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Internet-Draft This document defines an HTTP authentication scheme that 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 an acceptable Privacy Pass token.

Introduction Privacy Pass tokens are unlinkable, one-time-use authenticators that can be used to anonymously authorize a client (see ). 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. This document defines a common HTTP authentication scheme (), 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 authentiation 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 . 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.

Challenge-response redemption protocol flow | | | // Run issuance protocol <------- Authorization: Token ----------+ | | ]]>

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.

Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here. Unless otherwise specified, this document encodes protocol messages in TLS notation from , Section 3. This document uses the terms "Client", "Origin", "Issuer", "Issuance Protocol", and "Token" as defined in . 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.

HTTP Authentication Scheme Token redemption is performed using HTTP Authentication (), with the scheme "PrivateToken". Origins challenge clients to present a token from a specific issuer (). Once a client has received a token from that issuer, or already has a valid token available, it presents the token to the origin (). 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. 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 ). Clients can have implementation-specific policy to minimize the number of tokens that can be retrieved by origins, so origins are advised to only request tokens when necessary within a single session. See for more discussion on how to optimize token challenges to improve the user experience.

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:

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 .
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, . 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 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: ; 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.
• "issuer_name" is an ASCII string that identifies the issuer using the format of the authority portion of a URI as defined in . 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 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 . 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 TokenChallenge value. Since the length of the challenge is not fixed, the base64url value MUST include padding. As an Authentication Parameter (auth-param from ), 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. Since the length of the key is not fixed, the base64url value MUST include padding. As an Authentication Parameter (auth-param from ), 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: 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.

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 . 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: Origins should only include challenges for different types of issuance protocols with functionally equivalent properties. For instance, both issuance protocols in 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.

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 SHA256(current time window).
• Context bound to a client location: Construct redemption context as SHA256(client IP address prefix).
• Context bound to a given time window and location: Construct redemption context as SHA256(current time window, client IP address prefix).

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 does 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.

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 (). 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 , 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.

Token Redemption The output of the issuance protocol is a token that corresponds to the origin's challenge (see ). 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 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: 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, SHA256(TokenChallenge).
• "token_key_id" is an Nid-octet identifier for the 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 covers the preceding fields in the token. The value of this field is defined by the token_type and corresponding issuance protocol. The value of constant Nk depends on token_type, as defined in .

The authenticator value in the Token structure is computed over the token_type, nonce, challenge_digest, and token_key_id fields. When used for client authorization, the "PrivateToken" authentication scheme defines one parameter, "token", which contains the base64url-encoded Token struct. Since the length of the Token struct is not fixed, the base64url value MUST include padding. As an Authentication Parameter (auth-param from ), 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: For token types that support public verifiability, origins verify the token authenticator using the public key of the issuer, and validate that the signed message matches the concatenation of the client nonce and the hash of a valid TokenChallenge. For context-bound tokens, origins store or reconstruct the contexts of previous TokenChallenge structures in order to validate the token. A TokenChallenge MAY 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. This prevents clients from "replaying" 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.

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. Tokens 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. 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 on token authentication. Instead, token authentication can be used in similar ways to CAPTCHA validation today, but without the need for user interaction. If issuance is taking a long time, a website could show an indicator that it is waiting, or 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 an alternative type of challenge such as CAPTCHA for accessing a 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 servers SHOULD grease their behavior unless explicitly configured not to. In particular, clients SHOULD ignore token challenges with some non-zero probability. 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 ), with some non-zero probability.

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.

Replay attack example | | | +--- (replay challenge) ---> | <-- Authorization: Token --+ <----------- (replay token) ------------+ ]]>

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 , 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 an exact 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 "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 . As discussed in , 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 and for details about anti-replay mechanisms, as well as 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.

