RANDOM(4) OpenBSD Programmer's Manual RANDOM(4) NAME random, srandom, urandom, prandom, arandom - random data source devices SYNOPSIS #include <sys/types.h> #include <dev/rndvar.h> #include <dev/rndioctl.h> DESCRIPTION The various random devices produce random output data with different ran- dom qualities. Entropy data is collected from system activity (like disk and network device interrupts and such), and then run through various hash or message digest functions to generate the output. /dev/random This device is reserved for future support of hardware random generators. /dev/srandom Strong random data. This device returns reliable random data. If sufficient entropy is not currently available (i.e., the entropy pool quality starts to run low), the driver pauses while more of such data is collected. The entropy pool data is converted into output data using MD5. /dev/urandom Same as above, but does not guarantee the data to be strong. The entropy pool data is converted into output data using MD5. When the entropy pool quality runs low, the driver will continue to output data. /dev/prandom Simple pseudo-random generator. /dev/arandom As required, entropy pool data re-seeds an ARC4 generator, which then generates high-quality pseudo-random output da- ta. The arc4random(3) function in userland libraries seeds it- self from this device, providing a second level of ARC4 hashed data. FILES /dev/random /dev/srandom /dev/urandom /dev/prandom /dev/arandom SEE ALSO arc4random(3), md5(3), random(3), amdpm(4), glxsb(4), pchb(4), md5(9), random(9) HISTORY A random device first appeared in the Linux operating system. This is a cloned interface. ARC4 routines added by David Mazieres. BUGS No randomness testing suite provided. OpenBSD 4.4 May 18, 2008 1