In Unix-like operating systems, /dev/random is a special file that serves as a true random number generator or as a pseudorandom number generator. It allows access to environmental noise collected from device drivers and other sources. Not all operating systems implement the same semantics for /dev/random.

Linux was the first operating system to implement a true random number generator in this way. Random number generator from kernel space was first time implemented for Linux in 1994 by Theodore Ts'o. The implementation uses secure hashes rather than ciphers, as required to avoid legal restrictions that were in place when the generator was originally designed.

The implementation was also designed with the assumption that any given hash or cipher might eventually be found to be weak, and so the design is durable in the face of any such weaknesses. Fast recovery from pool compromise is not considered a requirement, because the requirements for pool compromise are sufficient for much easier and more direct attacks on unrelated parts of the operating system. In this implementation, the generator keeps an estimate of the number of bits of noise in the entropy pool.

From this entropy pool random numbers are created. When read, the /dev/random device will only return random bytes within the estimated number of bits of noise in the entropy pool. /dev/random should be suitable for uses that need very high quality randomness such as one-time pad or key generation.

When the entropy pool is empty, reads from /dev/random will block until additional environmental noise is gathered. (Source: Linux Programmer's Manual, section 4)

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