libskarnet
skalibs
Software
skarnet.org

The random library interface

librandom is a small library designed to provide an interface to some reasonable-quality pseudorandom number generation. Some libcs have a bad random() implementation; librandom is designed to use system pseudorandom number generation when it's provided via /dev/random and /dev/urandom, and to use a good default PRNG otherwise.

librandom also supports EGD. If you have built skalibs with --enable-egd, then the librandom primitives will try and connect to an EGD service to get random bytes if there is no kernel-based entropy generator such as /dev/random. If the EGD connection fails, a SURF PRNG is used.

Compiling

• Use #include <skalibs/random.h>

Programming

You should refer to the skalibs/random.h header for the exact function prototypes.

High quality, cryptographically strong random data

  unsigned char c ;
  unsigned int max ;
  unsigned int n ;
  unsigned int b ;
  char data[at least b] ;
  int r ;

  goodrandom_init() ;
  c = goodrandom_char() ;
  n = goodrandom_int(max) ;
  r = goodrandom_string(data, b) ;
  goodrandom_finish() ;

goodrandom_init() becomes optional with skalibs-0.43. It is recommended that you let the library perform cleanups after you have used it, by calling goodrandom_finish().

• goodrandom_char() returns a random character
• goodrandom_int(max) returns a random integer between 0 and max-1
• goodrandom_string(data, b) puts b random bytes in data, which must be preallocated. It returns b if it succeeds, or a non-negative integer lesser than b if it fails for any reason.

If you have neither /dev/random nor EGD, a software PRNG is used. This PRNG is based on the SURF function, which is unpredictable enough for most uses.

Lower quality random data

It works basically the same, by replacing goodrandom_* with badrandom_*. It uses /dev/urandom on systems that support it; on systems that do not, but support EGD, non-blocking calls to EGD are made ; if that is not enough, or EGD is not supported, the SURF generator is used.

The point of badrandom is to get random bytes instantly, even at the expense of quality; whereas goodrandom always returns high-quality random bytes, but may block if entropy is insufficient. In practice, in spite of its name, badrandom will return quite unpredictable pseudo-random data, so goodrandom should be used only when paranoia is the rule and blocking is an option.