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RANDOM(4) FreeBSD/i386 Kernel Interfaces Manual RANDOM(4)
NAME
random, urandom -- random number devices
DESCRIPTION
This device gathers environmental noise from device drivers, etc., and
returns good random numbers, suitable for cryptographic use. Besides the
obvious cryptographic uses, these numbers are also good for seeding TCP
sequence numbers, and other places where it is desirable to have numbers
which are not only random, but hard to predict by an attacker.
Theory of operation
Computers are very predictable devices. Hence it is extremely hard to
produce truly random numbers on a computer -- as opposed to pseudo-random
numbers, which can easily generated by using an algorithm. Unfortu-
nately, it is very easy for attackers to guess the sequence of pseudo-
random number generators, and for some applications this is not accept-
able. So instead, we must try to gather "environmental noise" from the
computer's environment, which must be hard for outside attackers to
observe, and use that to generate random numbers. In a Unix environment,
this is best done from inside the kernel.
Sources of randomness from the environment include inter-keyboard tim-
ings, inter-interrupt timings from some interrupts, and other events
which are both (a) non-deterministic and (b) hard for an outside observer
to measure. Randomness from these sources are added to an "entropy
pool", which is periodically mixed using the MD5 compression function in
CBC mode. As random bytes are mixed into the entropy pool, the routines
keep an estimate of how many bits of randomness have been stored into the
random number generator's internal state.
When random bytes are desired, they are obtained by taking the MD5 hash
of a counter plus the contents of the "entropy pool". The reason for the
MD5 hash is so that we can avoid exposing the internal state of random
number generator. Although the MD5 hash does protect the pool, each ran-
dom byte which is generated from the pool reveals some information which
was derived from the internal state, and thus increases the amount of
information an outside attacker has available to try to make some guesses
about the random number generator's internal state. For this reason, the
routine decreases its internal estimate of how many bits of "true random-
ness" are contained in the entropy pool as it outputs random numbers.
If this estimate goes to zero, the routine can still generate random num-
bers; however it may now be possible for an attacker to analyze the out-
put of the random number generator, and the MD5 algorithm, and thus have
some success in guessing the output of the routine. Phil Karn (who
devised this mechanism of using MD5 plus a counter to extract random num-
bers from an entropy pool) calls this "practical randomness", since in
the worst case this is equivalent to hashing MD5 with a counter and an
undisclosed secret. If MD5 is a strong cryptographic hash, this should
be fairly resistant to attack.
Exported interfaces -- output
There are three exported interfaces; the first is one designed to be used
from within the kernel:
void get_random_bytes(void *buf, int nbytes);
This interface will return the requested number of random bytes, and
place it in the requested buffer.
The two other interfaces are two character devices /dev/random and
/dev/urandom. The /dev/random device is suitable for use when very high
quality randomness is desired (e.g. for key generation), as it will only
return a maximum of the number of bits of randomness (as estimated by the
random number generator) contained in the entropy pool.
The /dev/urandom device does not have this limit, and will return as many
bytes as are requested. As more and more random bytes are requested
without giving time for the entropy pool to recharge, this will result in
lower quality random numbers. For many applications, however, this is
acceptable.
Exported interfaces -- input
The two current exported interfaces for gathering environmental noise
from the devices are:
void add_keyboard_randomness(unsigned char scancode);
void add_interrupt_randomness(int irq);
The first function uses the inter-keypress timing, as well as the scan-
code as random inputs into the "entropy pool".
The second function uses the inter-interrupt timing as random inputs to
the entropy pool. Note that not all interrupts are good sources of ran-
domness! For example, the timer interrupts is not a good choice, because
the periodicity of the interrupts is too regular, and hence predictable
to an attacker. Disk interrupts are a better measure, since the timing
of the disk interrupts are more unpredictable. The routines try to esti-
mate how many bits of randomness a particular interrupt channel offers,
by keeping track of the first and second order deltas in the interrupt
timings.
ACKNOWLEDGEMENTS
The original core code was written by Theodore Ts'o, and was intended for
the Linux platform. This was ported to FreeBSD by Mark Murray, who also
wrote the rndcontrol(8) utility.
Ideas for constructing this random number generator were derived from the
Pretty Good Privacy's random number generator, and from private discus-
sions with Phil Karn. This design has been further modified by myself,
so any flaws are solely my responsibility, and should not be attributed
to the authors of PGP or to Phil.
The code for MD5 transform was taken from Colin Plumb's implementation,
which has been placed in the public domain. The MD5 cryptographic check-
sum was devised by Ronald Rivest, and is documented in RFC 1321, "The MD5
Message Digest Algorithm".
Further background information on this topic may be obtained from RFC
1750, "Randomness Recommendations for Security", by Donald Eastlake,
Steve Crocker, and Jeff Schiller.
SEE ALSO
rndcontrol(8)
FILES
/dev/random
/dev/urandom
HISTORY
The random, urandom files appeared in FreeBSD 2.1.5.
FreeBSD 4.9 October 21, 1995 FreeBSD 4.9
Федотов А.М.
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