Oblivious HTTP Application Intermediation                     M. Thomson
Internet-Draft                                                   Mozilla
Intended status: Standards Track                              C. A. Wood
Expires: 9 February 2023                                      Cloudflare
                                                           8 August 2022


                             Oblivious HTTP
                        draft-ietf-ohai-ohttp-03

Abstract

   This document describes a system for forwarding encrypted HTTP
   messages.  This allows a client to make multiple requests to an
   origin server without that server being able to link those requests
   to the client or to identify the requests as having come from the
   same client, while placing only limited trust in the nodes used to
   forward the messages.

About This Document

   This note is to be removed before publishing as an RFC.

   The latest revision of this draft can be found at https://ietf-wg-
   ohai.github.io/oblivious-http/draft-ietf-ohai-ohttp.html.  Status
   information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-ohai-ohttp/.

   Discussion of this document takes place on the Oblivious HTTP
   Application Intermediation Working Group mailing list
   (mailto:ohai@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/ohai/.  Subscribe at
   https://www.ietf.org/mailman/listinfo/ohai/.

   Source for this draft and an issue tracker can be found at
   https://github.com/ietf-wg-ohai/oblivious-http.

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/.





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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Applicability . . . . . . . . . . . . . . . . . . . . . .   6
     2.2.  Conventions and Definitions . . . . . . . . . . . . . . .   7
   3.  Key Configuration . . . . . . . . . . . . . . . . . . . . . .   8
     3.1.  Key Configuration Encoding  . . . . . . . . . . . . . . .   9
     3.2.  Key Configuration Media Type  . . . . . . . . . . . . . .   9
   4.  HPKE Encapsulation  . . . . . . . . . . . . . . . . . . . . .   9
     4.1.  Encapsulation of Requests . . . . . . . . . . . . . . . .  11
     4.2.  Encapsulation of Responses  . . . . . . . . . . . . . . .  13
     4.3.  Request and Response Media Types  . . . . . . . . . . . .  14
   5.  HTTP Usage  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     5.1.  Informational Responses . . . . . . . . . . . . . . . . .  15
     5.2.  Errors  . . . . . . . . . . . . . . . . . . . . . . . . .  16
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
     6.1.  Client Responsibilities . . . . . . . . . . . . . . . . .  17
     6.2.  Relay Responsibilities  . . . . . . . . . . . . . . . . .  18
       6.2.1.  Differential Treatment  . . . . . . . . . . . . . . .  19
       6.2.2.  Denial of Service . . . . . . . . . . . . . . . . . .  20
       6.2.3.  Traffic Analysis  . . . . . . . . . . . . . . . . . .  20
     6.3.  Server Responsibilities . . . . . . . . . . . . . . . . .  21
     6.4.  Key Management  . . . . . . . . . . . . . . . . . . . . .  22
     6.5.  Replay Attacks  . . . . . . . . . . . . . . . . . . . . .  22
       6.5.1.  Use of Date for Anti-Replay . . . . . . . . . . . . .  23
       6.5.2.  Correcting Clock Differences  . . . . . . . . . . . .  24



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     6.6.  Forward Secrecy . . . . . . . . . . . . . . . . . . . . .  25
     6.7.  Post-Compromise Security  . . . . . . . . . . . . . . . .  26
     6.8.  Client Clock Exposure . . . . . . . . . . . . . . . . . .  26
   7.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  26
   8.  Operational and Deployment Considerations . . . . . . . . . .  27
     8.1.  Performance Overhead  . . . . . . . . . . . . . . . . . .  28
     8.2.  Resource Mappings . . . . . . . . . . . . . . . . . . . .  28
     8.3.  Network Management  . . . . . . . . . . . . . . . . . . .  28
   9.  Repurposing the Encapsulation Format  . . . . . . . . . . . .  29
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  29
     10.1.  message/ohttp-keys Media Type  . . . . . . . . . . . . .  29
     10.2.  message/ohttp-req Media Type . . . . . . . . . . . . . .  30
     10.3.  message/ohttp-res Media Type . . . . . . . . . . . . . .  31
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  32
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  32
     11.2.  Informative References . . . . . . . . . . . . . . . . .  33
   Appendix A.  Complete Example of a Request and Response . . . . .  36
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  38
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  39

1.  Introduction

   A HTTP request reveals information about the client's identity to the
   server.  Some of that information is in the request content, and
   therefore under the control of the client.  However, the source IP
   address of the underlying connection reveals information that the
   client has only limited control over.

   Even where an IP address is not directly associated with an
   individual, the requests made from it can be correlated over time to
   assemble a profile of client behavior.  In particular, connection
   reuse improves performance, but provides servers with the ability to
   correlate requests that share a connection.

   Client-configured HTTP proxies can provide a degree of protection
   against IP address tracking, and systems like virtual private
   networks and the Tor network [Dingledine2004] provide additional
   options for clients.

   However, even when IP address tracking is mitigated using one of
   these techniques, each request needs to be on a completely new TLS
   connection to avoid the connection itself being used to correlate
   behavior.  This imposes considerable performance and efficiency
   overheads, due to the additional round trip to the server (at a
   minumum), additional data exchanged, and additional CPU cost of
   cryptographic computations.





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   This document defines two kinds of HTTP resources -- Oblivious Relay
   Resources and Oblivious Gateway Resources -- that process
   encapsulated binary HTTP messages [BINARY] using Hybrid Public Key
   Encryption (HPKE; [HPKE]).  They can be composed to protect the
   content of encapsulated requests and responses, thereby separating
   the identity of a requester from the request.

   Although this scheme requires support for two new kinds of oblivious
   resources, it represents a performance improvement over options that
   perform just one request in each connection.  With limited trust
   placed in the Oblivious Relay Resource (see Section 6), clients are
   assured that requests are not uniquely attributed to them or linked
   to other requests.

2.  Overview

   A client must initially know the following:

   *  The identity of an Oblivious Gateway Resource.  This might include
      some information about what Target Resources the Oblivious Gateway
      Resource supports.

   *  The details of an HPKE public key that the Oblivious Gateway
      Resource accepts, including an identifier for that key and the
      HPKE algorithms that are used with that key.

   *  The identity of an Oblivious Relay Resource that will accept relay
      requests carrying an encapsulated request as its content and
      forward the content in these requests to a single Oblivious
      Gateway Resource.  See Section 8.2 for more information about the
      mapping between Oblivious Relay and Gateway Resources.

   This information allows the client to make a request of a Target
   Resource with that resource having only a limited ability to
   correlate that request with the client IP or other requests that the
   client might make to that server.















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   +---------+       +----------+      +----------+      +----------+
   | Client  |       | Relay    |      | Gateway  |      | Target   |
   |         |       | Resource |      | Resource |      | Resource |
   +----+----+       +----+-----+      +-------+--+      +----+-----+
        |                 |                    |              |
        | Relay           |                    |              |
        | Request         |                    |              |
        | [+ Encapsulated |                    |              |
        |    Request ]    |                    |              |
        +---------------->| Gateway            |              |
        |                 | Request            |              |
        |                 | [+ Encapsulated    |              |
        |                 |    Request ]       |              |
        |                 +------------------->| Request      |
        |                 |                    +------------->|
        |                 |                    |              |
        |                 |                    |     Response |
        |                 |            Gateway |<-------------+
        |                 |           Response |              |
        |                 |    [+ Encapsulated |              |
        |                 |         Response ] |              |
        |           Relay |<-------------------+              |
        |        Response |                    |              |
        | [+ Encapsulated |                    |              |
        |      Response ] |                    |              |
        |<----------------+                    |              |
        |                 |                    |              |

                    Figure 1: Overview of Oblivious HTTP

   In order to make a request to a Target Resource, the following steps
   occur, as shown in Figure 1:

   1.   The client constructs an HTTP request for a Target Resource.

   2.   The client encodes the HTTP request in a binary HTTP message and
        then encapsulates that message using HPKE and the process from
        Section 4.1.

   3.   The client sends a POST request to the Oblivious Relay Resource
        with the Encapsulated Request as the content of that message.

