Network Working Group T. Pauly Internet-Draft Apple Inc. Intended status: Standards Track S. Valdez Expires: 10 February 2024 Google LLC C. A. Wood Cloudflare 9 August 2023 The Privacy Pass HTTP Authentication Scheme draft-ietf-privacypass-auth-scheme-12 Abstract This document defines an HTTP authentication scheme for Privacy Pass, a privacy-preserving authentication mechanism used for authorization. The authentication scheme in this document can be used by clients to redeem Privacy Pass tokens with an origin. It can also be used by origins to challenge clients to present Privacy Pass tokens. 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 10 February 2024. Copyright Notice Copyright (c) 2023 IETF Trust and the persons identified as the document authors. All rights reserved. Pauly, et al. Expires 10 February 2024 [Page 1] Internet-Draft Privacy Pass Authentication August 2023 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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 2. HTTP Authentication Scheme . . . . . . . . . . . . . . . . . 4 2.1. Token Challenge . . . . . . . . . . . . . . . . . . . . . 4 2.1.1. Redemption Context Construction . . . . . . . . . . . 8 2.1.2. Token Caching . . . . . . . . . . . . . . . . . . . . 9 2.2. Token Redemption . . . . . . . . . . . . . . . . . . . . 10 2.2.1. Token Verification . . . . . . . . . . . . . . . . . 11 3. User Interaction . . . . . . . . . . . . . . . . . . . . . . 12 4. Security Considerations . . . . . . . . . . . . . . . . . . . 14 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 5.1. Authentication Scheme . . . . . . . . . . . . . . . . . . 15 5.2. Token Type Registry . . . . . . . . . . . . . . . . . . . 16 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 18 6.1. Normative References . . . . . . . . . . . . . . . . . . 18 6.2. Informative References . . . . . . . . . . . . . . . . . 19 Appendix A. Test Vectors . . . . . . . . . . . . . . . . . . . . 20 A.1. Challenge and Redemption Structure Test Vectors . . . . . 20 A.2. HTTP Header Test Vectors . . . . . . . . . . . . . . . . 23 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25 1. Introduction Privacy Pass tokens are unlinkable, one-time-use authenticators that can be used to anonymously authorize a client (see [ARCHITECTURE]). Tokens are generated by token issuers, on the basis of authentication, attestation, or some previous action such as solving a CAPTCHA. A client possessing such a token is able to prove that it was able to get a token issued, without allowing the relying party redeeming the client's token (the origin) to link it with the issuance flow. Different types of authenticators, using different token issuance protocols, can be used as Privacy Pass tokens. Pauly, et al. Expires 10 February 2024 [Page 2] Internet-Draft Privacy Pass Authentication August 2023 This document defines a common HTTP authentication scheme ([RFC9110], Section 11), PrivateToken, that allows clients to redeem various kinds of Privacy Pass tokens. Clients and relying parties (origins) interact using this scheme to perform the token challenge and token redemption flow. In particular, origins challenge clients for a token with an HTTP Authentication challenge (using the WWW-Authenticate response header field). Clients then respond to that challenge with an HTTP authentication response (using the Authorization request header field). Clients produce an authentication response based on the origin's token challenge by running the token issuance protocol [ISSUANCE]. The act of presenting a token in an Authorization request header is referred to as token redemption. This interaction between client and origin is shown below. +--------+ +--------+ | Origin | | Client | +---+----+ +---+----+ | | +-- WWW-Authenticate: TokenChallenge -->| | | // Run issuance protocol <------- Authorization: Token ----------+ | | Figure 1: Challenge-response redemption protocol flow In addition to working with different token issuance protocols, this scheme optionally supports use of tokens that are associated with origin-chosen contexts and specific origin names. Relying parties that request and redeem tokens can choose a specific kind of token, as appropriate for its use case. These options allow for different deployment models to prevent double-spending, and allow for both interactive (online challenges) and non-interactive (pre-fetched) tokens. 1.1. 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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. Unless otherwise specified, this document encodes protocol messages in TLS notation from [TLS13], Section 3. Pauly, et al. Expires 10 February 2024 [Page 3] Internet-Draft Privacy Pass Authentication August 2023 This document uses the terms "Client", "Origin", "Issuer", "Issuance Protocol", and "Token" as defined in [ARCHITECTURE]. It additionally uses the following terms in more specific ways: * Issuer key: Keying material that can be used with an issuance protocol to create a signed token. * Token challenge: A requirement for tokens sent from an origin to a client, using the "WWW-Authenticate" HTTP header field. This challenge is bound to a specific token issuer and issuance protocol, and may be additionally bound to a specific context or origin name. * Token redemption: An action by which a client presents a token to an origin in an HTTP request, using the "Authorization" HTTP header field. 2. HTTP Authentication Scheme Token redemption is performed using HTTP Authentication ([RFC9110], Section 11), with the scheme "PrivateToken". Origins challenge clients to present a token from a specific issuer (Section 2.1). Once a client has received a token from that issuer, or already has a valid token available, it presents the token to the origin (Section 2.2). Unlike many authentication schemes in which a client will present the same credentials across multiple requests, tokens used with the "PrivateToken" scheme are single-use credentials, and are not reused. Spending the same token value more than once allows the origin to link multiple transactions to the same client. In deployment scenarios where origins send token challenges to request tokens, origins ought to expect at most one request containing a token from the client in reaction to a particular challenge. The rest of this section describes the token challenge and redemption interactions in more detail. 