Network Working Group S. Celi Internet-Draft Cloudflare Intended status: Informational A. Davidson Expires: 23 August 2021 LIP A. Faz-Hernandez Cloudflare 19 February 2021 Privacy Pass Protocol Specification draft-ietf-privacypass-protocol-01 Abstract This document specifies the Privacy Pass protocol. This protocol provides anonymity-preserving authorization of clients to servers. In particular, client re-authorization events cannot be linked to any previous initial authorization. Privacy Pass is intended to be used as a performant protocol in the application-layer. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 23 August 2021. Copyright Notice Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved. Celi, et al. Expires 23 August 2021 [Page 1] Internet-Draft PP protocol February 2021 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.1. Motivating use-cases . . . . . . . . . . . . . . . . . . 4 3.2. Anonymity and security guarantees . . . . . . . . . . . . 5 3.3. Basic assumptions . . . . . . . . . . . . . . . . . . . . 5 4. Protocol description . . . . . . . . . . . . . . . . . . . . 5 4.1. Server setup . . . . . . . . . . . . . . . . . . . . . . 6 4.2. Client setup . . . . . . . . . . . . . . . . . . . . . . 6 4.3. Issuance phase . . . . . . . . . . . . . . . . . . . . . 6 4.4. Redemption phase . . . . . . . . . . . . . . . . . . . . 8 4.4.1. Client info . . . . . . . . . . . . . . . . . . . . . 8 4.4.2. Double-spend protection . . . . . . . . . . . . . . . 9 4.5. Handling errors . . . . . . . . . . . . . . . . . . . . . 9 5. Functionality . . . . . . . . . . . . . . . . . . . . . . . . 9 5.1. Data structures . . . . . . . . . . . . . . . . . . . . . 9 5.1.1. Ciphersuite . . . . . . . . . . . . . . . . . . . . . 9 5.1.2. Keys . . . . . . . . . . . . . . . . . . . . . . . . 9 5.1.3. CommitRequest . . . . . . . . . . . . . . . . . . . . 10 5.1.4. CommitResponse . . . . . . . . . . . . . . . . . . . 10 5.1.5. IssuanceInput . . . . . . . . . . . . . . . . . . . . 10 5.1.6. IssuanceResponse . . . . . . . . . . . . . . . . . . 10 5.1.7. RedemptionToken . . . . . . . . . . . . . . . . . . . 11 5.1.8. RedemptionRequest . . . . . . . . . . . . . . . . . . 11 5.1.9. RedemptionResponse . . . . . . . . . . . . . . . . . 11 5.2. API functions . . . . . . . . . . . . . . . . . . . . . . 12 5.2.1. Prepare . . . . . . . . . . . . . . . . . . . . . . . 12 5.2.2. Commit . . . . . . . . . . . . . . . . . . . . . . . 12 5.2.3. Generate . . . . . . . . . . . . . . . . . . . . . . 12 5.2.4. Issue . . . . . . . . . . . . . . . . . . . . . . . . 13 5.2.5. Process . . . . . . . . . . . . . . . . . . . . . . . 13 5.2.6. Redeem . . . . . . . . . . . . . . . . . . . . . . . 14 5.2.7. Verify . . . . . . . . . . . . . . . . . . . . . . . 14 5.3. Error types . . . . . . . . . . . . . . . . . . . . . . . 14 6. Security considerations . . . . . . . . . . . . . . . . . . . 15 6.1. Unlinkability . . . . . . . . . . . . . . . . . . . . . . 15 6.2. One-more unforgeability . . . . . . . . . . . . . . . . . 16 Celi, et al. Expires 23 August 2021 [Page 2] Internet-Draft PP protocol February 2021 6.3. Double-spend protection . . . . . . . . . . . . . . . . . 16 6.4. Additional token metadata . . . . . . . . . . . . . . . . 17 6.5. Maximum number of tokens issued . . . . . . . . . . . . . 17 7. VOPRF instantiation . . . . . . . . . . . . . . . . . . . . . 17 7.1. Recommended ciphersuites . . . . . . . . . . . . . . . . 17 7.2. Protocol contexts . . . . . . . . . . . . . . . . . . . . 18 7.3. Functionality . . . . . . . . . . . . . . . . . . . . . . 18 7.3.1. Generate . . . . . . . . . . . . . . . . . . . . . . 18 7.3.2. Issue . . . . . . . . . . . . . . . . . . . . . . . . 19 7.3.3. Process . . . . . . . . . . . . . . . . . . . . . . . 19 7.3.4. Redeem . . . . . . . . . . . . . . . . . . . . . . . 19 7.3.5. Verify . . . . . . . . . . . . . . . . . . . . . . . 19 7.4. Security justification . . . . . . . . . . . . . . . . . 20 8. Protocol ciphersuites . . . . . . . . . . . . . . . . . . . . 20 8.1. PP(OPRF2) . . . . . . . . . . . . . . . . . . . . . . . . 20 8.2. PP(OPRF4) . . . . . . . . . . . . . . . . . . . . . . . . 21 8.3. PP(OPRF5) . . . . . . . . . . . . . . . . . . . . . . . . 21 9. Extensions framework policy . . . . . . . . . . . . . . . . . 21 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 22 10.1. Normative References . . . . . . . . . . . . . . . . . . 22 10.2. Informative References . . . . . . . . . . . . . . . . . 