   4.   The Oblivious Relay Resource forwards this request to the
        Oblivious Gateway resource.

   5.   The Oblivious Gateway Resource receives this request and removes
        the HPKE protection to obtain an HTTP request.




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   6.   The Oblivious Gateway Resource makes an HTTP request that
        includes the target URI, method, fields, and content of the
        request it acquires.

   7.   The Target Resource answers this HTTP request with an HTTP
        response.

   8.   The Oblivious Gateway Resource encapsulates the HTTP response
        following the process in Section 4.2 and sends this in response
        to the request from the Oblivious Relay Resource.

   9.   The Oblivious Relay Resource forwards this response to the
        client.

   10.  The client removes the encapsulation to obtain the response to
        the original request.

2.1.  Applicability

   Oblivious HTTP has limited applicability.  Many uses of HTTP benefit
   from being able to carry state between requests, such as with cookies
   ([RFC6265]), authentication (Section 11 of [HTTP]), or even
   alternative services ([RFC7838]).  Oblivious HTTP removes linkage at
   the transport layer, which must be used in conjunction with
   applications that do not carry state between requests.

   Oblivious HTTP is primarily useful where privacy risks associated
   with possible stateful treatment of requests are sufficiently large
   that the cost of deploying this protocol can be justified.  Oblivious
   HTTP is simpler and less costly than more robust systems, like Prio
   ([PRIO]) or Tor ([Dingledine2004]), which can provide stronger
   guarantees at higher operational costs.

   Oblivious HTTP is more costly than a direct connection to a server.
   Some costs, like those involved with connection setup, can be
   amortized, but there are several ways in which Oblivious HTTP is more
   expensive than a direct request:

   *  Each request requires at least two regular HTTP requests, which
      adds latency.

   *  Each request is expanded in size with additional HTTP fields,
      encryption-related metadata, and AEAD expansion.

   *  Deriving cryptographic keys and applying them for request and
      response protection takes non-negligible computational resources.





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   Examples of where preventing the linking of requests might justify
   these costs include:

   *  DNS queries.  DNS queries made to a recursive resolver reveal
      information about the requester, particularly if linked to other
      queries.

   *  Telemetry submission.  Applications that submit reports about
      their usage to their developers might use Oblivious HTTP for some
      types of moderately sensitive data.

   These are examples of requests where there is information in a
   request that - if it were connected to the identity of the user -
   might allow a server to learn something about that user even if the
   identity of the user is pseudonymous.  Other examples include the
   submission of anonymous surveys, making search queries, or requesting
   location-specific content (such as retrieving tiles of a map
   display).

2.2.  Conventions and Definitions

   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.

   Client:  This document uses its own definition of client.  When
      referring to the HTTP definition of client (Section 3.3 of
      [HTTP]), the term "HTTP client" is used; see Section 5.

   Encapsulated Request:  An HTTP request that is encapsulated in an
      HPKE-encrypted message; see Section 4.1.

   Encapsulated Response:  An HTTP response that is encapsulated in an
      HPKE-encrypted message; see Section 4.2.

   Oblivious Relay Resource:  An intermediary that forwards encapsulated
      requests and responses between clients and a single Oblivious
      Gateway Resource.

   Oblivious Gateway Resource:  A resource that can receive an
      encapsulated request, extract the contents of that request,
      forward it to a Target Resource, receive a response, encapsulate
      that response, then return that response.

   Target Resource:  The resource that is the target of an encapsulated




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      request.  This resource logically handles only regular HTTP
      requests and responses and so might be ignorant of the use of
      Oblivious HTTP to reach it.

   Relay Request:  An HTTP request from Client to Relay that contains an
      encapsulated request as the content.

   Relay Response:  An HTTP response from Relay to Client that contains
      an encapsulated response as the content.

   Gateway Request:  An HTTP request from Relay to Gateway that contains
      an encapsulated request as the content.

   Gateway Response:  An HTTP response from Gateway to Relay that
      contains an encapsulated response as the content.

   This draft includes pseudocode that uses the functions and
   conventions defined in [HPKE].

   Encoding an integer to a sequence of bytes in network byte order is
   described using the function encode(n, v), where n is the number of
   bytes and v is the integer value.  ASCII [ASCII] encoding of a string
   s is indicated using the function encode_str(s).  The function len()
   returns the length of a sequence of bytes.

   Formats are described using notation from Section 1.3 of [QUIC].

3.  Key Configuration

   A client needs to acquire information about the key configuration of
   the Oblivious Gateway Resource in order to send encapsulated
   requests.  In order to ensure that clients do not encapsulate
   messages that other entities can intercept, the key configuration
   MUST be authenticated and have integrity protection.

   This document does not define how that acquisition occurs.  However,
   in order to help facilitate interoperability, it does specify a
   format for the keys.  This ensures that different client
   implementations can be configured in the same way and also enables
   advertising key configurations in a consistent format.  This format
   might be used, for example with HTTPS, as part of a system for
   configuring or discovering key configurations.  Note however that
   such a system needs to consider the potential for key configuration
   to be used to compromise client privacy; see Section 7.

   A client might have multiple key configurations to select from when
   encapsulating a request.  Clients are responsible for selecting a
   preferred key configuration from those it supports.  Clients need to



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   consider both the key encapsulation method (KEM) and the combinations
   of key derivation function (KDF) and authenticated encryption with
   associated data (AEAD) in this decision.

3.1.  Key Configuration Encoding

   A single key configuration consists of a key identifier, a public
   key, an identifier for the KEM that the public key uses, and a set
   HPKE symmetric algorithms.  Each symmetric algorithm consists of an
   identifier for a KDF and an identifier for an AEAD.

   Figure 2 shows a single key configuration.

   HPKE Symmetric Algorithms {
     HPKE KDF ID (16),
     HPKE AEAD ID (16),
   }

   OHTTP Key Config {
     Key Identifier (8),
     HPKE KEM ID (16),
     HPKE Public Key (Npk * 8),
     HPKE Symmetric Algorithms Length (16),
     HPKE Symmetric Algorithms (32..262140),
   }

                    Figure 2: A Single Key Configuration

   The definitions for the identifiers used in HPKE and the semantics of
   the algorithms they identify can be found in [HPKE].  The Npk
   parameter is determined by the choice of HPKE KEM, which can also be
   found in [HPKE].

3.2.  Key Configuration Media Type

   The "application/ohttp-keys" format is a media type that identifies a
   serialized collection of key configurations.  The content of this
   media type comprises one or more key configuration encodings (see
   Section 3.1) that are concatenated; see Section 10.1 for a definition
   of the media type.

   Evolution of the key configuration format is supported through the
   definition of new formats that are identified by new media types.

4.  HPKE Encapsulation

   HTTP message encapsulation uses HPKE for request and response
   encryption.



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   An encapsulated HTTP request contains a binary-encoded HTTP message
   [BINARY] and no other fields; see Figure 3.

   Request {
     Binary HTTP Message (..),
   }

                    Figure 3: Plaintext Request Content

   An Encapsulated Request is comprised of a key identifier and a HPKE-
   protected request message.  HPKE protection includes an encapsulated
   KEM shared secret (or enc), plus the AEAD-protected request message.
   An Encapsulated Request is shown in Figure 4.  Section 4.1 describes
   the process for constructing and processing an Encapsulated Request.

   Encapsulated Request {
     Key Identifier (8),
     KEM Identifier (16),
     KDF Identifier (16),
     AEAD Identifier (16),
     Encapsulated KEM Shared Secret (8*Nenc),
     AEAD-Protected Request (..),
   }

                       Figure 4: Encapsulated Request

   The Nenc parameter corresponding to the HpkeKdfId can be found in
   Section 7.1 of [HPKE].

   An encrypted HTTP response includes a binary-encoded HTTP message
   [BINARY] and no other content; see Figure 5.

   Response {
     Binary HTTP Message (..),
   }

                    Figure 5: Plaintext Response Content

   Responses are bound to requests and so consist only of AEAD-protected
   content.  Section 4.2 describes the process for constructing and
   processing an Encapsulated Response.

