.Dd September 6, 2001 .Dt RANDOM 4 .Os Darwin .Sh NAME .Nm random , .Nm urandom .Nd random data source devices. .Sh SYNOPSIS .Cd "pseudo-device random" .Sh DESCRIPTION The .Nm device produces uniformly distributed random byte values of potentially high quality. .Pp To obtain random bytes, open .Nm /dev/random for reading and read from it. .Pp To add entropy to the random generation system, open .Nm /dev/random for writing and write data that you believe to be somehow random. .Pp .Nm /dev/urandom is a compatibility nod to Linux. On Linux, .Nm /dev/urandom will produce lower quality output if the entropy pool drains, while .Nm /dev/random will prefer to block and wait for additional entropy to be collected. With Yarrow, this choice and distinction is not necessary, and the two devices behave identically. You may use either. .Sh OPERATION The .Nm device implements the .Nm Yarrow pseudo random number generator algorithm and maintains its entropy pool. Additional entropy is fed to the generator regularly by the .Nm SecurityServer daemon from random jitter measurements of the kernel. .Nm SecurityServer is also responsible for periodically saving some entropy to disk and reloading it during startup to provide entropy in early system operation. .Pp You may feed additional entropy to the generator by writing it to the .Nm device, though this is not required in a normal operating environment. .Sh LIMITATIONS AND WARNINGS .Nm Yarrow is a fairly resilient algorithm, and is believed to be resistant to non-root. The quality of its output is however dependent on regular addition of appropriate entropy. If the .Nm SecurityServer system daemon fails for any reason, output quality will suffer over time without any explicit indication from the .Nm device itself. .Pp Paranoid programmers can counteract this risk somewhat by collecting entropy of their choice (e.g. from keystroke or mouse timings) and seeding it into .Nm directly before obtaining important random numbers. .Sh FILES .Bl -tag -width /dev/urandom -compact .It Pa /dev/random .It Pa /dev/urandom .El .Sh HISTORY A .Nm device appeared in Linux operating system.