Network Working Group R. Robert Internet-Draft Phoenix R&D Intended status: Standards Track C. A. Wood Expires: 9 May 2025 Cloudflare 5 November 2024 Batched Token Issuance Protocol draft-ietf-privacypass-batched-tokens-03 Abstract This document specifies a variant of the Privacy Pass issuance protocol that allows for batched issuance of tokens. This allows clients to request more than one token at a time and for issuers to issue more than one token at a time. 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 9 May 2025. Copyright Notice Copyright (c) 2024 IETF Trust and the persons identified as the document authors. All rights reserved. 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 Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Robert & Wood Expires 9 May 2025 [Page 1] Internet-Draft Batched Tokens November 2024 Table of Contents 1. Change Log: . . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 4 4. Batched Privately Verifiable Token . . . . . . . . . . . . . 4 4.1. Client-to-Issuer Request . . . . . . . . . . . . . . . . 4 4.2. Issuer-to-Client Response . . . . . . . . . . . . . . . . 6 4.3. Finalization . . . . . . . . . . . . . . . . . . . . . . 8 5. Arbitrary Batched Token Issuance . . . . . . . . . . . . . . 10 5.1. Client-to-Issuer Request . . . . . . . . . . . . . . . . 10 5.2. Issuer-to-Client Response . . . . . . . . . . . . . . . . 11 5.3. Finalization . . . . . . . . . . . . . . . . . . . . . . 12 6. Security considerations . . . . . . . . . . . . . . . . . . . 12 6.1. Batched Privately Verifiable Tokens . . . . . . . . . . . 12 6.2. Arbitrary Batched Verifiable Tokens . . . . . . . . . . . 12 7. IANA considerations . . . . . . . . . . . . . . . . . . . . . 12 7.1. Token Type . . . . . . . . . . . . . . . . . . . . . . . 13 7.2. Media Types . . . . . . . . . . . . . . . . . . . . . . . 13 7.2.1. "application/ private-token-privately-verifiable-batch-request" media type . . . . . . . . . . . . . . . . . . . . . . . . 14 7.2.2. "application/ private-token-privately-verifiable-batch-response" media type . . . . . . . . . . . . . . . . . . . . . . . . 14 7.2.3. "application/private-token-arbitrary-batch-request" media type . . . . . . . . . . . . . . . . . . . . . 15 7.2.4. "application/private-token-arbitrary-batch-response" media type . . . . . . . . . . . . . . . . . . . . . 15 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 16 8.1. Normative References . . . . . . . . . . . . . . . . . . 16 8.2. Informative References . . . . . . . . . . . . . . . . . 16 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16 1. Change Log: RFC EDITOR PLEASE DELETE THIS SECTION. draft-03 * Arbitrary token types * Error code 400 aligned with RFC9578 and replaced by error code 422 draft-02 * Renaming TokenRequest to BatchTokenRequest and TokenResponse to BatchTokenResponse Robert & Wood Expires 9 May 2025 [Page 2] Internet-Draft Batched Tokens November 2024 * IANA: Media types for BatchTokenRequest and BatchTokenResponse * IANA: Expand Token Type registry entry * Various editorial fixes draft-01 * Initial WG document version 2. Introduction This document specifies two Privacy Pass issuance protocols (as defined in [RFC9576]) that allow for batched issuance of tokens. This allows clients to request more than one token at a time and for issuers to issue more than one token at a time. The base Privacy Pass issuance protocol [RFC9578] defines stateless anonymous tokens, which can either be publicly verifiable or not. While it is possible to run multiple instances of the issuance protocol in parallel, e.g., over a multiplexed transport such as HTTP/3 [HTTP3] or by orchestrating multiple HTTP requests, these ad- hoc solutions vary based on transport protocol support. In addition, in some cases, they cannot take advantage of cryptographic optimizations. The first variant of the issuance protocol builds upon the privately verifiable issuance protocol in [RFC9578] that uses VOPRF [OPRF], and allows for batched issuance of tokens. This allows clients to request more than one token at a time and for issuers to issue more than one token at a time. In effect, private batched issuance performance