Re: [Privacy-pass] Updated WG charter text

[Alex Davidson <[e-mail]>]

It has been mentioned off-list that the charter would benefit from having explicit milestones laid out in the document. To this end, here are the key tasks that need to be completed by the WG (in my opinion):

- Specification of protocol & protocol architecture.
- Specification of HTTP API (in coordination with W3C).
- Formal analysis of protocol.
- Concrete implementations.
- Any protocol extensions.

I think getting the first two milestones done for the core instantiation of the protocol (currently documented) will be important in facilitating the three steps afterwards. The final three steps can be done in parallel, since they do not depend on each other. In terms of a rough timeline, I would expect that achieving the first step would be doable by Spring 2021, and then the HTTP API could follow in mid-2021. The rest of the steps are slightly variable, but I would expect that the rest of the work could completed in a further 6-12 months.

Questions: Do these milestones reflect the scope of required work to everyone else? Does the proposed timeline make sense?

Next steps: Once we have agreement (/if no-one disagrees with what is written), then I’ll add it to the charter. We will then take this version of the charter forward in the WG formation process.

> On 28 Apr 2020, at 11:47, Alex Davidson <[e-mail]> wrote:
> 
> Thanks all for the feedback! I’ve pushed updated wording to the GitHub PR (https://github.com/alxdavids/privacy-pass-ietf/pull/12/commits/e987d2ba60fa7b0ba986a65c29214ce893d7b667 <https://github.com/alxdavids/privacy-pass-ietf/pull/12/commits/e987d2ba60fa7b0ba986a65c29214ce893d7b667>) and also added it at the bottom of this email. I’ve inlined some responses to the specific points explaining the changes below.
> 
>> On 23 Apr 2020, at 17:52, Christopher Wood <[e-mail]> wrote:
>> 
>> On Wed, Apr 22, 2020, at 5:32 PM, Martin Thomson wrote:
>>>> First, specify an extensible protocol for creating and redeeming
>>>> anonymous and transferrable tokens. The protocol should permit suitable
>>>> cryptographic ciphersuites and security parameterization for
>>>> cryptographic agility. Negotiation of cryptographic parameters is an
>>>> application-specific property and thus out of scope for the Working
>>>> Group. Specification of the underlying cryptographic algorithms or
>>> 
>>> This bit about choosing parameters seems to be in tension with the 
>>> previous sentence.  One of the problems with a system like this is 
>>> coordinating algorithm changes.  I think that we need a plan, not a big 
>>> punt.
>> 
>> I'm thinking about this more like the DoH effort, which focused first and foremost on the protocol itself and punted other concerns such as discovery and whatnot. 
> 
> Due to the restrictions on key material updates for client privacy, the ciphersuite is determined explicitly by the key that the server is currently using. Moreover, the supported ciphersuites should be specified by the underlying cryptographic protocol that is being used. Therefore, I think the choice of ciphersuite is a unilateral decision made by the server, and so the negotiation framework does not really belong to the protocol description.
> 
>>>> protocols is also out of scope. The Working Group will specify a
>>>> preliminary set of extensions, including Issuer-supplied metadata and
>>>> public verifiability, as well as any additional extensions that may
>>> 
>>> What does "public verifiability" mean here?
>> 
>> I think it means that someone beyond the party which issued the token (and holds the OPRF private key) can verify the authenticity of a token. As an example, blind signatures have this property. 
> 
> Yes, this is what we were alluding to. I’ve updated the wording to be more explicit on this.
> 
>> 
>>>> 1. Describing use cases and interfaces that allow the protocol to be
>>>> used for those use cases.
>>>> 2. Defining the privacy goals for each Client during protocol execution,
>>>> along with expectations placed on the Issuers and the ecosystem at
>>>> large.
>>>> 3. Describing parameterizations that control the Client privacy budget
>>>> and Issuer security parameters.
>>> 
>>> What is a "privacy budget"?  To be clear, I think that I know, and my 
>>> opinion is that it is not a good framing for this.  I also think that 
>>> it is not a good reference model to use in general.
> 
> This makes sense, I’ve changed the wording to directly refer to the size of the client’s anonymity set, which is what we were implicitly referring to here and is also used elsewhere in the charter.
> 
>>>> 4. Describing verification mechanisms for sanctioning or trusting
