UUID v4

What is UUID v4?

UUID (Universally Unique Identifier) v4 is a 128-bit identifier generated using random values. It is defined in RFC 4122. A UUID v4 looks like:

550e8400-e29b-41d4-a716-446655440000

Of the 128 bits, 4 bits indicate the version (4) and 2-3 bits indicate the variant. This leaves 122 bits of randomness, giving 2¹²² possible UUIDs — about 5.3 × 10³⁶.

Where UUIDs are used

UUIDs are widely used as identifiers in software:

Database primary keys
Request tracing IDs
File names and resource identifiers
Distributed system node IDs
Session tokens and correlation IDs

Why a UUID is not a secret

While UUID v4 values are random and unique, they are not suitable as secrets for several reasons:

UUIDs are often visible in URLs, logs, and databases
The 122 bits of entropy is good for uniqueness but may not be enough for security tokens
UUIDs have a recognizable format that makes them easy to identify
Some UUID implementations may use weaker randomness sources

For secrets, use the Hex Generator or Base64 Generator with at least 256 bits (32 bytes) of entropy. For identifiers, UUIDs are perfect.