UUID

A UUID (acronym for Universally Unique IDentifier) is an identifier that is intended to be unique and universal, defined by the ISO/IEC 9834-8:2008 standard. This is 128 bits of data usually displayed in hexadecimal in the format xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx.

Version 4 is made up of random bits based on the timestamp of the system that generates them. The timestamp is usually shown in microseconds (one thousandth of a second) since a specific date (called EPOCH). The result is therefore pseudo-random and presents a very probable uniqueness (not guaranteed). Each time a UUID v4 is generated, the code generated is different.

Version 3 is generated from an MD5 hash of a 2-part string: on one side a UUID named namespace , and on the other a character string containing the name to be encoded .

Version 5 is the same as version 3 but generated from a SHA-1 hash.

Each time a v3 or v5 UUID is generated, the generated code is always the same.

A version 4 UUID is completely random and contains nothing to decode.

A UUID version 3 or 5 can sometimes have been generated with a character string but it has been hashed with MD5 or SHA-1 cryptographic functions which are not intended to be decoded.

UUIDs are generally categorized into different versions, ranging from version 1 to version 5.

Version 1: contains a timestamp and MAC address

Version 2: a variant of V1 for DCE (Distributed Computing Environment) which includes POSIX UIDs

Version 3: MD5 hash

Version 4: randomly generated

Version 5: SHA-1 hash

UUIDs also have a variant number (3 bits) assigned to them:

0xx: Reserved for NCS (Network Computing System) for backward compatibility

10x: Default variant, see RFC 4122

110: Reserved for Microsoft for backward compatibility

111: Reserved for future definition

The UUID code contains 128 bits presented in the form of 32 hexadecimal characters divided into 8-4-4-4-12 digits (base 16) and separated by a dash -

Certain characters of the UUID string are marked, this is the case of the 13th byte (here denoted M) and the 17th byte (here denoted N) in a UUID xxxxxxxx-xxxx-Yxxx-Zxxx-xxxxxxxxxxxx

This is because Y and Z are hexadecimal digits specifying the version and variant of the UUID, respectively.

The value of Y corresponds directly to the version of the UUID

The first 3 bits of Z (coded on 4 bits) correspond to the variant.