Subject: [MMUSIC] confsing text in draft-ietf-mmusic-4572-update

From: "Cullen Jennings (fluffy)" <[email-masked]>
Date: Fri, 30 Sep 2016 17:49:40 +0000

It says 

   For each used certificate, an endpoint MUST be able to match at
    least one fingerprint, calculated using the hash function that the
    endpoint supports and considers most secure, with the used
    certificate. If the checked fingerprint does not match the used
    certificate, the endpoint MUST NOT establish the TLS connection. In
    addition, the endpoint MAY also check fingerprints calculated using
    other hash functions that it has received for a match. For each
    hash function checked, one of the received fingerprints calculated
    using the hash function MUST match the used certificate.

But I don't understand what implementations are supposed to do.  Lets say the SDP ha a sha-1 and sha-256 fingerprint and the sha-1 matches the TLS cert but the sha-256 does not. Is the TLS sessions accepted or not? The text in first half a para seems to imply it is not, while the text in the second half seem to imply it is. I think this needs to be cleared up.

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From: Paul Kyzivat <[email-masked]>
Date: Fri, 30 Sep 2016 14:16:58 -0400

> But I don't understand what implementations are supposed to do.  Lets say the SDP ha a sha-1 and sha-256 fingerprint and the sha-1 matches the TLS cert but the sha-256 does not. Is the TLS sessions…

If the recipient understands both sha-1 and sha-256, and considers 
sha-256 stronger, then it will first check that. If it fails, the TLS 
connection is not established. If the sha-256 check succeeds, that could 
be the end of checking. Alternately, the recipient MAY also then check 
sha-1, and if it fails also refuse to establish the connection.

But consider a recipient that doesn't support sha-256. It will then 
check sha-1. If it succeeds, then the TLS connection will be established.

What this does is tell the sender of the fingerprints that it had better 
not send bogus ones, because a stringent recipient might check them all. 
But at the same time it doesn't require recipients to check extra 
fingerprints if they have verified one they are happy with.

	Thanks,
	Paul