2.1. Token Challenge Origins send a token challenge to clients in an "WWW-Authenticate" header field with the "PrivateToken" scheme. This challenge includes a TokenChallenge message, along with information about what keys to use when requesting a token from the issuer. Origins that support this authentication scheme need to handle the following tasks: Pauly, et al. Expires 10 February 2024 [Page 4] Internet-Draft Privacy Pass Authentication August 2023 1. Select which issuer to use, and configure the issuer name and token-key to include in WWW-Authenticate challenges. 2. Determine a redemption context construction to include in the TokenChallenge, as discussed in Section 2.1.1. 3. Select the origin information to include in the TokenChallenge. This can be empty to allow fully cross-origin tokens, a single origin name that matches the origin itself, or a list of origin names containing the origin itself. All token challenges MUST begin with a 2-octet integer that defines the token type, in network byte order. This type indicates the issuance protocol used to generate the token and determines the structure and semantics of the rest of the structure. Values are registered in an IANA registry, Section 5.2. Client MUST ignore challenges with token_types they do not support. This document defines the default challenge structure that can be used across token types, although future token types MAY extend or modify the structure of the challenge; see Section 5.2 for the registry information which establishes and defines the relationship between "token_type" and the contents of the TokenChallenge message. Even when a given token type uses the default challenge, structure, the requirements on the presence or interpretation of the fields can differ across token types. For example, some token types might require that "origin_info" is non-empty, while others allow it to be empty. The default TokenChallenge message has the following structure: struct { uint16_t token_type; opaque issuer_name<1..2^16-1>; opaque redemption_context<0..32>; opaque origin_info<0..2^16-1>; } TokenChallenge; The structure fields are defined as follows: * "token_type" is a 2-octet integer, in network byte order, as described above. Pauly, et al. Expires 10 February 2024 [Page 5] Internet-Draft Privacy Pass Authentication August 2023 * "issuer_name" is an ASCII string that identifies the issuer using the format of the authority portion of a URI as defined in Section 3.2 of [URI]. This name identifies the issuer that is allowed to issue tokens that can be redeemed by this origin. The field that stores this string in the challenge is prefixed with a 2-octet integer indicating the length, in network byte order. * "redemption_context" is a field that is either 0 or 32 bytes, prefixed with a single octet indicating the length (either 0 or 32). If value is non-empty, it is a 32-byte value generated by the origin that allows the origin to require that clients fetch tokens bound to a specific context, as opposed to reusing tokens that were fetched for other contexts. See Section 2.1.1 for example contexts that might be useful in practice. Challenges with redemption_context values of invalid lengths MUST be ignored. * "origin_info" is an ASCII string that is either empty, or contains one or more origin names that allow a token to be scoped to a specific set of origins. Each origin name uses the format of the authority portion of a URI as defined in Section 3.2 of [URI]. The string is prefixed with a 2-octet integer indicating the length, in network byte order. If empty, any non-origin-specific token can be redeemed. If the string contains multiple origin names, they are delimited with commas "," without any whitespace. If this field is not empty, the Origin MUST include its own name as one of the names in the list. When used in an authentication challenge, the "PrivateToken" scheme uses the following parameters: * "challenge", which contains a base64url-encoded [RFC4648] TokenChallenge value. This document follows the default padding behavior described in Section 3.2 of [RFC4648], so the base64url value MUST include padding. As an Authentication Parameter (auth- param from [RFC9110], Section 11.2), the value can be either a token or a quoted-string, and might be required to be a quoted- string if the base64url string includes "=" characters. This challenge value MUST be unique for every 401 HTTP response to prevent replay attacks. This parameter is required for all challenges. * "token-key", which contains a base64url encoding of the public key for use with the issuance protocol indicated by the challenge. See [ISSUANCE] for more information about how this public key is used by the issuance protocols in that specification. The encoding of the public key is determined by the token type; see Section 5.2. As with "challenge", the base64url value MUST include padding. As an Authentication Parameter (auth-param from Pauly, et al. Expires 10 February 2024 [Page 6] Internet-Draft Privacy Pass Authentication August 2023 [RFC9110], Section 11.2), the value can be either a token or a quoted-string, and might be required to be a quoted-string if the base64url string includes "=" characters. This parameter MAY be omitted in deployments where clients are able to retrieve the issuer key using an out-of-band mechanism. * "max-age", an optional parameter that consists of the number of seconds for which the challenge will be accepted by the origin. Clients MAY ignore the challenge, e.g., because the token-key is invalid or otherwise