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random(4) [osx man page]

RANDOM(4)						   BSD Kernel Interfaces Manual 						 RANDOM(4)

random , urandom -- random data source devices. SYNOPSIS
pseudo-device random DESCRIPTION
The random device produces uniformly distributed random byte values of potentially high quality. To obtain random bytes, open /dev/random for reading and read from it. To add entropy to the random generation system, open /dev/random for writing and write data that you believe to be somehow random. /dev/urandom is a compatibility nod to Linux. On Linux, /dev/urandom will produce lower quality output if the entropy pool drains, while /dev/random will prefer to block and wait for additional entropy to be collected. With Yarrow, this choice and distinction is not necessary, and the two devices behave identically. You may use either. OPERATION
The random device implements the Yarrow pseudo random number generator algorithm and maintains its entropy pool. Additional entropy is fed to the generator regularly by the SecurityServer daemon from random jitter measurements of the kernel. SecurityServer is also responsible for periodically saving some entropy to disk and reloading it during startup to provide entropy in early system operation. You may feed additional entropy to the generator by writing it to the random device, though this is not required in a normal operating envi- ronment. LIMITATIONS AND WARNINGS
Yarrow is a fairly resilient algorithm, and is believed to be resistant to non-root. The quality of its output is however dependent on regu- lar addition of appropriate entropy. If the SecurityServer system daemon fails for any reason, output quality will suffer over time without any explicit indication from the random device itself. Paranoid programmers can counteract this risk somewhat by collecting entropy of their choice (e.g. from keystroke or mouse timings) and seed- ing it into random directly before obtaining important random numbers. FILES
/dev/random /dev/urandom HISTORY
A random device appeared in the Linux operating system. Darwin September 6, 2001 Darwin

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RANDOM(4)						     Linux Programmer's Manual							 RANDOM(4)

random, urandom - kernel random number source devices DESCRIPTION
The character special files /dev/random and /dev/urandom (present since Linux 1.3.30) provide an interface to the kernel's random number generator. File /dev/random has major device number 1 and minor device number 8. File /dev/urandom has major device number 1 and minor device number 9. The random number generator gathers environmental noise from device drivers and other sources into an entropy pool. The generator also keeps an estimate of the number of bit of the 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 to /dev/random will block until additional environmental noise is gathered. When read, /dev/urandom device will return as many bytes as are requested. As a result, if there is not sufficient entropy in the entropy pool, the returned values are theoretically vulnerable to a cryptographic attack on the algorithms used by the driver. Knowledge of how to do this is not available in the current non-classified literature, but it is theoretically possible that such an attack may exist. If this is a concern in your application, use /dev/random instead. CONFIGURING
If your system does not have /dev/random and /dev/urandom created already, they can be created with the following commands: mknod -m 644 /dev/random c 1 8 mknod -m 644 /dev/urandom c 1 9 chown root:root /dev/random /dev/urandom When a Linux system starts up without much operator interaction, the entropy pool may be in a fairly predictable state. This reduces the actual amount of noise in the entropy pool below the estimate. In order to counteract this effect, it helps to carry entropy pool informa- tion across shut-downs and start-ups. To do this, add the following lines to an appropriate script which is run during the Linux system start-up sequence: echo "Initializing kernel random number generator..." # Initialize kernel random number generator with random seed # from last shut-down (or start-up) to this start-up. Load and # then save 512 bytes, which is the size of the entropy pool. if [ -f /var/random-seed ]; then cat /var/random-seed >/dev/urandom fi dd if=/dev/urandom of=/var/random-seed count=1 Also, add the following lines in an appropriate script which is run during the Linux system shutdown: # Carry a random seed from shut-down to start-up for the random # number generator. Save 512 bytes, which is the size of the # random number generator's entropy pool. echo "Saving random seed..." dd if=/dev/urandom of=/var/random-seed count=1 FILES
/dev/random /dev/urandom AUTHOR
The kernel's random number generator was written by Theodore Ts'o ( SEE ALSO
mknod (1) RFC 1750, "Randomness Recommendations for Security" Linux 1997-08-01 RANDOM(4)
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