Kerberos 5 TGS-REP etype 23 Hash

What is Kerberos 5 TGS-REP etype 23?

Kerberos 5’s TGS-REP etype 23 hash format represents a legacy cryptographic mechanism designed for authenticating Kerberos tickets using the **DES-CBC-MD5** algorithm (also colloquially referred to as "etype 23" in Kerberos parlance). Introduced in the early 1990s as part of the Kerberos v5 protocol (RFC 4120), this format was intended to provide a lightweight, interoperable alternative to stronger encryption types for environments constrained by legacy hardware or compliance requirements. Structurally, the TGS-REP etype 23 hash consists of a **64-bit DES key** (or a concatenation of two 64-bit keys for double DES) used in CBC mode with a 128-bit IV, followed by the **MD5 hash** of the concatenated ticket data (including the client’s identity, server identity, timestamp, and session key). The resulting 128-bit hash is then XORed with the IV to produce the final 128-bit output, which is appended to the Kerberos ticket for verification. This format was primarily deployed in environments where full-strength encryption (e.g., AES or RSA) was impractical, often in legacy Unix/Linux systems or legacy government/military networks.

The vulnerability of etype 23 stems from its reliance on **DES-CBC-MD5**, a cryptographically broken combination that suffers from fundamental flaws in both components. DES’s 56-bit key space is trivially susceptible to brute-force attacks using optimized hardware—modern GPU clusters can crack DES keys in minutes via tools like **John the Ripper** or **Hashcat**, especially when leveraging **Meet-in-the-Middle (MITM)** attacks that reduce the search space. MD5’s collision resistance is also compromised, though its primary weakness here lies in its deterministic nature: without a salt, rainbow tables can precompute hashes for all possible DES keys, enabling near-instant lookups. Additionally, the lack of integrity protection (unlike modern hash types like etype 18/19/21) means that even if the hash isn’t cracked, an attacker could forge tickets by manipulating input fields. Finally, the format’s reliance on CBC mode with a fixed IV (or predictable IVs) enables **chosen-plaintext attacks** to derive session keys, further exacerbating its security flaws. These vulnerabilities render etype 23 obsolete in modern networks, with Kerberos deployments now strongly discouraged from using it in favor of AES or RSA-based etypes.

Identifying Kerberos 5 TGS-REP etype 23 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 TGS-REP etype 23, security researchers analyze the hash against a strict regular expression (regex) signature (shown above) to find structural matches.

How to Crack Kerberos 5 TGS-REP etype 23

If you need to recover the plaintext password for a Kerberos 5 TGS-REP etype 23 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.