UUID / GUID v4 Generator (RFC 4122 Cryptographic)

A Version 4 UUID is a 36-character hexadecimal string formatted into 5 canonical groups:

$$\underbrace{\text{xxxxxxxx}}_{8\text{ chars}} - \underbrace{\text{xxxx}}_{4\text{ chars}} - \underbrace{4\text{xxx}}_{4\text{ chars}} - \underbrace{\text{yxxx}}_{4\text{ chars}} - \underbrace{\text{xxxxxxxxxxxx}}_{12\text{ chars}}$$

• Version Nibble (Position 13): Fixed to the character 4, identifying it as a Version 4 randomly generated UUID.
• Variant Bits (Position 17): The character y must be one of 8, 9, a, or b (representing binary 10xx for the standard Leach-Salz variant).
• 122 Bits of Cryptographic Entropy: With 6 bits reserved for version and variant metadata, $128 - 6 = 122$ bits of pure entropy remain ($2^{122} \approx 5.3 \times 10^{36}$ unique identifiers).
• Collision Probability: To have a 50% probability of a single collision, one would need to generate 1 billion UUIDs per second for 85 consecutive years, making accidental collisions impossible in practice.

• Database Primary Keys: Generating decentralized primary keys in PostgreSQL, MySQL, and MongoDB without auto-increment bottlenecks.
• Distributed Tracing: Assigning unique correlation IDs (X-Request-ID) across microservice HTTP headers.
• Session & Transaction Tokens: Identifying user sessions, idempotency keys, and payment webhooks.