[Pkg-gnutls-maint] Help with exim4 #390712, interaction with mobile phones

[James Westby]

On (21/12/06 15:14), Marc Haber wrote:
> On Wed, Dec 13, 2006 at 06:46:00PM +0000, James Westby wrote:
> > It looks that way. As there is no way to use AES without SHA1
> 
> so AES without SHA1 is not standard compliant? That's an explanation
> why we fall back to RC4 when SHA1 is forbidden.

I meant in this case, sorry. I don't know the whole list of supported
pairs, but the phone and GnuTLS obviously can't negotiate AES with
another MAC.

<looks up in OpenSSL man page ciphers(1ssl)>

Yes, that doesn't list AES with anything but SHA.

> > I fear though that for etch users of these phones are going to have to
> > find a workaround for the problem. Assuming that there is a bug in
> > GnuTLS there's no guarantee we can find it, let alone fix it, before the
> > release.
> 
> I understand that. Unfortunately, both known workarounds (OpenSSL and
> forbidding SHA1 via source code modification) mean rebuilding exim.

Yes that's true. Maybe its something the users can do locally, but
obviously that has problems and is far from ideal, so hopefully we can
find another workaround.

> > I think the only debugging that we can do from here is to verify the
> > MACs and then the plaintext/ciphertext pairs of one of the sessions.
> > This is going to be a bit of work, but I'll look in to setting up a
> > debug copy of the library soon.
> > 
> > There's one more thing that I have been meaning to mention wireshark
> > (ethereal) has some support for watching SSL handshakes. It might be
> > worth checking that it's idea of what is going on is the same as
> > GnuTLS'. It doesn't tell us what the phone thinks, but it is a quick
> > sanity check.
> 
> I have sent you tcpdumps in private mail of one successful connection
> and one failed connection. In my opinion, the wireshark analysis of
> these dump is quite inconclusive, but you know much more about TLS
> than I do.

Thanks for those. I will try and look at them soon. I didn't expect they
would show much, but I thought it was worth a look.

> > Thanks for your help,
> 
> I appreciate your efforts and thank you for leading me through the
> debugging process, which has been so far a great experience for me and
> has greatly improved my knowledge of the tools.

It has greatly increased my knowledge as well. I am going to draft a
mail to upstream now to see if they have any bright ideas, but I don't
think there will be a quick response at this time, and I wont be around
for a few days to work with them if they do respond.

Thanks,

James

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  James Westby   --    GPG Key ID: B577FE13    --     http://jameswestby.net/
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