Re: [rtcweb] draft-ietf-rtcweb-security-arch: Questions on SDP identity attribute

[Christer Holmberg <[e-mail]>]

Resending, as it seems like my reply to Cullen yesterday got stucked somewhere on the Internet.

Regards,

Christer

From: Christer Holmberg <[e-mail]<mailto:[e-mail]>>
Date: Thursday 17 May 2018 at 21:03
To: Cullen Jennings <[e-mail]<mailto:[e-mail]>>
Cc: "[e-mail]<mailto:[e-mail]>" <[e-mail]<mailto:[e-mail]>>
Subject: RE: [rtcweb] draft-ietf-rtcweb-security-arch: Questions on SDP identity attribute


Hi,

A few questions/comments on the SDP identity attribute.

Q1:
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The draft says that, at minimum, the fingerprint needs to be bound to the identity.

Does this mean that, in order to include an SDP identity attribute in an offer/answer, the offer/answer MUST also contain at least one SDP fingerprint attribute? Based on the text about generating the identity it seems like that, but it is not very clear in the SDP procedures.


JSEP requires DTLS/SRTP which requires adding the fingerprint so I think this is already required. Also JSEP points out lack of fingerprint will cause negotiation failure.

This is not about JSEP, but the definition of the SDP identity attribute. As this attribute can (I assume) be used on the wire, it needs to defined as such.

The USAGE of the attribute can then be scoped to WebRTC, JSEP, or whatever.


Q2:
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Section 5.6.4.1 says:

   "The "a=identity" attribute MUST include all

   fingerprint values that are included in "a=fingerprint" lines."

First, related to the generation of the assertion value, it is unclear how this is implemented. For example, is there a separate JSON object (section 5.6.4) provided to the IdP for each fingerprint? The text talks about “single” fingerprint in the object. Or, are all fingerprints included in the same JSON object?

Yah, I think it would get a Identity token for each fingerprint via a Generate Assertion for each fingerprint. But I think that is largely WebRTC issue.

While it may be an WebRTC issue how to use multiple fingerprints in general, it is an SDP issue how to encode the attribute.


Second, it is unclear what “attribute MUST include all fingerprint values” means. I assume that the attribute will only include a single attribute assertion value, but that the value will be generated by the IdP based on all fingerprint values?

So if there were multiple fingerprint, the SDP would have multiple Identity lines.


In that case I don’t understand the statement in section 5.6.4.2, that says:

   “The semantics of multiple identity attributes are undefined.”

Also, to me “attribute MUST include all fingerprint values” sounds like a single attribute with multiple fingerprint values.



Q3:
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In the example in Section 5.6.4.1 the SDP fingerprint attribute is included as a session-level attribute. However, it is a media-level attribute. AFAIR, media-level attributes cannot be included as session-level attributes.


I think fingerprint is valid as both session-level or media-level so I suspect this is fine. JSEP also says that if the certs are the same, then it can be session level but if different then must be media level. So I think this is covered in JSEP.

JSEP does not define the attribute. If the attribute can be used both on session- and media-level it needs to be defined in such way in draft-ietf-rtcpweb-security-arch.

Regards,

Christer