>>>> Issuers and their corresponding keying material.
>>> 
>>> Sanctioning implies punishment, which we can't write a protocol for.
>> 
>> That's a good point. Maybe we should just say, "Describing verification mechanisms for trusting Issuers and their corresponding keying material”?
> 
> Agree, incorporated this wording.
> 
>>> If this is about providing technical safeguards against the potential 
>>> for issuers to abuse key management for a covert channel, then say that.
>>> 
>>>> 5. Describing where key material is stored and how it is accessed.
>>> 
>>> I don't think that we need this in a charter.
> 
> Removed this point.
> 
>>>> 6. Specifying mechanisms for ensuring that Issuers are not acting
>>>> maliciously.
>>> 
>>> This seems a bit open-ended.  Will this extend to audits of their 
>>> processes?  I don't think we have a protocol for that either.
>> 
>> Maybe we could refine this by specifying particular attacks we're concerned about, such as key tagging?
> 
> I removed this point as I think it is now covered by what is written in point 4. However, I mentioned specifically the attack surface that is being considered with the verification mechanisms that we will specify.
> 
> Thanks,
> Alex
> 
> Updated Charter text
> ================
> 
> The Privacy Pass protocol provides a performant, application-layer
> mechanism for anonymous token creation and redemption. Servers (Issuers)
> create and later verify tokens that are redeemed by an ecosystem of
> clients, such that:
> 
> - Any token granted by a given Issuer is unlinkable with all other
>   tokens granted by the same Issuer.
> - Clients can verify that a token granted by an Issuer corresponds to a
>   committed keypair.
> - Tokens are unforgeable.
> - The token issuance and redemption mechanisms are efficient.
> 
> The primary purpose of the Privacy Pass Working Group is to develop and
> standardize a protocol that meets these requirements, influenced by
> applications that have arisen from the wider community. The aims of the
> Working Group can be split into three distinct goals:
> 
> First, specify an extensible protocol for creating and redeeming
> anonymous and transferrable tokens. The protocol should permit suitable
> cryptographic ciphersuites and security parameterization for
> cryptographic agility. Negotiation of cryptographic parameters during
> the protocol is an application-specific property and thus out of scope
> for the Working Group. Specification of the underlying cryptographic
> algorithms or protocols is also out of scope. The Working Group will
> specify a preliminary set of extensions, including Issuer-supplied
> metadata and alternative cryptographic instantiations that support
> public verifiability of Issued tokens, as well as any additional
> extensions that may arise in the future. Security and privacy properties
> of the protocol shall be well-documented.
> 
> Second, describe and develop protocol use cases and properties thereof.
> This includes, though is not limited to:
> 
> 1. Describing use cases and interfaces that allow the protocol to be
>    used for those use cases.
> 2. Defining the privacy goals for each Client during protocol execution,
>    along with expectations placed on the Issuers and the ecosystem at
>    large.
> 3. Describing recommended parameterizations of variables associated with
>    the protocol ecosystem that control the size of the anonymity set
>    that the client belongs to.
> 4. Describing verification mechanisms for trusting Issuers and their
>    corresponding keying material. Such mechanisms should prevent Issuers
>    from presenting any key material that could be used to deanonymize
>    clients.
> 5. Describing the procedure for including small amounts of metadata with
>    Issued tokens, as well as the associated impacts on privacy.
> 6. Describing the risk and possible ramifications of Issuer
>    centralization, and exploring possible mechanisms to mitigate these
>    risks.
> 
> Third, and finally, specify a HTTP-layer API for the protocol. This
> includes a common understanding of how Privacy Pass is integrated with
> HTTP requests and responses for web-based applications.
> 
> Note that the specifications developed by this working group will be
> informed by the following initial drafts:
> 
> - draft-davidson-pp-protocol-00;
> - draft-davidson-pp-architecture-00;
> - draft-svaldez-pp-http-api-00.
> 
> These existing drafts may be further developed into the core
> deliverables of the working group, supplemented by any additional
> extensions. Alternatively, they may contribute indirectly to a future
> set of documents that meet the core goals of the working group.
> 
>