untrusted. The header field MAY also include the standard "realm" parameter, if desired. Issuance protocols MAY require other parameters. Clients SHOULD ignore unknown parameters in challenges, except if otherwise specified by issuance protocols. As an example, the WWW-Authenticate header field could look like this: WWW-Authenticate: PrivateToken challenge="abc...", token-key="123..." Upon receipt of this challenge, a client validates the TokenChallenge before responding to it. Validation requirements are as follows: * The token_type is recognized and supported by the client; * The TokenChallenge structure is well-formed; and * If the origin_info field is non-empty, the name of the origin that issued the authentication challenge is included in the list of origin names. Comparison of the origin name that issued the authentication challenge against elements in the origin_info list is done via case-insensitive equality checks. If validation fails, the client MUST NOT process or respond to the challenge. Clients MAY have further restrictions and requirements around validating when a challenge is considered acceptable or valid. For example, clients can choose to ignore challenges that list origin names for which current connection is not authoritative (according to the TLS certificate). Caching and pre-fetching of tokens is discussed in Section 2.1.2. Pauly, et al. Expires 10 February 2024 [Page 7] Internet-Draft Privacy Pass Authentication August 2023 Note that it is possible for the WWW-Authenticate header field to include multiple challenges. This allows the origin to indicate support for different token types, issuers, or to include multiple redemption contexts. For example, the WWW-Authenticate header field could look like this: WWW-Authenticate: PrivateToken challenge="abc...", token-key="123...", PrivateToken challenge="def...", token-key="234..." Origins should only include challenges for different types of issuance protocols with functionally equivalent properties. For instance, both issuance protocols in [ISSUANCE] have the same functional properties, albeit with different mechanisms for verifying the resulting tokens during redemption. Since clients are free to choose which challenge they want to consume when presented with options, mixing multiple challenges with different functional properties for one use case is nonsensical. If the origin has a preference for one challenge over another (for example, if one uses a token type that is faster to verify), it can sort it to be first in the list of challenges as a hint to the client. 2.1.1. Redemption Context Construction The TokenChallenge redemption context allows the origin to determine the context in which a given token can be redeemed. This value can be a unique per-request nonce, constructed from 32 freshly generated random bytes. It can also represent state or properties of the client session. Some example properties and methods for constructing the corresponding context are below. This list is not exhaustive. * Context bound to a given time window: Construct redemption context as F(current time window), where F is a pseudorandom function. * Context bound to a client network: Construct redemption context as F(client ASN), where F is a pseudorandom function. * Context bound to a given time window and client network: Construct redemption context as F(current time window, client ASN), where F is a pseudorandom function. Pauly, et al. Expires 10 February 2024 [Page 8] Internet-Draft Privacy Pass Authentication August 2023 An empty redemption context is not bound to any property of the client session. Preventing double spending on tokens requires the origin to keep state associated with the redemption context. The size of this state varies based on the size of the redemption context. For example, double spend state for unique, per-request redemption contexts only needs to exist within the scope of the request connection or session. In contrast, double spend state for empty redemption contexts must be stored and shared across all requests until token-key expiration or rotation. Origins that share redemption contexts, i.e., by using the same redemption context, choosing the same issuer, and providing the same origin_info field in the TokenChallenge, must necessarily share state required to enforce double spend prevention. Origins should consider the operational complexity of this shared state before choosing to share redemption contexts. Failure to successfully synchronize this state and use it for double spend prevention can allow Clients to redeem tokens to one Origin that were issued after an interaction with another Origin that shares the context. 2.1.2. Token Caching Clients can generate multiple tokens from a single TokenChallenge, and cache them for future use. This improves privacy by separating the time of token issuance from the time of token redemption, and also allows clients to avoid any overhead of receiving new tokens via the issuance protocol. Cached tokens can only be redeemed when they match all of the fields in the TokenChallenge: token_type, issuer_name, redemption_context, and origin_info. Clients ought to store cached tokens based on all of these fields, to avoid trying to redeem a token that does not match. Note that each token has a unique client nonce, which is sent in token redemption (Section 2.2). If a client fetches a batch of multiple tokens for future use that are bound to a specific redemption context (the redemption_context in the TokenChallenge was not empty), clients SHOULD discard these tokens upon flushing state such as HTTP cookies [COOKIES], or changing networks. Using these tokens in a context that otherwise would not be linkable to the original context could allow the origin to recognize a client. Pauly, et al. Expires 10 February 2024 [Page 9] Internet-Draft Privacy Pass Authentication August 2023 2.2. Token Redemption The output of the issuance protocol is a token that corresponds to the origin's