NIST Special Publication 800-90A

The National Institute of Standards and Technology Special Publication 800-90A: "Recommendation for Random Number Generation Using Deterministic Random Bit Generators" specifies techniques for the generation of random bits that may then be used directly or converted to random numbers when random values are required by applications using cryptography. There are two fundamentally different strategies for generating random bits. One strategy is to produce bits non-deterministically, where every bit of output is based on a physical process that is unpredictable; this class of random bit generators (RBGs) is commonly known as non-deterministic random bit generators (NRBGs). The other strategy is to compute bits deterministically using an algorithm; this class of RBGs is known as Deterministic Random Bit Generators (DRBGs). A DRBG is based on a DRBG mechanism as specified in this Recommendation and includes a source of entropy input. A DRBG mechanism uses an algorithm (i.e., a DRBG algorithm) that produces a sequence of bits from an initial value that is determined by a seed that is determined from the entropy input. Once the seed is provided and the initial value is determined, the DRBG is said to be instantiated and may be used to produce output. Because of the deterministic nature of the process, a DRBG is said to produce pseudorandom bits, rather than random bits. The seed used to instantiate the DRBG must contain sufficient entropy to provide an assurance of randomness. If the seed is kept secret, and the algorithm is well designed, the bits output by the DRBG will be unpredictable, up to the instantiated security strength of the DRBG. The security provided by an RBG that uses a DRBG mechanism is a system implementation issue; both the DRBG mechanism and its source of entropy input must be considered when determining whether the RBG is appropriate for use by consuming applications.