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Security Privacy Pass This document specifies an instantiation of Privacy Pass Architecture that allows for a "reverse" flow from the Origin to the Client. It describes a method for an Origin to issue new tokens in response to a request in which a token is redeemed. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the Privacy Pass Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction This document specifies an instantiation of Privacy Pass Architecture that allows for a reverse flow from the Origin to the Client. In other words, it specifies a way for an Origin to act as an Attester + Issuer.

Motivation With Privacy Pass issuance as described in , once a token is sent by a Client, it is considered spent and cannot be reused in order to guarantee unlinkability. If a token were to be spent twice, the two requests would be linkable by the Origin. However, requiring that all tokens are spent only once means that Clients might need to request more tokens for new requests, even if the request that included the original token doesn't need to "spend" that token from the Origin's perspective (due to the request ending up being insignificant to the Origin). This draft provides a mechanism for an Origin to provide tokens, allowing reuse without reaching out to the initial Attester and Issuer.

Refunding tokens Certain Origin use Privacy Pass tokens to rate-limit requests they receive over a certain time window because of resource constraints. If a Client sends a request that can be served without utilising that resource, the Origin would like to authorise them to do a second request. This is the case for request requiring compute and the compute is low, or when the request leads to a redirection instead of content generation for instance. With a reverse flow, a Client that has already been authorised by an Origin can maintain that authorization, without losing the unlinkability property provided by Privacy Pass.

Bootstraping issuer An Origin wants to grant 30 access for Clients that solved a CAPTCHA. To do so, it consumes a type 0x0002 public veriable token from an initial issuer that guarantees a CAPTCHA has been solved, and use it to issue 30 type 0x0001 private tokens. Without a reverse flow, the Origin would have to require 30 0x0002 issuer tokens, which have lower performance and a higher number of requests going to the issuer.

Attester feedback loop In , a Client gets a token from an Issuer and redeems it at an Origin. However, if the Client's request is deemed unwanted by the Origin at redemption time, there is no mechanism that prevents the Client from going back to the initial Issuer to get a new token and be authorized again. With a reverse flow, the initial Issuer may require Clients to present an Origin-issued token before providing them with a second token. This allows for a feedback loop between the Origin and the initial Issuer, without breaking Client unlinkability.

Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here. We reuse terminology from . The following terms are used throughout this document:

Flow:

Direction from PrivateToken issuance to its redemption. The entity starting the flow acts as an Issuer, while the end of the flow acts as an Origin. The Client is always included, as it finalises the TokenResponse, and coordinate interactions.

Initial Flow:

Issuer -> Attester -> Client -> Origin. This flow produces a PrivateToken that is used by the Origin to kickstart a Reverse Flow.

Reverse Flow:

Issuer <- Attester <- Client <- Origin. This flow allows Origin to issues PrivateToken. In the reverse flow, the Origin operates one or more Issuer, and the Client MAY provide these tokens either to the Initial Attester/Issuer, or use them against the Origin

Initial Attester/Issuer:

Attester/Issuer part of the Initial Flow

Origin Issuer:

Issuer operated by the Origin

Origin PrivateToken:

PrivateToken issued by the Origin

Reverse Origin:

An entity that consumes the Origin PrivateToken. It can be the Origin, or the Initial Attester/Issuer

Architecture overview Along with sending their PrivateToken for authentication (as specified in ), Client sends TokenRequest

Architec ture of Privacy Pass with a Reverse Flow | | | | | |<== Attestation ==>| | | | | | | | | +--------- TokenRequest ------->| | | |<-------- TokenResponse -------+ | | | | | | | Finalisation | | | | | | | | |<-- Request+Token+TokenRequest(Origin) ---+ | | |<-- TokenRequest ---+ | | | +--- TokenResponse ->| | | | | |---- Response+TokenResponse(Origin) ----->| | | | | | | | | | Finalisation | | | | | | | ]]>

The initial flow matches the one defined by . A Client gets challenged when accessing a resource on an Origin. The Client goes to the Attester to get issue a Token. Through configuration mechanism not defined in this document, the Client is aware the Origin acts as a Reverse Flow issuer. This is an extension of . The redemption flow of a Privacy Pass token is defined in . Reverse flow extends this so that redemption flow is interleaved with the issuance flow described in . This is denoted in the diagram above by the Client sending Request+Token+TokenRequest(Origin Issuer). The Origin runs the issuance protocol, and returns Response+TokenResponse(Origin Issuer). Such flow can be performed through various means. This document introduces one to serve as example and first basis.

TokenRequest, TokenResponse, and Finalisation In , the Client sends an TokenRequest and receives an TokenResponse. These are meant to abstract request from different protocol to the Issuer. As specified in ,

• The structure and semantics of the TokenRequest and TokenResponse messages depend on the issuance protocol and token type being used.