   Encapsulated Response {
     Nonce (Nk),
     AEAD-Protected Response (..),
   }

                      Figure 6: Encapsulated Response



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   The Nenc and Nk parameters corresponding to the HpkeKdfId can be
   found in [HPKE].  Nenc refers to the size of the encapsulated KEM
   shared secret, in bytes; Nk refers to the size of the AEAD key for
   the HPKE ciphersuite, in bits.

4.1.  Encapsulation of Requests

   Clients encapsulate a request request using values from a key
   configuration:

   *  the key identifier from the configuration, keyID, with the
      corresponding KEM identified by kemID,

   *  the public key from the configuration, pkR, and

   *  a selected combination of KDF, identified by kdfID, and AEAD,
      identified by aeadID.

   The client then constructs an Encapsulated Request, enc_request, from
   a binary encoded HTTP request, request, as follows:

   1.  Construct a message header, hdr, by concatenating the values of
       keyID, kemID, kdfID, and aeadID, as one 8-bit integer and three
       16-bit integers, respectively, each in network byte order.

   2.  Build info by concatenating the ASCII-encoded string "message/
       bhttp request", a zero byte, and the header.

   3.  Create a sending HPKE context by invoking SetupBaseS()
       (Section 5.1.1 of [HPKE]) with the public key of the receiver pkR
       and info.  This yields the context sctxt and an encapsulation key
       enc.

   4.  Encrypt request by invoking the Seal() method on sctxt
       (Section 5.2 of [HPKE]) with empty associated data aad, yielding
       ciphertext ct.

   5.  Concatenate the values of hdr, enc, and ct, yielding an Encrypted
       Request enc_request.

   Note that enc is of fixed-length, so there is no ambiguity in parsing
   this structure.

   In pseudocode, this procedure is as follows:







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   hdr = concat(encode(1, keyID),
                encode(2, kemID),
                encode(2, kdfID),
                encode(2, aeadID))
   info = concat(encode_str("message/bhttp request"),
                 encode(1, 0),
                 hdr)
   enc, sctxt = SetupBaseS(pkR, info)
   ct = sctxt.Seal([], request)
   enc_request = concat(hdr, enc, ct)

   Servers decrypt an Encapsulated Request by reversing this process.
   Given an Encapsulated Request enc_request, a server:

   1.  Parses enc_request into keyID, kemID, kdfID, aeadID, enc, and ct
       (indicated using the function parse() in pseudocode).  The server
       is then able to find the HPKE private key, skR, corresponding to
       keyID.

       a.  If keyID does not identify a key matching the type of kemID,
       the server returns an error.

       b.  If kdfID and aeadID identify a combination of KDF and AEAD
       that the server is unwilling to use with skR, the server returns
       an error.

   2.  Build info by concatenating the ASCII-encoded string "message/
       bhttp request", a zero byte, keyID as an 8-bit integer, plus
       kemID, kdfID, and aeadID as three 16-bit integers.

   3.  Create a receiving HPKE context by invoking SetupBaseR()
       (Section 5.1.1 of [HPKE]) with skR, enc, and info.  This produces
       a context rctxt.

   4.  Decrypt ct by invoking the Open() method on rctxt (Section 5.2 of
       [HPKE]), with an empty associated data aad, yielding request or
       an error on failure.  If decryption fails, the server returns an
       error.

   In pseudocode, this procedure is as follows:











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   keyID, kemID, kdfID, aeadID, enc, ct = parse(enc_request)
   info = concat(encode_str("message/bhttp request"),
                 encode(1, 0),
                 encode(1, keyID),
                 encode(2, kemID),
                 encode(2, kdfID),
                 encode(2, aeadID))
   rctxt = SetupBaseR(enc, skR, info)
   request, error = rctxt.Open([], ct)

4.2.  Encapsulation of Responses

   Given an HPKE context context, a request message request, and a
   response response, servers generate an Encapsulated Response
   enc_response as follows:

   1.  Export a secret secret from context, using the string "message/
       bhttp response" as context.  The length of this secret is max(Nn,
       Nk), where Nn and Nk are the length of AEAD key and nonce
       associated with context.

   2.  Generate a random value of length max(Nn, Nk) bytes, called
       response_nonce.

   3.  Extract a pseudorandom key prk using the Extract function
       provided by the KDF algorithm associated with context.  The ikm
       input to this function is secret; the salt input is the
       concatenation of enc (from enc_request) and response_nonce

   4.  Use the Expand function provided by the same KDF to extract an
       AEAD key key, of length Nk - the length of the keys used by the
       AEAD associated with context.  Generating key uses a label of
       "key".

   5.  Use the same Expand function to extract a nonce nonce of length
       Nn - the length of the nonce used by the AEAD.  Generating nonce
       uses a label of "nonce".

   6.  Encrypt response, passing the AEAD function Seal the values of
       key, nonce, empty aad, and a pt input of request, which yields
       ct.

   7.  Concatenate response_nonce and ct, yielding an Encapsulated
       Response enc_response.  Note that response_nonce is of fixed-
       length, so there is no ambiguity in parsing either response_nonce
       or ct.

   In pseudocode, this procedure is as follows:



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   secret = context.Export("message/bhttp response", Nk)
   response_nonce = random(max(Nn, Nk))
   salt = concat(enc, response_nonce)
   prk = Extract(salt, secret)
   aead_key = Expand(prk, "key", Nk)
   aead_nonce = Expand(prk, "nonce", Nn)
   ct = Seal(aead_key, aead_nonce, "", response)
   enc_response = concat(response_nonce, ct)

   Clients decrypt an Encapsulated Response by reversing this process.
   That is, they first parse enc_response into response_nonce and ct.
   They then follow the same process to derive values for aead_key and
   aead_nonce.

   The client uses these values to decrypt ct using the Open function
   provided by the AEAD.  Decrypting might produce an error, as follows:

   reponse, error = Open(aead_key, aead_nonce, "", ct)

4.3.  Request and Response Media Types

   Media types are used to identify Encapsulated Requests and Responses;
   see Section 10.2 and Section 10.3 for definitions of these media
   types.

   Evolution of the format of Encapsulated Requests and Responses is
   supported through the definition of new formats that are identified
   by new media types.

5.  HTTP Usage

   A client interacts with the Oblivious Relay Resource by constructing
   an Encapsulated Request.  This Encapsulated Request is included as
   the content of a POST request to the Oblivious Relay Resource.  This
   request MUST only contain those fields necessary to carry the
   Encapsulated Request: a method of POST, a target URI of the Oblivious
   Relay Resource, a header field containing the content type (see
   (Section 10.2), and the Encapsulated Request as the request content.
   In the request to the Oblivious Relay Resource, clients MAY include
   additional fields.  However, those fields MUST be independent of the
   Encapsulated Request and MUST be fields that the Oblivious Relay
   Resource will remove before forwarding the Encapsulated Request
   towards the target, such as the Connection or Proxy-Authorization
   header fields [SEMANTICS].

   The client role in this protocol acts as an HTTP client both with
   respect to the Oblivious Relay Resource and the Target Resource.  For
   the request the clients makes to the Target Resource, this diverges



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   from typical HTTP assumptions about the use of a connection (see
   Section 3.3 of [HTTP]) in that the request and response are encrypted
   rather than sent over a connection.  The Oblivious Relay Resource and
   the Oblivious Gateway Resource also act as HTTP clients toward the
   Oblivious Gateway Resource and Target Resource respectively.

   The Oblivious Relay Resource interacts with the Oblivious Gateway
   Resource as an HTTP client by constructing a request using the same
   restrictions as the client request, except that the target URI is the
   Oblivious Gateway Resource.  The content of this request is copied
   from the client.  The Oblivious Relay Resource MUST NOT add
   information to the request without the client being aware of the type
   of information that might be added; see Section 6.2 for more
   information on relay responsibilities.

   When a response is received from the Oblivious Gateway Resource, the
   Oblivious Relay Resource forwards the response according to the rules
   of an HTTP proxy; see Section 7.6 of [HTTP].