22 Appendix A. Document contributors . . . . . . . . . . . . . . . 23 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23 1. Introduction A common problem on the Internet is providing an effective mechanism for servers to derive trust from clients that they interact with. Typically, this can be done by providing some sort of authorization challenge to the client. But this also negatively impacts the experience of clients that regularly have to solve such challenges. To mitigate accessibility issues, a client that correctly solves the challenge can be provided with a cookie. This cookie can be presented the next time the client interacts with the server, instead of performing the challenge. However, this does not solve the problem of reauthorization of clients across multiple domains. Using current tools, providing some multi-domain authorization token would allow linking client browsing patterns across those domains, and severely reduces their online privacy. The Privacy Pass protocol provides a set of cross-domain authorization tokens that protect the client's anonymity in message exchanges with a server. This allows clients to communicate an attestation of a previously authenticated server action, without having to reauthenticate manually. The tokens retain anonymity in the sense that the act of revealing them cannot be linked back to the session where they were initially issued. Celi, et al. Expires 23 August 2021 [Page 3] Internet-Draft PP protocol February 2021 This document lays out the generic description of the protocol, along with the data and message formats. We detail an implementation of the protocol functionality based on the description of a verifiable oblivious pseudorandom function [I-D.irtf-cfrg-voprf]. This document DOES NOT cover the architectural framework required for running and maintaining the Privacy Pass protocol in the Internet setting. In addition, it DOES NOT cover the choices that are necessary for ensuring that client privacy leaks do not occur. Both of these considerations are covered in a separate document [draft-davidson-pp-architecture]. In addition, [draft-svaldez-pp-http-api] provides an instantiation of this protocol intended for the HTTP setting. 2. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. The following terms are used throughout this document. * Server: A service that provides the server-side functionality required by the protocol. May be referred to as the issuer. * Client: An entity that seeks authorization from a server that supports interactions in the Privacy Pass protocol. * Key: The secret key used by the server for authorizing client data. We assume that all protocol messages are encoded into raw byte format before being sent. We use the TLS presentation language [RFC8446] to describe the structure of protocol data types and messages. 3. Background We discuss the core motivation behind the protocol along with the guarantees and assumptions that we make in this document. 3.1. Motivating use-cases The Privacy Pass protocol was originally developed to provide anonymous authorization of Tor users. In particular, the protocol allows clients to reveal authorization tokens that they have been issued without linking the authorization to the actual issuance event. This means that the tokens cannot be used to link the browsing patterns of clients that reveal tokens. Celi, et al. Expires 23 August 2021 [Page 4] Internet-Draft PP protocol February 2021 Beyond these uses-cases, the Privacy Pass protocol is used in a number of practical applications. See [DGSTV18], [TrustTokenAPI], [PrivateStorage], [OpenPrivacy], and [Brave] for examples. 3.2. Anonymity and security guarantees Privacy Pass provides anonymity-preserving authorization tokens for clients. Throughout this document, we use the terms "anonymous", "anonymous-preserving" and "anonymity" to refer to the core security guarantee of the protocol. Informally, this guarantee means that any token issued by a server key and subsequently redeemed is indistinguishable from any other token issued under the same key. Privacy Pass also prohibits clients from forging tokens, as otherwise the protocol would have little value as an authorization protocol. Informally, this means any client that is issued "N" tokens under a given server key cannot redeem more than "N" valid tokens. Section 6 elaborates on these protocol anonymity and security requirements. 