challenge (see Section 2.1). A token is a structure that begins with a two-octet field that indicates a token type, which MUST match the token_type in the TokenChallenge structure. This value determines the structure and semantics of the rest of token structure. This document defines the default token structure that can be used across token types, although future token types MAY extend or modify the structure of the token; see Section 5.2 for the registry information which establishes and defines the relationship between "token_type" and the contents of the Token structure. The default Token message has the following structure: struct { uint16_t token_type; uint8_t nonce[32]; uint8_t challenge_digest[32]; uint8_t token_key_id[Nid]; uint8_t authenticator[Nk]; } Token; The structure fields are defined as follows: * "token_type" is a 2-octet integer, in network byte order, as described above. * "nonce" is a 32-octet value containing a client-generated random nonce. * "challenge_digest" is a 32-octet value containing the hash of the original TokenChallenge, SHA-256(TokenChallenge), where SHA-256 is as defined in [SHS]. Changing the hash function to something other than SHA-256 would require defining a new token type and token structure (since the contents of challenge_digest would be computed differently), which can be done in a future specification. * "token_key_id" is a Nid-octet identifier for the token authentication key. The value of this field is defined by the token_type and corresponding issuance protocol. * "authenticator" is a Nk-octet authenticator that is cryptographically bound to the preceding fields in the token; see Section 2.2.1 for more information about how this field is used in Pauly, et al. Expires 10 February 2024 [Page 10] Internet-Draft Privacy Pass Authentication August 2023 verifying a token. The token_type and corresponding issuance protocol determine the value of the authenticator field and how it is computed. The value of constant Nk depends on token_type, as defined in Section 5.2. The authenticator value in the Token structure is computed over the token_type, nonce, challenge_digest, and token_key_id fields. A token is considered a valid if token verification using succeeds; see Section 2.2.1 for details about verifying the token and its authenticator value. When used for client authorization, the "PrivateToken" authentication scheme defines one parameter, "token", which contains the base64url- encoded Token struct. As with the challenge parameters (Section 2.1), the base64url value MUST include padding. As an Authentication Parameter (auth-param from [RFC9110], Section 11.2), the value can be either a token or a quoted-string, and might be required to be a quoted-string if the base64url string includes "=" characters. All unknown or unsupported parameters to "PrivateToken" authentication credentials MUST be ignored. Clients present this Token structure to origins in a new HTTP request using the Authorization header field as follows: Authorization: PrivateToken token="abc..." For context-bound tokens, origins store or reconstruct the contexts of previous TokenChallenge structures in order to validate the token. A TokenChallenge can be bound to a specific TLS session with a client, but origins can also accept tokens for valid challenges in new sessions. Origins SHOULD implement some form of double-spend prevention that prevents a token with the same nonce from being redeemed twice. Double-spend prevention ensures that clients cannot replay tokens for previous challenges. For context-bound tokens, this double-spend prevention can require no state or minimal state, since the context can be used to verify token uniqueness. If a client is unable to fetch a token, it MUST react to the challenge as if it could not produce a valid Authorization response. 2.2.1. Token Verification A token consists of some input cryptographically bound to an authenticator value, such as a digital signature. Verifying a token consists of checking that the authenticator value is correct. Pauly, et al. Expires 10 February 2024 [Page 11] Internet-Draft Privacy Pass Authentication August 2023 The authenticator value is as computed when running and finalizing the issuance protocol corresponding to the token type with the following value as the input: struct { uint16_t token_type; uint8_t nonce[32]; uint8_t challenge_digest[32]; uint8_t token_key_id[Nid]; } AuthenticatorInput; The value of these fields are as described in Section 2.2. The cryptographic verification check depends on the token type; see Section 5.4 of [ISSUANCE] and Section 6.4 of [ISSUANCE] for verification instructions for the issuance protocols described in [ISSUANCE]. As such, the security properties of the token, e.g., the probability that one can forge an authenticator value without invoking the issuance protocol, depend on the cryptographic algorithm used by the issuance protocol as determined by the token type. 3. User Interaction When used in contexts like websites, origins that challenge clients for tokens need to consider how to optimize their interaction model to ensure a good user experience. Origins SHOULD minimize the number of challenges sent on a particular client session, such as a unique TLS session between a client and origin (referred to as the "redemption context" in [ARCHITECTURE]). Similarly, clients SHOULD have some implementation-specific policy to minimize the number of tokens that can be retrieved by origins. One possible implementation of this policy is to bound the number of token challenges a given origin can provide for a given session. Token challenges can be performed without explicit user involvement, depending on the issuance protocol. If tokens are scoped to a specific origin, there is no need for per-challenge user interaction. Note that the issuance protocol may separately involve user interaction if the client needs to be newly validated. Pauly, et al. Expires 10 February 2024 [Page 