Network Working Group                                         M. Thomson
Internet-Draft                                                   Mozilla
Intended status: Standards Track                             G. Eriksson
Expires: May 3, 2017                                         C. Holmberg
                                                        October 30, 2016

             Caching Secure HTTP Content using Blind Caches


   A mechanism is described whereby a server can use client-selected
   shared cache.

Status of This Memo

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   This Internet-Draft will expire on May 3, 2017.

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Table of Contents

   1.  Shared Caching for HTTPS  . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  Same-Host Secure Content Delegation . . . . . . . . . . . . .   3
     2.1.  Signaling Presence of a Proxy . . . . . . . . . . . . . .   3
     2.2.  Proxy Identification and Authentication . . . . . . . . .   4
   3.  Performance Optimizations . . . . . . . . . . . . . . . . . .   5
     3.1.  Proxy Cache Priming . . . . . . . . . . . . . . . . . . .   5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Shared Caching for HTTPS

   Shared caches allow an HTTP server to offload the responsibility for
   delivering certain content.  Content in the shared cache can be
   accessed efficiently by multiple clients, saving the origin server
   from having to serve those requests and ensuring that clients receive
   responses to cached requests more quickly.

   Proxy caching is the most common configuration for shared caching.  A
   proxy cache is either explicitly configured by a client, discovered
   as a result of being automatically configured.

   HTTPS [RFC2818] prevents the use of proxies by creating an
   authenticated end-to-end connection to the origin server or its
   gateway that is authenticated.  This provides a critical protection
   against man-in-the-middle attacks, but it also prevents the proxy
   from acting as a shared cache.

   Clients do not direct queries for "https" URIs to proxies.  Clients
   configured with a proxy use the CONNECT pseudo-method (Section 4.3.6
   of [RFC7231]) with any explicitly configured or discovered proxies to
   create an end-to-end tunnel.  Transparent proxies are unable to
   intercept connections that are protected with TLS.

   This document describes a method that enables shared caching for a
   limited set of "https" resources, as selected by the server.  The
   server conditionally delegates the hosting of secure content to
   itself.  This delegation includes a marker that signals permission
   for a client to send a request for an "https" resource via a proxy
   rather than insisting on an end-to-end TLS connection.

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1.1.  Notational Conventions

   The words "MUST", "MUST NOT", "SHOULD", and "MAY" are used in this
   document.  It's not shouting; when they are capitalized, they have
   the special meaning defined in [RFC2119].

   This document uses the term "proxy cache" to refer to a proxy
   [RFC7230] that operates an HTTP cache [RFC7234].

2.  Same-Host Secure Content Delegation

   The secure content delegation mechanism defined in [SCD] is used to
   create a separate resource that contains encrypted and integrity
   protected content.  To enable caching, the primary and secondary
   servers can be the same server.

   A client that signals a willingness to support delegation is provided
   with a response that uses a proxy-enabled out-of-band encoding that
   behaves identically to the out-of-band encoding defined in
   [I-D.reschke-http-oob-encoding].  The out-of-band encoding identifies
   a secondary resource and implicitly indicates that the client is
   willing to use a proxy and that the server allows this use.  The
   client is then able to request the secondary resource from a proxy
   cache rather than directly to the origin server.

   In this document, the origin server is able to act in the role of the
   secondary server in [SCD].  However, all of the considerations that
   apply to having a secondary server host content apply instead to the
   proxy cache.  Thus, integrity and confidentiality protections against
   the proxy cache are the primary consideration.

2.1.  Signaling Presence of a Proxy

   Without a clear signal from the client that a caching proxy is
   present, an origin server is unable to send a response with out-of-
   band encoding.  A value of "out-of-band" in the Accept-Encoding
   header field only indicates willingness to use the secure content
   delegation mechanism.

   A new "oobp" content encoding is defined.  The "oobp" content
   encoding is identical to the "out-of-band" content encoding, with the
   following additional conditions:

   o  A client MUST NOT signal support for "oobp" content encoding
      unless it is using a proxy cache and it is willing to direct
      requests to that proxy.

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   o  A server MUST NOT encode a response using the "oobp" content
      encoding unless it permits the request to be made to a proxy

   o  The "oobp" content encoding MUST NOT be used to encode the
      contents of a request.  The "out-of-band" content encoding is
      sufficient for that purpose.

   Using a different content encoding name means that a resource using
   secure content delegation to a secondary server [SCD] does not
   inadvertently trigger a request via a proxy.

