DRBG

Deterministic Random Bit Generator (DRBG) is a cryptographic component that produces a sequence of bits that appear random, given a secret internal state and a seed. It is deterministic: the same seed and internal state yield the same output. DRBGs rely on an entropy source to seed or reseed and are used to generate cryptographic keys, nonces, and other randomness requirements in software and hardware.

Common DRBG designs include Hash DRBG, HMAC DRBG, and CTR DRBG, defined in NIST SP 800-90A. Hash DRBG uses a hash function to transform the state into output bits; HMAC DRBG uses an HMAC construction; CTR DRBG uses a block cipher in counter mode. All rely on a state update function and an entropy input. Reseeding introduces fresh entropy to refresh the internal state and protect against state exhaustion.

Security properties and usage: The security of a DRBG hinges on the entropy of the seed and the unpredictability of the internal state. If the seed or state is compromised, past and future outputs may be at risk unless proper protections are in place. DRBGs include health tests to detect failures and are designed to support reseeding at intervals or when entropy is available. They are used in protocols and systems such as TLS, PKI, and secure key generation, and are specified in SP 800-90A/B/C and related standards.

Limitations and considerations: The quality of DRBG output depends on entropy input and correct implementation. Inadequate entropy, improper seeding, or misconfiguration can weaken security. Regular validation, adherence to standards, and proper entropy sources are essential.