12] Internet-Draft Privacy Pass Authentication August 2023 If a client cannot use cached tokens to respond to a challenge, either because it has run out of cached tokens or the associated context is unique, the token issuance process can add user- perceivable latency. Origins need not block useful work such as loading the contents of a web page on token authentication. Instead, token authentication can be used in similar ways to existing CAPTCHA validation flows, wherein validation sometimes proceeds alongside useful work, e.g., when loading contents of a web page, but without the need for user interaction. If issuance is taking a long time, an origin can fall back to another method of user validation. An origin MUST NOT use more than one redemption context value for a given token type and issuer per client request. If an origin issues a large number of challenges with unique contexts, such as more than once for each request, this can indicate that the origin is either not functioning correctly or is trying to attack or overload the client or issuance server. In such cases, a client MUST ignore redundant token challenges for the same request and SHOULD alert the user if possible. Origins MAY include multiple challenges, where each challenge refers to a different issuer or a different token type, to allow clients to choose a preferred issuer or type. An origin MUST NOT assume that token challenges will always yield a valid token. Clients might experience issues running the issuance protocol, e.g., because the attester or issuer is unavailable, or clients might simply not support the requested token type. Origins SHOULD account for such operational or interoperability failures by offering clients a fallback challenge such as CAPTCHA for accessing a resource. Failure to provide a fallback will likely mean that some clients fail authentication and cannot perform the desired action, such as loading a web page or accessing some other resource. For example, consider a scenario in which the client is a web browser, and the origin can accept either a token or a solution to a puzzle intended to determine if the client is a real human user. The origin would send clients a 401 HTTP response that contains a token challenge in a "WWW-Authenticate" header field along with content that contains the puzzle to display to the user. Clients that are able to respond with a token will be able to automatically return the token and not show the puzzle, while clients that either do not support tokens or are unable to fetch tokens at a particular time can present the user with the puzzle. To mitigate the risk of deployments becoming dependent on tokens, clients and origins SHOULD grease their behavior unless explicitly configured not to. In particular, clients SHOULD ignore token Pauly, et al. Expires 10 February 2024 [Page 13] Internet-Draft Privacy Pass Authentication August 2023 challenges with some non-zero probability. From the origin's perspective, ignoring a token challenge is indistinguishable from the issuance protocol failing for arbitrary reasons (excluding what can be inferred from latency between the client and origin interaction). Likewise, origins SHOULD randomly choose to not challenge clients for tokens with some non-zero probability. Moreover, origins SHOULD include random token types, from the Reserved list of "greased" types (defined in Section 5.2), with some non-zero probability. 4. Security Considerations The security properties of token challenges vary depending on whether the challenge contains a redemption context or not, as well as whether the challenge is per-origin or not. For example, cross- origin tokens with empty contexts can be replayed from one party by another, as shown below. +--------+ +----------+ +--------+ | Origin | | Attacker | | Client | +---+----+ +-----+----+ +---+----+ | | | +-- WWW-Authenticate: TokenChallenge -->| | | +--- (replay challenge) ---> | <-- Authorization: Token --+ <----------- (replay token) ------------+ Figure 2: Replay attack example Moreover, when a Client holds cross-origin tokens with empty contexts, it is possible for any Origin in the cross-origin set to deplete that Client set of tokens. To prevent this from happening, tokens can be scoped to single Origins (with non-empty origin_info) such that they can only be redeemed for a single Origin. Alternatively, if tokens are cross-Origin, Clients can use alternate methods to prevent many tokens from being redeemed at once. For example, if the Origin requests an excess of tokens, the Client could choose to not present any tokens for verification if a redemption had already occurred in a given time window. Token challenges that include non-empty origin_info bind tokens to one or more specific origins. As described in Section 2.1, clients only accept such challenges from origin names listed in the origin_info string. Even if multiple origins are listed, a token can only be redeemed for an origin if the challenge has a match for the origin_info. For example, if "a.example.com" issues a challenge with an origin_info string of "a.example.com,b.example.com", a client could redeem a token fetched for this challenge if and only if "b.example.com" also included an origin_info string of Pauly, et al. Expires 10 February 2024 [Page 14] Internet-Draft Privacy Pass Authentication August 2023 "a.example.com,b.example.com". On the other hand, if "b.example.com" had an origin_info string of "b.example.com" or "b.example.com,a.example.com" or "a.example.com,b.example.com,c.example.com", the string would not match and the client would need to use a different token. Context-bound token challenges require clients to obtain matching tokens when challenged, rather than presenting a token that was obtained from a different context in the past. This can make it more likely that issuance and redemption events will occur at approximately the same time. For example, if a client is challenged for a token with a unique context at time