   An Oblivious Gateway Resource, if it receives any response from the
   Target Resource, sends a single 200 response containing the
   encapsulated response.  Like the request from the client, this
   response MUST only contain those fields necessary to carry the
   encapsulated response: a 200 status code, a header field indicating
   the content type, and the encapsulated response as the response
   content.  As with requests, additional fields MAY be used to convey
   information that does not reveal information about the encapsulated
   response.

   An Oblivious Gateway Resource acts as a gateway for requests to the
   Target Resource (see Section 7.6 of [HTTP]).  The one exception is
   that any information it might forward in a response MUST be
   encapsulated, unless it is responding to errors it detects before
   removing encapsulation of the request; see Section 5.2.

5.1.  Informational Responses

   This encapsulation does not permit progressive processing of
   responses.  Though the binary HTTP response format does support the
   inclusion of informational (1xx) status codes, the AEAD encapsulation
   cannot be removed until the entire message is received.

   In particular, the Expect header field with 100-continue (see
   Section 10.1.1 of [HTTP]) cannot be used.  Clients MUST NOT construct
   a request that includes a 100-continue expectation; the Oblivious
   Gateway Resource MUST generate an error if a 100-continue expectation
   is received.




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5.2.  Errors

   A server that receives an invalid message for any reason MUST
   generate an HTTP response with a 4xx status code.

   Errors detected by the Oblivious Relay Resource and errors detected
   by the Oblivious Gateway Resource before removing protection
   (including being unable to remove encapsulation for any reason)
   result in the status code being sent without protection in response
   to the POST request made to that resource.

   Errors detected by the Oblivious Gateway Resource after successfully
   removing encapsulation and errors detected by the Target Resource
   MUST be sent in an Encapsulated Response.

6.  Security Considerations

   In this design, a client wishes to make a request of a server that is
   authoritative for the Target Resource.  The client wishes to make
   this request without linking that request with either:

   1.  The identity at the network and transport layer of the client
       (that is, the client IP address and TCP or UDP port number the
       client uses to create a connection).

   2.  Any other request the client might have made in the past or might
       make in the future.

   In order to ensure this, the client selects a relay (that serves the
   Oblivious Relay Resource) that it trusts will protect this
   information by forwarding the Encapsulated Request and Response
   without passing it to the server (that serves the Oblivious Gateway
   Resource).

   In this section, a deployment where there are three entities is
   considered:

   *  A client makes requests and receives responses

   *  A relay operates the Oblivious Relay Resource

   *  A server operates both the Oblivious Gateway Resource and the
      Target Resource








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   To achieve the stated privacy goals, the Oblivious Relay Resource
   cannot be operated by the same entity as the Oblivious Gateway
   Resource.  However, colocation of the Oblivious Gateway Resource and
   Target Resource simplifies the interactions between those resources
   without affecting client privacy.

   As a consequence of this configuration, Oblivious HTTP prevents
   linkability described above.  Informally, this means:

   1.  Requests and responses are known only to clients and targets in
       possession of the corresponding response encapsulation key and
       HPKE keying material.  In particular, the Oblivious Relay knows
       the origin and destination of an Encapsulated Request and
       Response, yet does not know the decrypted contents.  Likewise,
       targets know only the Oblivious Gateway origin, i.e., the relay,
       and the decrypted request.  Only the client knows both the
       plaintext request and response.

   2.  Targets cannot link requests from the same client in the absence
       of unique per-client keys.

   Traffic analysis that might affect these properties are outside the
   scope of this document; see Section 6.2.3.

   A formal analysis of Oblivious HTTP is in [OHTTP-ANALYSIS].

6.1.  Client Responsibilities

   Clients MUST ensure that the key configuration they select for
   generating Encapsulated Requests is integrity protected and
   authenticated so that it can be attributed to the Oblivious Gateway
   Resource; see Section 3.

   Since clients connect directly to the relay instead of the target,
   application configurations wherein clients make policy decisions
   about target connections, e.g., to apply certificate pinning, are
   incompatible with Oblivious HTTP.  In such cases, alternative
   technologies such as HTTP CONNECT (Section 9.3.6 of [HTTP]) can be
   used.  Applications could implement related policies on key
   configurations and relay connections, though these might not provide
   the same properties as policies enforced directly on target
   connections.  When this difference is relevant, applications can
   instead connect directly to the target at the cost of either privacy
   or performance.







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   Clients MUST NOT include identifying information in the request that
   is encrypted.  Identifying information includes cookies [COOKIES],
   authentication credentials or tokens, and any information that might
   reveal client-specific information such as account credentials.

   Clients cannot carry connection-level state between requests as they
   only establish direct connections to the relay responsible for the
   Oblivious Relay resource.  However, clients need to ensure that they
   construct requests without any information gained from previous
   requests.  Otherwise, the server might be able to use that
   information to link requests.  Cookies [COOKIES] are the most obvious
   feature that MUST NOT be used by clients.  However, clients need to
   include all information learned from requests, which could include
   the identity of resources.

   Clients MUST generate a new HPKE context for every request, using a
   good source of entropy ([RANDOM]) for generating keys.  Key reuse not
   only risks requests being linked, reuse could expose request and
   response contents to the relay.

   The request the client sends to the Oblivious Relay Resource only
   requires minimal information; see Section 5.  The request that
   carries the Encapsulated Request and is sent to the Oblivious Relay
   Resource MUST NOT include identifying information unless the client
   ensures that this information is removed by the relay.  A client MAY
   include information only for the Oblivious Relay Resource in header
   fields identified by the Connection header field if it trusts the
   relay to remove these as required by Section 7.6.1 of [HTTP].  The
   client needs to trust that the relay does not replicate the source
   addressing information in the request it forwards.

   Clients rely on the Oblivious Relay Resource to forward Encapsulated
   Requests and responses.  However, the relay can only refuse to
   forward messages, it cannot inspect or modify the contents of
   Encapsulated Requests or responses.

6.2.  Relay Responsibilities

   The relay that serves the Oblivious Relay Resource has a very simple
   function to perform.  For each request it receives, it makes a
   request of the Oblivious Gateway Resource that includes the same
   content.  When it receives a response, it sends a response to the
   client that includes the content of the response from the Oblivious
   Gateway Resource.







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   When forwarding a request, the relay MUST follow the forwarding rules
   in Section 7.6 of [HTTP].  A generic HTTP intermediary implementation
   is suitable for the purposes of serving an Oblivious Relay Resource,
   but additional care is needed to ensure that client privacy is
   maintained.

   Firstly, a generic implementation will forward unknown fields.  For
   Oblivious HTTP, a Oblivious Relay Resource SHOULD NOT forward unknown
   fields.  Though clients are not expected to include fields that might
   contain identifying information, removing unknown fields removes this
   privacy risk.

   Secondly, generic implementations are often configured to augment
   requests with information about the client, such as the Via field or
   the Forwarded field [FORWARDED].  A relay MUST NOT add information
   when forwarding requests that might be used to identify clients, with
   the exception of information that a client is aware of.

   Finally, a relay can also generate responses, though it assumed to
   not be able to examine the content of a request (other than to
   observe the choice of key identifier, KDF, and AEAD), so it is also
   assumed that it cannot generate an Encapsulated Response.

6.2.1.  Differential Treatment

   A relay MAY add information to requests if the client is aware of the
   nature of the information that could be added.  The client does not
   need to be aware of the exact value added for each request, but needs
   to know the range of possible values the relay might use.
   Importantly, information added by the relay - beyond what is already
   revealed through encapsulated requests from clients - can reduce the
   size of the anonymity set of clients at a gateway.

   Moreover, relays MAY apply differential treatment to clients that
   engage in abusive behavior, e.g., by sending too many requests in
   comparison to other clients, or as a response to rate limits
   signalled from the gateway.  Any such differential treatment can
   reveal information to the gateway that would not be revealed
   otherwise and therefore reduce the size of the anonymity set of
   clients using a gateway.  For example, if a relay chooses to rate
   limit or block an abusive client, this means that any client requests
   which are not treated this way are known to be non-abusive by the
   gateway.  Clients should consider the likelihood of such differential
   treatment and the privacy risks when using a relay.