scales better than linearly. The second variant of the issuance protocol introduces a new Client- Issuer communication method, which allows for batched issuance of arbitrary token types. This allows clients to request more than one token at a time and for issuers to issue more than one token at a time. This variant has no other effect than batching requests and responses and the issuance performance remains linear. This batched issuance protocol registers one new token type (Section 7.1), to be used with the PrivateToken HTTP authentication scheme defined in [AUTHSCHEME]. Robert & Wood Expires 9 May 2025 [Page 3] Internet-Draft Batched Tokens November 2024 3. Motivation Privacy Pass tokens (as defined in [RFC9576] and [RFC9578]) are unlinkable during issuance and redemption. The basic issuance protocols defined in [RFC9578] however only allow for a single token to be issued at a time for every challenge. In some cases, especially where a large number of clients need to fetch a large number of tokens, this may introduce performance bottlenecks. Batched token issuance improves upon the basic Privately Verifiable Token issuance protocol in the following key ways: 1. Issuing multiple tokens at once in response to a single TokenChallenge, thereby reducing the size of the proofs required for multiple tokens. 2. Improving server and client issuance efficiency by amortizing the cost of the VOPRF proof generation and verification, respectively. For all Verifiable Token issuance protocol, it allows for a single TokenRequest to be sent that encompasses multiple token requests. This enables the issuance of tokens for more than one key in one round trip between the Client and the Issuer. The cost remains linear. 4. Batched Privately Verifiable Token This section describes a batched issuance protocol for select token types, including 0x0001 (defined in [RFC9578]) and 0xF91A (defined in this document). This variant is more efficient than Arbitary Batch Token Issuance defined below. It does so by requiring the same key to be used by all token requests. 4.1. Client-to-Issuer Request Except where specified otherwise, the client follows the same protocol as described in [RFC9578], Section 5.1. The Client first creates a context as follows: client_context = SetupVOPRFClient(ciphersuiteID, pkI) ciphersuiteID is the ciphersuite identifier from [OPRF] corresponding to the ciphersuite being used for this token version. SetupVOPRFClient is defined in [OPRF], Section 3.2. Robert & Wood Expires 9 May 2025 [Page 4] Internet-Draft Batched Tokens November 2024 Nr denotes the number of tokens the clients wants to request. For every token, the Client then creates an issuance request message for a random value nonce with the input challenge and Issuer key identifier as described below: nonce_i = random(32) challenge_digest = SHA256(challenge) token_input = concat(token_type, nonce_i, challenge_digest, token_key_id) blind_i, blinded_element_i = client_context.Blind(token_input) token_type corresponds to the 2-octet integer in the challenge. The above is repeated for each token to be requested. Importantly, a fresh nonce MUST be sampled each time. The Client then creates a BatchTokenRequest structured as follows: struct { uint8_t blinded_element[Ne]; } BlindedElement; struct { uint16_t token_type; uint8_t truncated_token_key_id; BlindedElement blinded_elements<0..2^16-1>; } BatchTokenRequest; The structure fields are defined as follows: * "token_type" is a 2-octet integer, which matches the type in the challenge. * "truncated_token_key_id" is the least significant byte of the token_key_id in network byte order (in other words, the last 8 bits of token_key_id). * "blinded_elements" is a list of Nr serialized elements, each of length Ne bytes and computed as SerializeElement(blinded_element_i), where blinded_element_i is the i-th output sequence of Blind invocations above. Ne is as defined in [OPRF], Section 4. The Client then generates an HTTP POST request to send to the Issuer Request URL, with the BatchTokenRequest as the content. The media type for this request is "application/private-token-privately- verifiable-batch-request". An example