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Key Consistency for Oblivious HTTP by Double-Checking
draft-schwartz-ohai-consistency-doublecheck-00

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draft-schwartz-ohai-consistency-doublecheck-00
ohai                                                      B. M. Schwartz
Internet-Draft                                                Google LLC
Intended status: Standards Track                            7 April 2022
Expires: 9 October 2022

         Key Consistency for Oblivious HTTP by Double-Checking
             draft-schwartz-ohai-consistency-doublecheck-00

Abstract

   The assurances provided by Oblivious HTTP depend on the client's
   ability to verify that it is using the same Request Resource and
   KeyConfig as many other users.  This specification defines a protocol
   to enable this verification.

About This Document

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

   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-schwartz-ohai-consistency-
   doublecheck/.

   Source for this draft and an issue tracker can be found at
   https://github.com/bemasc/access-services.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 9 October 2022.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  Oblivious Request Resource  . . . . . . . . . . . . . . .   4
     4.2.  Oblivious Proxy . . . . . . . . . . . . . . . . . . . . .   4
     4.3.  Client  . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Example: Oblivious DoH  . . . . . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
     6.1.  In scope  . . . . . . . . . . . . . . . . . . . . . . . .   8
     6.2.  Out of scope  . . . . . . . . . . . . . . . . . . . . . .   9
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  12
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   Oblivious HTTP [I-D.ietf-ohai-ohttp] presumes at least three parties
   to each exchange: the client, the proxy, and the target (formally,
   the Oblivious Request Resource).  When used properly, Oblivious HTTP
   enables the client to send requests to the target in such a way that
   the target cannot tell whether two requests came from the same client
   and the proxy cannot see the contents of the requests.

   Oblivious HTTP's threat model assumes that at least one of the proxy
   and the target is acting properly, i.e. complying with the protocol
   and keeping certain information confidential.  If either proxy or
   target misbehaves, the only effect must be a denial of service.

   In order for these security guarantees to hold, several preconditions
   must be met:

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   1.  The client must be one of many users who might be using the
       proxy.  Otherwise, use of the proxy reveals the user's identity
       to the target.

   2.  The client must hold an authentic KeyConfig for the target.
       Otherwise, they could be speaking to the proxy, impersonating the
       target.

   3.  All users of this proxy must be equally likely to use this URI
       and KeyConfig for this target, regardless of their prior
       activity.  Otherwise, the encrypted request identifies the user
       to the target.

   4.  (optional) The target must not learn the IP addresses of the
       clients, collectively.  Otherwise, the target might be able to
       deanonymize requests by correlating them with external
       information about the clients.

   This specification defines behaviors for the client, proxy, and
   target that achieve preconditions 2-4.  (This specification does not
   address precondition 1.)

   This draft is an instantiation of the "Single Proxy Discovery"
   architecture for key consistency, defined in Section 4.2 of
   [I-D.wood-key-consistency].

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.

3.  Overview

   In the Key Consistency Double-Check procedure, the Client emits two
   requests: one to the Proxy, and one through the Proxy to the Target.
   The Proxy will forward the first request to the Target if the
   response is not in cache.

                   +--------+       +-------+      +--------+
                   |        |<=====>|       |<---->|        |
                   | Client |       | Proxy |      | Target |
                   |        |<====================>|        |
                   +--------+       +-------+      +--------+

             Figure 1: Overview of Key-Consistency Double-Check

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   The proxy caches the response, ensuring that all clients share it
   during its freshness lifetime.  The client checks this against the
   authenticated response from the Target, preventing forgeries.

4.  Requirements

4.1.  Oblivious Request Resource

   The Oblivious Request Resource MUST publish an Access Description
   [I-D.schwartz-masque-access-descriptions] over HTTP/3 containing the
   ohttp.request key, e.g.:

   {
     "ohttp": {
       "request": {
         "uri": "https://example.com/ohttp/",
         "key": "(KeyConfig in Base64)"
       }
     }
   }

   The Oblivious Request Resource MUST include a "strong validator" ETag
   (Section 2 of [RFC7232]) in any response to a GET request for this
   access description, and MUST support the "If-Match" HTTP request
   header (Section 3 of [RFC7232]).  The response MUST indicate "Cache-
   Control: public, no-transform, s-maxage=(...), immutable"
   [I-D.ietf-httpbis-cache][RFC8246].  For efficiency reasons, the max
   age SHOULD be at least 60 seconds, and preferably much longer.

   If this Access Description changes, and the resource receives a
   request whose "If-Match" header identifies a previously served
   version that has not yet expired, it MUST return a success response
   containing the previous version.  This response MAY indicate "Cache-
   Control: private".

4.2.  Oblivious Proxy

   The Oblivious Proxy MUST publish an Access Description that includes
   the ohttp.proxy and udp keys, indicating support for CONNECT-UDP
   [I-D.ietf-masque-connect-udp].  It SHOULD also contain the dns key,
   indicating support for DNS over HTTPS [RFC8484], to enable the use of
   HTTPS records with CONNECT-UDP.

