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RANDOM(4)            FreeBSD/i386 Kernel Interfaces Manual           RANDOM(4)

     random, urandom -- random number devices

     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

   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

     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.



     The random, urandom files appeared in FreeBSD 2.1.5.

FreeBSD 4.10                   October 21, 1995                   FreeBSD 4.10


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