RANDOM(4) BSD Programmer's Manual RANDOM(4)
random, srandom, urandom, prandom, wrandom, arandom - random data source devices
#include <sys/types.h> #include <dev/rndvar.h> #include <dev/rndioctl.h>
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/wrandom This device is actually the same as /dev/prandom, but can be written to by regular users, even if this interface is simulated using pipes or other means on other operating systems where /dev/prandom can only be read from. Its pur- pose is to allow anything from userspace or other not 100% trustworthy sources to contribute even fractional bit amounts of entropy into the kernel pool by hashing, col- lapsing, and rate-limiting. /dev/arandom As required, entropy pool data re-seeds an ARC4 generator, which then generates high-quality pseudo-random output data. The arc4random(3) function in userland libraries seeds it- self from this device, providing a second level of ARC4 hashed data. The arc4random_pushb_fast(3) function, any write access to the KERN_ARND sysctl(3) and writes to /dev/wrandom feed data back to the kernel as described above and in arc4random(9).
/dev/random /dev/srandom /dev/urandom /dev/prandom /dev/arandom
arc4random(3), md5(3), random(3), amdpm(4), pchb(4), md5(9), random(9)
A random device first appeared in Linux operating system. This is a cloned interface. ARC4 routines added by David Mazieres. Rewritten by Thorsten Glaser.
No randomness testing suite provided. The ioctl(2) interface is not described in this manual page. MirOS BSD #10-current September 24, 2010 1
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