Pseudo Random Number Generators

  A Pseudo Random Number Generator (PRNG) provides applications with a stream of numbers which have certain important properties for system security:

• It should be impossible for an outsider to predict the output of the random number generator even with knowledge of previous output.
• The generated numbers should not have repeating patterns which means the PRNG should have a very long cycle length.
• A PRNG is normally just an algorithm where the same initial starting values will yield the same sequence of outputs.

Some applications have criteria which affect the type of PRNG which is needed. For instance, later on we will discuss IP datagram IDs and DNS [30] query-IDs, both of these issues have qualities which make it extremely desirable to have a PRNG which makes efforts to avoid emitting repetitions (thus ruling out use of a true-random source).

Many other operating systems also have random number device drivers and other related mechanisms, but largely make no use of them. Some such systems even provide such support only as optional device drivers, therefore discouraging use (i.e., reliance). OpenBSD deviates by actually using these mechanisms in numerous ways. A few major interfaces or techniques are used:

• /dev/?random and similar kernel interfaces
• arc4random(3) in libc
• non-repeating PRNG

Each of these, and their uses in OpenBSD, will be covered in the following sections.