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**10/11/2021 - Nicolas F**
Xen0rInspire
<style> section { font-family: 'Century Gothic', serif !important; font-size: 26pt } </style>

Random Number Generation on Linux Systems

How does it work ?

Technical Terminology

  • Entropy
  • Special files on Linux distributions
  • Pseudorandom number generation (PRNG)
  • Seed (in a random number generator context)

What is the entropy ?

The entropy means a measurable physical property that is particularly associated with a state of disorder, randomness, uncertainty or even chaos.

Example in computer science


String 1 : aiK!l0bUMud5?2

String 2 : aaaAbkkKmmB99b

Example in computer science


String 1 : aiK!l0bUMud5?2 ← Better entropy

String 2 : aaaAbkkKmmB99b

What are the special files on Linux distributions ?

A device file or special file is an interface to a device driver that appears in a file system as if it were an ordinary file.


Block devices

/dev/sda
/dev/nvme
/dev/vda
/dev/cdrom

Characters devices

/dev/random
/dev/urandom
/dev/zero
/dev/ttyX # where X is a number

Why we talk about "pseudo-random" generation ?

Pseudo-random number generation creates a sequence of numbers that approximates the properties of perfect random numbers as closely as possible. It's based on mathematical algorithms.

What is a seed in pseudo-random number generation ?

The seed is the entry point of the pseudo-random generation algorithm. This value is defined explicitly by the user or directly with a default value like the system timestamp for example. The purpose of the seed is to allow the user to lock the pseudo-random number generator, to prevent replicable analysis.

/dev/random vs /dev/urandom

The most common special files to generate random numbers

/dev/random

  • High entropy quality → Blocks the reading process if the entropy isn't enough
  • Can rely on hardware peripherals

Used to generate SSH keys, for LUKS encryption...

/dev/urandom

  • Lower entropy quality → Doesn't block the reading process no matter how much entropy
  • Better to use for long processes

Used to wipe disks (shred command for example)

Other random data source devices

Not implemented on every Linux distributions

  • /dev/arandom : Generates high-quality pseudo-random output data (based on RC4)

  • /dev/prandom : Simple pseudo-random generator

  • /dev/srandom : This device returns reliable random data even if sufficient entropy is not currently available (based on MD5)

bg fit right

Any questions ?