FCS-16 Hash
Technical details, regex signatures, and cracking commands for the FCS-16 algorithm.
What is FCS-16?
The **FCS-16** (Frame Check Sequence, 16-bit) is a lightweight error-detection algorithm derived from the **CRC-16** family, originally standardized in **IEEE 802** protocols (e.g., Ethernet, HDLC) for link-layer integrity verification. Structurally, it operates on a fixed 16-bit polynomial divisor (e.g., `0x8005` or `0x1021`), where the input data is treated as a binary polynomial multiplied by `x^16`, followed by modular reduction via XOR operations with the divisor. Unlike cryptographic hashes, FCS-16 prioritizes **real-time efficiency** over collision resistance, producing a checksum rather than a one-way function. Its primary use-case lies in **network packet validation**, where a minimal 16-bit footprint ensures low-latency error detection during transmission, though it lacks the properties required for security applications.
FCS-16 is inherently vulnerable to cracking due to its **deterministic, non-cryptographic design**. The absence of a **salt** or **randomness** makes it susceptible to **precomputed collision attacks**, as the same input polynomial guarantees identical outputs. Its **linear structure** enables **birthday-bound collisions** (O(2⁸) complexity for 16-bit), and modern **GPU/FPGA acceleration** (e.g., via SIMD optimizations) trivializes brute-force searches. Additionally, the **predictable polynomial** allows adversaries to exploit **mathematical inversions** or **bit-flipping attacks** to manipulate checksums without altering payloads, rendering it unsuitable for integrity or authentication in security contexts.
Identifying FCS-16 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 FCS-16, security researchers analyze the hash against a strict regular expression (regex) signature (shown above) to find structural matches.
How to Crack FCS-16
If you need to recover the plaintext password for a FCS-16 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.