   Some patterns of abuse cannot be detected without access to the
   request that is made to the target.  This means that only the gateway
   or target are in a position to identify abuse.  A gateway MAY send



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   signals toward the relay to provide feedback about specific requests.
   For example, a gateway could respond differently to requests it
   cannot decapsulate, as mentioned in Section 5.2.  A relay that acts
   on this feedback could - either inadvertently or by design - lead to
   client deanonymization.

6.2.2.  Denial of Service

   As there are privacy benefits from having a large rate of requests
   forwarded by the same relay (see Section 6.2.3), servers that operate
   the Oblivious Gateway Resource might need an arrangement with
   Oblivious Relay Resources.  This arrangement might be necessary to
   prevent having the large volume of requests being classified as an
   attack by the server.

   If a server accepts a larger volume of requests from a relay, it
   needs to trust that the relay does not allow abusive levels of
   request volumes from clients.  That is, if a server allows requests
   from the relay to be exempt from rate limits, the server might want
   to ensure that the relay applies a rate limiting policy that is
   acceptable to the server.

   Servers that enter into an agreement with a relay that enables a
   higher request rate might choose to authenticate the relay to enable
   the higher rate.

6.2.3.  Traffic Analysis

   This document assumes that all communication between different
   Oblivious Client, Oblivious Relay Resource, and Oblivious Gateway
   Resource is protected by HTTPS.  This protects information about
   which resources are the subject of request and prevents a network
   observer from being able to trivially correlate messages on either
   side of a relay.  However, it does not mitigate traffic analysis by
   such network observers.

   The time at which Encapsulated Request or response messages are sent
   can reveal information to a network observer.  Though messages
   exchanged between the Oblivious Relay Resource and the Oblivious
   Gateway Resource might be sent in a single connection, traffic
   analysis could be used to match messages that are forwarded by the
   relay.

   A relay could, as part of its function, add delays in order to
   increase the anonymity set into which each message is attributed.
   This could latency to the overall time clients take to receive a
   response, which might not be what some clients want.




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   A relay that forwards large volumes of exchanges can provide better
   privacy by providing larger sets of messages that need to be matched.

   Traffic analysis is not restricted to network observers.  A malicious
   Oblivious Relay Resource could use traffic analysis to learn
   information about otherwise encrypted requests and responses relayed
   between clients and gateways.  An Oblivious Relay Resource terminates
   TLS connections from clients, so they see message boundaries.  This
   privileged position allows for richer feature extraction from
   encrypted data, which might improve traffic analysis.

   Clients can use padding to reduce the effectiveness of traffic
   analysis.  Padding is a capability provided by binary HTTP messages;
   see Section 3.8 of [BINARY].

6.3.  Server Responsibilities

   The Oblivious Gateway Resource can be operated by a different entity
   than the Target Resource.  However, this means that the client needs
   to trust the Oblivious Gateway Resource not to modify requests or
   responses.  This analysis concerns itself with a deployment scenario
   where a single server provides both the Oblivious Gateway Resource
   and Target Resource.

   A server that operates both Oblivious Gateway and Target Resources is
   responsible for removing request encryption, generating a response to
   the Encapsulated Request, and encrypting the response.

   Servers should account for traffic analysis based on response size or
   generation time.  Techniques such as padding or timing delays can
   help protect against such attacks; see Section 6.2.3.

   If separate entities provide the Oblivious Gateway Resource and
   Target Resource, these entities might need an arrangement similar to
   that between server and relay for managing denial of service; see
   Section 6.2.2.  It is also necessary to provide confidentiality
   protection for the unprotected requests and responses, plus
   protections for traffic analysis; see Section 6.2.3.

   Nonsecure requests - such those with the "http" scheme as opposed to
   the "https" scheme - SHOULD NOT be used if the Oblivious Gateway and
   Target Resources are operated by different entities as that would
   expose both requests and response to modification or inspection by a
   network attacker.







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6.4.  Key Management

   An Oblivious Gateway Resource needs to have a plan for replacing
   keys.  This might include regular replacement of keys, which can be
   assigned new key identifiers.  If an Oblivious Gateway Resource
   receives a request that contains a key identifier that it does not
   understand or that corresponds to a key that has been replaced, the
   server can respond with an HTTP 422 (Unprocessable Content) status
   code.

   A server can also use a 422 status code if the server has a key that
   corresponds to the key identifier, but the Encapsulated Request
   cannot be successfully decrypted using the key.

   A server MUST ensure that the HPKE keys it uses are not valid for any
   other protocol that uses HPKE with the "message/bhttp request" label.
   Designers of protocols that reuse this encryption format, especially
   new versions of this protocol, can ensure key diversity by choosing a
   different label in their use of HPKE.  The "message/bhttp response"
   label was chosen for symmetry only as it provides key diversity only
   within the HPKE context created using the "message/bhttp request"
   label; see Section 9.

6.5.  Replay Attacks

   A server is responsible for either rejecting replayed requests or
   ensuring that the effect of replays does not adversely affect clients
   or resources.

   Encrypted requests can be copied and replayed by the Oblivious Relay
   resource.  The threat model for Oblivious HTTP allows the possibility
   that an Oblivious Relay Resource might replay requests.  Furthermore,
   if a client sends an Encapsulated Request in TLS early data (see
   Section 8 of [TLS] and [RFC8470]), a network-based adversary might be
   able to cause the request to be replayed.  In both cases, the effect
   of a replay attack and the mitigations that might be employed are
   similar to TLS early data.

   It is the responsibility of the application that uses Oblivious HTTP
   to either reject replayed requests or to ensure that replayed
   requests have no adverse affects on their operation.  This section
   describes some approaches that are universally applicable and
   suggestions for more targeted techniques.

   A client or Oblivious Relay Resource MUST NOT automatically attempt
   to retry a failed request unless it receives a positive signal
   indicating that the request was not processed or forwarded.  The
   HTTP/2 REFUSED_STREAM error code (Section 8.1.4 of [RFC7540]), the



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   HTTP/3 H3_REQUEST_REJECTED error code (Section 8.1 of [QUIC-HTTP]),
   or a GOAWAY frame with a low enough identifier (in either protocol
   version) are all sufficient signals that no processing occurred.
   Connection failures or interruptions are not sufficient signals that
   no processing occurred.

   The anti-replay mechanisms described in Section 8 of [TLS] are
   generally applicable to Oblivious HTTP requests.  The encapsulated
   keying material (or enc) can be used in place of a nonce to uniquely
   identify a request.  This value is a high-entropy value that is
   freshly generated for every request, so two valid requests will have
   different values with overwhelming probability.

   The mechanism used in TLS for managing differences in client and
   server clocks cannot be used as it depends on being able to observe
   previous interactions.  Oblivious HTTP explicitly prevents such
   linkability.

   The considerations in [RFC8470] as they relate to managing the risk
   of replay also apply, though there is no option to delay the
   processing of a request.

   Limiting requests to those with safe methods might not be
   satisfactory for some applications, particularly those that involve
   the submission of data to a server.  The use of idempotent methods
   might be of some use in managing replay risk, though it is important
   to recognize that different idempotent requests can be combined to be
   not idempotent.

   Even without replay prevention, the server-chosen response_nonce
   field ensures that responses have unique AEAD keys and nonces even
   when requests are replayed.

6.5.1.  Use of Date for Anti-Replay

   Clients SHOULD include a Date header field in Encapsulated Requests,
   unless the Oblivious Gateway Resource does not use Date for anti-
   replay purposes.

   Though HTTP requests often do not include a Date header field, the
   value of this field might be used by a server to limit the amount of
   requests it needs to track if it needs to prevent replay attacks.

