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SubjectRe: [PATCH]
On Mon, Aug 09, 2004 at 02:43:24PM -0400, Theodore Ts'o wrote:
> On Sun, Aug 08, 2004 at 11:38:46AM -0400, Jean-Luc Cooke wrote:
> > In our paper (I'm testing the patch now) we'll be proposing using the Fortuna
> > PRNG inplace of the current design. It only uses SHA256 and AES256 (or any
> > message digest & block cipher combo). The primary advantages of this design
> > would be:
> > - It's simpler
> > - It's faster
> > - It doesn't "rool your own" crypto
> SHA is not going to be faster than the cut-down MD4 --- and you can't
> use a pure random sequence number for the initial TCP sequence number,
> lest a packet from a previous TCP connection get mistaken as belong to
> the newly created TCP stream. See Bellovin's recommendations for
> secure TCP sequnce number genreation for a discussion of the
> constraints.
> > If you pass all input data into a Yarrow-type PRNG - like the Fortuna PRNG
> > we're going to propose - you will never care about this since the current
> > state of the pools are always based on all the previous input.
> The Yarrow-type PRNG suffers from the problem that the entropy pool is
> pathetically small. It fundamentally assumes that the crypto checksum
> is secure, and it is really much more of a *P*RNG than anything else.
> The scheme used in the current /dev/random design is much closer to
> that used by GPG, and relies on a large pool so that we can collect as
> much entropy as possible from the hardware sources available to the
> kernel. I'm not familiar with the Fortuna PRNG that you're going to
> propose, but my guess is that it will have a similar, much heavier
> dependence on the crypto algorithms in terms of its assumptions.

Fortuna overcomes the small pool size problem. For the most part, I really
like how the current random.c collects data.

The only parts we're proposing to replace are:
- The pooling (thus, mixing) mechanizums as per in Fortuna by Schnier and
- The random data output mechanizum (Fortuna)
- TCP sequence number generation (AES in CTR mode truncated to 32 bits, has
period of 2^32 and cannot be easily determined from pervious sequences)
I will read your reference and learn from the implementation in random.c
- TCP SYNcookies to encrypt 8bit "count" value and the 24bit "data" (MSS)
value with AES in CTR mode. I can prove that this is exactly what is
being done now with the "two SHA1 hashes and a bunch of addition" method.

Crutial to my proposal will be not "not rock the boat". That is, keeping all
important interfaces intact to not break anything else in the kernel.

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