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SubjectRe: Using Yarrow in /dev/random
   Date: Tue, 12 Sep 2000 09:56:12 +0000
From: Pravir Chandra <pchandra@rstcorp.com>

i agree that the yarrow generator does place some faith on the crypto
cipher and the accumulator uses a hash, but current /dev/random
places faith on a crc and urandom uses a hash.

No, not true. The mixing into the entropy pool uses a twisted LFSR, but
all outputs from the pool (to either /dev/random or /dev/urandom)
filters the output through SHA-1 as a whitener. The key here, though,
and what makes this fundamentally different from yarrow, is that since
we're feeding the entire (large) entropy pool through SHA-1, even if the
SHA-1 algorithm is very badly broken (say as in what's been happening
with MD5), as long as there's sufficient entropy in the pool, the
adversary can define but minimal information about the pool, since the
8192 -> 160 bit transform has to lose information by definition.

i also agree that the entropy pools are small, but the nature of the
hash preserves the amount of entropy that has been uses to create the
state of the pools. basically, if the pool size is 160 bits (hash
output) its state can be built by more than 160 bits of entropy, its
just that adding entropy after that increases the unguessability
(conventional attacks) of the state but brute forcing the state is
still 2^160.

Which is just another way of saying that yarrow is only a cryptographic
random number generator whose maxmum entropy storage is 160 bits.....

- Ted
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