request for the Issuer Request URL "https://issuer.example.net/request" is shown below. Robert & Wood Expires 9 May 2025 [Page 5] Internet-Draft Batched Tokens November 2024 POST /request HTTP/1.1 Host: issuer.example.net Accept: application/private-token-privately-verifiable-batch-response Content-Type: application/private-token-privately-verifiable-batch-request Content-Length: 4.2. Issuer-to-Client Response Except where specified otherwise, the client follows the same protocol as described in [RFC9578], Section 5.2. Upon receipt of the request, the Issuer validates the following conditions: * The BatchTokenRequest contains a supported token_type equal to one of the batched token types defined in this document. * The BatchTokenRequest.truncated_token_key_id corresponds to a key ID of a Public Key owned by the issuer. * Nr, as determined based on the size of BatchTokenRequest.blinded_elements, is less than or equal to the number of tokens that the issuer can issue in a single batch. If any of these conditions is not met, the Issuer MUST return an HTTP 422 (Unprocessable Content) error to the client. The Issuer then tries to deseralize the i-th element of BatchTokenRequest.blinded_elements using DeserializeElement from Section 2.1 of [OPRF], yielding blinded_element_i of type Element. If this fails for any of the BatchTokenRequest.blinded_elements values, the Issuer MUST return an HTTP 422 (Unprocessable Content) error to the client. Otherwise, if the Issuer is willing to produce a token to the Client, the issuer forms a list of Element values, denoted blinded_elements, and computes a blinded response as follows: server_context = SetupVOPRFServer(ciphersuiteID, skI, pkI) evaluated_elements, proof = server_context.BlindEvaluateBatch(skI, blinded_elements) Robert & Wood Expires 9 May 2025 [Page 6] Internet-Draft Batched Tokens November 2024 ciphersuiteID is the ciphersuite identifier from [OPRF] corresponding to the ciphersuite being used for this token version. SetupVOPRFServer is defined in [OPRF], Section 3.2. The issuer uses a list of blinded elements to compute in the proof generation step. The BlindEvaluateBatch function is a batch-oriented version of the BlindEvaluate function described in [OPRF], Section 3.3.2. The description of BlindEvaluateBatch is below. Input: Element blindedElements[Nr] Output: Element evaluatedElements[Nr] Proof proof Parameters: Group G Scalar skS Element pkS def BlindEvaluateBatch(blindedElements): evaluatedElements = [] for blindedElement in blindedElements: evaluatedElements.append(skS * blindedElement) proof = GenerateProof(skS, G.Generator(), pkS, blindedElements, evaluatedElements) return evaluatedElements, proof The Issuer then creates a BatchTokenResponse structured as follows: struct { uint8_t evaluated_element[Ne]; } EvaluatedElement; struct { EvaluatedElement evaluated_elements<0..2^16-1>; uint8_t evaluated_proof[Ns + Ns]; } BatchTokenResponse; The structure fields are defined as follows: Robert & Wood Expires 9 May 2025 [Page 7] Internet-Draft Batched Tokens November 2024 * "evaluated_elements" is a list of Nr serialized elements, each of length Ne bytes and computed as SerializeElement(evaluate_element_i), where evaluate_element_i is the i-th output of BlindEvaluate. * "evaluated_proof" is the (Ns+Ns)-octet serialized proof, which is a pair of Scalar values, computed as concat(SerializeScalar(proof[0]), SerializeScalar(proof[1])), where Ns is as defined in [OPRF], Section 4. The Issuer generates an HTTP response with status code 200 whose content consists of TokenResponse, with the content type set as "application/private-token-privately-verifiable-batch-response". HTTP/1.1 200 OK Content-Type: application/private-token-privately-verifiable-batch-response Content-Length: 4.3. Finalization Upon receipt, the Client handles the response and, if successful, deserializes the body values TokenResponse.evaluate_response and TokenResponse.evaluate_proof, yielding evaluated_elements and proof. If deserialization of either value fails, the Client aborts the protocol. Otherwise, the Client processes the response as follows: authenticator_values = client_context.FinalizeBatch(token_input, blind, evaluated_elements, blinded_elements, proof) The FinalizeBatch function is a batched variant of the Finalize function as defined in [OPRF], Section 3.3.2. FinalizeBatch accepts lists of evaluated elements and blinded elements as input parameters, and is implemented as described below: Robert & Wood Expires 9 May 2025 [Page 8] Internet-Draft Batched Tokens November 2024 Input: PrivateInput input Scalar blind Element evaluatedElements[Nr] Element blindedElements[Nr] Proof proof Output: opaque output[Nh * Nr] Parameters: Group G Element pkS Errors: VerifyError def FinalizeBatch(input, blind, evaluatedElements, blindedElements, proof): if VerifyProof(G.Generator(), pkS, blindedElements, evaluatedElements, proof) == false: raise VerifyError output = nil for evaluatedElement in evaluatedElements: N = G.ScalarInverse(blind) * evaluatedElement unblindedElement = G.SerializeElement(N) hashInput = I2OSP(len(input), 2) || input || I2OSP(len(unblindedElement), 2) || unblindedElement || "Finalize" output = concat(output, Hash(hashInput)) return output If this succeeds, the Client then constructs Nr Token values, where authenticator is the i-th Nh-byte length slice of authenticator_values that corresponds to nonce, the i-th nonce that was sampled in Section 4.1: struct { uint16_t token_type; uint8_t nonce[32]; uint8_t challenge_digest[32]; uint8_t token_key_id[32]; uint8_t authenticator[Nh]; } Token; Robert & Wood Expires 9 May 2025 [Page 9] Internet-Draft Batched Tokens November 2024 If the FinalizeBatch function fails, the Client aborts the protocol. Token verification works exactly as specified in [RFC9578]. 5. Arbitrary Batched Token Issuance This section describes an issuance protocol mechanism for issuing multiple tokens in one round trip between Client and Issuer. An arbitrary batched token request can contain token requests for any token type. 5.1. Client-to-Issuer Request The Client first creates all TokenRequest it wants to batch. To do so, the client follows protocol describing issuance, such as [RFC9578], Section 5.1 or [RFC9578], Section 6.1. The Client then creates a BatchedTokenRequest structured as follows: struct { uint16_t token_type; select (token_type) { case (0x0001): /* Type VOPRF(P-384, SHA-384), RFC 9578 */ uint8_t truncated_token_key_id; uint8_t blinded_msg[Ne]; case (0x0002): /* Type Blind RSA (2048-bit), RFC 9578 */ uint8_t truncated_token_key_id; uint8_t blinded_msg[Nk]; } } TokenRequest; struct { TokenRequest token_requests<0..2^16-1>; } BatchTokenRequest The structure fields are defined as follows: * "token_type" is a 2-octet integer. TokenRequest MUST be prefixed with a uint16 "token_type" indicating the token type. The rest of the structure follows based on that type, within the inner opaque token_request attribute. The above definition corresponds to TokenRequest from [RFC9578]. For TokenRequest not defined in [RFC9578], they MAY be used as long as they are prefixed with a 2-octet token_type. Robert & Wood Expires 9 May 2025 [Page 10] Internet-Draft Batched Tokens November 2024 * "token_requests" are serialized TokenRequests, in network byte order. The number of token_requests, as a 2-octet integer, is prepended to the serialized TokenRequests. In addition, the 2-octet integer length of each TokenRequest is prepended to the serialized TokenRequests. The Client then generates an HTTP POST request to send to the Issuer Request URL, with the BatchTokenRequest as the content. The media type for this request is "application/private-token-arbitrary-batch- request". An example request for the Issuer Request URL "https://issuer.example.net/request" is shown below. POST /request HTTP/1.1 Host: issuer.example.net Accept: application/private-token-arbitrary-batch-response Content-Type: application/private-token-arbitrary-batch-request Content-Length: 5.2. Issuer-to-Client Response Upon receipt of the request, the Issuer validates the following conditions: * The Content-Type is application/private-token-arbitrary-batch- request as registered with IANA. If this condition is not met, the Issuer MUST return an HTTP 422 (Unprocessable Content) error to the client. The Issuer then tries