Re: [TLS] New Algorithm identifier for EDH > 1024 bits?

mrex@sap.com (Martin Rex) Mon, 30 September 2013 19:28 UTC

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To: Peter Gutmann <pgut001@cs.auckland.ac.nz>
Date: Mon, 30 Sep 2013 21:28:05 +0200 (CEST)
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From: mrex@sap.com (Martin Rex)
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Subject: Re: [TLS] New Algorithm identifier for EDH > 1024 bits?
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Peter Gutmann wrote:
> Phillip Hallam-Baker <hallam@gmail.com> writes:
> 
> >My understanding of the 1024 bit Ephemeral DH key issue is that it is not
> >currently possible to use longer keys because a certain number of deployed
> >Web servers will abort the connection if a client presents a longer key.
> >
> >Hmmm, wonder who made that decision...
> 
> Could that be because of all of the TLS_DHE_DSS_* suites, with DSS limited to
> 1024 bits so implementers also limited the matching DH to 1024 bits?  Just
> wondering what other reason there could possibly be for artificially limiting
> the size to 1024 bits.

The delay incurred by key generation maybe?

The app-supplied credentials are either RSA or DSA.  So for DHE,
_everything_ has to be newly generated after process start and potentially
in-band during the TLS handshake, unless the application caller has
written some code that does generation out-of-band and installs/updates
the DHE keying material regularly.

-Martin