combining entropy
Jack Lloyd
lloyd at randombit.net
Fri Oct 24 16:12:32 EDT 2008
On Fri, Oct 24, 2008 at 10:23:07AM -0500, Thierry Moreau wrote:
> Do you really trust that no single source of entropy can have knowledge of
> the other source's output, so it can surreptitiously correlate its own?
>
> I.e, you are are also assuming that these sources are *independent*.
I do not think one means the other here.
An omniscient malicious RNG source seems quite unlikely in most threat
models. However that is a very different statement from saying that
lacking such an attacker, you can safely assume your 'pools of
entropy' (to quote the original question) are independent in the
information-theoretic sense.
Say you execute (on a Linux machine) two commands, like ifconfig -a
and netstat -s (which print ASCII text with statistics about network
interfaces and network protocols, resp), capturing the output as two
of your entropy sources.
Both have some amount of entropy (perhaps zero if an attacker is on
the machine and runs his commands at the same time as yours - and
perhaps quite a bit more if the local machine happens to be safe). But
they are certainly not statistically independent! Information in one
will be somewhat reflected in the other (packet counters), and of
course at the macro level all your inputs have high bit unset, so if
you combined via XOR your output will have at best .875 bits of
entropy per bit.
To address IanG's question more directly, my first thought would be to
use something like the design Hugo Krawczyk describes in "On
Extract-then-Expand Key Derivation Functions and an HMAC-based KDF"
(http://www.ee.technion.ac.il/~hugo/kdf/kdf.pdf) or one of the related
PRNG designs he references. Then use the output of the HMAC PRF to
feed the DT vector of an X9.31 PRNG (using block cipher du jour), a
trick AFAIK invented by Peter Gutmann which has always seemed like a
good worst-case-scenario trick to me (for instance, if the code for
the hash's compression function is miscompiled), though at the cost of
extra code/design complexity (and thus points of failure) - as always
there are tradeoffs to make.
-Jack (IANAC)
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