/dev/random vs. /dev/urandom and the krb5 test suite

ghudson@MIT.EDU ghudson at MIT.EDU
Thu Jun 18 00:48:28 EDT 2009

The krb5 test suite generates a lot of random keying material.  Under
the assumption that cryptographic keys require "strong" randomness,
most of that material comes from /dev/random.  This can create stalls
in some environments; for instance, my Linux box spends a few extra
minutes per testing run if I don't use a local hack to make lib/crypto
use /dev/urandom instead.

Some other developers have run into this recently as well, so we may
do something about it.  An obvious choice is to add a context flag and
krb5.conf setting to always use /dev/urandom in favor of /dev/random.

My question is whether to put big red flashing lights around the
option name or not.  It's possible that some high-load production
environments are seeing /dev/random stalls as well, although I have no
particular reason to believe this is so.  If we use a relatively
innocuous name, people might legitimately find the option useful for
such cases--or people might set it out of ignorance when it's not
needed and could cause harm.

If the system PRNG is weak, or if it has been seeded with very little
true random data (say, 64 bits or less), then using /dev/urandom would
be disastrous for a system's security.  However, both cases are a bit
far-fetched today in my opinion.  What I'd like input on is whether
there are practical risks in the expected cases of using /dev/urandom
for keys.  For example, say you generate a hundred 128-bit keys with
your system's /dev/urandom when there are 600 bits of true randomness
in your system's pool, instead of using 12800 bits of "true"
randomness from /dev/random as one might like.  An attacker can
compromise all 12800 bits of keying material with a 2^600 workload, at
least in theory, but this is not a very interesting attack, especially
since one of those 128-bit keys is probably of higher value (like your
TGS long-term key) and would be more interesting to break in
isolation.  Are there other attacks of interest in this scenario?

Sorry if this is a little long-winded; I'm just trying to choose
between a judgemental name like "insecure_random" or something more
bland like "always_use_urandom" for the override.  The default will
certainly remain the way it is.

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