Kerberos 5 AS-REP etype 23 Hash

What is Kerberos 5 AS-REP etype 23?

The Kerberos 5 AS-REP etype 23 hash format represents an encrypted timestamp response (AS-REP) using the **RC4-HMAC-MD5** (also known as *etype 23* or *Kerberos 5 etype 23*) cipher suite, a legacy encryption mechanism defined in early Kerberos implementations (pre-Kerberos 5.0). Structurally, this format encodes a client’s authentication response by hashing a timestamp and session key using **RC4** (Ron’s Code 4) in a keyed-HMAC fashion with **MD5**, where the RC4 key is derived from the user’s password hashed with MD5. The output is a 32-byte (256-bit) ciphertext block containing the encrypted timestamp, which the Key Distribution Center (KDC) verifies to validate the client’s identity. This etype was widely deployed in older Microsoft Active Directory deployments (e.g., Windows Server 2003/2008) and other Kerberos implementations before stronger encryption types (e.g., AES or RC4-HMAC-SHA1) became mandatory. Its primary use-case was to authenticate clients in Kerberos v5 environments where backward compatibility with older password hashing schemes was required, though it was never designed for modern security standards.

The vulnerability of etype 23 stems from its reliance on **RC4’s deterministic behavior** when used in HMAC mode with a weak key derivation (MD5), combined with **no salt in the password hashing process** and **inefficient resistance to brute-force attacks**. RC4’s output is pseudo-random but exhibits biases and predictability, particularly when fed with weak passwords (e.g., short, dictionary-based credentials), allowing attackers to exploit **rainbow tables** or **precomputed hash lookups** due to the lack of salting. Additionally, modern GPU/CPU optimizations (e.g., **CUDA-accelerated MD5 cracking** or **hashcat’s RC4-HMAC-MD5 attacks**) enable rapid offline brute-forcing of plaintext passwords from captured AS-REP packets. The absence of collision resistance in MD5 further exacerbates risks, as identical passwords across systems produce identical hashes, enabling large-scale credential reuse attacks. Finally, the **256-bit ciphertext size** (though fixed) does not mitigate brute-force feasibility, as the effective search space remains constrained by password entropy rather than block size, making it susceptible to **dictionary attacks** and **hybrid cracking** (e.g., combining rule-based mutations with brute-force).

Identifying Kerberos 5 AS-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 AS-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 AS-REP etype 23

If you need to recover the plaintext password for a Kerberos 5 AS-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.