linux - Is /dev/random considered truly random?

Question

For instance, could it be used to generate a one-time pad key?
Also, what are its sources and how could it be used to generate a random number between x and y?

Answer 1

Strictly speaking, /dev/random is not really completely random. /dev/random feeds on hardware sources which are assumed to be impredictible in some way; then it mixes such data using functions (hash functions, mostly) which are also assumed to be one-way. So the "true randomness" of /dev/random is thus relative to the inherent security of the mixing functions, security which is no more guaranteed than that of any other cryptographic primitive, in particular the PRNG hidden in /dev/urandom.

The difference between /dev/random and /dev/urandom is that the former will try to maintain an estimate (which means "a wild guess") of how much entropy it has gathered, and will refuse to output more bits than that. On the other hand, /dev/urandom will happily produce megabytes of data from the entropy it has.

The security difference between the two approaches is meaningless unless you assume that "classical" cryptographic algorithms can be broken, and you use one of the very few information-theoretic algorithms (e.g. OTP or Shamir's secret sharing); and, even then, /dev/random may be considered as more secure than /dev/urandom only if the mixing functions are still considered to be one-way, which is not compatible with the idea that a classical cryptographic algorithm can be broken. So, in practice and even in theory, no difference whatsoever. You can use the output of /dev/urandom for an OTP and it will not be broken because of any structure internal to /dev/urandom -- actual management of the obtained stream will be the weak point (especially long-time storage). On the other hand, /dev/random has very real practical issues, namely that it can block at untimely instants. It is really irksome when an automated OS install blocks (for hours !) because SSH server key generation insists on using /dev/random and needlessly stalls for entropy.

There are many applications which read /dev/random as a kind of ritual, as if it was "better" than /dev/urandom, probably on a karmic level. This is plain wrong, especially when the alea is to be used with classical cryptographic algorithms (e.g. to generate a SSH server public key). Do not do that. Instead, use /dev/urandom and you will live longer and happier. Even for one-time pad.

(Just for completeness, there is a quirk with /dev/urandom as implemented on Linux: it will never block, even if it has not gathered any entropy at all since previous boot. Distributions avoid this problem by creating a "random seed" at installation time, with /dev/random, and using that seed at each boot to initialize the PRNG used by /dev/urandom; a new random seed is regenerated immediately, for next boot. This ensures that /dev/urandom always works over a sufficiently big internal seed. The FreeBSD implementation of /dev/urandom will block until a given entropy threshold is reached, which is safer.)