RANDOM(4)

NAME
random, urandom - random number devices

DESCRIPTION
This device gathers environmental noise from device drivers, etc., and returns good random numbers, suitable for cryptographic use. Besides the obvious cryptographic uses, these numbers are also good for seeding TCP sequence numbers, and other places where it is desirable to have numbers which are not only random, but hard to predict by an attacker.

  Theory of operation
Computers are very predictable devices. Hence it is extremely hard to produce truly random numbers on a computer — as opposed to pseudo-random numbers, which can easily be generated by using an algorithm. Unfortunately, it is very easy for attackers to guess the sequence of pseudo-random number generators, and for some applications this is not acceptable. So instead, we must try to gather "environmental noise" from the computer's environment, which must be hard for outside attackers to observe, and use that to generate random numbers. In a Unix environment, this is best done from inside the kernel.

Previous and contemporary random implementations typically used an "entropy" pool which was processed with a cryptographic hash function such as MD5. However, at the time of this writing security issues have been discovered in some of these functions (MD4, MD5, SHA0, SHA1).

This implementation uses a CSPRNG (Cryptographically Secure Pseudo Random Number Generator) which is continuously reseeded as described above.

The user interface consists of two character devices /dev/random and /dev/urandom. The /dev/random device is suitable for use when very high quality randomness is desired (e.g. for key generation). Previous implementations of this device attempted to limit the number of returned bytes based on a guess as to the secureness of the pool. However, this resulted in the interface being so undependable that most programs just started using /dev/urandom instead of /dev/random. The current DragonFly implementation will return all requested bytes but the system reserves the right in the future to limit the transfer rate to maintain the high quality of randomness requested.

The /dev/urandom device uses a different and much faster algorithm, but one which is not considered to be as secure (though for all practical purposes it probably is good enough).

Root may write entropy to /dev/random to seed the random number generator only if the securelevel is less than or equal to zero and the kern.seedenable sysctl is non-zero. A certain degree of entropy is added by RC scripts during the boot sequence.

FILES
/dev/random
/dev/urandom

SEE ALSO
arc4random(3), drand48(3), getentropy(3), rand(3), RAND_add(3), RAND_bytes(3), random(3), rndcontrol(8)

HISTORY
The random, urandom files appeared in FreeBSD 2.1.5.

AUTHORS
-nosplit Mark Murray wrote the rndcontrol(8) utility for FreeBSD.

The IBAA CSPRNG was developed by Bob Jenkins and is used by /dev/urandom. The DragonFly implementation was ported by Matthew Dillon from initial work done by Robin Carey. The /dev/random generator uses a choice of IBAA and a Fortuna -based CSPRNG implemented by Alex Hornung.