Re: [TLS] New cipher suites for SRP

Hubert Kario <hkario@redhat.com> Mon, 29 June 2015 11:31 UTC

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From: Hubert Kario <hkario@redhat.com>
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Date: Mon, 29 Jun 2015 13:31:18 +0200
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Cc: Geoffrey Keating <geoffk@geoffk.org>
Subject: Re: [TLS] New cipher suites for SRP
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On Friday 26 June 2015 20:39:24 Geoffrey Keating wrote:
> Dave Garrett <davemgarrett@gmail.com> writes:
> > On Friday, June 26, 2015 07:48:01 pm Attila Molnar wrote:
> > > Currently SRP cannot be used with newer crypto primitives such as
> > > ciphers in AEAD mode or SHA-2 due to the lack of cipher suites enabling
> > > these. There's only 3DES and AES-CBC with SHA-1.
> > > 
> > > Would there be support for expanding the SRP cipher suites?
> > 
> > I don't think it's a good idea to add new SRP cipher suites.
> > 
> > Instead, I think redefining SRP as an extension to PSK would make more
> > sense. Use (EC)DHE_PSK cipher suites with an updated SRP extension to get
> > similar capabilities. This would make updating SRP to use newer crypto
> > much easier, as modern PSK cipher suites are easier to get standardized.
> > The current SRP spec actually already appears to rely on PSK identity
> > alert codes.
> The problem with that is that there are surely many use cases where
> you're willing to do SRP, or if no SRP you can do a regular ECDHE and
> prompt for the username/password, but PSK is too insecure.
> 
> I've been thinking an improved SRP would be useful.  It should:
> 
> - Specify Modern cipher and hash algorithms as mentioned above
> 
> - Replace the existing SHA1+seed with a password whitening function
>   like PBKDF2, so that in the event of a compromised server cracking
>   the password is harder, and also making online password guessing
>   attacks (sending lots of username+password pairs to the server) more
>   expensive*
> 
> - Deprecate the 1024-bit and the 1536-bit group, and the previous SRP
>   ciphersuites; and say that these should only be chosen if the server
>   has a legacy verifier for a particular username which requires
>   them.

+1, provided we do two more things:

 - Change the negotiation so that user name is not exchanged in the clear
 - Change key exchange to do PFS

-- 
Regards,
Hubert Kario
Quality Engineer, QE BaseOS Security team
Web: www.cz.redhat.com
Red Hat Czech s.r.o., Purky┼łova 99/71, 612 45, Brno, Czech Republic