3.3. Basic assumptions We make only a few minimal assumptions about the environment of the clients and servers supporting the Privacy Pass protocol. * At any one time, we assume that the server uses only one configuration containing their ciphersuite choice along with their secret key data. This ensures that all clients are issued tokens under the single key associated with any given epoch. * We assume that the client has access to a global directory of the current public parts of the configurations used the server. The wider ecosystem that this protocol is employed in is described in [draft-davidson-pp-architecture]. 4. Protocol description The Privacy Pass protocol is split into two phases that are built upon the functionality described in Section 5 later. Celi, et al. Expires 23 August 2021 [Page 5] Internet-Draft PP protocol February 2021 The first phase, "issuance", provides the client with unlinkable tokens that can be used to initiate re-authorization with the server in the future. The second phase, "redemption", allows the client to redeem a given re-authorization token with the server that it interacted with during the issuance phase. The protocol must satisfy two cryptographic security requirements known as "unlinkability" and "unforgeability". These requirements are covered in Section 6. 4.1. Server setup Before the protocol takes place, the server chooses a ciphersuite and generates a keypair by running "(pkS, skS) = KeyGen()". This configuration must be available to all clients that interact with the server (for the purpose of engaging in a Privacy Pass exchange). We assume that the server has a public (and unique) identity that the client uses to retrieve this configuration. 4.2. Client setup The client initialises a global storage system "store" that allows it store the tokens that are received during issuance. The storage system is a map of server identifiers ("server.id") to arrays of stored tokens. We assume that the client knows the server public key "pkS" ahead of time. The client picks a value "m" of tokens to receive during the issuance phase. In [draft-davidson-pp-architecture] we discuss mechanisms that the client can use to ensure that this public key is consistent across the entire ecosystem. 4.3. Issuance phase The issuance phase is a two-round protocol that allows the client to receive "m" anonymous authorization tokens from the server. The first round sees the server generate a commitment. The second round sees the server issue a token to the client. Celi, et al. Expires 23 August 2021 [Page 6] Internet-Draft PP protocol February 2021 Client(pkS, m, info) Server(skS, pkS) ------------------------------------------------------------ commit_req = Prepare(info) commit_req -------------------> commit_resp = Commit(skS, pkS, commit_req) commit_resp <------------------- cInput = Generate(m, commit_resp) req = cInput.req req -------------------> issueResp = Issue(pkS, skS, req) serverResp <------------------- tokens = Process(pkS, cInput, serverResp) store[server.id].push(tokens) Note that the first round of the protocol is only necessitated for certain ciphersuites that require client and servers commit to some value. When such commitment "commit_resp" is generated and sent to the client, the client returns "commit_resp" with the "IssuanceRequest" message. The server MUST check that the commitment corresponds to "commit_resp" that was previously committed. This requires the commitment to either be a reference to some commitment on the server, or the commitment be an encrypted (and authenticated) blob that the server can use to recover commitment. The mechanism by which servers handle this commitment is implementation specific, and similar to how TLS session resumption state is managed; see [RFC8446] for details. In addition, the "Commit" function is implementation- specific and MUST be defined by the underlying ciphersuite. When the server does not need to generate this commitment, the client instead DOES NOT send the "CommitRequest" message, and runs: cInput = Generate(m, "") Celi, et al. Expires 23 August 2021 [Page 7] Internet-Draft PP protocol February 2021 A server that is expecting some non-empty "commit_resp" to be passed must abort the protocol on receiving a request containing an empty "commit_resp" value. Note: currently, no ciphersuites are supported that support working with empty commitment messages. 