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Versions: 00 01 02

Network Working Group                                         M. Thomson
Internet-Draft                                                   Mozilla
Intended status: Standards Track                             G. Eriksson
Expires: September 22, 2016                                  C. Holmberg
                                                                Ericsson
                                                          March 21, 2016


        An Architecture for Secure Content Delegation using HTTP
                       draft-thomson-http-scd-00

Abstract

   An architecture is described for content distribution via third-party
   content distribution networks with reduced privileges.  This
   architecture allows an origin server to delegate the responsibility
   for delivery of the payload of an HTTP response to a third party.
   That party is unable to modify this content.  The content is
   encrypted, which in some cases will prevent the third party from
   learning about the content.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on September 22, 2016.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   carefully, as they describe your rights and restrictions with respect



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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
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Table of Contents

   1.  Content Distribution Security . . . . . . . . . . . . . . . .   2
     1.1.  Secure Content Delegation . . . . . . . . . . . . . . . .   3
     1.2.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  Out-of-Band Content Encoding  . . . . . . . . . . . . . . . .   4
     2.1.  Performance Trade-Off . . . . . . . . . . . . . . . . . .   4
     2.2.  Confidentiality of Resource Identity  . . . . . . . . . .   5
   3.  Content Integrity . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Content Confidentiality . . . . . . . . . . . . . . . . . . .   6
   5.  Resource Map  . . . . . . . . . . . . . . . . . . . . . . . .   7
   6.  Error Handling  . . . . . . . . . . . . . . . . . . . . . . .   7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
     7.1.  Confidentiality Protection Limitations  . . . . . . . . .   8
     7.2.  Cross-Origin Access . . . . . . . . . . . . . . . . . . .   8
     7.3.  Traffic Analysis  . . . . . . . . . . . . . . . . . . . .   9
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Content Distribution Security

   The distribution of content on the web at scale is necessarily highly
   distributed.  Large amounts of content needs large numbers of
   servers.  And distributing those servers closer to clients has a
   significant, positive impact on performance.

   This has led to large networks of content distribution servers, in
   some cases operated by web sites, but often run by third parties in
   service to sites that don't have the resources to operate a network
   at very large scales.  A content distribution network (CDN) aims to
   dramatically improve the performance of content delivery by moving
   content closer to clients and by offloading the delivery
   responsibility to servers that are highly optimized for that task.

   A major drawback of existing solutions for content distribution is
   that an origin is required to yield control over their content to the
   CDN.  A CDN is able to see and modify content that they distribute.
   In some cases, expediency dictates that the CDN be given control over
   the entire origin.



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   There are few technical mechanisms in place to limit the capabilities
   of a CDN.  Mechanisms like content security policy [CSP] and sub-
   resource integrity [SRI] can be used to prevent modification of
   resources in some contexts, but these mechanisms are limited in what
   they can protect and they can impose certain operational costs.  For
   the most part, operators of origin servers either limit the content
   that is served using the CDN or rely on non-technical measures such
   as contracts and courts to proscribe bad behavior.

1.1.  Secure Content Delegation

   This document describes how an origin server might securely delegate
   the responsibility for serving content to a CDN.  The solution
   comprises three basic components:

   o  A delegation component allows an origin server to delegate
      specific resources to another server.

   o  Integrity attributes ensure that the content cannot be modified by
      the delegated server.

   o  Confidentiality protection limits the ability of the delegated
      server to learn what the content holds.

   Note that the guarantees provided by confidentiality protection are
   not strong, see Section 4 for details.

   In addition to these basic components, a fourth mechanism provides a
   client with the ability to learn resource metadata from the origin
   prior to making a request for specific resources.  This can
   dramatically improve performance where a client needs to acquire
   multiple delegated resources.

   No new mechanisms are described in this document; the application of
   several existing and separately-proposed protocol mechanisms to this
   problem is described.  An origin server can use these mechanisms to
   take advantage of content distribution networks where concerns about
   security might have otherwise prevented their use.  This might be for
   content that was previously considered too sensitive for third-party
   distribution, or for a CDN provider that might not otherwise be used.

1.2.  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].





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   This document uses the terms client, CDN, and origin server.  These
   terms refer to the three roles played in this architecture.  Note
   that "origin server" as used in this document encompasses both origin
   servers and gateways as defined in [RFC7230].

2.  Out-of-Band Content Encoding

   The out-of-band content encoding [I-D.reschke-http-oob-encoding]
   provides the basis for delegation of content distribution.  A request
   is made to the origin server, but in place of the complete response
   only response header fields and an out-of-band content encoding is
   provided.  The out-of-band content encoding directs the client to
   retrieve content from another resource.