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   {
     "dns": {
       "template": "https://doh.example.com/dns-query{?dns}",
     },
     "udp": {
       "template":
           "https://proxy.example.org/masque{?target_host,target_port}"
     },
     "ohttp": {
       "proxy": {
         "template": "https://proxy.example.org/ohttp{?request_uri}"
       }
     }
   }

                 Figure 2: Example Proxy Access Description

   The Oblivious Proxy Resources MUST allow use of the GET method to
   retrieve small JSON responses, and SHOULD make ample cache space
   available in order to cache Access Descriptions.  Each proxy instance
   (as defined by its external-facing network interface) MUST share
   cache state among all clients to ensure that they use the same Access
   Descriptions for each Oblivious Request Resource.

   Oblivious Proxies MUST preserve the ETag response header on cached
   responses, and MUST add an Age header ([I-D.ietf-httpbis-cache-19],
   Section 5.1) to all proxied responses.  Oblivious Proxies MUST
   respect the "Cache-Control: immutable" directive, never revalidating
   these cached entries, and MUST NOT accept PUSH_PROMISE frames from
   the target.

   Proxies SHOULD employ defenses against malicious attempts to fill the
   cache.  Some possible defenses include:

   *  Rate-limiting each client's use of GET requests.

   *  Prioritizing preservation of cache entries that have been served
      to many clients, if eviction is required.

   Oblivious Proxies that are not intended for general-purpose proxy
   usage MAY impose strict transfer limits or rate limits on HTTP
   CONNECT and CONNECT-UDP usage.

4.3.  Client

   The Client is assumed to know an "https" URI of an Oblivious Request
   Resource's Access Description.  To use that Request Resource, it MUST
   perform the following "double-check" procedure:

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   1.  Send a GET request to the Oblivious Proxy's template with
       request_uri set to the Access Description URI.

   2.  Record the response (A).

   3.  Check that response A's "Cache-Control" values indicates "public"
       and "immutable".

   4.  Fetch the Access Description URI from its origin via CONNECT-UDP,
       with "If-Match" set to response A's ETag.

   5.  Record the response (B).

   6.  Check that responses A and B were successful and the contents are
       identical, otherwise fail.

   This procedure ensures that the Access Description is authentic and
   will be shared by all users of this proxy.  Once response A or B
   expires, the client MUST refresh it before continuing to use this
   Access Description, and MUST repeat the "double-check" process if
   either response changes.

5.  Example: Oblivious DoH

   In this example, the client has been configured with an Oblivious DoH
   server and an Oblivious Proxy.  The Oblivious DoH server is
   identified by an Access Description at "https://doh.example.com/
   config.json" with the following contents:

   {
     "dns": {
       "template": "https://doh.example.com/dns-query{?dns}",
     },
     "ohttp": {
       "request": {
         "uri": "https://example.com/ohttp/",
         "key": "(KeyConfig in Base64)"
       }
     }
   }

   The Oblivious Proxy is identified as "proxy.example.org", which
   implies an Access Description at "https://proxy.example.org/.well-
   known/access-services".  This resource's contents are:

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   {
     "dns": {
       "template": "https://proxy.example.org/dns-query{?dns}",
     },
     "udp": {
       "template":
           "https://proxy.example.org/masque{?target_host,target_port}"
     },
     "ohttp": {
       "proxy": {
         "template": "https://proxy.example.org/ohttp{?request_uri}"
       }
     }
   }

   The following exchanges then occur between the client and the proxy:

  HEADERS
  :method = GET
  :scheme = https
  :authority = proxy.example.org
  :path = /ohttp?request_uri=https%3A%2F%2Fdoh.example.com%2Fconfig.json
  accept: application/json

                                HEADERS
                                :status = 200
                                cache-control: public, immutable, \
                                    no-transform, s-maxage=86400
                                age: 80000
                                etag: ABCD1234
                                content-type: application/json
                                [Access Description contents here]

  HEADERS
  :method = CONNECT
  :protocol = connect-udp
  :scheme = https
  :authority = proxy.example.org
  :path = /masque?target_host=doh.example.com,target_port=443
  capsule-protocol = ?1

                                HEADERS
                                :status = 200
                                capsule-protocol = ?1

   The client now has a CONNECT-UDP tunnel to doh.example.com, over
   which it performs the following request using HTTP/3:

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   HEADERS
   :method = GET
   :scheme = https
   :authority = doh.example.com
   :path = /config.json
   if-match = ABCD1234

                                 HEADERS
                                 :status = 200
                                 cache-control: public, immutable, \
                                     no-transform, s-maxage=86400
                                 etag: ABCD1234
                                 content-type: application/json
                                 [Access Description contents here]

   Having successfully fetched the Access Description from both
   locations, the client confirms that:

   *  The responses are identical.

   *  The cache-control response from the proxy contained the "public"
      and "immutable" directives.

   The client can now use the KeyConfig in this Access Description to
   reach the Oblivious DoH server, by forming Binary HTTP requests for
   "https://doh.example.com/dns-query" and delivering the encapsulated
   requests to "https://example.com/ohttp/" via the proxy.

