Crypt16 Hash

What is Crypt16?

**Crypt16** is a lightweight cryptographic hash format derived from the **MD4** family of hash functions, originally proposed as an alternative to SHA-1 in constrained environments such as embedded systems or legacy password storage. Structurally, it maintains a 128-bit (16-byte) output hash, but deviates from MD4’s standard by incorporating a truncated intermediate state during the compression function—specifically, it processes input data in 512-bit blocks while only retaining the first 128 bits of the chaining variables after each round, effectively discarding the upper 256 bits. This truncation introduces a non-standard behavior where the final hash is computed as `H = (A ⊕ B ⊕ C ⊕ D) mod 2¹²⁸`, where `A–D` are the final 128-bit chaining variables, rather than the full 512-bit state. Its primary use-case was in systems requiring fast, fixed-length hashing (e.g., lightweight authentication or checksums), though it was never standardized and remains obscure outside niche implementations.

The vulnerability of **Crypt16** stems from its **truncated state propagation**, which enables **length-extension attacks** and **collision generation** with minimal computational overhead. Unlike MD4, which relies on a full 512-bit state, Crypt16’s premature truncation reduces the effective collision resistance to approximately **2⁶⁴** (due to the 64-bit XOR operations on truncated variables), making it susceptible to **birthday-bound attacks** with feasible GPU/FPGA acceleration. Additionally, the absence of salting or iterative hashing (e.g., like bcrypt) allows for **rainbow table precomputation**, as the hash space is too small for practical key stretching. Optimizations like **parallel block processing** and **bit-slicing** further exacerbate weaknesses, enabling attackers to brute-force or invert hashes at rates exceeding **10⁹ hashes/second** on modern hardware, rendering it unsuitable for security-critical applications.

Identifying Crypt16 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 Crypt16, security researchers analyze the hash against a strict regular expression (regex) signature (shown above) to find structural matches.

How to Crack Crypt16

If you need to recover the plaintext password for a Crypt16 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.