T1 and then subsequently obtains a token at time T2, a colluding issuer and origin can link this to the same client if T2 is unique to the client. This linkability is less feasible as the number of issuance events at time T2 increases. Depending on the "max-age" token challenge parameter, clients MAY try to augment the time between getting challenged then redeeming a token so as to make this sort of linkability more difficult. For more discussion on correlation risks between token issuance and redemption, see [ARCHITECTURE]. As discussed in Section 2.1, clients SHOULD discard any context-bound tokens upon flushing cookies or changing networks, to prevent an origin using the redemption context state as a cookie to recognize clients. Applications SHOULD constrain tokens to a single origin unless the use case can accommodate such replay attacks. Replays are also possible if the client redeems a token sent as part of 0-RTT data. If successful token redemption produces side effects, origins SHOULD implement an anti-replay mechanism to mitigate the harm of such replays. See [TLS13], Section 8 and [RFC9001], Section 9.2 for details about anti-replay mechanisms, as well as [RFC8470], Section 3 for discussion about safety considerations for 0-RTT HTTP data. All random values in the challenge and token MUST be generated using a cryptographically secure source of randomness. 5. IANA Considerations 5.1. Authentication Scheme This document registers the "PrivateToken" authentication scheme in the "Hypertext Transfer Protocol (HTTP) Authentication Scheme Registry" defined in [RFC9110], Section 16.4. Authentication Scheme Name: PrivateToken Pauly, et al. Expires 10 February 2024 [Page 15] Internet-Draft Privacy Pass Authentication August 2023 Pointer to specification text: Section 2 of this document 5.2. Token Type Registry IANA is requested to create a new "Privacy Pass Token Type" registry in a new "Privacy Pass Parameters" page to list identifiers for issuance protocols defined for use with the Privacy Pass token authentication scheme. These identifiers are two-byte values, so the maximum possible value is 0xFFFF = 65535. Template: * Value: The two-byte identifier for the algorithm * Name: Name of the issuance protocol * Token Structure: The contents of the Token structure in Section 2.2 * Token Key Encoding: The encoding of the "token-key" parameter in Section 2.2 * TokenChallenge Structure: The contents of the TokenChallenge structure in Section 2.1 * Publicly Verifiable: A Y/N value indicating if the output tokens are publicly verifiable * Public Metadata: A Y/N value indicating if the output tokens can contain public metadata. * Private Metadata: A Y/N value indicating if the output tokens can contain private metadata. * Nk: The length in bytes of an output authenticator * Nid: The length of the token key identifier * Reference: Where this algorithm is defined * Notes: Any notes associated with the entry New entries in this registry are subject to the Specification Required registration policy ([RFC8126], Section 4.6). Designated experts need to ensure that the token type is sufficiently clearly defined to be used for both token issuance and redemption, and meets the common security and privacy requirements for issuance protocols defined in Section 3.2 of [ARCHITECTURE]. Pauly, et al. Expires 10 February 2024 [Page 16] Internet-Draft Privacy Pass Authentication August 2023 Values 0xFF00-0xFFFF are reserved for private use. Implementers can use values in this range for experimentation with new token type protocols, as well as other proprietary uses that do not require interoperability. This document defines several Reserved values, which can be used by clients and servers to send "greased" values in token challenges and responses to ensure that implementations remain able to handle unknown token types gracefully (this technique is inspired by [RFC8701]). Implementations SHOULD select reserved values at random when including them in greased messages. Servers can include these in TokenChallenge structures, either as the only challenge when no real token type is desired, or as one challenge in a list of challenges that include real values. Clients can include these in Token structures when they are not able to present a real token response. The contents of the Token structure SHOULD be filled with random bytes when using greased values. The initial contents for this registry consist of the following Values. For each Value, the Name is "RESERVED", the Publicly Verifiable, Public Metadata, Private Metadata, Nk, and Nid attributes are all assigned "N/A", the Reference is this document, and the Notes attribute is "None". The initial list of Values is as follows: * 0x0000 * 0x02AA * 0x1132 * 0x2E96 * 0x3CD3 * 0x4473 * 0x5A63 * 0x6D32 * 0x7F3F * 0x8D07 * 0x916B * 0xA6A4 Pauly, et al. Expires 10 February 2024 [Page 17] Internet-Draft Privacy Pass Authentication August 2023 * 0xBEAB * 0xC3F3 * 0xDA42 * 0xE944 * 0xF057 Additionally, the registry is to be initialized with the following entry for Private Use. * Value: 0xFF00-0xFFFF * Name: Private Use * Token Structure: The contents of the Token structure in Section 2.2 * Token Key Encoding: N/A * TokenChallenge Structure: The contents of the TokenChallenge structure in Section 2.1 * Publicly Verifiable: N/A * Public Metadata: N/A * Private Metadata: N/A * Nk: N/A * Nid: N/A * Reference: This document * Notes: None [ISSUANCE] defines other non-grease entries for this registry. 6. References 6.1. Normative References Pauly, et al. Expires 10 February 2024 [Page 18] Internet-Draft Privacy Pass Authentication August 2023 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006, . [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, June 2017, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC9110] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP Semantics", STD 97, RFC 9110, DOI 10.17487/RFC9110, June 2022, . [SHS] Dang, Q. H. and National Institute of Standards and Technology, "Secure Hash Standard", DOI 10.6028/nist.fips.180-4, July 2015, . [TLS13] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, . [URI] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, January 2005, . 