Kerberos 5, etype 17, Pre-Auth (with salt) Hash

What is Kerberos 5, etype 17, Pre-Auth (with salt)?

Kerberos 5’s etype 17 (also known as **RC4-HMAC-DES** or **RC4-SHA1** in some contexts, though historically misattributed) refers to a legacy authentication mechanism where the **Pre-Authentication (PA) phase** incorporates a salted hash of the user’s password to generate a session key. This format emerged in early Kerberos implementations (pre-Kerberos 5.0) as a transitional measure to support DES-based encryption while introducing basic protection against plaintext password transmission. The structure involves a **two-part hash**: first, the user’s password is hashed with a salt (typically a timestamp or random value) using **MD4** (or later SHA-1 in some variants) to produce a **PA-data** blob. This blob is then encrypted under the Kerberos realm’s key (often the KDC’s DES key) and sent to the client during authentication. The primary use-case was to mitigate offline password guessing by obscuring the password’s raw form, though it remained vulnerable to classical cryptanalysis due to its reliance on weak hashing primitives and predictable salts.

The vulnerability stems from **three critical flaws**: (1) **Weak hashing primitives**—MD4/SHA-1 are susceptible to collision attacks and do not meet modern cryptographic standards, allowing for **rainbow table** precomputation or **brute-force acceleration** via GPU/FPGA clusters (e.g., Hashcat’s **modes 1000–1099** for salted hashes). (2) **Predictable salts**—early implementations used timestamps or fixed values, reducing entropy and enabling **offline cracking** via dictionary attacks or hybrid methods. (3) **RC4’s inherent flaws**—while not directly part of the hash format, the subsequent RC4 key derivation from the salted hash is vulnerable to **Bleichenbacher attacks** and **timing-side-channel exploits**, further compromising session integrity. Additionally, the **lack of key stretching** (e.g., no iterative hashing) allows attackers to evaluate millions of passwords per second on modern hardware, rendering the mechanism obsolete by contemporary standards.

Identifying Kerberos 5, etype 17, 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 17, 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 17, Pre-Auth (with salt)

If you need to recover the plaintext password for a Kerberos 5, etype 17, 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.