4.4. Redemption phase The redemption phase allows the client to anonymously reauthenticate to the server, using data that it has received from a previous issuance phase. Client(info) Server(skS, pkS) ------------------------------------------------------------ token = store[server.id].pop() req = Redeem(token, info) req ------------------> if (dsIdx.includes(req.data)) { raise ERR_DOUBLE_SPEND } resp = Verify(pkS, skS, req) if (resp.success) { dsIdx.push(req.data) } resp <------------------ Output resp 4.4.1. Client info The client input "info" is arbitrary byte data that is used for linking the redemption request to the specific session. We RECOMMEND that "info" is constructed as the following concatenated byte-encoded data: len(aux) || aux || len(server.id) || server.id || current_time() where "len(x)" is the length of "x" in bytes, and "aux" is arbitrary auxiliary data chosen by the client. The usage of "current_time()" allows the server to check that the redemption request has happened in an appropriate time window. Celi, et al. Expires 23 August 2021 [Page 8] Internet-Draft PP protocol February 2021 4.4.2. Double-spend protection To protect against clients that attempt to spend a value "req.data" more than once, the server uses an index, "dsIdx", to collect valid inputs it witnesses. Since this store needs to only be optimized for storage and querying, a structure such as a Bloom filter suffices. The storage should be parameterized to live as long as the server keypair that is in use. See Section 6 for more details. 4.5. Handling errors It is possible for the API functions from Section 5.2 to return one of the errors indicated in Section 5.3 rather than their expected value. In these cases, we assume that the entire protocol aborts. 5. Functionality This section details the data types and API functions that are used to construct the protocol in Section 4. We provide an explicit instantiation of the Privacy Pass API in Section 7.3, based on the public API provided in [I-D.irtf-cfrg-voprf]. 5.1. Data structures The following data structures are used throughout the Privacy Pass protocol and are written in the TLS presentation language [RFC8446]. It is intended that any of these data structures can be written into widely-adopted encoding schemes such as those detailed in TLS [RFC8446], CBOR [RFC7049], and JSON [RFC7159]. 5.1.1. Ciphersuite The "Ciphersuite" enum provides identifiers for each of the supported ciphersuites of the protocol. Some initial values that are supported by the core protocol are described in Section 8. Note that the list of supported ciphersuites may be expanded by extensions to the core protocol description in separate documents. 5.1.2. Keys We use the following types to describe the public and private keys used by the server. opaque PublicKey<1..2^16-1> opaque PrivateKey<1..2^16-1> Celi, et al. Expires 23 August 2021 [Page 9] Internet-Draft PP protocol February 2021 5.1.3. CommitRequest The "CommitRequest" struct is simply a fixed message allowing opaque metadata. struct { opaque info<1..2^16-1> } CommitRequest; 5.1.4. CommitResponse The "CommitResponse" struct is contains an opaque set of bytes that correspond to some commitment that the server has generated. The structure and format of this value is implementation specific depending on whether the server is stateful. struct { opaque commitment<1..2^16-1> } CommitResponse; 5.1.5. IssuanceInput The "IssuanceInput" struct describes the data that is initially generated by the client during the issuance phase. Firstly, we define sequences of bytes that partition the client input. opaque Internal<1..2^16-1> opaque IssuanceRequest<1..2^16-1> These data types represent members of the wider "IssuanceInput" data type. struct { Internal data[m] IssuanceRequest req[m] } IssuanceInput; Note that a "IssuanceInput" contains equal-length arrays of "Internal" and "IssuanceRequest" types corresponding to the number of tokens that should be issued. 5.1.6. IssuanceResponse Firstly, the "IssuedToken" type corresponds to a single sequence of bytes that represents a single issued token received from the server. Celi, et al. Expires 23 August 2021 [Page 10] Internet-Draft PP protocol February 2021 opaque IssuedToken<1..2^16-1> Then an "IssuanceResponse" corresponds to a collection of "IssuedTokens" as well as a sequence of bytes "proof". struct { IssuedToken tokens[m] opaque proof<1..2^16-1> } The value of "m" is equal to the length of the "IssuanceRequest" vector sent by the client. 