Kerberos 5, etype 17, Pre-Auth Hash

What is Kerberos 5, etype 17, Pre-Auth?

Kerberos 5’s etype 17 (pre-authentication) mechanism utilizes the **DES-CBC-CRC** (Data Encryption Standard in Cipher Block Chaining mode with CRC-32 checksum) algorithm, a legacy cryptographic primitive designed for early Kerberos implementations (pre-Kerberos 5v5). Structurally, it consists of a 64-bit DES key (or triple-DES in some variants) applied in CBC mode to encrypt a 64-bit block containing the client’s timestamp and random nonce, followed by a 32-bit CRC-32 checksum appended to the ciphertext. This format was standardized in RFC 1510 (1993) as a transitional solution before stronger encryption types (e.g., AES or RC4) were adopted. Its primary use-case was in Kerberos authentication to prevent replay attacks during the initial client-server handshake, where the server verifies the client’s possession of the shared secret (ticket-granting ticket) by decrypting the pre-authenticator and validating the embedded timestamp and nonce.

The vulnerability of DES-CBC-CRC to cracking stems from its inherent cryptographic weaknesses, exacerbated by implementation choices. The algorithm lacks a salt, making brute-force attacks trivial via rainbow tables or precomputed lookups for common DES keys. The CRC-32 checksum, while providing basic integrity, is computationally weak—modern GPUs and FPGA-based hardware (e.g., Hashcat or John the Ripper with DES optimizations) can crack 56-bit DES keys in minutes using brute-force or dictionary attacks. Additionally, the CBC mode’s vulnerability to chosen-plaintext attacks (e.g., via padding oracle exploits) allows adversaries to derive the key from partial ciphertexts. The absence of key stretching or iterative hashing further accelerates cracking, as parallelized hardware can evaluate billions of candidates per second, rendering DES-CBC-CRC obsolete for secure authentication.

Identifying Kerberos 5, etype 17, 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 17, 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 17, Pre-Auth

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