Pre-Auth?

Kerberos 5’s etype 18 (pre-authentication) mechanism employs the **DES-CBC-MD5** (Data Encryption Standard in Cipher Block Chaining Mode with Message Digest 5) hash format, a hybrid cryptographic construct originally designed for Kerberos v5’s legacy authentication framework. Structurally, this format combines a **DES-based encryption** of a timestamp and nonce with an **MD5 hash** of the user’s password concatenated with the timestamp, serving as a challenge-response proof. The primary use-case lies in Kerberos v5’s pre-authentication phase (AS-REQ/AS-REP exchange), where the client and Key Distribution Center (KDC) validate mutual possession of the secret key without transmitting it over the network. The DES component encrypts the timestamp under the session key derived from the user’s password hash, while MD5 ensures integrity of the password-derived key material, though the latter is never directly exposed. This design predates modern cryptographic best practices, relying on a 56-bit DES key for encryption and an unsalted MD5 hash for password representation, making it susceptible to offline attacks.

The vulnerability stems from three critical flaws: **weak cryptographic strength, lack of salting, and computational efficiency**. DES-CBC-MD5’s 56-bit key space is trivially broken via brute-force (e.g., via **DES crackers** like John the Ripper’s `des-cbc-md5` module), while the absence of a salt in MD5 password storage enables **rainbow table attacks** on precomputed hash databases. Additionally, the format’s reliance on **CPU-bound DES operations** (unlike GPU-accelerated hashes like bcrypt) makes it vulnerable to **optimized dictionary attacks** on modern hardware, where attackers can parallelize brute-force attempts across thousands of cores. Collision resistance is further compromised by MD5’s known weaknesses, though Kerberos’ use of it here is primarily for integrity rather than collision resistance. The combination of these factors—**predictable hashing, no salt, and brute-force feasibility**—makes etype 18 a prime target for offline password recovery tools, despite its historical role in securing Kerberos authentication.

Identifying Kerberos 5, etype 18, Pre-Auth Hashes

Identifying a hash blindly is often difficult because multiple algorithms can output strings of the exact same length. For example, a standard 32-character hexadecimal string could be MD5, NTLM, MD4, or LM. To definitively identify Kerberos 5, etype 18, Pre-Auth, security researchers analyze the hash against a strict regular expression (regex) signature (shown above) to find structural matches.

How to Crack Kerberos 5, etype 18, Pre-Auth

If you need to recover the plaintext password for a Kerberos 5, etype 18, Pre-Auth hash during a penetration test or forensics investigation, you must supply the correct module ID to your cracking software. Use the exact commands provided at the top of this page to target the hash successfully.