5.1.7. RedemptionToken The "RedemptionToken" struct contains the data required to generate the client message in the redemption phase of the Privacy Pass protocol. struct { opaque data<1..2^16-1>; opaque issued<1..2^16-1>; } RedemptionToken; 5.1.8. RedemptionRequest The "RedemptionRequest" struct consists of the data that is sent by the client during the redemption phase of the protocol. struct { opaque data<1..2^16-1>; opaque tag<1..2^16-1>; opaque info<1..2^16-1>; } RedemptionRequest; 5.1.9. RedemptionResponse The "RedemptionResponse" struct corresponds to a boolean value that indicates whether the "RedemptionRequest" sent by the client is valid. It can also contain any associated data. struct { boolean success; opaque ad<1..2^16-1>; } RedemptionResponse; Celi, et al. Expires 23 August 2021 [Page 11] Internet-Draft PP protocol February 2021 5.2. API functions The following functions wrap the core of the functionality required in the Privacy Pass protocol. For each of the descriptions, we essentially provide the function signature, leaving the actual contents to be defined by specific instantiations or extensions of the protocol. 5.2.1. Prepare A function run by the client to prepare for a commitment will used during the issuance flow of the Privacy Pass protocol. Inputs: "info": An opaque byte application-specific byte string. Outputs: "commit_req": A "CommitRequest" struct. This function should be implemented by any ciphersuites that require a two-phase issuance protocol ("COMMIT=true"). 5.2.2. Commit A function run by the server that generates a commitment in the first phase of the issuance protocol. Inputs: * "skS": A server "PrivateKey". * "pkS": A server "PublicKey". * "commit_req": A "CommitRequest" struct Outputs: * "commit_resp": A "CommitResponse" struct. This function should be implemented by any ciphersuites that require a two-phase issuance protocol ("COMMIT=true"). 5.2.3. Generate A function run by the client to generate the initial data that is used as its input in the Privacy Pass protocol. Celi, et al. Expires 23 August 2021 [Page 12] Internet-Draft PP protocol February 2021 Inputs: * "m": A "uint8" value corresponding to the number of Privacy Pass tokens to generate. Outputs: * "input": An "IssuanceInput" struct. 5.2.4. Issue A function run by the server to issue valid redemption tokens to the client. Inputs: * "pkS": A server "PublicKey". * "skS": A server "PrivateKey". * "req": An "IssuanceRequest" struct. Outputs: * "resp": An "IssuanceResponse" struct. Throws: * "ERR_FAILED_COMMITMENT" (Section 5.3) 5.2.5. Process Run by the client when processing the server response in the issuance phase of the protocol. Inputs: * "pkS": An server "PublicKey". * "input": An "IssuanceInput" struct. * "resp": An "IssuanceResponse" struct. Outputs: * "tokens": A vector of "RedemptionToken" structs, whose length is equal to length of the internal "ServerEvaluation" vector in the "IssuanceResponse" struct. Celi, et al. Expires 23 August 2021 [Page 13] Internet-Draft PP protocol February 2021 Throws: * "ERR_PROOF_VALIDATION" (Section 5.3) 5.2.6. Redeem Run by the client in the redemption phase of the protocol to generate the client's message. Inputs: * "token": A "RedemptionToken" struct. * "info": An "opaque<1..2^16-1>" type corresponding to data that is linked to the redemption. See Section 4.4.1 for advice on how to construct this. Outputs: * "req": A "RedemptionRequest" struct. 5.2.7. Verify Run by the server in the redemption phase of the protocol. Determines whether the data sent by the client is valid. Inputs: * "pkS": An server "PublicKey". * "skS": An server "PrivateKey". * "req": A "RedemptionRequest" struct. Outputs: * "resp": A "RedemptionResponse" struct. 5.3. Error types * "ERR_PROOF_VALIDATION": Error occurred when a client attempted to verify the proof that is part of the server's response. * "ERR_DOUBLE_SPEND": Error occurred when a client has attempted to redeem a token that has already been used for authorization. Celi, et al. Expires 23 August 2021 [Page 14] Internet-Draft PP protocol February 2021 * "ERR_FAILED_COMMITMENT": Error occurs during issuance phase if non-empty commitment does not match the commitment generated in the first round. 6. Security considerations We discuss the security requirements that are necessary to uphold when instantiating the Privacy Pass protocol. In particular, we focus on the security requirements of "unlinkability", and "unforgeability". Informally, the notion of unlinkability is required to preserve the anonymity of the client in the redemption phase of the protocol. The notion of unforgeability is to protect against an adversarial client that may look to subvert the security of the protocol. Both requirements are modelled as typical cryptographic security games, following the formats laid out in [DGSTV18] and [KLOR20]. Note that the privacy requirements of the protocol are covered in the architectural framework document [draft-davidson-pp-architecture]. 