Re: [TLS] Inclusion of OCB mode in TLS 1.3

Aaron Zauner <> Sat, 24 January 2015 19:31 UTC

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Date: Sat, 24 Jan 2015 20:31:19 +0100
From: Aaron Zauner <>
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Subject: Re: [TLS] Inclusion of OCB mode in TLS 1.3
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Hi Peter,

Peter Gutmann wrote:
> Not necessarily.  When I did my PSK draft I added the discussion of embedded
> systems use mostly as a red herring to deal with the people who complained
> that since we already have PKI in all its infinite wonder and perfection to
> secure TLS, there's no need for anything involving shared secrets.  The
> mention of use in embedded systems was to deal with the PKI zombies, not
> embedded systems.  RFC 4279 doesn't mention embedded systems at all.
>> It would be really good to have someone doing TLS crypto in the embedded
>> world comment on this topic.
> I've got lots of TLS running in embedded systems.  I'm not aware of anything
> currently using pure PSK, it's always DHE with PSK.  To deal with lower-power
> CPUs, you use smaller DH parameters.  Even for older stuff from before it was
> ARM everywhere (with a side order of MIPS, PPC, NIOS, and whatnot everywhere),
> the standard approach was still to use smaller RSA/DH values rather than pure
> PSK.  I support non-PFS PSK, but only after all of the PFS PSK suites, so I
> doubt that ever gets used.

Thanks for your commentary and insight. I find this (personally) very
interesting. As I've pointed out in private correspondence: all the PSK
ciphersuites will stay in the document unchanged. Due to good points
being raised during the discussion for them not to be removed.

> At a pinch you can even do DHE-RSA in an embedded device, you just memcpy()
> out a pre-encoded certificate chain that's generated on install so you don't
> have to have any certificate-processing code present.  Problem is that (a)
> this doesn't give you mutual auth and (b) more importantly certificates
> inherently don't work with embedded devices which can't be identified or
> provisioned in any manner that's expected for certificates.  At best you can
> hardcode in a fixed certificate with a meaningless identifier and infinite
> lifetime at system build time, but that's just going through the motions of
> using certs.

That's a nice trick but as you point out pretty useless in terms of a
PKI. Although I'm pretty sure some vendors will ship uniform client
certificates on devices; that /really/ should not be recommended anywhere :)

I've still gotten no feedback if SRP will still be supported with TLS
1.3. I really think SRP is a valuable protocol but rarely used. I have
not included SRP ciphersuites in my ID so far.