   An Oblivious Gateway Resource can maintain state for requests for a
   small window of time over which it wishes to accept requests.  The
   Oblivious Gateway Resource can store all requests it processes within
   this window.  Storing just the enc field of a request, which should
   be unique to each request, is sufficient.  The Oblivious Gateway



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   Resource then rejects requests if the request is the same as one that
   was previously answered within that time window, or if the Date
   header field from the decrypted request is outside of the current
   time window.

   Oblivious Gateway Resources SHOULD allow for the time it takes
   requests to arrive from the client, with a time window that is large
   enough to allow for differences in clocks.

   Oblivious Gateway Resources MUST NOT treat the time window as secret
   information.  An attacker can actively probe with different values
   for the Date field to determine the time window over which the server
   will accept responses.

6.5.2.  Correcting Clock Differences

   An Oblivious Gateway Resource can reject requests that contain a Date
   value that is outside of its active window with a 400 series status
   code.  The problem type [PROBLEM] of "https://iana.org/assignments/
   http-problem-types#date" is defined to allow the server to signal
   that the Date value in the request was unacceptable.

   Figure 7 shows an example response in HTTP/1.1 format.

   HTTP/1.1 400 Bad Request
   Date: Mon, 07 Feb 2022 00:28:05 GMT
   Content-Type: application/problem+json
   Content-Length: 128

   {"type":"https://iana.org/assignments/http-problem-types#date",
   "title": "date field in request outside of acceptable range"}

             Figure 7: Example Rejection of Request Date Field

   Disagreements about time are unlikely if both client and Oblivious
   Gateway Resource have a good source of time; see [NTP].  However,
   clock differences are known to be commonplace; see Section 7.1 of
   [CLOCKSKEW].

   Including a Date header field in the response allows the client to
   correct clock errors by retrying the same request using the value of
   the Date field provided by the Oblivious Gateway Resource.  The value
   of the Date field can be copied if the request is fresh, with an
   adjustment based on the Age field otherwise.  When retrying a
   request, the client MUST create a fresh encryption of the modified
   request, using a new HPKE context.





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   +---------+       +-------------------+      +----------+
   | Client  |       | Relay and Gateway |      | Target   |
   |         |       |     Resources     |      | Resource |
   +----+----+       +----+-----------+--+      +----+-----+
        |                 |           |              |
        |                 |           |              |
        |  Request        |           |              |
        +============================>+------------->|
        |                 |           |              |
        |                 |           | 400 Response |
        |                 |           |       + Date |
        |<============================+<-------------+
        |                 |           |              |
        |  Request        |           |              |
        |  + Updated Date |           |              |
        +============================>+------------->|
        |                 |           |              |

                Figure 8: Retrying with an Update Date Field

   Intermediaries can sometimes rewrite the Date field when forwarding
   responses.  This might cause problems if the Oblivious Gateway
   Resource and intermediary clocks differ by enough to cause the retry
   to be rejected.  Therefore, clients MUST NOT retry a request with an
   adjusted date more than once.

   Oblivious Gateway Resources that condition their responses on the
   Date header field SHOULD either ensure that intermediaries do not
   cache responses (by including a Cache-Control directive of no-store)
   or designate the response as conditional on the value of the Date
   request header field (by including the token "date" in a Vary header
   field).

   Clients MUST NOT use the date provided by the Oblivious Gateway
   Resource for any other purpose, including future requests to any
   resource.  Any request that uses information provided by the
   Oblivious Gateway Resource might be correlated using that
   information.

6.6.  Forward Secrecy

   This document does not provide forward secrecy for either requests or
   responses during the lifetime of the key configuration.  A measure of
   forward secrecy can be provided by generating a new key configuration
   then deleting the old keys after a suitable period.






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6.7.  Post-Compromise Security

   This design does not provide post-compromise security for responses.

   A client only needs to retain keying material that might be used
   compromise the confidentiality and integrity of a response until that
   response is consumed, so there is negligible risk associated with a
   client compromise.

   A server retains a secret key that might be used to remove protection
   from messages over much longer periods.  A server compromise that
   provided access to the Oblivious Gateway Resource secret key could
   allow an attacker to recover the plaintext of all requests sent
   toward affected keys and all of the responses that were generated.

   Even if server keys are compromised, an adversary cannot access
   messages exchanged by the client with the Oblivious Relay Resource as
   messages are protected by TLS.  Use of a compromised key also
   requires that the Oblivious Relay Resource cooperate with the
   attacker or that the attacker is able to compromise these TLS
   connections.

   The total number of affected messages affected by server key
   compromise can be limited by regular rotation of server keys.

6.8.  Client Clock Exposure

   Including a Date field in requests reveals some information about the
   client clock.  This might be used to fingerprint clients [UWT] or to
   identify clients that are vulnerable to attacks that depend on
   incorrect clocks.

   Clients can randomize the value that they provide for Date to obscure
   the true value of their clock and reduce the chance of linking of
   requests over time.  However, this increases the risk that their
   request is rejected as outside the acceptable window.

7.  Privacy Considerations

   One goal of this design is that independent client requests are only
   linkable by their content.  However, the choice of client
   configuration might be used to correlate requests.  A client
   configuration includes the Oblivious Relay Resource URI, the
   Oblivious Gateway key configuration (KeyConfig), and Oblivious
   Gateway Resource URI.  A configuration is active if clients can
   successfully use it for interacting with with a target.





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   Oblivious Relay and Gateway Resources can identify when requests use
   the same configuration by matching KeyConfig.key\_id or the Oblivious
   Gateway Resource URI.  The Oblivious Gateway Resource might use the
   source address of requests to correlate requests that use an
   Oblivious Relay Resource run by the same operator.  If the Oblivious
   Gateway Resource is willing to use trial decryption, requests can be
   further separated into smaller groupings based on the keys that are
   used.

   Each active client configuration partitions the client anonymity set.
   In practice, it is infeasible to reduce the number of active
   configurations to one.  Enabling diversity in choice of Oblivious
   Relay Resource naturally increases the number of active
   configurations.  A small number of configurations might need to be
   active to allow for key rotation and server maintenance.

   Client privacy depends on having each configuration used by many
   other clients.  It is critical prevent the use of unique client
   configurations, which might be used to track of individual clients,
   but it is also important to avoid creating small groupings of clients
   that might weaken privacy protections.

   A specific method for a client to acquire configurations is not
   included in this specification.  Applications using this design MUST
   provide accommodations to mitigate tracking using client
   configurations.  [CONSISTENCY] provides options for ensuring that
   client configurations are consistent between clients.

   The content of requests or responses, if used in forming new
   requests, can be used to correlate requests.  This includes obvious
   methods of linking requests, like cookies [COOKIES], but it also
   includes any information in either message that might affect how
   subsequent requests are formulated.  For example, [FIELDING]
   describes how interactions that are individually stateless can be
   used to build a stateful system when a client acts on the content of
   a response.

8.  Operational and Deployment Considerations

   This section discusses various operational and deployment
   considerations.










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8.1.  Performance Overhead

   Using Oblivious HTTP adds both cryptographic and latency to requests
   relative to a simple HTTP request-response exchange.  Deploying relay
   services that are on path between clients and servers avoids adding
   significant additional delay due to network topology.  A study of a
   similar system [ODoH] found that deploying proxies close to servers
   was most effective in minimizing additional latency.

8.2.  Resource Mappings

   This protocol assumes a fixed, one-to-one mapping between the
   Oblivious Relay Resource and the Oblivious Gateway Resource.  This
   means that any encrypted request sent to the Oblivious Relay Resource
   will always be forwarded to the Oblivious Gateway Resource.  This
   constraint was imposed to simplify relay configuration and mitigate
   against the Oblivious Relay Resource being used as a generic relay
   for unknown Oblivious Gateway Resources.  The relay will only forward
   for Oblivious Gateway Resources that it has explicitly configured and
   allowed.

   It is possible for a server to be configured with multiple Oblivious
   Relay Resources, each for a different Oblivious Gateway Resource as
   needed.  If the goal is to support a large number of Oblivious
   Gateway Resources, clients might be provided with a URI template
   [TEMPLATE], from which multiple Oblivious Relay Resources could be
   constructed.