6.1. Unlinkability Formally speaking the security model is the following: * The adversary runs the server setup and generates a keypair "(pkS, skS)". * The adversary specifies a number "Q" of issuance phases to initiate, where each phase "i in range(Q)" consists of "m_i" Issue evaluations. * The adversary runs "Issue" using the keypair that it generated on each of the client messages in the issuance phase. * When the adversary wants, it stops the issuance phase, and a random number "l" is picked from "range(Q)". * A redemption phase is initiated with a single token with index "i" randomly sampled from "range(m_l)". * The adversary guesses an index "l_guess" corresponding to the index of the issuance phase that it believes the redemption token was received in. * The adversary succeeds if "l == l_guess". Celi, et al. Expires 23 August 2021 [Page 15] Internet-Draft PP protocol February 2021 The security requirement is that the adversary has only a negligible probability of success greater than "1/Q". 6.2. One-more unforgeability The one-more unforgeability requirement states that it is hard for any adversarial client that has received "m" valid tokens from the issuance phase to redeem "m+1" of them. In essence, this requirement prevents a malicious client from being able to forge valid tokens based on the Issue responses that it sees. The security model roughly takes the following form: * The adversary specifies a number "Q" of issuance phases to initiate with the server, where each phase "i in range(Q)" consists of "m_i" server evaluation. Let "m = sum(m_i)" where "i in range(Q)". * The adversary receives "Q" responses, where the response with index "i" contains "m_i" individual tokens. * The adversary initiates "m_adv" redemption sessions with the server and the server verifies that the sessions are successful (return true), and that each request includes a unique token. The adversary succeeds in "m_succ =< m_adv" redemption sessions. * The adversary succeeds if "m_succ > m". The security requirement is that the adversarial client has only a negligible probability of succeeding. Note that [KLOR20] strengthens the capabilities of the adversary, in comparison to the original work of [DGSTV18]. In [KLOR20], the adversary is provided with oracle access that allows it to verify that the server responses in the issuance phase are valid. 6.3. Double-spend protection All issuing servers should implement a robust, global storage-query mechanism for checking that tokens sent by clients have not been spent before. Such tokens only need to be checked for each server individually. This prevents clients from "replaying" previous requests, and is necessary for achieving the unforgeability requirement. Celi, et al. Expires 23 August 2021 [Page 16] Internet-Draft PP protocol February 2021 6.4. Additional token metadata Some use-cases of the Privacy Pass protocol benefit from associating a limited amount of metadata with tokens that can be read by the server when a token is redeemed. Adding metadata to tokens can be used as a vector to segment the anonymity of the client in the protocol. Therefore, it is important that any metadata that is added is heavily limited. Any additional metadata that can be added to redemption tokens should be described in the specific protocol instantiation. Note that any additional metadata will have to be justified in light of the privacy concerns raised above. For more details on the impacts associated with segmenting user privacy, see [draft-davidson-pp-architecture]. Any metadata added to tokens will be considered either "public" or "private". Public metadata corresponds to unmodifiable bits that a client can read. Private metadata corresponds to unmodifiable private bits that should be obscured to the client. Note that the instantiation in Section 7 provides randomized redemption tokens with no additional metadata for an server with a single key. 6.5. Maximum number of tokens issued Servers SHOULD impose a hard ceiling on the number of tokens that can be issued in a single issuance phase to a client. If there is no limit, malicious clients could abuse this and cause excessive computation, leading to a Denial-of-Service attack. 