5.5. UUID Version 5

UUIDv5 is meant for generating UUIDs from "names" that are drawn from, and unique within, some "namespace" as per Section 6.5.

UUIDv5 values are created by computing an SHA-1 hash [FIPS180-4] over a given Namespace ID value (Section 6.6) concatenated with the desired name value after both have been converted to a canonical sequence of octets, as defined by the standards or conventions of its namespace, in network byte order. The most significant, leftmost 128 bits of the SHA-1 value are then used to populate all 128 bits of the UUID layout, and the remaining 32 least significant, rightmost bits of SHA-1 output are discarded. The UUID version and variant then replace the respective bits as defined by Sections 4.2 and 4.1. An example of this bit substitution and discarding excess bits can be found in Appendix A.4.

Information around selecting a desired name's canonical format within a given namespace can be found in Section 6.5 under the heading "A note on names".

There may be scenarios, usually depending on organizational security policies, where SHA-1 libraries may not be available or may be deemed unsafe for use. As such, it may be desirable to generate name-based UUIDs derived from SHA-256 or newer SHA methods. These name-based UUIDs MUST NOT utilize UUIDv5 and MUST be within the UUIDv8 space defined by Section 5.8. An illustrative example of UUIDv8 for SHA-256 name-based UUIDs is provided in Appendix B.2.

For more information on SHA-1 security considerations, see [RFC6194].

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           sha1_high                           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         sha1_high             |  ver  |      sha1_mid         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|var|                       sha1_low                            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           sha1_low                            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 9: UUIDv5 Field and Bit Layout

Field Definitions:

sha1_high: The first 48 bits of the layout are filled with the most significant, leftmost 48 bits from the computed SHA-1 value. Occupies bits 0 through 47 (octets 0-5).

ver: The 4-bit version field as defined by Section 4.2, set to 0b0101 (5). Occupies bits 48 through 51 of octet 6.

sha1_mid: 12 more bits of the layout consisting of the least significant, rightmost 12 bits of 16 bits immediately following sha1_high from the computed SHA-1 value. Occupies bits 52 through 63 (octets 6-7).

var: The 2-bit variant field as defined by Section 4.1, set to 0b10. Occupies bits 64 and 65 of octet 8.

sha1_low: The final 62 bits of the layout immediately following the var field to be filled by skipping the two most significant, leftmost bits of the remaining SHA-1 hash and then using the next 62 most significant, leftmost bits. Any leftover SHA-1 bits are discarded and unused. Occupies bits 66 through 127 (octets 8-15).

5.5. UUID Version 5​

5.5. UUID Version 5