Re: [MMUSIC] WGLC on draft-ietf-mmusic-dtls-sdp-06.txt

Martin Thomson <martin.thomson@gmail.com> Wed, 24 February 2016 22:01 UTC

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Date: Wed, 24 Feb 2016 14:01:54 -0800
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From: Martin Thomson <martin.thomson@gmail.com>
To: Paul Kyzivat <pkyzivat@alum.mit.edu>
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Cc: Jonathan Lennox <jonathan@vidyo.com>, "mmusic@ietf.org" <mmusic@ietf.org>, Christer Holmberg <christer.holmberg@ericsson.com>
Subject: Re: [MMUSIC] WGLC on draft-ietf-mmusic-dtls-sdp-06.txt
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On 24 February 2016 at 13:47, Paul Kyzivat <pkyzivat@alum.mit.edu> wrote:
> If we are going to make normative references to multiple fingerprints we
> need some place authoritative to reference for the details.

I recommend judicious use of copy and paste.  Feel free to steal text
that I wrote.

FWIW, my reading of 4572 is that it permits multiple fingerprints: one
for each certificate that might be used.  Its failing is that a) it
doesn't allow for hash agility, and b) it's unclear, perhaps in the
extreme.

However, I see this:

   Endpoints MUST support SHA-256 for generating and verifying the
   fingerprint value associated with the DTLS association.  The use of
   SHA-256 is preferred.

Which doesn't mention that this requires an update of 4572.

And this:

   The certificate received during the DTLS handshake MUST match the
   fingerprint received in the SDP "fingerprint" attribute.

Which should say '*a* fingerprint', to allow for there being multiple.

And this:

   [...]  In addition, the offerer
   MUST insert an SDP 'setup' attribute according to the procedures in
   [RFC4145], and an SDP 'fingerprint' attribute according to the
   procedures in [RFC4572], in the offer.

Which doesn't deal with multiple certificates.

I also see:

   The
   subjectAltName is not an important component of the certificate
   verification.

Which is true but insufficient; the text can simply say that the
certificate is only a receptacle for a public key and authentication
is tied to an a=fingerprint line in the SDP.

And this:

   This offer includes, as part of the SDP payload, the fingerprint of
   the certificate that the endpoint wants to use.  The endpoint SHOULD

Which should be plural fingerprint*s*.

The pattern repeats throughout.