/dev/random  In Unix-like operating systems, ', ' and are special files that serve as pseudorandom number generators. They allow access to environmental noise collected from device drivers and other sources. typically blocks if there is less entropy available than requested; typically never blocks, even if the pseudorandom number generator seed was not fully initialized with entropy since boot. blocks after boot until the seed has been securely initialized with enough entropy, and then never blocks again. Not all operating systems implement the same methods for and and only a few provide .  > hexdump -C -n 8 /dev/random 0000000 79 5d 13 c2 b4 fe ca d7 |y].Â´þÊ×| > hexdump -C -n 8 /dev/random 0000000 bd f1 6d 48 10 f8 25 3c |..mH..% in 1994 by Theodore Ts'o. The implementation used secure hashes rather than ciphers, to avoid cryptography export 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 Ts'o's 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 device will only return random bytes within the estimated number of bits of noise in the entropy pool. When the entropy pool is empty, reads from will block until additional environmental noise is gathered. The intent is to serve as a cryptographically secure pseudorandom number generator, delivering output with entropy as large as possible. This is suggested by the authors for use in generating cryptographic keys for high-value or long-term protection. A counterpart to is ("unlimited"/non-blocking random source) which reuses the internal pool to produce more pseudo-random bits. This means that the call will not block, but the output may contain less entropy than the corresponding read from . While is still intended as a pseudorandom number generator suitable for most cryptographic purposes, the authors of the corresponding man page note that, theoretically, there may exist an as-yet-unpublished attack on the algorithm used by , and that users concerned about such an attack should use instead.