Random Number Devices

• The special devices /dev/random and /dev/urandom provide access to the Linux kernel's built-in random number generation facility.
• Most software functions for generating random numbers, such as the rand function in the standard C library, actually generate pseudorandom numbers.
• Although these numbers satisfy some properties of random numbers, they are reproducible: If you start with the same seed value, you'll obtain the same sequence of pseudorandom numbers every time.
• By measuring the time delay between your input actions, such as keystrokes and mouse movements, Linux is capable of generating an unpredictable stream of high-quality random numbers.You can access this stream by reading from /dev/random and /dev/urandom. The data that you read is a stream of randomly generated bytes.

  $ od -t a (d2,x1) /dev/random
  $ od -t x1 /dev/urandom
  $ man od
  

• Using random numbers from /dev/random in a program is easy, too. The http://siber.cankaya.edu.tr/SystemsProgramming/cfiles/random_number.cprogram in Fig. 6.15 presents a function that generates a random number using bytes read from in /dev/random.
• Remember that /dev/random blocks a read until there is enough randomness available to satisfy it; you can use /dev/urandom instead if fast execution is more important and you can live with the potential lower quality of random numbers.

  Figure 6.15: Function to Generate a Random Number.