6.  Security Considerations

6.1.  In scope

   A malicious proxy could attempt to learn the contents of the
   oblivious request by forging an Access Description containing its own
   KeyConfig.  This is prevented by the client's requirement that the
   KeyConfig be served to it by the configured origin over HTTPS
   (Section 4.3).

   A malicious target could attempt to link multiple requests together
   by issuing each user a unique, persistent KeyConfig.  This attack is
   prevented by the client's requirement that the KeyConfig be fresh
   according to the proxy's cache (Section 4.3).

   A malicious target could attempt to rotate its entry in the proxy's
   cache in several ways:

   *  Using HTTP PUSH_PROMISE frames.  This attack is prevented by
      disabling PUSH_PROMISE at the proxy (Section 4.2).

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   *  By also acting as a client and sending requests designed to
      replace the Access Description in the cache before it expires:

      -  By sending requests with a "Cache-Control: no-cache" or similar
         directive.  This is prevented by the response's "Cache-Control:
         public, immutable" directives, which are verified by the client
         (Section 4.3).

      -  By filling the cache with new entries, causing its previous
         Access Description to be evicted.  Section 4.2 describes some
         possible mitigations.

   A malicious client could use the proxy to send abusive traffic to any
   destination on the internet.  Abuse concerns can be mitigated by
   imposing a rate limit at the proxy (Section 4.2).

6.2.  Out of scope

   This specification assumes that the client starts with identities of
   the proxy and target that are authentic and widely shared.  If these
   identities are inauthentic, or are unique to the user, then the
   security goals of this specification are not achieved.

   This specification assumes that at most a small fraction of clients
   are acting on behalf of a malicious target.  If a large fraction of
   the clients are malicious, they could conspire to flood the proxy
   cache with entries that seem popular, leading to rapid eviction of
   the malicious target's Access Descriptions.  Similar concerns apply
   if a malicious target can compel naive clients to fetch a very large
   number of Access Descriptions.

7.  IANA Considerations

   IANA is requested to open a Specification Required registry entitled
   "HTTP Access Service Descriptors", with the following initial
   contents:

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                        +=======+=================+
                        | Key   | Specification   |
                        +=======+=================+
                        | dns   | (This document) |
                        +-------+-----------------+
                        | udp   | (This document) |
                        +-------+-----------------+
                        | ip    | (This document) |
                        +-------+-----------------+
                        | ohttp | (This document) |
                        +-------+-----------------+

                                  Table 1

   IANA is requested to add the following entry to the "Well-Known URIs"
   registry

   +=================+============+=========+===========+==============+
   | URI Suffix      | Change     |Reference| Status    | Related      |
   |                 | Controller |         |           | Information  |
   +=================+============+=========+===========+==============+
   | access-services | IETF       |(This    | permanent | Sub-registry |
   |                 |            |document)|           | at (link)    |
   +-----------------+------------+---------+-----------+--------------+

                                  Table 2

8.  References

8.1.  Normative References

   [I-D.ietf-httpbis-cache]
              Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
              Caching", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-cache-19, 12 September 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              cache-19>.

   [I-D.ietf-httpbis-cache-19]
              Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
              Caching", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-cache-19, 12 September 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              cache-19>.

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   [I-D.ietf-masque-connect-udp]
              Schinazi, D., "UDP Proxying Support for HTTP", Work in
              Progress, Internet-Draft, draft-ietf-masque-connect-udp-
              08, 21 March 2022, <https://datatracker.ietf.org/doc/html/
              draft-ietf-masque-connect-udp-08>.

   [I-D.ietf-ohai-ohttp]
              Thomson, M. and C. A. Wood, "Oblivious HTTP", Work in
              Progress, Internet-Draft, draft-ietf-ohai-ohttp-01, 15
              February 2022, <https://datatracker.ietf.org/doc/html/
              draft-ietf-ohai-ohttp-01>.

   [I-D.schwartz-masque-access-descriptions]
              Schwartz, B. M., "HTTP Access Service Description
              Objects", Work in Progress, Internet-Draft, draft-
              schwartz-masque-access-descriptions-00, 7 April 2022,
              <https://datatracker.ietf.org/doc/html/draft-schwartz-
              masque-access-descriptions-00>.

   [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>.

   [RFC7232]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Conditional Requests", RFC 7232,
              DOI 10.17487/RFC7232, June 2014,
              <https://www.rfc-editor.org/rfc/rfc7232>.

   [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>.

   [RFC8246]  McManus, P., "HTTP Immutable Responses", RFC 8246,
              DOI 10.17487/RFC8246, September 2017,
              <https://www.rfc-editor.org/rfc/rfc8246>.

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
              <https://www.rfc-editor.org/rfc/rfc8484>.

8.2.  Informative References

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   [I-D.wood-key-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>.

Acknowledgments

   TODO acknowledge.

Author's Address

   Benjamin M. Schwartz
   Google LLC
   Email: bemasc@google.com

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