to deserialize the first 2 bytes of the i-th element of BatchTokenRequest.token_requests. If this is not a token type registered with IANA, the Issuer MUST return an HTTP 422 (Unprocessable Content) error to the client. The issuer creates a BatchTokenResponse structured as follows: struct { TokenResponse token_response<0..2^16-1>; /* Defined by token_type */ } OptionalTokenResponse; struct { OptionalTokenResponse token_responses<0..2^16-1>; } BatchTokenResponse Robert & Wood Expires 9 May 2025 [Page 11] Internet-Draft Batched Tokens November 2024 BatchTokenResponse.token_responses is a vector of OptionalTokenResponses, length prefixed with two bytes. OptionalTokenResponse.token_response is a length-prefix-encoded TokenResponse, where a length of 0 indicates that the Issuer failed or refused to issue the associated TokenRequest. The Issuer generates an HTTP response with status code 200 whose content consists of TokenResponse, with the content type set as "application/private-token-arbitrary-batch-response". If the Issuer issues some tokens but not all, it MUST return an HTTP 206 to the client and continue processing further requests. HTTP/1.1 200 OK Content-Type: application/private-token-arbitrary-batch-response Content-Length: 5.3. Finalization The Client tries to deserialize the i-th element of BatchTokenResponse.token_responses using the protocol associated to BatchTokenRequest.token_type. If the element has a size of 0, the Client MUST ignore this token, and continue processing the next token. The Client finalizes each deserialized TokenResponse using the matching TokenRequest according to the corresponding finalization procedure defined by the token type. 6. Security considerations 6.1. Batched Privately Verifiable Tokens Implementors SHOULD be aware of the security considerations described in [OPRF], Section 6.2.3 and implement mitigation mechanisms. Application can mitigate this issue by limiting the number of clients and limiting the number of token requests per client per key. 6.2. Arbitrary Batched Verifiable Tokens Implementors SHOULD be aware of the inherent linear cost of this token type. An Issuer MAY ignore TokenRequest if the number of tokens per request past a limit. 7. IANA considerations This section contains IANA codepoint allocation requests. Robert & Wood Expires 9 May 2025 [Page 12] Internet-Draft Batched Tokens November 2024 7.1. Token Type This document updates the "Token Type" Registry ([AUTHSCHEME]) with the following entry: * Value: 0xF91A * Name: VOPRF (ristretto255, SHA-512) * Token Structure: As defined in Section 2.2 of [AUTHSCHEME] * Token Key Encoding: Serialized using SerializeElement from Section 2.1 of [OPRF] * TokenChallenge Structure: As defined in Section 2.1 of [AUTHSCHEME] * Publicly Verifiable: N * Public Metadata: N * Private Metadata: N * Nk: 32 * Nid: 32 * Reference: [RFC9578], Section 5 * Notes: None 7.2. Media Types The following entries should be added to the IANA "media types" registry: * "application/private-token-privately-verifiable-batch-request" * "application/private-token-privately-verifiable-batch-response" * "application/private-token-arbitrary-batch-request" * "application/private-token-arbitrary-batch-response" The templates for these entries are listed below and the reference should be this RFC. Robert & Wood Expires 9 May 2025 [Page 13] Internet-Draft Batched Tokens November 2024 7.2.1. "application/private-token-privately-verifiable-batch-request" media type Type name: application Subtype name: private-token-request Required parameters: N/A Optional parameters: N/A Encoding considerations: "binary" Security considerations: see Section 6 Interoperability considerations: N/A Published specification: this specification Applications that use this media type: Applications that want to issue or facilitate issuance of Privacy Pass tokens, including Privacy Pass issuer applications themselves. Fragment identifier considerations: N/A Additional information: Magic number(s): N/A Deprecated alias names for this type: N/A File extension(s): N/A Macintosh file type code(s): N/A Person and email address to contact for further information: see Aut hors' Addresses