UUID v5 is a name-based universally unique identifier that uses SHA-1 hashing to generate deterministic UUIDs from a namespace and name combination. Unlike random UUID v4, UUID v5 produces the same identifier every time you use the same namespace and name inputs, making it perfect for creating reproducible, consistent identifiers across distributed systems.

The UUID v5 specification follows RFC 4122 standards and generates 128-bit identifiers in the standard format: xxxxxxxx-xxxx-5xxx-yxxx-xxxxxxxxxxxx, where the '5' indicates version 5. This deterministic nature makes UUID v5 ideal for creating stable identifiers based on URLs, domain names, or any string-based data.

UUID v5 is ideal when you need deterministic, reproducible identifiers. Use UUID v5 for API endpoints that need consistent IDs based on resource names, database records where the same logical entity should always have the same UUID, caching systems that benefit from predictable keys, or distributed systems requiring synchronized identifiers without central coordination.

Examples include generating user IDs from email addresses, creating resource identifiers from URLs, generating consistent test data, implementing content-addressable storage, or building federation systems where multiple nodes need to independently arrive at the same UUID for the same entity.

UUID v5 generation follows a precise algorithm: First, the namespace UUID is converted to its 16-byte binary representation. The name string is encoded as UTF-8 bytes. These are concatenated together and hashed using SHA-1, producing a 160-bit hash. The first 128 bits of this hash become the UUID, with specific version and variant bits set according to RFC 4122 standards.

The version bits (4 bits set to '0101' for version 5) are placed in the time_hi_and_version field. The variant bits (2 bits set to '10') are placed in the clock_seq_hi_and_reserved field. This ensures the resulting UUID is RFC 4122 compliant and universally recognizable as a version 5 UUID.

While UUID v5 uses SHA-1 hashing, it's not designed for cryptographic security. The deterministic nature means anyone who knows the namespace and name can generate the same UUID. Don't use UUID v5 for security tokens, passwords, or any security-critical identifiers where unpredictability is required.

UUID v5 is perfect for identification and consistency, not for security. For security-sensitive applications, use UUID v4 (random) or dedicated cryptographic methods. The SHA-1 algorithm, while considered weak for collision resistance in security contexts, is sufficient for UUID v5's intended purpose of creating reproducible identifiers.