Network Working Group R. Robert Internet-Draft Phoenix R&D Intended status: Standards Track C. A. Wood Expires: 12 March 2024 Cloudflare 9 September 2023 Batched Token Issuance Protocol draft-ietf-privacypass-batched-tokens-00 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 isse 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 12 March 2024. Copyright Notice Copyright (c) 2023 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 12 March 2024 [Page 1] Internet-Draft Batched Tokens September 2023 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 2 3. Client-to-Issuer Request . . . . . . . . . . . . . . . . . . 3 4. Issuer-to-Client Response . . . . . . . . . . . . . . . . . . 4 5. Finalization . . . . . . . . . . . . . . . . . . . . . . . . 6 6. Security considerations . . . . . . . . . . . . . . . . . . . 8 7. IANA considerations . . . . . . . . . . . . . . . . . . . . . 8 7.1. Token Type . . . . . . . . . . . . . . . . . . . . . . . 8 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 8.1. Normative References . . . . . . . . . . . . . . . . . . 8 8.2. Informative References . . . . . . . . . . . . . . . . . 9 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9 1. Introduction This document specifies a variant of the Privacy Pass issuance protocol (as defined in [ARCH]) that allows for batched issuance of tokens. This allows clients to request more than one token at a time and for issuers to isse more than one token at a time. The base Privacy Pass issuance protocol [ISSUANCE] 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], the cost of doing so scales linearly with the number of instances. This variant builds upon the privately verifiable issuance protocol 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, batched issuance performance scales better than linearly. This issuance protocol registers the batched token type (Section 7.1), to be used with the PrivateToken HTTP authentication scheme defined in [AUTHSCHEME]. 2. Motivation Privately Verifiable Tokens (as defines in [ISSUANCE]) offer a simple way to unlink the issuance from the redemption. The base protocol however only allows 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. The Batched Token Issuance Protocol improves upon the basic Privately Verifiable Token issuance protocol Robert & Wood Expires 12 March 2024 [Page 2] Internet-Draft Batched Tokens September 2023 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. 3. Client-to-Issuer Request Except where specified otherwise, the client follows the same protocol as described in [ISSUANCE], Section 5.1. The Client first creates a context as follows: client_context = SetupVOPRFClient("ristretto255-SHA512", pkI) Here, "ristretto255-SHA512" is the identifier corresponding to the OPRF(ristretto255, SHA-512) ciphersuite in [OPRF]. SetupVOPRFClient is defined in [OPRF], Section 3.2. 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(0xF91A, nonce_i, challenge_digest, key_id) blind_i, blinded_element_i = client_context.Blind(token_input) The above is repeated for each token to be requested. Importantly, a fresh nonce MUST be sampled each time. The Client then creates a TokenRequest structured as follows: struct { uint8_t blinded_element[Ne]; } BlindedElement; struct { uint16_t token_type = 0xF91A; uint8_t token_key_id; BlindedElement blinded_elements<0..2^16-1>; } TokenRequest; Robert & Wood Expires 12 March 2024 [Page 3] Internet-Draft Batched Tokens September 2023 The structure fields are defined as follows: * "token_type" is a 2-octet integer, which matches the type in the challenge. * "token_key_id" is the least significant byte of the key_id in network byte order (in other words, the last 8 bits of 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. Upon receipt of the request, the Issuer validates the following conditions: * The TokenRequest contains a supported token_type equal to 0xF91A. * The TokenRequest.token_key_id corresponds to a key ID of a Public Key owned by the issuer. * Nr, as determined based on the size of TokenRequest.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 400 error to the client. 4. Issuer-to-Client Response Except where specified otherwise, the client follows the same protocol as described in [ISSUANCE], Section 5.2. Upon receipt of a TokenRequest, the Issuer tries to deseralize the i-th element of TokenRequest.blinded_elements using DeserializeElement from Section 2.1 of [OPRF], yielding blinded_element_i of type Element. If this fails for any of the TokenRequest.blinded_elements values, the Issuer MUST return an HTTP 400 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("ristretto255-SHA512", skI, pkI) evaluated_elements, proof = server_context.BlindEvaluateBatch(skI, blinded_elements) Robert & Wood Expires 12 March 2024 [Page 4] Internet-Draft Batched Tokens September 2023 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 TokenResponse structured as follows: struct { uint8_t evaluated_element[Ne]; } EvaluatedElement; struct { EvaluatedElement evaluated_elements<0..2^16-1>; uint8_t evaluated_proof[Ns + Ns]; } TokenResponse; The structure fields are defined as follows: * "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. Robert & Wood Expires 12 March 2024 [Page 5] Internet-Draft Batched Tokens September 2023 * "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. 5. 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 12 March 2024 [Page 6] Internet-Draft Batched Tokens September 2023 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 as follows, 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 3: struct { uint16_t token_type = 0xF91A uint8_t nonce[32]; uint8_t challenge_digest[32]; uint8_t token_key_id[32]; uint8_t authenticator[Nh]; } Token; Robert & Wood Expires 12 March 2024 [Page 7] Internet-Draft Batched Tokens September 2023 If the FinalizeBatch function fails, the Client aborts the protocol. 6. Security considerations 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. 7. IANA considerations 7.1. Token Type This document updates the "Token Type" Registry ([AUTHSCHEME]) with the following value: +======+================+==========+========+========+==+=========+ |Value | Name |Publicly |Public |Private |Nk|Reference| | | |Verifiable|Metadata|Metadata| | | +======+================+==========+========+========+==+=========+ |0xF91A| Batched Token |N |N |N |32|This | | | VOPRF | | | | |document | | | (ristretto255, | | | | | | | | SHA-512) | | | | | | +------+----------------+----------+--------+--------+--+---------+ Table 1: Token Types 8. References 8.1. Normative References [ARCH] Davidson, A., Iyengar, J., and C. A. Wood, "The Privacy Pass Architecture", Work in Progress, Internet-Draft, draft-ietf-privacypass-architecture-14, 9 August 2023, . [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-12, 9 August 2023, . Robert & Wood Expires 12 March 2024 [Page 8] Internet-Draft Batched Tokens September 2023 [ISSUANCE] Celi, S., Davidson, A., Valdez, S., and C. A. Wood, "Privacy Pass Issuance Protocol", Work in Progress, Internet-Draft, draft-ietf-privacypass-protocol-12, 16 August 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, . 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 Christopher A. Wood Cloudflare Email: caw@heapingbits.net Robert & Wood Expires 12 March 2024 [Page 9]