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SubjectRe: [RFC][PATCH] Make cryptoapi non-optional?
Jamie Lokier  wrote:
>If you return xy, you are returning a strong digest of the pool state.
>Even with the backtrack-prevention, if the attacker reads 20 bytes
>from /dev/random and sees a _recognised_ pattern, they immediately
>know the entire state of the secondary pool.

Irrelevant. I think you missed something.

If you pick a pattern in advance, the chance that your pattern appears
at the output of SHA1 is about 2^-160 (assuming SHA1 is secure). If you
pick 2^50 patterns (that takes an awfully big RAID array to store them
all!), then the chance that your pattent appears at the output of SHA1
is 2^-110. If you pick 2^50 patterns and poll /dev/urandom 2^50 times
to get 2^50 outputs, the chance that one of your patterns appears as one
of the /dev/urandom outputs is only 2^-60. In other words, your attack
has a success probability that is truly negligible.

You might as well argue that "if a cosmic ray hits memory and turns off
/dev/urandom, then things will fail". This is such an unlikely event
that we all ignore it. Likewise, the risk of special patterns appearing
at the output of SHA1 is also so unlikely that it can also be ignored.
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