Re: [dispatch] New I-D - SPIN - on voice/video interop between app providers

Brian Rosen <> Fri, 22 July 2022 19:06 UTC

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From: Brian Rosen <>
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Date: Fri, 22 Jul 2022 15:05:59 -0400
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Subject: Re: [dispatch] New I-D - SPIN - on voice/video interop between app providers
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IVC was never about replacing VRS, although it would have significantly changed how it worked if IVC ever became a reality.  There was a clear path forward, and we may yet take it, but I understand the issues of ever getting it adopted.  

I think if you don’t trust the platform vendor, you can’t trust any system that depends on a telephone number.  The fact that an SMS arrives with the TN that purportedly sent it depends completely on the platform correctly sending it and the one correctly receiving it (as well as the network operators).  It makes no sense to me not to trust it.  We HAVE to trust it. 


> On Jul 22, 2022, at 1:08 PM, Richard Shockey <> wrote:
> <SIGH>
> OK btw here is the report the NANC produced on interoperable Video Calling at the request of the Hearing Impaired community to ultimately replace the US Video Relay System.  Of course the FCC never acted on the report. Eric you are correct the issue is ultimately regulatory since control of telephone numbers are fundamentally a nation state issue by statue. In the US BTW is USC 251 (e) 1 
> <>
> More in line.
> From: dispatch < <>> on behalf of Eric Rescorla < <>>
> Date: Friday, July 22, 2022 at 11:21 AM
> To: Jonathan Rosenberg < <>>
> Cc: DISPATCH < <>>
> Subject: Re: [dispatch] New I-D - SPIN - on voice/video interop between app providers
> Thanks for starting this conversation.
> I agree with a number of the the assumptions underlying
> this proposal, specifically:
> - What makes this potentially possible where previous efforts have
>   failed is the force of regulation, specifically the DMA.
> - Forward message routing is the most practical way
>   to establish who is entitled to a specific number.
> However, it seems to me that the specific design you describe
> has a number of suboptimal properties. In particular:
> - It requires the sending and receiving endpoints to be jointly
>   online. This is not unreasonable for voice calling but is
>   undesirable for messaging.
> - It makes the OS vendors certificate authorities, which (a) they may
>   not to be (b) gives users no real choices in their trust decisions
>   (specifically, even if I am an Apple user, I need to trust Android!)
>   and (c) is incompatible with purely open source systems.
> - It requires each individual relying party (caller) to make their own
>   verification, which makes the kinds of transparency mechanisms we
>   ordinarily use to detect impersonation or misissuance/misrouting
>   much more difficult, if not impossible [0].
> It seems to me that there are alternative designs which do not have
> this problem.
> As an intuition pump, consider a system in which we have a single
> central Resolution Service (RS).
> RS>  Been there done that  ☺  
> - When a user installs a communications application on their device,
>   that application contacts the RS, demonstrates control of the relevant
>   number via SMS answerback (i.e., the RS sends them a challenge via
>   SMS) [1]. The application is then able to store a record at the
>   RS with the relevant contact information. If there are multiple
>   applications, there would be multiple records. 
> - The RS issues Alice a credential (e.g., a certificate) which she can
>   use to authenticate ownership of her number.
> - When Alice wants to call Bob, she (or rather the calling/messaging
>   application) looks up Bob's phone number in the registration
>   service, retrieves the appropriate records, and is able to select
>   whichever one is appropriate to complete the communication.  Alice
>   uses her credential to authenticate the call.
> This system addresses most of my objections above. Specifically:
> - It doesn't require the endpoints to be jointly online.
> - It is fully compatible with open source because it doesn't
>   require trusting the OS or OS vendor on the other end.
>   It doesn't give the user choices about who to trust because
>   they have to trust the RS (but see below).
> - It doesn't require online user verification, and so is
>   compatible with Certificate Transparency type systems,
>   audit of the RS, etc.
> I do want to flag one potential privacy issue with this class of
> design, which is that it allows the calling party to determine which
> messaging/calling applications a given user uses.  By contrast, a
> design like the one in SPIN allows for filitering on the receiving
> side (though that doesn't seem to be in the document). I'm not sure
> how big an issue this is, given that you can often join each service
> and then try to connect, but it's not ideal.  I do have some handwavy
> ideas for how to address this (e.g., ACLs uploaded the RS), but
> they're not fully fleshed out. I do think it's possible to address,
> however.
> RS> There are serious privacy issues here in exposing the capabilities of end points.