The introducion of Privacy Pass issuance protocol based on Anonymous credential, such as or , modifies TokenRequest to use CredentialRequest instead. Upon receiving an TokenResponse, the Client has to finalise the Token so it can be presented to an origin. This may be a Finalisation for type 0x0002 as defined in , a presentation for , or even a refund for .

Reverse flow with an HTTP header This section defines a Reverse Flow, as presented in , leveraging a new HTTP headers. TokenRequest(Origin) and TokenResponse(Origin) happen through a new HTTP Header PrivacyPass-Reverse. PrivacyPass-Reverse is a base64url () encoded GenericBatchTokenRequest as defined in .

• The use of generic batch tokens as defined in is because this already provides encoding for request and response, error wrapping, and a concise format. One could use binary http or a new format

Client behaviour Along with sending PrivateToken from the Initial Issuer to the Origin, the Client sends a TokenRequest as defined in , , or . In all these definitions, TokenRequest MUST prepended by a uint16_t representing the token type. The Client SHOULD consider Privacy Pass Reverse Flow like the initial flow. The Client is responsible to coordinate between the different entities. Specifically, if the Reverse Origin is the Initial Attester/Issuer, the Client SHOULD account for possible privacy leakage.

Origin/Issuer/Attester deployment In this model, the Origin, Attester, and Issuer are all operated by the same entity, as shown in . The Reverse Flow is the same as the Initial Flow, except for the request/response encapsulation. The Origin is the Reverse Origin.

Shared Deployment Model | | | | | | | +----------- TokenRequest --------| | |<---------- TokenResponse -------+ | | | | | | | +------- Token+TokenRequest(Origin) ------->| |<--------- TokenResponse(Origin) ----------+ | | ]]>

Similar to the original Shared Deployment Model, the Attester, Issuer, and Origin share the attestation, issuance, and redemption contexts. Even if this context changes between the Initial and Reverse Flow, attestation mechanism that can uniquely identify a Client are not appropriate as they could lead to unlinkability violations.

• This model allows for private verifiability. Even though no optimised scheme is available at the time of writting, the author recommends to follow advances of anonymous credential within the Privacy Pass group. Specifically
  These scheme allow for optimisation of Token finalisation by the Client.

Split Origin-Attester deployment In this model, the Attester and Issuer are operated by the same entity that is separate from the Origin. The Origin trusts the joint Attester and Issuer to perform attestation and issue Tokens. Origin Tokens can then be sent by Client on new requests, as long as the Reverse Origin trusts the Origin to perform attestation and issue Tokens.

Joint Attester and Issuer Deployment Model | | | | | |<== Attestation ==>| | | | | | | | | +--------- TokenRequest ------->| | | |<-------- TokenResponse -------+ | |<-- Request+Token+TokenRequest(Origin) --+ | | |<-- TokenRequest ---+ | | | +-- TokenResponse -->| | | | | +--- Response+TokenResponse(Origin) ----->| | | | | | | | ]]>

The Origin Issuer MUST NOT issue privately verifiable tokens, as this would lead to secret material being shared between the Origin and the Reverse Origin. A particular deployment model is when the Reverse Origin is the Attester/Issuer. This model is described in

Joint Attester and Issuer Deployment Model with reverse | | | | | |<== Attestation ==>| | | | | | | | | +--------- TokenRequest ------->| | | |<-------- TokenResponse -------+ | |<-- Request+Token+TokenRequest(Origin) --+ | | |<-- TokenRequest ---+ | | | +--- TokenResponse ->| | | | | +--- Response+TokenResponse(Origin) ----->| | | | | +------------ Token ----------->| | | | | | ]]>

This deployment SHOULD not allow the Reverse Origin to infer the request made to the Origin, as it would break unlinkability.

Privacy Considerations Privacy Pass states

• In general, limiting the amount of metadata permitted helps limit the extent to which metadata can uniquely identify individual Clients. Failure to bound the number of possible metadata values can therefore lead to a reduction in Client privacy. Most token types do not admit any metadata, so this bound is implicitly enforced.

In Privacy Pass with a reverse flow, Clients are provided with new PrivateTokens depending on their request. They can spend these tokens to continue making further requests. While the token are still unlinkable, the token_key_id associated to them represent metadata. It leaks some information about the Client. The following subsections discuss the issues that influence the anonymity set, and possible mitigations/safeguards to protect against this underlying problem.

