UUID v5 Generator

A version-5 UUID is a hash, not a draw. SHA-1 is run over a namespace UUID followed by a name, the first sixteen bytes become the identifier, and six bits are overwritten to record the version and variant. The consequence is that the same namespace and the same name always produce the same UUID, on any machine, in any language, forever — which is the whole point, and the opposite of version 4's randomness. Choose one of the four standard namespaces or paste your own, type a name, and the hashing happens locally through the Web Crypto API.

The determinism is what makes it useful. If every service in a system derives a record's identifier from a stable natural key — a URL, a domain, a file path, an account number — they all arrive at the same UUID without coordinating, without a shared counter and without a round trip to ask what the ID is. Re-importing the same data twice produces the same identifiers, so an idempotent upsert becomes possible without a lookup table mapping old keys to new ones.

Two details decide whether your UUIDs match everyone else's. The namespace is hashed as its sixteen raw bytes, not as the thirty-six-character text with hyphens — hashing the string is the single most common way an implementation ends up producing plausible-looking UUIDs that no other library agrees with. And the name is hashed as bytes too, so the encoding matters: the same name in UTF-8 and UTF-16 gives two different UUIDs, and the RFC leaves the choice to the namespace's own convention rather than mandating one. This tool uses UTF-8, which is what every common library does.

What version 5 is not is a security mechanism. The input is recoverable in the sense that anyone who can guess the name can compute the UUID, so deriving one from an email address does not conceal the address from someone checking a guess. SHA-1 is also no longer collision-resistant against a determined adversary, which does not matter for identifiers you generate from your own data and does matter if an attacker chooses the names.