6.2. Informative References [ARCHITECTURE] Davidson, A., Iyengar, J., and C. A. Wood, "The Privacy Pass Architecture", Work in Progress, Internet-Draft, draft-ietf-privacypass-architecture-13, 15 June 2023, . Pauly, et al. Expires 10 February 2024 [Page 19] Internet-Draft Privacy Pass Authentication August 2023 [COOKIES] Bingler, S., West, M., and J. Wilander, "Cookies: HTTP State Management Mechanism", Work in Progress, Internet- Draft, draft-ietf-httpbis-rfc6265bis-12, 10 May 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-11, 26 June 2023, . [RFC8470] Thomson, M., Nottingham, M., and W. Tarreau, "Using Early Data in HTTP", RFC 8470, DOI 10.17487/RFC8470, September 2018, . [RFC8701] Benjamin, D., "Applying Generate Random Extensions And Sustain Extensibility (GREASE) to TLS Extensibility", RFC 8701, DOI 10.17487/RFC8701, January 2020, . [RFC9001] Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021, . Appendix A. Test Vectors This section includes test vectors for the HTTP authentication scheme specified in this document. It consists of the following types of test vectors: 1. Test vectors for the challenge and redemption protocols. Implementations can use these test vectors for verifying code that builds and encodes TokenChallenge structures, as well as code that produces a well-formed Token bound to a TokenChallenge. 2. Test vectors for the HTTP headers used for authentication. Implementations can use these test vectors for validating whether they parse HTTP authentication headers correctly to produce TokenChallenge structures and the other associated parameters, such as the token-key and max-age values. A.1. Challenge and Redemption Structure Test Vectors This section includes test vectors for the challenge and redemption functionalities described in Section 2.1 and Section 2.2. Each test vector lists the following values: Pauly, et al. Expires 10 February 2024 [Page 20] Internet-Draft Privacy Pass Authentication August 2023 * token_type: The type of token issuance protocol, a value from Section 5.2. For these test vectors, token_type is 0x0002, corresponding to the issuance protocol in [ISSUANCE]. * issuer_name: The name of the issuer in the TokenChallenge structure, represented as a hexadecimal string. * redemption_context: The redemption context in the TokenChallenge structure, represented as a hexadecimal string. * origin_info: The origin info in the TokenChallenge structure, represented as a hexadecimal string. * nonce: The nonce in the Token structure, represented as a hexadecimal string. * token_key: The public token-key, encoded based on the corresponding token type, represented as a hexadecimal string. * token_authenticator_input: The values in the Token structure used to compute the Token authenticator value, represented as a hexadecimal string. Test vectors are provided for each of the following TokenChallenge configurations: 1. TokenChallenge with a single origin and non-empty redemption context 2. TokenChallenge with a single origin and empty redemption context 3. TokenChallenge with an empty origin and redemption context 4. TokenChallenge with an empty origin and non-empty redemption context 5. TokenChallenge with a multiple origins and non-empty redemption context These test vectors are below. // Test vector 1: // token_type(0002), issuer_name(issuer.example), // origin_info(origin.example), redemption_context(non-empty) token_type: 0002 issuer_name: 6973737565722e6578616d706c65 redemption_context: 476ac2c935f458e9b2d7af32dacfbd22dd6023ef5887a789f1abe004e79bb5bb Pauly, et al. Expires 10 February 2024 [Page 21] Internet-Draft Privacy Pass Authentication August 2023 origin_info: 6f726967696e2e6578616d706c65 nonce: e01978182c469e5e026d66558ee186568614f235e41ef7e2378e6f202688abab token_key_id: ca572f8982a9ca248a3056186322d93ca147266121ddeb5632c07f1f71cd2708 token_authenticator_input: 0002e01978182c469e5e026d66558ee1865686 14f235e41ef7e2378e6f202688abab8e1d5518ec82964255526efd8f9db88205a 8ddd3ffb1db298fcc3ad36c42388fca572f8982a9ca248a3056186322d93ca147 266121ddeb5632c07f1f71cd2708 // Test vector 2: // token_type(0002), issuer_name(issuer.example), // origin_info(origin.example), redemption_context(empty) token_type: 0002 issuer_name: 6973737565722e6578616d706c65 redemption_context: origin_info: 6f726967696e2e6578616d706c65 nonce: e01978182c469e5e026d66558ee186568614f235e41ef7e2378e6f202688abab token_key_id: ca572f8982a9ca248a3056186322d93ca147266121ddeb5632c07f1f71cd2708 token_authenticator_input: 0002e01978182c469e5e026d66558ee1865686 14f235e41ef7e2378e6f202688abab11e15c91a7c2ad02abd66645802373db1d8 23bea80f08d452541fb2b62b5898bca572f8982a9ca248a3056186322d93ca147 266121ddeb5632c07f1f71cd2708 // Test vector 3: // token_type(0002), issuer_name(issuer.example), // origin_info(), redemption_context(empty) token_type: 0002 issuer_name: 6973737565722e6578616d706c65 redemption_context: origin_info: nonce: e01978182c469e5e026d66558ee186568614f235e41ef7e2378e6f202688abab token_key_id: ca572f8982a9ca248a3056186322d93ca147266121ddeb5632c07f1f71cd2708 token_authenticator_input: 0002e01978182c469e5e026d66558ee1865686 14f235e41ef7e2378e6f202688ababb741ec1b6fd05f1e95f8982906aec161289 6d9ca97d53eef94ad3c9fe023f7a4ca572f8982a9ca248a3056186322d93ca147 266121ddeb5632c07f1f71cd2708 // Test vector 4: // token_type(0002), issuer_name(issuer.example), // origin_info(), redemption_context(non-empty) token_type: 0002 issuer_name: 6973737565722e6578616d706c65 redemption_context: Pauly, et al. Expires 10 February 2024 [Page 22] Internet-Draft Privacy Pass Authentication August 2023 476ac2c935f458e9b2d7af32dacfbd22dd6023ef5887a789f1abe004e79bb5bb origin_info: nonce: e01978182c469e5e026d66558ee186568614f235e41ef7e2378e6f202688abab token_key_id: