Kerberos 5, etype 18, Pre-Auth (with salt) Hash

What is Kerberos 5, etype 18, Pre-Auth (with salt)?

The Kerberos 5 etype 18 (rc4_hmac_etype) Pre-Authentication (with salt) hash format is derived from the RC4 stream cipher combined with HMAC-MD5, a cryptographic construct historically used for Kerberos authentication in versions predating modern best practices. Introduced in the early 2000s as part of MIT Kerberos 5 (krb5) implementations, this mechanism was designed to provide a lightweight alternative to stronger algorithms like AES while maintaining compatibility with legacy systems. The structure consists of a salted HMAC-MD5 key derivation function (KDF) applied over the user’s password, followed by an RC4-encrypted timestamp (or "pre-authentication data") to verify session integrity. The salt, typically a 8-byte random value, is appended to the password before hashing to mitigate rainbow table attacks, though its effectiveness depends on implementation consistency. This format was primarily deployed in enterprise environments where backward compatibility with older Kerberos clients (e.g., Windows 2000/2003) was critical, though it was never intended for high-security contexts due to inherent cryptographic weaknesses.

The vulnerability of rc4_hmac_etype stems from its reliance on RC4’s inherent flaws—such as keystream predictability, bias in byte distribution, and susceptibility to timing attacks—as well as the cryptographic limitations of HMAC-MD5. The algorithm’s reliance on a 128-bit RC4 keystream for pre-authentication data encryption makes it susceptible to offline brute-force attacks, particularly when combined with weak passwords or poorly managed salts. Modern GPU-accelerated cracking tools (e.g., Hashcat) can efficiently brute-force RC4-encrypted pre-auth blocks due to the cipher’s parallelizable nature, even when salts are present. Additionally, the use of MD5 in HMAC introduces collision vulnerabilities and weak collision resistance, while the salt’s size (often insufficiently randomized) fails to deter rainbow table precomputation. Compoundingly, RC4’s linear feedback shift register (LFSR) design allows for partial keystream recovery via statistical analysis, further weakening the pre-auth integrity. These flaws render rc4_hmac_etype obsolete in contemporary security standards, despite its historical role in Kerberos authentication.

Identifying Kerberos 5, etype 18, Pre-Auth (with salt) 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 (with salt), 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 (with salt)

If you need to recover the plaintext password for a Kerberos 5, etype 18, Pre-Auth (with salt) 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.