Kerberos 5 TGS-REP etype 18 (AES256-CTS-HMAC-SHA1-96) Hash

What is Kerberos 5 TGS-REP etype 18 (AES256-CTS-HMAC-SHA1-96)?

The Kerberos 5 TGS-REP etype 18 (AES256-CTS-HMAC-SHA1-96) is a cryptographic hash format derived from the AES-256 block cipher combined with a counter-based authenticated encryption (CTS) mode and HMAC-SHA1-96 for integrity verification. Introduced as part of Kerberos 5’s extensible encryption type (etype) framework, this format was standardized to support stronger encryption than the legacy DES-based etypes while maintaining backward compatibility with the protocol’s authentication infrastructure. Structurally, it consists of a 128-bit initialization vector (IV) followed by a counter (CTR) stream encrypted under AES-256 in CTS mode, with the final 96-bit output of HMAC-SHA1 applied over the encrypted payload and key to ensure message authenticity. Its primary use-case lies in securing Kerberos Ticket-Granting Service (TGS) responses, where it encrypts session keys and authentication data exchanged between clients and servers, mitigating eavesdropping and replay attacks in networked environments.

Despite its adoption for enhanced security, AES256-CTS-HMAC-SHA1-96 remains vulnerable to cracking due to several architectural limitations. The reliance on SHA1 for HMAC generation renders it susceptible to length-extension attacks and collision weaknesses, as SHA1’s pre-image resistance has been compromised, enabling forgery of partial hashes. Additionally, the absence of a cryptographic salt in the key derivation process exposes the format to offline brute-force attacks, particularly when combined with optimized GPU-accelerated implementations like Hashcat or John the Ripper’s AES modules. The CTS mode, while providing authenticated encryption, lacks the forward secrecy guarantees of authenticated encryption with associated data (AEAD) schemes, and its reliance on AES-256’s block size (128-bit) introduces potential padding oracle vulnerabilities if improperly implemented. Furthermore, the fixed IV and predictable counter values in some deployments can be exploited to deduce plaintext patterns, exacerbating recovery attacks when combined with weak password-based key derivation.

Identifying Kerberos 5 TGS-REP etype 18 (AES256-CTS-HMAC-SHA1-96) 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 18 (AES256-CTS-HMAC-SHA1-96), 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 18 (AES256-CTS-HMAC-SHA1-96)

If you need to recover the plaintext password for a Kerberos 5 TGS-REP etype 18 (AES256-CTS-HMAC-SHA1-96) 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.