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Terms & Definitions

pseudo-random number generator

A pseudo-random number generator, or PRNG, is a random number generator that produces a sequence of values based on a seed and a current state. Given the same seed, a PRNG will always output the same sequence of values.

The term pseudo is not intended to be a pejorative term. There exist PRNG's such as the Blum-Blum-Shub PRNG that have been proved to be cryptographically strong.

PRNGs do have a problem that non-deterministic random number generators to not have: the selection and security of the seed.

To achieve output unpredictability of high quality PRNG, one must start with an unpredictable seed. Using second PRNG to create the seed does not help because that simply transfers the problem of seeding onto the second PRNG. Worse yet, using a PRNG to repeatedly seed another (or even worse the same) PRNG usually degrades the quality of the output.

One way to resolve this ''Chicken & Egg'' problem is to use a non-pseudo random number generator such as a hardware random number generator, to produce a seed for a PRNG.

The seed of a PRNG is analogous to a key of a cipher. In fact, some ciphers (e.g., RC4) use a PRNG internally, seeded by the key, to mix pseudo-random output with plain-text to produce cipher text. The discovery of a PRNG's seed is analogous to the discovery of a cipher's key. One must protect the PRNG seed from unauthorized disclosure. One must select the PRNG seed in such a way that it is not easily guessed.

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