UUID Version 5 (name-based, SHA-1)

UUIDv5 is a name-based UUID derived deterministically by SHA-1-hashing a namespace UUID concatenated with a name, truncating to 128 bits, then overwriting the version (5) and variant bits. Like v3 it is stable — same namespace + name always yields the same UUID — but uses SHA-1 instead of MD5 and is the RFC 9562-recommended choice for name-based ids.

identifier

spec: RFC 9562 §5.5

length 128 bits not sortable hex with dashes (8-4-4-4-12)

example:

2ed6657d-e927-568b-95e1-2665a8aea6a2

field	bits	meaning
sha1_high	48	High 48 bits of SHA-1(namespace_uuid \|\| name) (digest truncated to 128 bits).
ver	4	version = 0b0101 (5), overwriting hash bits.
sha1_mid	12	Next 12 bits of the truncated digest.
var	2	variant = 0b10, overwriting hash bits.
sha1_low	62	Remaining 62 bits of the truncated digest.

Deterministic: UUIDv5(namespace, name) = set_version_variant( SHA1( bytes(namespace) || bytes(name) )[0:16], 5 ). SHA-1 output is 160 bits, truncated to the leading 128.
Example uses the DNS namespace (6ba7b810-9dad-11d1-80b4-00c04fd430c8) with name 'www.example.com', yielding 2ed6657d-e927-568b-95e1-2665a8aea6a2 (RFC 9562 Appendix).
SHA-1 is used only for namespacing here, not as a security primitive; RFC 9562 recommends v5 over v3.

agent: curl -H 'accept: application/json' wire.phall.io/identifier/uuidv5 or /identifier/uuidv5.json

UUID Version 5 (name-based, SHA-1)

identifier

provenance

see also