7. VOPRF instantiation In this section, we show how to instantiate the functional API in Section 5 with the VOPRF protocol described in [I-D.irtf-cfrg-voprf]. Moreover, we show that this protocol satisfies the security requirements laid out in Section 6, based on the security proofs provided in [DGSTV18] and [KLOR20]. 7.1. Recommended ciphersuites The RECOMMENDED server ciphersuites are as follows: detailed in [I-D.irtf-cfrg-voprf]: * OPRF(decaf448, SHA-512) (ID = 0x0002); * OPRF(P-384, SHA-512) (ID = 0x0004); Celi, et al. Expires 23 August 2021 [Page 17] Internet-Draft PP protocol February 2021 * OPRF(P-521, SHA-512) (ID = 0x0005). We deliberately avoid the usage of smaller ciphersuites (associated with P-256 and ristretto255) due to the potential to reduce security to unfavourable levels via static Diffie Hellman attacks. See [I-D.irtf-cfrg-voprf] for more details. 7.2. Protocol contexts Note that we must run the verifiable version of the protocol in [I-D.irtf-cfrg-voprf]. Therefore the "server" takes the role of the "Server" running in "modeVerifiable". In other words, the "server" runs "ctxtS = SetupVerifiableServer(suite, skS, pkS)"; where "suite" is one of the ciphersuites in Section 7.1, "skS" and "pkS" is the server's secret and public key respectively (generated by calling "KeyGen"). It returns "ctxtS", which is the Server context. Likewise, run "ctxtC = SetupVerifiableClient(suite, pkS)" to generate the Client context. 7.3. Functionality We instantiate each functions using the API functions in [I-D.irtf-cfrg-voprf]. Note that we use the framework mentioned in the document to allow for batching multiple tokens into a single VOPRF evaluation. For the explicit signatures of each of the functions, refer to Section 5. 7.3.1. Generate The generate functionality generates an initial set of tokens and blinded representation on the client-side. The function also takes an optional (possibly empty) value for a commitment "com" committed to by the server. def Generate(m, com): tokens = [] blindedTokens = [] for i in range(m): x = random_bytes() (token, blindedToken) = Blind(x, com) tokens[i] = token blindedTokens[i] = blindedToken return IssuanceInput { data: tokens, req: blindedTokens, } Celi, et al. Expires 23 August 2021 [Page 18] Internet-Draft PP protocol February 2021 7.3.2. Issue For this functionality, note that we supply multiple tokens in "req" to "Evaluate". This allows batching a single proof object for multiple evaluations. While the construction in [I-D.irtf-cfrg-voprf] only permits a single input, we follow the advice for providing vectors of inputs. def Issue(pkS, skS, req): elements, proof = Evaluate(skS, pkS, req) return IssuanceResponse { tokens: elements, proof: proof, } 7.3.3. Process Similarly to "Issue", we follow the advice for providing vectors of inputs to the "Unblind" function for verifying the batched proof object. Process(pkS, input, resp): unblindedTokens = Unblind(input.data, resp.elements, input.req, pkS, resp.proof) redemptionTokens = [] for bt in unblindedTokens: rt = RedemptionToken { data: input.data, issued: bt } redemptionTokens[i] = rt return redemptionTokens 7.3.4. Redeem def Redeem(token, info): tag = Finalize(token.data, token.issued, info) return RedemptionRequest { data: data, tag: tag, info: info, } 7.3.5. Verify def Verify(pkS, skS, req): resp = VerifyFinalize(skS, req.data, req.info, req.tag) Output RedemptionResponse { success: resp } Celi, et al. Expires 23 August 2021 [Page 19] Internet-Draft PP protocol February 2021 7.4. Security justification The protocol devised in Section 4, coupled with the API instantiation in Section 7.3, are equivalent to the protocol description in [DGSTV18] and [KLOR20] from a security perspective. In [DGSTV18], it is proven that this protocol satisfies the security requirements of unlinkability (Section 6.1) and unforgeability (Section 6.2). The unlinkability property follows unconditionally as the view of the adversary in the redemption phase is distributed independently of the issuance phase. The unforgeability property follows from the one- more decryption security of the ElGamal cryptosystem [DGSTV18]. In [KLOR20] it is also proven that this protocol satisfies the stronger notion of unforgeability, where the adversary is granted a verification oracle, under the chosen-target Diffie-Hellman assumption. Note that the existing security proofs do not leverage the VOPRF primitive as a black-box in the security reductions. Instead, it relies on the underlying operations in a non-black-box manner. Hence, an explicit reduction from the generic VOPRF primitive to the Privacy Pass protocol would strengthen these security guarantees. 