Kerberos 5 TGS-REP etype 17 (AES128-CTS-HMAC-SHA1-96) Hash

What is Kerberos 5 TGS-REP etype 17 (AES128-CTS-HMAC-SHA1-96)?

The Kerberos 5 TGS-REP etype 17 (AES128-CTS-HMAC-SHA1-96) cipher suite represents a hybrid encryption and authentication mechanism designed for the Kerberos network authentication protocol, standardized in RFC 4120 and later extensions. Originating from the need to secure Kerberos communications against evolving cryptographic threats, this format combines **AES-128 in CTS mode** for authenticated encryption of the Ticket-Granting Service (TGS) reply, paired with **HMAC-SHA1-96** for keyed-hash message authentication. Structurally, it encodes the encrypted ticket and auxiliary data (e.g., sequence numbers, timestamps) under AES-CTS, while the HMAC-SHA1-96 generates a 96-bit tag derived from the SHA-1 hash of the encrypted payload and a shared secret key, ensuring integrity and authenticity. This etype was introduced to replace weaker algorithms like DES-CBC-MD5, leveraging AES’s improved security properties while retaining compatibility with Kerberos’ ticket-based delegation model. Its primary use-case lies in high-security environments (e.g., enterprise networks, military systems) where resistance to replay attacks and eavesdropping is critical, though its reliance on SHA-1 introduces long-term vulnerabilities.

The vulnerability of AES128-CTS-HMAC-SHA1-96 stems from **SHA-1’s cryptographic obsolescence**, which renders it susceptible to collision attacks (e.g., SHA-1 preimage attacks with sub-exponential complexity) and weak collision resistance, violating modern security standards like NIST’s SP 800-131A. While AES-128 itself remains secure against brute-force under ideal conditions, the **lack of a salt or iterative hashing** in HMAC-SHA1-96 enables **rainbow table attacks** if plaintext-ciphertext pairs are leaked, and its **96-bit tag length** is insufficient for modern security requirements (NIST recommends ≥128 bits for HMAC). Furthermore, the **CTS mode’s vulnerability to chosen-plaintext attacks** (e.g., via padding oracle exploits) and **GPU/ASIC acceleration** of SHA-1 hashing (e.g., via tools like Hashcat) enable practical cracking of weak implementations, particularly when combined with **credential stuffing** or **offline dictionary attacks** on leaked keys. The absence of forward secrecy and reliance on static keys also exposes the system to long-term compromise if the Kerberos keytab is compromised.

Identifying Kerberos 5 TGS-REP etype 17 (AES128-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 17 (AES128-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 17 (AES128-CTS-HMAC-SHA1-96)

If you need to recover the plaintext password for a Kerberos 5 TGS-REP etype 17 (AES128-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.