8.3.  Network Management

   Oblivious HTTP might be incompatible with network interception
   regimes, such as those that rely on configuring clients with trust
   anchors and intercepting TLS connections.  While TLS might be
   intercepted successfully, interception middleboxes devices might not
   receive updates that would allow Oblivious HTTP to be correctly
   identified using the media types defined in Section 10.2 and
   Section 10.3.

   Oblivious HTTP has a simple key management design that is not
   trivially altered to enable interception by intermediaries.  Clients
   that are configured to enable interception might choose to disable
   Oblivious HTTP in order to ensure that content is accessible to
   middleboxes.








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9.  Repurposing the Encapsulation Format

   The encrypted payload of an OHTTP request and response is a binary
   HTTP message [BINARY].  Client and target agree on this encrypted
   payload type by specifying the media type "message/bhttp" in the HPKE
   info string and HPKE export context string for request and response
   encryption, respectively.

   Future specifications may repurpose the encapsulation mechanism
   described in Section 4, provided that the content type of the
   encrypted payload is appropriately reflected in the HPKE info and
   context strings.  For example, if a future specification were to use
   the encryption mechanism in this specification for DNS messages,
   identified by the "application/dns-message" media type, then the HPKE
   info string SHOULD be "application/dns-message request" for request
   encryption, and the HPKE export context string should be
   "application/dns-message response" for response encryption.

10.  IANA Considerations

   Please update the "Media Types" registry at
   https://www.iana.org/assignments/media-types
   (https://www.iana.org/assignments/media-types) for the media types
   "application/ohttp-keys" (Section 10.1), "message/ohttp-req"
   (Section 10.2), and "message/ohttp-res" (Section 10.3).

10.1.  message/ohttp-keys Media Type

   The "application/ohttp-keys" media type identifies a key
   configuration used by Oblivious HTTP.

   Type name:  application

   Subtype name:  ohttp-keys

   Required parameters:  N/A

   Optional parameters:  None

   Encoding considerations:  only "8bit" or "binary" is permitted

   Security considerations:  see Section 6

   Interoperability considerations:  N/A

   Published specification:  this specification

   Applications that use this media type:  Oblivious HTTP and



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      applications that use Oblivious HTTP

   Fragment identifier considerations:  N/A

   Additional information:  Magic number(s):  N/A

                            Deprecated alias names for this type:  N/A

                            File extension(s):  N/A

                            Macintosh file type code(s):  N/A

   Person and email address to contact for further information:  see Aut
      hors' Addresses section

   Intended usage:  COMMON

   Restrictions on usage:  N/A

   Author:  see Authors' Addresses section

   Change controller:  IESG

10.2.  message/ohttp-req Media Type

   The "message/ohttp-req" identifies an encrypted binary HTTP request.
   This is a binary format that is defined in Section 4.1.

   Type name:  message

   Subtype name:  ohttp-req

   Required parameters:  N/A

   Optional parameters:  None

   Encoding considerations:  only "8bit" or "binary" is permitted

   Security considerations:  see Section 6

   Interoperability considerations:  N/A

   Published specification:  this specification

   Applications that use this media type:  Oblivious HTTP and
      applications that use Oblivious HTTP

   Fragment identifier considerations:  N/A



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   Additional information:  Magic number(s):  N/A

                            Deprecated alias names for this type:  N/A

                            File extension(s):  N/A

                            Macintosh file type code(s):  N/A

   Person and email address to contact for further information:  see Aut
      hors' Addresses section

   Intended usage:  COMMON

   Restrictions on usage:  N/A

   Author:  see Authors' Addresses section

   Change controller:  IESG

10.3.  message/ohttp-res Media Type

   The "message/ohttp-res" identifies an encrypted binary HTTP response.
   This is a binary format that is defined in Section 4.2.

   Type name:  message

   Subtype name:  ohttp-res

   Required parameters:  N/A

   Optional parameters:  None

   Encoding considerations:  only "8bit" or "binary" is permitted

   Security considerations:  see Section 6

   Interoperability considerations:  N/A

   Published specification:  this specification

   Applications that use this media type:  Oblivious HTTP and
      applications that use Oblivious HTTP

   Fragment identifier considerations:  N/A

   Additional information:  Magic number(s):  N/A

                            Deprecated alias names for this type:  N/A



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                            File extension(s):  N/A

                            Macintosh file type code(s):  N/A

   Person and email address to contact for further information:  see Aut
      hors' Addresses section

   Intended usage:  COMMON

   Restrictions on usage:  N/A

   Author:  see Authors' Addresses section

   Change controller:  IESG

   IANA are requested to create a new entry in the "HTTP Problem Type"
   registry established by [PROBLEM].

   Type URI:  https://iana.org/assignments/http-problem-types#date

   Title:  Date Not Acceptable

   Recommended HTTP Status Code:  400

   Reference:  Section 6.5.2 of this document

11.  References

11.1.  Normative References

   [ASCII]    Cerf, V., "ASCII format for network interchange", STD 80,
              RFC 20, DOI 10.17487/RFC0020, October 1969,
              <https://www.rfc-editor.org/rfc/rfc20>.

   [BINARY]   Thomson, M. and C. A. Wood, "Binary Representation of HTTP
              Messages", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-binary-message-06, 6 July 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              binary-message-06>.

   [HPKE]     Barnes, R., Bhargavan, K., Lipp, B., and C. Wood, "Hybrid
              Public Key Encryption", RFC 9180, DOI 10.17487/RFC9180,
              February 2022, <https://www.rfc-editor.org/rfc/rfc9180>.

   [HTTP]     Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,
              <https://www.rfc-editor.org/rfc/rfc9110>.



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   [PROBLEM]  Nottingham, M., Wilde, E., and S. Dalal, "Problem Details
              for HTTP APIs", Work in Progress, Internet-Draft, draft-
              ietf-httpapi-rfc7807bis-03, 25 May 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpapi-
              rfc7807bis-03>.

   [QUIC]     Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,
              <https://www.rfc-editor.org/rfc/rfc9000>.

   [QUIC-HTTP]
              Bishop, M., "HTTP/3", Work in Progress, Internet-Draft,
              draft-ietf-quic-http-34, 2 February 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-quic-
              http-34>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <https://www.rfc-editor.org/rfc/rfc7540>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

   [RFC8470]  Thomson, M., Nottingham, M., and W. Tarreau, "Using Early
              Data in HTTP", RFC 8470, DOI 10.17487/RFC8470, September
              2018, <https://www.rfc-editor.org/rfc/rfc8470>.

   [TLS]      Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/rfc/rfc8446>.

11.2.  Informative References











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   [CLOCKSKEW]
              Acer, M., Stark, E., Felt, A., Fahl, S., Bhargava, R.,
              Dev, B., Braithwaite, M., Sleevi, R., and P. Tabriz,
              "Where the Wild Warnings Are: Root Causes of Chrome HTTPS
              Certificate Errors", Proceedings of the 2017 ACM SIGSAC
              Conference on Computer and Communications Security,
              DOI 10.1145/3133956.3134007, October 2017,
              <https://doi.org/10.1145/3133956.3134007>.

   [CONSISTENCY]
              Davidson, A., Finkel, M., Thomson, M., and C. A. Wood,
              "Key Consistency and Discovery", Work in Progress,
              Internet-Draft, draft-wood-key-consistency-02, 4 March
              2022, <https://datatracker.ietf.org/doc/html/draft-wood-
              key-consistency-02>.

   [COOKIES]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011,
              <https://www.rfc-editor.org/rfc/rfc6265>.

   [Dingledine2004]
              Dingledine, R., Mathewson, N., and P. Syverson, "Tor: The
              Second-Generation Onion Router", August 2004,
              <https://svn.torproject.org/svn/projects/design-paper/tor-
              design.html>.

   [FIELDING] Fielding, R. T., "Architectural Styles and the Design of
              Network-based Software Architectures", 2000,
              <https://www.ics.uci.edu/~fielding/pubs/dissertation/
              fielding_dissertation.pdf>.