section Intended usage: COMMON Restrictions on usage: N/A Author: see Authors' Addresses section Change controller: IETF 7.2.2. "application/private-token-privately-verifiable-batch-response" media type Type name: application Subtype name: private-token-response Required parameters: N/A Optional parameters: N/A Encoding considerations: "binary" Security considerations: see Section 6 Interoperability considerations: N/A Published specification: this specification Applications that use this media type: Applications that want to issue or facilitate issuance of Privacy Pass tokens, including Privacy Pass issuer applications themselves. Fragment identifier considerations: N/A Additional information: Magic number(s): N/A Deprecated alias names for this type: N/A File extension(s): N/A Macintosh file type code(s): N/A Person and email address to contact for further information: see Aut hors' Addresses section Intended usage: COMMON Robert & Wood Expires 9 May 2025 [Page 14] Internet-Draft Batched Tokens November 2024 Restrictions on usage: N/A Author: see Authors' Addresses section Change controller: IETF 7.2.3. "application/private-token-arbitrary-batch-request" media type Type name: application Subtype name: private-token-request Required parameters: N/A Optional parameters: N/A Encoding considerations: "binary" Security considerations: see Section 6 Interoperability considerations: N/A Published specification: this specification Applications that use this media type: Applications that want to issue or facilitate issuance of Privacy Pass tokens, including Privacy Pass issuer applications themselves. Fragment identifier considerations: N/A Additional information: Magic number(s): N/A Deprecated alias names for this type: N/A File extension(s): N/A Macintosh file type code(s): N/A Person and email address to contact for further information: see Aut hors' Addresses section Intended usage: COMMON Restrictions on usage: N/A Author: see Authors' Addresses section Change controller: IETF 7.2.4. "application/private-token-arbitrary-batch-response" media type Type name: application Subtype name: private-token-response Required parameters: N/A Optional parameters: N/A Encoding considerations: "binary" Security considerations: see Section 6 Interoperability considerations: N/A Published specification: this specification Applications that use this media type: Applications that want to issue or facilitate issuance of Privacy Pass tokens, including Privacy Pass issuer applications themselves. Fragment identifier considerations: N/A Additional information: Magic number(s): N/A Deprecated alias names for this type: N/A File extension(s): N/A Macintosh file type code(s): N/A Person and email address to contact for further information: see Aut Robert & Wood Expires 9 May 2025 [Page 15] Internet-Draft Batched Tokens November 2024 hors' Addresses section Intended usage: COMMON Restrictions on usage: N/A Author: see Authors' Addresses section Change controller: IETF 8. References 8.1. Normative References [AUTHSCHEME] Pauly, T., Valdez, S., and C. A. Wood, "The Privacy Pass HTTP Authentication Scheme", Work in Progress, Internet- Draft, draft-ietf-privacypass-auth-scheme-15, 23 October 2023, . [OPRF] Davidson, A., Faz-Hernandez, A., Sullivan, N., and C. A. Wood, "Oblivious Pseudorandom Functions (OPRFs) using Prime-Order Groups", Work in Progress, Internet-Draft, draft-irtf-cfrg-voprf-21, 21 February 2023, . [RFC9576] Davidson, A., Iyengar, J., and C. A. Wood, "The Privacy Pass Architecture", Work in Progress, Internet-Draft, draft-ietf-privacypass-architecture-16, 25 September 2023, . [RFC9578] Celi, S., Davidson, A., Valdez, S., and C. A. Wood, "Privacy Pass Issuance Protocol", Work in Progress, Internet-Draft, draft-ietf-privacypass-protocol-16, 3 October 2023, . 8.2. Informative References [HTTP3] Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114, June 2022, . Authors' Addresses Raphael Robert Phoenix R&D Email: ietf@raphaelrobert.com Robert & Wood Expires 9 May 2025 [Page 16] Internet-Draft Batched Tokens November 2024 Christopher A. Wood Cloudflare Email: caw@heapingbits.net Robert & Wood Expires 9 May 2025 [Page 17]