Issuer face values When setting up a reverse flow deployment, an Origin MAY operate multiple Issuers, and assign them some metadata to them. The amount of possible metadata grows as 2^(origin_issuers). We RECOMMEND that:

1. Origin defines their anonimity sets, and deploy no more than log2(#anonimity_sets). This bounds the possible anonimity sets by design.
2. Client to only send 1 PrivateToken per request. This is inline with RFC9577 and RFC (Web Authentication) which only allows one challenge response to be provided as part of Authorization HTTP header.
3. Issuers metadata to be publicly disclosed via an origin endpoint, and externally monitored

Token for specific Clients In Privacy Pass with a reverse flow, an Origin MAY operate multiple Issuers, with arbitrary metadata associated to them. A malicious Origin MAY uses this opportunity to associate certain token values to a specific set of Clients. Let's consider the following deployment: the Origin operates two issuers A and B. The Client sends Token_A, and (TokenRequest_A, TokenRequest_B). Issuer B is associated to croissant aficionados. If a Client requests croissant, or sends Token_B, the origin provides TokenResponse_B. If not, it provides TokenResponse_A. Over time, this means the Origin is able to track croissants aficionados. To mitigate this, we RECOMMEND:

1. The initial PrivateToken to be provided by an Issuer not in control of the Origin. The joint Origin/Attester/Issuer model SHOULD NOT be used.
2. Clients to reset their state regularly with the initial Issuer.

Sending more than one token While that's not part of Privacy Pass with a reverse flow, some deployment might consider allowing Clients to send multiple PrivateToken, similar to how normal Privacy Pass deployment allow two distinct PrivateToken to be sent. In Privacy Pass with a reverse flow deployment, there are as many bits as Issuers; each token is one bit. We RECOMMEND to have a maximum of 6 Origin operated Issuers, bounding Client information to 2^6 = 64. Accounting for the initial Issuer, this means a total of log2(64)+1=7 issuers. Origin should have sufficient traffic to not single-out particular Client based on timings of requests.

Swap endpoint and its privacy implication With multiple Issuers, a Client MAY end up with a bunch of tokens, for various Issuers. Origins MAY propose a swap endpoint at which a Client can exchange one or more Origin tokens against one or more new Origin tokens. The Origin SHOULD ensure this endpoint receives enough traffic to not reduce the anonymity sets.

IANA Considerations This document has no IANA actions.

References Normative References Phoenix R&D Cloudflare Cloudflare Inc. This document specifies two variants of the Privacy Pass issuance protocol that allow for batched issuance of tokens. These allow clients to request more than one token at a time and for issuers to issue more than one token at a time. This document describes the commonly used base 64, base 32, and base 16 encoding schemes. It also discusses the use of line-feeds in encoded data, use of padding in encoded data, use of non-alphabet characters in encoded data, use of different encoding alphabets, and canonical encodings. [STANDARDS-TRACK] This document specifies the Privacy Pass architecture and requirements for its constituent protocols used for authorization based on privacy-preserving authentication mechanisms. It describes the conceptual model of Privacy Pass and its protocols, its security and privacy goals, practical deployment models, and recommendations for each deployment model, to help ensure that the desired security and privacy goals are fulfilled. This document specifies two variants of the two-message issuance protocol for Privacy Pass tokens: one that produces tokens that are privately verifiable using the Issuer Private Key and one that produces tokens that are publicly verifiable using the Issuer Public Key. Instances of "issuance protocol" and "issuance protocols" in the text of this document are used interchangeably to refer to the two variants of the Privacy Pass issuance protocol. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References n.d. Apple, Inc. Apple, Inc. This document specifies the issuance and redemption protocols for tokens based on the Anonymous Rate-Limited Credential (ARC) protocol. Akamai Technologies Existing token types in privacy pass conflate attribution with rate limiting. This document describes a token type where the issuer attests to a set of properties of the client, which the client can then selectively prove to the origin. Repeated showings of the same credential are unlinkable, unlike other token types in privacy pass. The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document describes the overall architecture of HTTP, establishes common terminology, and defines aspects of the protocol that are shared by all versions. In this definition are core protocol elements, extensibility mechanisms, and the "http" and "https" Uniform Resource Identifier (URI) schemes. This document updates RFC 3864 and obsoletes RFCs 2818, 7231, 7232, 7233, 7235, 7538, 7615, 7694, and portions of 7230.

Acknowledgments The author would like to thank Tommy Pauly, Chris Wood, Raphael Robert, and Armando Faz Hernandez for helpful discussion on Privacy Pass architecture and its considerations.

Changelog v01

• Editorial pass on the introduction
• Add a motivation section: refunding tokens, bootstraping issuer, attester feeback loop
• Split protocol overview via HTTP headers in its own section
• Add consideration about anonymous credentials in joint origin/issuer deployment

v00

• Initial draft
• Possibility of a new HTTP request for inlining request
• Privacy considerations about additional metadata