Re: [rtcweb] End-to-end encryption vs end-to-end authentication (DTLS-SRTP / SDES-SRTP)

"Fabio Pietrosanti (naif)" <lists@infosecurity.ch> Thu, 05 April 2012 17:07 UTC

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Date: Thu, 05 Apr 2012 19:07:11 +0200
From: "Fabio Pietrosanti (naif)" <lists@infosecurity.ch>
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Subject: Re: [rtcweb] End-to-end encryption vs end-to-end authentication (DTLS-SRTP / SDES-SRTP)
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On 4/5/12 5:49 PM, Igor Faynberg wrote:
> Fabio,
> 
> I am finding these definitions hard to understand.  I am afraid that
> somehow the activity (encryption) gets mixed with the services
> (confidentiality and authentication).
> 
>  I think your major point is that effective encryption must rely on
> symmetric keys, and that the key distribution is crucial to ensuring
> confidentiality provably.  I agree with that.
> 
> I also agree with sepating the case when session keys are handed out by
> another entity (such as KDC) and the case where the keys are known only
> to the end-points.  The latter can be achieved with signed
> Diffie-Hellman (using PKI, or PAK, or EKE).
> 
> Confidentiality, of course, relies on authentication: If I encrypt
> something with the key I share only with you, I'd better know that I
> share this key ONLY with you.
> 
> But I don't get the point of the taxonomy you propose...

Well, the main point is that DTLS-SRTP claim to provide end-to-end
encryption, however the user is not able to deliver "end-to-end security" .

end-to-end encryption != end-to-end security .

In the encryption technologies landscape, when we refer to end-to-end
encryption we always refer to cryptographic architecture where the users
communicating are guaranteed respect to the underlying transport and
communication architecture.

This bring a misleading concept on DTLS-SRTP in WebRTC because it's
referred as end-to-end encryption, but it doesn't provide end-to-end
security like other encryption technologies does (ZRTP, OpenPGP, OTR, etc).

DTLS-SRTP rely on third party for security verification.

DTLS-SRTP = end-to-site security
SRTP-SDES = end-to-site security
ZRTP = end-to-end security

This means that DTLS-SRTP, from a trust-model point of view, does not
provide end-to-end security because there will always be a trusted third
party able to authorize Man in the Middle to do eavesdropping.

>From that point of view SDES-SRTP provide very similar warranty, except
that you have to trust only 1 third party rather than 2 third party.

Fabio