   [FORWARDED]
              Petersson, A. and M. Nilsson, "Forwarded HTTP Extension",
              RFC 7239, DOI 10.17487/RFC7239, June 2014,
              <https://www.rfc-editor.org/rfc/rfc7239>.

   [NTP]      Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
              <https://www.rfc-editor.org/rfc/rfc5905>.

   [ODoH]     Singanamalla, S., Chunhapanya, S., Vavrusa, M., Verma, T.,
              Wu, P., Fayed, M., Heimerl, K., Sullivan, N., and C. A.
              Wood, "Oblivious DNS over HTTPS (ODoH): A Practical
              Privacy Enhancement to DNS", 7 January 2021,
              <https://www.petsymposium.org/2021/files/papers/issue4/
              popets-2021-0085.pdf>.




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   [ODOH]     Kinnear, E., McManus, P., Pauly, T., Verma, T., and C.A.
              Wood, "Oblivious DNS over HTTPS", RFC 9230,
              DOI 10.17487/RFC9230, June 2022,
              <https://www.rfc-editor.org/rfc/rfc9230>.

   [OHTTP-ANALYSIS]
              Hoyland, J., "Tamarin Model of Oblivious HTTP", 23 August
              2021, <https://github.com/cloudflare/ohttp-analysis>.

   [PRIO]     Corrigan-Gibbs, H. and D. Boneh, "Prio: Private, Robust,
              and Scalable Computation of Aggregate Statistics", 14
              March 2017, <https://crypto.stanford.edu/prio/paper.pdf>.

   [RANDOM]   Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <https://www.rfc-editor.org/rfc/rfc4086>.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011,
              <https://www.rfc-editor.org/rfc/rfc6265>.

   [RFC7838]  Nottingham, M., McManus, P., and J. Reschke, "HTTP
              Alternative Services", RFC 7838, DOI 10.17487/RFC7838,
              April 2016, <https://www.rfc-editor.org/rfc/rfc7838>.

   [SEMANTICS]
              Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,
              <https://www.rfc-editor.org/rfc/rfc9110>.

   [TEMPLATE] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
              and D. Orchard, "URI Template", RFC 6570,
              DOI 10.17487/RFC6570, March 2012,
              <https://www.rfc-editor.org/rfc/rfc6570>.

   [UWT]      Nottingham, M., "Unsanctioned Web Tracking", 17 July 2015,
              <https://www.w3.org/2001/tag/doc/unsanctioned-tracking/>.

   [X25519]   Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
              for Security", RFC 7748, DOI 10.17487/RFC7748, January
              2016, <https://www.rfc-editor.org/rfc/rfc7748>.








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Appendix A.  Complete Example of a Request and Response

   A single request and response exchange is shown here.  Binary values
   (key configuration, secret keys, the content of messages, and
   intermediate values) are shown in hexadecimal.  The request and
   response here are minimal; the purpose of this example is to show the
   cryptographic operations.  In this example, the client is configured
   with the Oblivious Relay Resource URI of
   https://proxy.example.org/request.example.net/proxy, and the proxy is
   configured to map requests to this URI to the Oblivious Gateway URI
   https://example.com/oblivious/request.  The Target Resource URI,
   i.e., the resource the client ultimately wishes to query, is
   https://example.com.

   To begin the process, the Oblivious Gateway Resource generates a key
   pair.  In this example the server chooses DHKEM(X25519, HKDF-SHA256)
   and generates an X25519 key pair [X25519].  The X25519 secret key is:

   3c168975674b2fa8e465970b79c8dcf09f1c741626480bd4c6162fc5b6a98e1a

   The Oblivious Gateway Resource constructs a key configuration that
   includes the corresponding public key as follows:

   01002031e1f05a740102115220e9af918f738674aec95f54db6e04eb705aae8e
   79815500080001000100010003

   This key configuration is somehow obtained by the client.  Then when
   a client wishes to send an HTTP request of a GET request to the
   target https://example.com, it constructs the following binary HTTP
   message:

   00034745540568747470730b6578616d706c652e636f6d012f

   The client then reads the Oblivious Gateway Resource key
   configuration and selects a mutually supported KDF and AEAD.  In this
   example, the client selects HKDF-SHA256 and AES-128-GCM.  The client
   then generates an HPKE sending context that uses the server public
   key.  This context is constructed from the following ephemeral secret
   key:

   bc51d5e930bda26589890ac7032f70ad12e4ecb37abb1b65b1256c9c48999c73

   The corresponding public key is:

   4b28f881333e7c164ffc499ad9796f877f4e1051ee6d31bad19dec96c208b472

   And an info parameter of:




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   6d6573736167652f626874747020726571756573740001002000010001

   Applying the Seal operation from the HPKE context produces an
   encrypted message, allowing the client to construct the following
   Encapsulated Request:

   010020000100014b28f881333e7c164ffc499ad9796f877f4e1051ee6d31bad1
   9dec96c208b4726374e469135906992e1268c594d2a10c695d858c40a026e796
   5e7d86b83dd440b2c0185204b4d63525

   The client then sends this to the Oblivious Relay Resource in a POST
   request, which might look like the following HTTP/1.1 request:

   POST /request.example.net/proxy HTTP/1.1
   Host: proxy.example.org
   Content-Type: message/ohttp-req
   Content-Length: 78

   <content is the Encapsulated Request above>

   The Oblivious Relay Resource receives this request and forwards it to
   the Oblivious Gateway Resource, which might look like:

   POST /oblivious/request HTTP/1.1
   Host: example.com
   Content-Type: message/ohttp-req
   Content-Length: 78

   <content is the Encapsulated Request above>

   The Oblivous Gateway Resource receives this request, selects the key
   it generated previously using the key identifier from the message,
   and decrypts the message.  As this request is directed to the same
   server, the Oblivious Gateway Resource does not need to initiate an
   HTTP request to the Target Resource.  The request can be served
   directly by the Target Resource, which generates a minimal response
   (consisting of just a 200 status code) as follows:

   0140c8

   The response is constructed by extracting a secret from the HPKE
   context:

   62d87a6ba569ee81014c2641f52bea36

   The key derivation for the Encapsulated Response uses both the
   encapsulated KEM key from the request and a randomly selected nonce.
   This produces a salt of:



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   4b28f881333e7c164ffc499ad9796f877f4e1051ee6d31bad19dec96c208b472
   c789e7151fcba46158ca84b04464910d

   The salt and secret are both passed to the Extract function of the
   selected KDF (HKDF-SHA256) to produce a pseudorandom key of:

   979aaeae066cf211ab407b31ae49767f344e1501e475c84e8aff547cc5a683db

   The pseudorandom key is used with the Expand function of the KDF and
   an info field of "key" to produce a 16-byte key for the selected AEAD
   (AES-128-GCM):

   5d0172a080e428b16d298c4ea0db620d

   With the same KDF and pseudorandom key, an info field of "nonce" is
   used to generate a 12-byte nonce:

   f6bf1aeb88d6df87007fa263

   The AEAD Seal() function is then used to encrypt the response, which
   is added to the randomized nonce value to produce the Encapsulated
   Response:

   c789e7151fcba46158ca84b04464910d86f9013e404feea014e7be4a441f234f
   857fbd

   The Oblivious Gateway Resource constructs a response with the same
   content:

   HTTP/1.1 200 OK
   Date: Wed, 27 Jan 2021 04:45:07 GMT
   Cache-Control: private, no-store
   Content-Type: message/ohttp-res
   Content-Length: 38

   <content is the Encapsulated Response>

   The same response might then be generated by the Oblivious Relay
   Resource which might change as little as the Date header.  The client
   is then able to use the HPKE context it created and the nonce from
   the Encapsulated Response to construct the AEAD key and nonce and
   decrypt the response.

Acknowledgments

   This design is based on a design for Oblivious DoH, described in
   [ODOH].  David Benjamin, Mark Nottingham, and Eric Rescorla made
   technical contributions.



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Authors' Addresses

   Martin Thomson
   Mozilla
   Email: mt@lowentropy.net


   Christopher A. Wood
   Cloudflare
   Email: caw@heapingbits.net









































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