[TLS] Antw: Re: Truncated HMAC: what to do with the MAC key?

"Andreas Walz" <andreas.walz@hs-offenburg.de> Sat, 08 July 2017 13:55 UTC

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Date: Sat, 08 Jul 2017 15:54:40 +0200
From: "Andreas Walz" <andreas.walz@hs-offenburg.de>
To: <tls@ietf.org>
References: <595F99DA020000AC00136830@gwia2.rz.hs-offenburg.de> <1499488687918.75643@cs.auckland.ac.nz> <201707080115.17663.davemgarrett@gmail.com>
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Subject: [TLS] Antw: Re: Truncated HMAC: what to do with the MAC key?
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Thanks for your feedback.

One other thing I could image is that truncated_hmac is mostly used in closed systems where one and the same implementation is used on both sides. 

@Peter: We are developing software for Smart Metering in Germany where TLS is used over the (wireless) Metering Bus. The corresponding specification [1] says about truncated_hmac: "servers shall support...".


[1] https://www.bsi.bund.de/SharedDocs/Downloads/DE/BSI/Publikationen/TechnischeRichtlinien/TR03109/TR-03109-1_Anlage_Feinspezifikation_Drahtlose_LMN-Schnittstelle-Teil2.pdf?__blob=publicationFile&v=1

>>> Dave Garrett <davemgarrett@gmail.com>; 08.07.17 7.15 Uhr >>>
On Saturday, July 08, 2017 12:38:18 am Peter Gutmann wrote:
> Andreas Walz <andreas.walz@hs-offenburg.de>; writes:
> >different TLS implementations do not seem to agree on how to implement
> >truncated HMAC
> It also says something about the status of this capability if three of the
> four known implementations can't interoperate.  If it's taken fourteen years
> (RFC 3546 was 2003) for someone to notice that the implementations don't
> work/interoperate then maybe the capability should be marked as deprecated or
> obsolete or unused or something.

In progress; the Truncated HMAC TLS extension is prohibited in implementations that support TLS 1.3+ as of the current draft.