Re: [TLS] RSA-PSS in TLS 1.3

"Scott Fluhrer (sfluhrer)" <sfluhrer@cisco.com> Fri, 04 March 2016 13:49 UTC

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From: "Scott Fluhrer (sfluhrer)" <sfluhrer@cisco.com>
To: Nikos Mavrogiannopoulos <nmav@redhat.com>, =?utf-8?B?SGFubm8gQsO2Y2s=?= <hanno@hboeck.de>, "Blumenthal, Uri - 0553 - MITLL" <uri@ll.mit.edu>, "tls@ietf.org" <tls@ietf.org>
Thread-Topic: [TLS] RSA-PSS in TLS 1.3
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Date: Fri, 4 Mar 2016 13:49:11 +0000
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References: <20160303152945.18296912.40009.55386@ll.mit.edu> <20160303171117.12e627b3@pc1> <1457078973.19296.1.camel@redhat.com>
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Subject: Re: [TLS] RSA-PSS in TLS 1.3
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> -----Original Message-----
> From: TLS [mailto:tls-bounces@ietf.org] On Behalf Of Nikos
> Mavrogiannopoulos
> Sent: Friday, March 04, 2016 3:10 AM
> To: Hanno Böck; Blumenthal, Uri - 0553 - MITLL; tls@ietf.org
> Subject: Re: [TLS] RSA-PSS in TLS 1.3
> 
> On Thu, 2016-03-03 at 17:11 +0100, Hanno Böck wrote:
> > It may be worth asking the authors what's their opinion of FDH vs
> > > PSS
> > > in view of the state of the art *today*.
> > You may do that, but I doubt that changes much.
> >
> > I think FDH really is not an option at all here. It may very well be
> > that there are better ways to do RSA-padding, but I don't think that
> > this is viable for TLS 1.3 (and I don't think FDH is better).
> > PSS has an RFC (3447) and has been thoroughly analyzed by research. I
> > think there has been far less analyzing effort towards FDH (or any
> > other construction) and it is not in any way specified in a standards
> > document. If one would want to use FDH or anything else one would imho
> > first have to go through some standardization process (which could be
> > CFRG or NIST or someone else) and call for a thorough analysis of it
> > by the cryptographic community. Which would take at least a couple of
> > years.
> >
> > Given that there probably is no long term future for RSA anyway
> > (people want ECC and postquantum is ahead) I doubt anything else than
> > the primitives we already have in standards will ever be viable.
> 
> On the contrary. If we have a future with quantum computers available, the
> only thing that we have now and would work is RSA with larger keys, not ECC.

RSA isn't *that* much more secure against a Quantum Computer than ECC.  It would appear to take a larger Quantum Computer to break RSA than it would to break ECC (for reasonable moduli/curve sizes), however not that much more.  It is possible that, at one stage, we'll be able to build a QC that's just large enough to break EC curves, but not larger RSA keys - however, we would be likely to be able to scale up our QC to be a bit larger; possibly in a few months, quite likely in a year or two.  Hence, moving back to RSA would appear likely to buy us only a short window...

I agree with Hanno; if we're interested in defending against a Quantum Computer, post Quantum algorithms are the way to go