IANA Considerations

Authentication Scheme This document registers the "PrivateToken" authentication scheme in the "Hypertext Transfer Protocol (HTTP) Authentication Scheme Registry" defined in . Authentication Scheme Name: PrivateToken Pointer to specification text: of this document

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
• TokenChallenge Structure: The contents of the TokenChallenge structure in
• 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 (). 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 . This registry also will also allow provisional registrations to allow for experimentation with protocols being developed. Designated experts review, approve, and revoke provisional registrations. Values 0xFF00-0xFFFF are reserved for private use, to enable proprietary uses and limited experimentation. 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 ). Implemenations 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 iniital list of Values is as follows:

• 0x0000
• 0x02AA
• 0x1132
• 0x2E96
• 0x3CD3
• 0x4473
• 0x5A63
• 0x6D32
• 0x7F3F
• 0x8D07
• 0x916B
• 0xA6A4
• 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
• TokenChallenge Structure: The contents of the TokenChallenge structure in
• Publicly Verifiable: N/A
• Public Metadata: N/A
• Private Metadata: N/A
• Nk: N/A
• Nid: N/A
• Reference: This document
• Notes: None

defines other non-grease entries for this registry.

References Normative References The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document describes the overall architecture of HTTP, establishes common terminology, and defines aspects of the protocol that are shared by all versions. In this definition are core protocol elements, extensibility mechanisms, and the "http" and "https" Uniform Resource Identifier (URI) schemes. This document updates RFC 3864 and obsoletes RFCs 2818, 7231, 7232, 7233, 7235, 7538, 7615, 7694, and portions of 7230. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. This document updates RFCs 5705 and 6066, and obsoletes RFCs 5077, 5246, and 6961. This document also specifies new requirements for TLS 1.2 implementations. A Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource. This specification defines the generic URI syntax and a process for resolving URI references that might be in relative form, along with guidelines and security considerations for the use of URIs on the Internet. The URI syntax defines a grammar that is a superset of all valid URIs, allowing an implementation to parse the common components of a URI reference without knowing the scheme-specific requirements of every possible identifier. This specification does not define a generative grammar for URIs; that task is performed by the individual specifications of each URI scheme. [STANDARDS-TRACK] This document describes the commonly used base 64, base 32, and base 16 encoding schemes. It also discusses the use of line-feeds in encoded data, use of padding in encoded data, use of non-alphabet characters in encoded data, use of different encoding alphabets, and canonical encodings. [STANDARDS-TRACK] Many protocols make use of points of extensibility that use constants to identify various protocol parameters. To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper. For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA). To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed. This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry. This is the third edition of this document; it obsoletes RFC 5226. Informative References LIP Fastly Cloudflare This document specifies the Privacy Pass architecture and requirements for its constituent protocols used for constructing privacy-preserving authentication mechanisms. It provides recommendations on how the architecture should be deployed to ensure the privacy of clients and the security of all participating entities. Brave Software Brave Software Cloudflare Google LLC Cloudflare This document specifies two variants of the two-message issuance protocol for Privacy Pass tokens: one that produces tokens that are privately verifiable using the issuance private key, and another that produces tokens that are publicly verifiable using the issuance public key. Google LLC Google LLC Apple, Inc This document defines the HTTP Cookie and Set-Cookie header fields. These header fields can be used by HTTP servers to store state (called cookies) at HTTP user agents, letting the servers maintain a stateful session over the mostly stateless HTTP protocol. Although cookies have many historical infelicities that degrade their security and privacy, the Cookie and Set-Cookie header fields are widely used on the Internet. This document obsoletes RFC 6265. This document describes how Transport Layer Security (TLS) is used to secure QUIC. Using TLS early data creates an exposure to the possibility of a replay attack. This document defines mechanisms that allow clients to communicate with servers about HTTP requests that are sent in early data. Techniques are described that use these mechanisms to mitigate the risk of replay. This document describes GREASE (Generate Random Extensions And Sustain Extensibility), a mechanism to prevent extensibility failures in the TLS ecosystem. It reserves a set of TLS protocol values that may be advertised to ensure peers correctly handle unknown values.

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.

Challenge and Redemption Structure Test Vectors This section includes test vectors for the challenge and redemption functionalities described in and . Each test vector lists the following values:

• token_type: The type of token issuance protocol, a value from . For these test vectors, token_type is 0x0002, corresponding to the issuance protocol in .
• 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:

• TokenChallenge with a single origin and non-empty redemption context
• TokenChallenge with a single origin and empty redemption context
• TokenChallenge with an empty origin and redemption context
• TokenChallenge with an empty origin and non-empty redemption context
• TokenChallenge with a multiple origins and non-empty redemption context

These test vectors are below.

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.