Why length alone can never be a definite answer

A cryptographic hash's output length is determined by the algorithm, but that mapping isn't one-to-one — MD5, NTLM, and MD4 all happen to produce a 32-character hex digest, and SHA-1 and RIPEMD-160 both produce 40. Two different algorithms landing on the same output length isn't a coincidence to route around; it's baked into how these algorithms were designed, so no amount of clever pattern-matching on the string alone can distinguish them without additional context — like knowing which system or tool actually generated the hash in the first place.

Prefixed formats are the exception

Formats like bcrypt (`$2b$...`), Unix crypt (`$6$...`), and Argon2 (`$argon2id$...`) sidestep the ambiguity entirely by embedding the algorithm name directly in the string as a prefix — that's a deliberate design choice in those formats, meant to make a hash self-describing so a verifier doesn't need out-of-band knowledge of which algorithm produced it. Whenever a hash you're working with has one of these prefixes, treat the identification as certain; it's only bare hex strings with no format markers where this tool's answer is necessarily a shortlist of possibilities rather than a single confirmed algorithm.