Telechat Review of draft-ietf-perc-private-media-framework-10
review-ietf-perc-private-media-framework-10-secdir-telechat-roca-2019-05-16-00

Request Review of draft-ietf-perc-private-media-framework
Requested rev. no specific revision (document currently at 12)
Type Telechat Review
Team Security Area Directorate (secdir)
Deadline 2019-03-05
Requested 2019-02-22
Draft last updated 2019-05-16
Completed reviews Secdir Last Call review of -08 by Vincent Roca (diff)
Genart Last Call review of -08 by Linda Dunbar (diff)
Tsvart Last Call review of -08 by Gorry Fairhurst (diff)
Secdir Telechat review of -10 by Vincent Roca (diff)
Genart Telechat review of -09 by Linda Dunbar (diff)
Assignment Reviewer Vincent Roca
State Completed
Review review-ietf-perc-private-media-framework-10-secdir-telechat-roca-2019-05-16
Posted at https://mailarchive.ietf.org/arch/msg/secdir/obAGHwP651Rcp3AfcGKeWBCFy8s
Reviewed rev. 10 (document currently at 12)
Review result Ready
Review completed: 2019-05-16

Review
review-ietf-perc-private-media-framework-10-secdir-telechat-roca-2019-05-16

Hello Paul,

Okay, I now get it. I was not considering an attacker stupid enough to fail authentication but
who would continue sending trafic with the same identifiers. So I’m okay with your explanations
and new version of the paragraph.

Summary: Ready

Thanks,

  Vincent


> Le 14 mai 2019 à 02:21, Paul E. Jones <paulej@packetizer.com> a écrit :
> 
> Vincent,
> 
> Please see inline below:
> 
> ------ Original Message ------
> From: "Vincent Roca" <vincent.roca@inria.fr <mailto:vincent.roca@inria.fr>>
> To: "Paul E. Jones" <paulej@packetizer.com <mailto:paulej@packetizer.com>>
> Cc: "Vincent Roca" <vincent.roca@inria.fr <mailto:vincent.roca@inria.fr>>; "The IESG" <iesg@ietf.org <mailto:iesg@ietf.org>>; "David Benham" <dabenham@gmail.com <mailto:dabenham@gmail.com>>; draft-ietf-perc-private-media-framework.all@ietf.org <mailto:draft-ietf-perc-private-media-framework.all@ietf.org>; aamelnikov@fastmail.fm <mailto:aamelnikov@fastmail.fm>; secdir@ietf.org <mailto:secdir@ietf.org>
> Sent: 5/13/2019 5:47:16 AM
> Subject: Re: Secdir last call review of draft-ietf-perc-private-media-framework
> 
>> Hello Paul, all,
>> 
>> 
>>> Le 12 mai 2019 à 06:34, Paul E. Jones <paulej@packetizer.com <mailto:paulej@packetizer.com>> a écrit :
>>> 
>>> Vincent,
>>> 
>>> Once again, thanks for the feedback.  Please allow me to reply inline and, if you're OK with the text, I can prepare a new revision.
>>> 
>>>> ** In section 8.1 it is said:
>>>> 
>>>>    "While confidentiality
>>>>    would not be compromised by failing to implement mutual
>>>>    authentication, employing it helps mitigate against denial of service
>>>>    attacks wherein a false entity sends a stream of packets that the
>>>>    would force a legitimate entity to spend time attempting to decrypt."
>>>> 
>>>> This is true only if authenticating a received packet is cheap, which is
>>>> not necessarily the case. And in section 5 « Authentication » you say that
>>>> "details of this are outside the scope of specification", so we are not
>>>> able to answer the above question: is authenticated really so cheap?
>>>> With certain authenticated encryption technics (e.g. MAC-then-Encrypt),
>>>> decrypting is required before checking data authenticity… So please
>>>> clarify.
>>> 
>>> Actually, after reading that original text, I think it is even misleading to suggest confidentiality is not compromised.  In terms of encryption, that would be true, but in terms of net effect, it would not: without verifying the certificate, an unwanted party could potentially engage in the conference.  I think this would be better text (replacing the entire paragraph):
>>> 
>>> Use of mutual DTLS authentication (as required by DTLS-SRTP) ensures that a
>>> false endpoint or false Media Distributor cannot interact with a legitimate
>>> Media Distributor or endpoint.  This helps mitigate against denial of service
>>> attacks wherein a false entity sends a stream of packets that would force
>>> a legitimate entity to spend time attempting to decrypt.
>>> 
>>> With respect to whether the DoS mitigation is true or not, I think it is.  If mutual authentication fails, then no media packets would flow at all, thus no wasted time decrypting packets; packets would likely get rejected without inspection.  If mutual authentication succeeds, then the cost isn't at issue here, anyway, since the point of the paragraph is to talk about a side benefit of mutual authentication.  While the specifics are outside the scope of the document, the assumption is that some mechanism is employed to enable checking the validity of certificates.
>> 
>> [VR] Yes for the first aspect.
>> 
>> Concerning DoS mitigation, wether DTLS authentication helps reducing decryption overhead at a receiver
>> really depends on how authentication is achieved. Looking at RFC 5764 (I imagine this is where DTLS-SRTP
>> is defined), I read in Section 5.1.2. « Reception », item 3., that decryption and authentication are performed
>> at the same step. This is in line, although no technical detail is provided, with my comment on authenticated
>> encryption (see also https://en.wikipedia.org/wiki/Authenticated_encryption <https://en.wikipedia.org/wiki/Authenticated_encryption>). In some cases, decrypting is 
>> required before checking data authenticity, so authentication won’t be of any help if your goal is to avoid
>> « spend[ing] time attempting to decrypt », what you're saying. 
>> RFC 5764, Section 7.4. « Decryption Cost », discusses decryption cost but does not presents authentication
>> as a solution to mitigate it.
>> 
>> What you’re saying in your answer, above (whether or not mutual authentication fails) is something else.
>> In section 8.1 we are considering 3rd party attacks, from entities that will never be authenticated.
> 
> This paragraph does deal with third parties, so I think it's appropriately placed. But clearly the wording still isn't quite right, as it's not conveying the intended meaning.  By using mutual authentication, a receiving endpoint would know if the sender is or is not a valid sender.  There is no need to authenticate the media packets received, because the DTLS handshake failed.  Receivers could just discard those packets without inspection.  So, the receiver never even gets to the point where it's dealing with the authenticated decryption of packets.
> 
> I revised that paragraph again.  Here is the new text
> 
> 
> Use of mutual DTLS authentication (as required by DTLS-SRTP) also helps to
> prevent a denial-of-service attack by preventing a false endpoint or false
> Media Distributor from successfully participating as a perceived valid media
> sender that could otherwise carry out an on-path attack.  When mutual
> authentication fails, a receiving endpoint would know that it could safely
> discard media packets received from the endpoint without inspection.
> 
> 
> How is that?
> 
> This says nothing about the other forms of attacks, such as a "false" endpoint sending bogus media packets to a receiver that carry the IP address of a valid sender.  In that case, the receiver would have no way of knowing if the packet is valid or not until it goes through the authenticated decryption process.  PERC (by its use of HBH authentication) would allow the receiver to detect such bogus packets, but that is a valid DoS attack vector addressed in prior paragraphs in that section.
> 
> Paul
> 
>