8. Protocol ciphersuites The ciphersuites that we describe for the Privacy Pass protocol are derived from the core instantiations of the protocol (such as in Section 7). In each of the ciphersuites below, the maximum security provided corresponds to the maximum difficulty of computing a discrete logarithm in the group. Note that the actual security level MAY be lower. See the security considerations in [I-D.irtf-cfrg-voprf] for examples. The COMMIT parameter refers to whether the first round of the issuance phase of the protocol is necessary. When this is false, the client ignores the first message and uses an empty value for the commitment parameter to "Generate". 8.1. PP(OPRF2) * OPRF2 = OPRF(decaf448, SHA-512) * ID = 0x0001 * COMMIT = false Celi, et al. Expires 23 August 2021 [Page 20] Internet-Draft PP protocol February 2021 * Maximum security provided: 224 bits 8.2. PP(OPRF4) * OPRF4 = OPRF(P-384, SHA-512) * ID = 0x0002 * COMMIT = false * Maximum security provided: 192 bits 8.3. PP(OPRF5) * OPRF5 = OPRF(P-521, SHA-512) * ID = 0x0003 * COMMIT = false * Maximum security provided: 256 bits 9. Extensions framework policy The intention with providing the Privacy Pass API in Section 5 is to allow new instantiations of the Privacy Pass protocol. These instantiations may provide either modified VOPRF constructions, or simply implement the API in a completely different way. Extensions to this initial draft SHOULD be specified as separate documents taking one of two possible routes: * Produce new VOPRF-like primitives that use the same public API provided in [I-D.irtf-cfrg-voprf] to implement the Privacy Pass API, but with different internal operations. * Implement the Privacy Pass API in a different way to the proposed implementation in Section 7. If an extension requires changing the generic protocol description as described in Section 4, then the change may have to result in changes to the draft specification here also. Each new extension that modifies the internals of the protocol in either of the two ways MUST re-justify that the extended protocol still satisfies the security requirements in Section 6. Protocol extensions MAY put forward new security guarantees if they are applicable. Celi, et al. Expires 23 August 2021 [Page 21] Internet-Draft PP protocol February 2021 The extensions MUST also conform with the extension framework policy as set out in the architectural framework document. For example, this may concern any potential impact on client anonymity that the extension may introduce. 10. References 10.1. Normative References [draft-davidson-pp-architecture] Davidson, A., "Privacy Pass: Architectural Framework", n.d., . [draft-svaldez-pp-http-api] Valdez, S., "Privacy Pass: HTTP API", n.d., . [I-D.irtf-cfrg-voprf] Davidson, A., Faz-Hernandez, A., Sullivan, N., and C. Wood, "Oblivious Pseudorandom Functions (OPRFs) using Prime-Order Groups", Work in Progress, Internet-Draft, draft-irtf-cfrg-voprf-05, 2 November 2020, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, . 10.2. Informative References [Brave] "Brave Rewards", n.d., . [DGSTV18] "Privacy Pass, Bypassing Internet Challenges Anonymously", n.d., . [KLOR20] "Anonymous Tokens with Private Metadata Bit", n.d., . Celi, et al. Expires 23 August 2021 [Page 22] Internet-Draft PP protocol February 2021 [OpenPrivacy] "Token Based Services - Differences from PrivacyPass", n.d., . [PrivateStorage] Steininger, L., "The Path from S4 to PrivateStorage", n.d., . [RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049, October 2013, . [RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March 2014, . [TrustTokenAPI] WICG, ., "Trust Token API", n.d., . Appendix A. Document contributors * Alex Davidson (alex.davidson92@gmail.com) * Sofia Celi (cherenkov@riseup.net) * Christopher Wood (caw@heapingbits.net) Authors' Addresses Sofía Celi Cloudflare Lisbon Portugal Email: sceli@cloudflare.com Alex Davidson LIP Lisbon Portugal Email: alex.davidson92@gmail.com Celi, et al. Expires 23 August 2021 [Page 23] Internet-Draft PP protocol February 2021 Armando Faz-Hernandez Cloudflare 101 Townsend St San Francisco, United States of America Email: armfazh@cloudflare.com Celi, et al. Expires 23 August 2021 [Page 24]