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[ To: Perry's Crypto List, James, Ben, Bram ##
Date: 07/25/99 ##
Subject: Re: depleting the random number generator ]
>Date: Sun, 25 Jul 1999 11:01:00 -0400
>To: "James A. Donald" <[EMAIL PROTECTED]>, Ben Laurie
> <[EMAIL PROTECTED]>, bram <[EMAIL PROTECTED]>
>From: "Arnold G. Reinhold" <[EMAIL PROTECTED]>
>Subject: Re: depleting the random number generator
>Cc: cryptography <[EMAIL PROTECTED]>
>One nice advantage of using RC4 as a nonce generator is that
>you can easily switch back and forth between key setup and
>code byte generation. You can even do both at the same time.
>(There is no need to reset the index variables.) This allows
>you to intersperse entropy deposits and withdrawals at will.
Has anyone looked at this from a cryptanalytic point of
view? I think there are chosen-input attacks available if
you do this in the straightforward way. That is, if I get
control over some of your inputs, I may be able to alternate
looking at your outputs and sending in new inputs, and mount
an attack that isn't possible at all against RC4 as it's
normally used. (This comes out of conversations with Jon
Callas, Dave Wagner, and Niels Ferguson, from a time when I
considered designing a Yarrow-variant using RC4 as the
underlying engine.)
>In particular, if you deposit the time of each entropy
>withdrawal, the proposed denial of service attack that
>started this thread would actually replenish a few bits of
>entropy with each service request.
This isn't a bad idea, but I'd be careful about assuming
that those times hold much entropy. After all, a given
piece of code which has thirty calls to the PRNG probably
runs in about the same amount of time every time, barring
disk or network I/O.
>Arnold Reinhold
- --John Kelsey, Counterpane Internet Security, [EMAIL PROTECTED]
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