   The security properties of delegation via a secondary server and via
   a caching proxy are similar only to the extent that a third party is
   involved.  However, it might be the case that a secondary server has
   a stronger relationship with the primary server and additional
   constraints on its actions, such as contractual limitations.  Such
   constraints might make it feasible to delegate to a secondary server
   selected by the primary server.  A caching proxy might not be
   considered acceptable in the same way.

   The "oobp" content encoding clearly indicates that the client is
   permitted to retrieve content from a proxy-cache.

   Servers that use the "oobp" content encoding MUST include header
   fields for message integrity and encryption, such as the M-I header
   field [I-D.thomson-http-mice] or the Crypto-Key header field
   [I-D.ietf-httpbis-encryption-encoding].  Clients MUST NOT send a
   request via a proxy if these headers are not present.  Absence of
   these header fields indicate an error on the part of the origin
   server, since integrity and confidentiality protection are mandatory.

2.2.  Proxy Identification and Authentication

   This mechanism does not work with a transparent caching proxy.  Since
   the request is made over end-to-end HTTPS in the absence of a proxy,
   the feature will not be used unless the proxy is known to the client.

   A proxy cache MUST therefore be expressly configured or discovered.
   This produces a name and possibly a port number for the proxy.  The
   proxy MUST be contacted using HTTPS [RFC2818] and authenticated using
   the configured or discovered domain name.

   Issue:  What signal do we need from the proxy cache that it supports
      receiving requests with an "https://" scheme?  Can we expect that
      a proxy cache will happily accept a request for an HTTPS URL?
      What if they ignore the scheme and send the request in the clear?

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3.  Performance Optimizations

   As noted in [SCD], the secondary request required by out-of-band
   content encoding imposes a performance penalty.  This can be
   mitigated by priming clients with information about the location and
   disposition of resources prior to the client making a request.  A
   resource map described in [SCD] might be provided to clients to
   eliminate the latency involved in making requests of the origin
   server for resources that might be cached.

3.1.  Proxy Cache Priming

   A client that makes a request of an origin server via an unprimed
   proxy cache will suffer additional latency as a consequence of the
   cache having to make a request to the origin server.

   The following options are possible:

   o  Clients can speculatively make requests to a proxy cache based on
      information it learns from a resource map, or from hints like the
      "prefetch" link relation [HINTS].  To avoid a potential waste of
      resources as a result of receiving complete responses, speculative
      requests might be limited to HEAD requests; alternatively, HTTP/2
      [RFC7540] flow control might be used to allow only limited
      information to be sent.

   o  The origin server might provide the proxy cache with "prefetch"
      link relations in responses to requests for secondary resources.
      These link relations might identify other resources that the proxy
      might retrieve speculatively.  This does not improve the latency
      of the initial request, but could improve subsequent requests.

4.  Security Considerations

   All the considerations of [SCD] apply.  In particular, content that
   is distributed with the assistance of a proxy cache MUST include
   integrity and confidentiality protection.  That means that the M-I
   header field [I-D.thomson-http-mice] and the Crypto-Key header field
   [I-D.ietf-httpbis-encryption-encoding] or equivalent information MUST
   be present in responses that include an out-of-band content encoding.

   Clients that receive a response without the information necessary to
   ensure integrity and confidentiality protection against a proxy cache
   MUST NOT make a request to a proxy to retrieve that response.
   Clients could treat such a response as failed.  Clients MAY then make
   the request directly to the origin server, or - if request can be
   safely retried - retry a request without the out-of-band token in the
   Accept-Encoding header field.

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5.  IANA Considerations

   This document has no IANA actions.  It should.

6.  References

6.1.  Normative References

              Thomson, M., "Encrypted Content-Encoding for HTTP", draft-
              ietf-httpbis-encryption-encoding-02 (work in progress),
              June 2016.

              Thomson, M., "Merkle Integrity Content Encoding", draft-
              thomson-http-mice-01 (work in progress), June 2016.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,

   [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
              RFC 7234, DOI 10.17487/RFC7234, June 2014,

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,

   [SCD]      Ericsson, G., Holmberg, C., and M. Thomson, "An
              Architecture for Secure Content Delegation using HTTP",
              February 2016.

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6.2.  Informative References

   [HINTS]    Grigorik, I., "Resource Hints", W3C TR , May 2015.

              Reschke, J. and S. Loreto, "'Out-Of-Band' Content Coding
              for HTTP", draft-reschke-http-oob-encoding-07 (work in
              progress), July 2016.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,

Authors' Addresses

   Martin Thomson


   Goeran AP Eriksson


   Christer Holmberg


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