> Obviously, one giant RS isn't that desirable (although as I understand
> it, this is effectively how Local Number Portability works in the
> NANP). With that said, one view of the current SPIN proposal is that
> it has two big RSes, one run by Apple and one by Google: as described
> in S 5, the originating party has already done effectively the
> registration flow I describe above:
> RS> Yes the NPAC is a big hub and spoke database system that is very secure. Ultimately the NPAC data is cached within carrier networks for more distributed access.   Its also the way nearly every nation state actually does LNP Canada France Brazil etc. There are endless ancillary numbering databases that are also incorporated that address things like Do Not Call, the NANP itself, Reclaimed numbers etc that would have to be incorporated.  Needless to say I get at least one call every 6 months or so about 6116 and I have been aware that there are some lunatics out there that have actually suggested Hyperledger for this kind of thing..
>    There are two ways in which the originating OS can obtain such a
>    certificate.  In one approach, the mobile OS would perform SMS
>    verification (again, invisibly, by absorbing the SMS it sends to
>    itself), and add an additional check of comparing it agaisnt the
>    mobile numnber the user claimed they owned during provisioning time
>    of the device.  The mobile OS vendor would be a valid CA, and then
>    generte a certificate valid for that individual phone number.  In an
>    alternative model, the telco uses certificate delegation [RFC9060],
>    and generates a certificate that is handed to the phone during device
>    provisioning.  The latter approach is more secure in some ways (as it
>    would no longer depend on SMS forward routability for authentication
>    of a user), but is much harder to deploy.
> In fact, one could design something with roughly similar security
> properties to the current draft by simply having Apple and Google
> expose an RS API for the endpoints which had already registered as
> above. The caller could then look up the target number in both Apple
> and Google APIs and skip the forward SMS pieces entirely. This seems
> less desirable than a single RS, but it would have a number of the
> same advantages, such as not requiring both endpoints to be online
> and being compatible with transparency mechanisms.
> With that said, we can also do better than a single central RS.  I
> don't have a complete design, but some thoughts are below.
> First, it seems like authentication and discovery are separate
> services, so we could have multiple CAs for telephone numbers that
> each do SMS verification (a similar structure to the WebPKI) but a
> single directory service. This would allow users (or really client
> applications) to make their own decisions about who to trust.
> RS>  I would agree with this. 
> One could also imagine having multiple RSes which stored phone number
> records as long as there was some mechanism for determining which RS
> had a given number. That mapping could then be on a single service or
> just replicated to each application vendor (it's really not that
> big). This would allow a diversity of RSs but with a single central
> reference point so the originating party wouldn't need to poll all of
> them.
> At any rate, I think this type of architecture is worth considering
> as an alternative to the design in this specification.
> -Ekr
> [0] As an example of this point, consider a nation-state attacker who
> controls the PSTN and wishes to covertly intercept Alice and Bob's
> communications: it reroutes the SMS messages from their communication
> and then completes the call itself. In the analogous context in the
> WebPKI, this creates a record in the CT log which can then be
> detected, but that is not the case here.
> [1] This might require some OS affordances, but I don't think
> they would be that hard to design.
> On Tue, Jul 12, 2022 at 7:13 AM Jonathan Rosenberg < <>> wrote:
>> Hi fellow dispatchers - 
>> I wanted to call attention to the following draft submitted yesterday:
>> <>
>> Abstract:
>> This document introduces a framework and a protocol for facilitating
>>    voice, video and messaging interoperability between application
>>    providers.  This work is motivated by the recent passage of
>>    regulation in the European Union - the Digital Markets Act (DMA) -
>>    which, amongst many other provisions, requires that vendors of
>>    applications with a large number of users enable interoperability
>>    with applications made by other vendors.  While such interoperability
>>    is broadly present within the public switched telephone network, it
>>    is not yet commonplace between over-the-top applications, such as
>>    Facetime, WhatsApp, and Facebook Messenger.  This document
>>    specifically defines the Simple Protocol for Inviting Numbers (SPIN)
>>    which is used to deliver invitations to mobile phone numbers that can
>>    bootstrap subsequent communications over the Internet.
>> Right now, we're looking to see if there is interest in working on this. Comments welcome.
>> Thx,
>> Jonathan R.
>> -- 
>> Jonathan Rosenberg, Ph.D.
>> <>
>> <>
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