ca572f8982a9ca248a3056186322d93ca147266121ddeb5632c07f1f71cd2708 token_authenticator_input: 0002e01978182c469e5e026d66558ee1865686 14f235e41ef7e2378e6f202688ababb85fb5bc06edeb0e8e8bdb5b3bea8c4fa40 837c82e8bcaf5882c81e14817ea18ca572f8982a9ca248a3056186322d93ca147 266121ddeb5632c07f1f71cd2708 // Test vector 5: // token_type(0002), issuer_name(issuer.example), // origin_info(foo.example,bar.example), // redemption_context(non-empty) token_type: 0002 issuer_name: 6973737565722e6578616d706c65 redemption_context: 476ac2c935f458e9b2d7af32dacfbd22dd6023ef5887a789f1abe004e79bb5bb origin_info: 666f6f2e6578616d706c652c6261722e6578616d706c65 nonce: e01978182c469e5e026d66558ee186568614f235e41ef7e2378e6f202688abab token_key_id: ca572f8982a9ca248a3056186322d93ca147266121ddeb5632c07f1f71cd2708 token_authenticator_input: 0002e01978182c469e5e026d66558ee1865686 14f235e41ef7e2378e6f202688ababa2a775866b6ae0f98944910c8f48728d8a2 735b9157762ddbf803f70e2e8ba3eca572f8982a9ca248a3056186322d93ca147 266121ddeb5632c07f1f71cd2708 A.2. HTTP Header Test Vectors This section includes test vectors the contents of the HTTP authentication headers. Each test vector consists of one or more challenges that comprise a WWW-Authenticate header. For each challenge, the token-type, token-key, max-age, and token-challenge parameters are listed. Each challenge also includes an unknown (not specified) parameter that implementations are meant to ignore. The parameters for each challenge are indexed by their position in the WWW-Authentication challenge list. For example, token-key-0 denotes the token-key parameter for the first challenge in the list, whereas token-key-1 denotes the token-key for the second challenge in the list. The resulting wire-encoded WWW-Authentication header based on this list of challenges is then listed at the end. Pauly, et al. Expires 10 February 2024 [Page 23] Internet-Draft Privacy Pass Authentication August 2023 token-type-0: 0x0002 token-key-0: 30820152303d06092a864886f70d01010a3030a00d300b060960864 8016503040202a11a301806092a864886f70d010108300b060960864801650304020 2a2030201300382010f003082010a0282010100cb1aed6b6a95f5b1ce013a4cfcab2 5b94b2e64a23034e4250a7eab43c0df3a8c12993af12b111908d4b471bec31d4b6c9 ad9cdda90612a2ee903523e6de5a224d6b02f09e5c374d0cfe01d8f529c500a78a2f 67908fa682b5a2b430c81eaf1af72d7b5e794fc98a3139276879757ce453b526ef9b f6ceb99979b8423b90f4461a22af37aab0cf5733f7597abe44d31c732db68a181c6c bbe607d8c0e52e0655fd9996dc584eca0be87afbcd78a337d17b1dba9e828bbd81e2 91317144e7ff89f55619709b096cbb9ea474cead264c2073fe49740c01f00e109106 066983d21e5f83f086e2e823c879cd43cef700d2a352a9babd612d03cad02db134b7 e225a5f0203010001 max-age-0: 10 token-challenge-0: 0002000e6973737565722e6578616d706c65208a3e83a33d9 8005d2f30bef419fa6bf4cd5c6005e36b1285bbb4ccd40fa4b383000e6f726967696 e2e6578616d706c65 WWW-Authenticate: PrivateToken challenge="AAIADmlzc3Vlci5leGFtcGxlII o-g6M9mABdLzC-9Bn6a_TNXGAF42sShbu0zNQPpLODAA5vcmlnaW4uZXhhbXBsZQ==", token-key="MIIBUjA9BgkqhkiG9w0BAQowMKANMAsGCWCGSAFlAwQCAqEaMBgGCSqG SIb3DQEBCDALBglghkgBZQMEAgKiAwIBMAOCAQ8AMIIBCgKCAQEAyxrta2qV9bHOATpM _KsluUsuZKIwNOQlCn6rQ8DfOowSmTrxKxEZCNS0cb7DHUtsmtnN2pBhKi7pA1I-beWi JNawLwnlw3TQz-Adj1KcUAp4ovZ5CPpoK1orQwyB6vGvcte155T8mKMTknaHl1fORTtS bvm_bOuZl5uEI7kPRGGiKvN6qwz1cz91l6vkTTHHMttooYHGy75gfYwOUuBlX9mZbcWE 7KC-h6-814ozfRex26noKLvYHikTFxROf_ifVWGXCbCWy7nqR0zq0mTCBz_kl0DAHwDh CRBgZpg9IeX4PwhuLoI8h5zUPO9wDSo1Kpur1hLQPK0C2xNLfiJaXwIDAQAB",unknow nChallengeAttribute="ignore-me", max-age="10" token-type-0: 0x0002 token-key-0: 30820152303d06092a864886f70d01010a3030a00d300b060960864 8016503040202a11a301806092a864886f70d010108300b060960864801650304020 2a2030201300382010f003082010a0282010100cb1aed6b6a95f5b1ce013a4cfcab2 5b94b2e64a23034e4250a7eab43c0df3a8c12993af12b111908d4b471bec31d4b6c9 ad9cdda90612a2ee903523e6de5a224d6b02f09e5c374d0cfe01d8f529c500a78a2f 67908fa682b5a2b430c81eaf1af72d7b5e794fc98a3139276879757ce453b526ef9b f6ceb99979b8423b90f4461a22af37aab0cf5733f7597abe44d31c732db68a181c6c bbe607d8c0e52e0655fd9996dc584eca0be87afbcd78a337d17b1dba9e828bbd81e2 91317144e7ff89f55619709b096cbb9ea474cead264c2073fe49740c01f00e109106 066983d21e5f83f086e2e823c879cd43cef700d2a352a9babd612d03cad02db134b7 e225a5f0203010001 max-age-0: 10 token-challenge-0: 0002000e6973737565722e6578616d706c65208a3e83a33d9 8005d2f30bef419fa6bf4cd5c6005e36b1285bbb4ccd40fa4b383000e6f726967696 e2e6578616d706c65 token-type-1: 0x0001 token-key-1: ebb1fed338310361c08d0c7576969671296e05e99a17d7926dfc28a 53fabd489fac0f82bca86249a668f3a5bfab374c9 max-age-1: 10 Pauly, et al. Expires 10 February 2024 [Page 24] Internet-Draft Privacy Pass Authentication August 2023 token-challenge-1: 0001000e6973737565722e6578616d706c65208a3e83a33d9 8005d2f30bef419fa6bf4cd5c6005e36b1285bbb4ccd40fa4b383000e6f726967696 e2e6578616d706c65 WWW-Authenticate: PrivateToken challenge="AAIADmlzc3Vlci5leGFtcGxlII o-g6M9mABdLzC-9Bn6a_TNXGAF42sShbu0zNQPpLODAA5vcmlnaW4uZXhhbXBsZQ==", token-key="MIIBUjA9BgkqhkiG9w0BAQowMKANMAsGCWCGSAFlAwQCAqEaMBgGCSqG SIb3DQEBCDALBglghkgBZQMEAgKiAwIBMAOCAQ8AMIIBCgKCAQEAyxrta2qV9bHOATpM _KsluUsuZKIwNOQlCn6rQ8DfOowSmTrxKxEZCNS0cb7DHUtsmtnN2pBhKi7pA1I-beWi JNawLwnlw3TQz-Adj1KcUAp4ovZ5CPpoK1orQwyB6vGvcte155T8mKMTknaHl1fORTtS bvm_bOuZl5uEI7kPRGGiKvN6qwz1cz91l6vkTTHHMttooYHGy75gfYwOUuBlX9mZbcWE 7KC-h6-814ozfRex26noKLvYHikTFxROf_ifVWGXCbCWy7nqR0zq0mTCBz_kl0DAHwDh CRBgZpg9IeX4PwhuLoI8h5zUPO9wDSo1Kpur1hLQPK0C2xNLfiJaXwIDAQAB",unknow nChallengeAttribute="ignore-me", max-age="10", PrivateToken challeng e="AAEADmlzc3Vlci5leGFtcGxlIIo-g6M9mABdLzC-9Bn6a_TNXGAF42sShbu0zNQPp LODAA5vcmlnaW4uZXhhbXBsZQ==", token-key="67H-0zgxA2HAjQx1dpaWcSluBem aF9eSbfwopT-r1In6wPgryoYkmmaPOlv6s3TJ",unknownChallengeAttribute="ig nore-me", max-age="10" Authors' Addresses Tommy Pauly Apple Inc. One Apple Park Way Cupertino, California 95014, United States of America Email: tpauly@apple.com Steven Valdez Google LLC Email: svaldez@chromium.org Christopher A. Wood Cloudflare Email: caw@heapingbits.net Pauly, et al. Expires 10 February 2024 [Page 25]