      Client           Cache/CDN           Server
        |                  |                 |
        | Request          |                 |
        +----------------------------------->|
        |                  |                 |
        |                  Response + OOB CE |
        |<-----------------------------------+
        |                  |                 |
        |GET               |                 |
        +----------------->|                 |
        |                  |                 |
        |              200 |                 |
        |<-----------------+                 |
        |                  |                 |

               Figure 1: Using Out-of-Band Content Encoding

   Out-of-band content encoding behaves much like a redirect.  In fact,
   a redirect was considered as part of the early design, but rejected
   because without defining a new set of 3xx status codes it would
   change the effective origin [RFC6454] of the resource.  Furthermore,
   the content encoding specifically preserves header fields sent by the
   origin server, rejecting any unauthenticated header fields that might
   be provided by the CDN.

2.1.  Performance Trade-Off

   An additional request is necessary to retrieve content.  This has a
   negative impact on latency.  However, if the CDN server is positioned
   close to the client, there are several potential benefits:

   Fewer bit-miles:  Content hosted in the CDN that is nearby can be
      served to those clients without having to traverse a long network
      path.



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   Better server resource allocation:  Using a dedicated content
      delivery server reduces the load on the origin server, allowing it
      more capacity for serving other requests.

   Lower time to last byte:  For some resources, the proximity of the
      CDN might also permit greater network throughput.  Increased
      bandwidth can counteract the added latency cost of the extra
      requests, and potentially reduce the time needed to retrieve the
      entire resource.

   Better throughput:  If a CDN is closer to a client, more bandwidth
      might be available for delivery of content when compared with the
      link between client and origin server.

   The problems of providing integrity protection for content delivered
   in this fashion is discussed in Section 3; confidentiality protection
   and its limitations is described in Section 4; and reducing the
   latency impact of making multiple requests for each resource is
   described in Section 5.

2.2.  Confidentiality of Resource Identity

   The URL used to acquire a resource from a CDN can be unrelated to the
   URL of the resource that refers to its contents.  This allows an
   origin server to hide the relationship between content in a CDN and
   the original resources that is served by that content.

   Any entity SHOULD be unable to determine the URL of the original
   resource based on the URL of the CDN resource alone.  This can be
   achieved by having randomized URLs for CDN resources and maintaining
   a mapping table, or by using a fixed mapping function with a secret
   input such as HMAC [RFC2104].

   Without other information, this would prevent the CDN from learning
   which resources are requested from the origin server by observing the
   requests that it serves for out-of-band content.  Note that in some
   cases, information about the resource is readily obtainable by the
   CDN cache, see Section 4, an unpredictable mapping ensures that other
   protection mechanisms can be effective if possible.

3.  Content Integrity

   Ensuring that content is not modified by the CDN is crucial to
   ensuring that content cannot be improperly modified.  Information
   that is acquired from the CDN is not integrity protected and
   therefore MUST NOT be used without being authenticated.





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   Several options are available for authenticating content provided by
   the CDN:

   o  A cryptographic hash over the content sent in the initial response
      could be compared against a hash of the content delivered by the
      CDN.  This is the basic design of [SRI].

   o  A digital signature could cover the content stored by the CDN.  A
      design that uses this approach is described in
      [I-D.thomson-http-content-signature].

   A progressive integrity mechanism like the one described in
   [I-D.thomson-http-mice] is perhaps the most flexible approach.  It
   allows for progressive consumption of content without losing
   integrity protection.  This supports both hash- and signature-based
   integrity protection for out-of-band content:

   o  A response from the origin server could include an M-I header
      field with an integrity proof, allowing the content to be
      delivered out-of-band without any additional header fields.

   o  A response from the origin server could include a Crypto-Key
      header field with a signature public key.  The out-of-band
      response could then include a M-I header field with the signature
      in addition to the content.

4.  Content Confidentiality

   Confidentiality protection for content is provided by applying an
   encryption content encoding [I-D.ietf-httpbis-encryption-encoding] to
   content before that content is provided to a CDN.

   Much of the value provided by a CDN derives from its ability to
   deliver the same content to multiple nearby clients.  The more
   clients that can be delivered the same resource, the greater the
   efficiency gains.  As a result, resources that are provided to many
   or all clients are the ones that benefit most from caching.

   This means that unless a resource has access control mechanisms that
   would prevent the CDN from accessing a resource, the confidentiality
   protections provided by encrypting content is limited.  A CDN need
   only independently request resources from the origin server in order
   to learn everything about the content it is serving, including the
   mapping of origin server URLs to CDN resource URLs.  For instance,
   employing a web crawler on a web site might reveal the identity of
   numerous resources and the location of the any out-of-band content
   for those resources.




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   Nonetheless, confidentiality protection MUST be used to prevent
   cross-origin theft of confidential data (see Section 7.2).

5.  Resource Map

   Learning about header fields and out-of-band cache locations for
   resources in advance of needing to make requests to those resources
   allows a client to avoid making requests to the origin server.  This
   can greatly improve the performance of applications that make
   multiple requests of the same server, such as web browsing or video
   streaming.

   Without defining any new additional protocol mechanisms, HTTP/2
   server push [RFC7540] can be used to provide requests, responses and
   the out-of-band content encoding information describing resources.
   Since no actual content is included, this requires relatively little
   data to describe a number of resources.  Once this information is
   available, the client no longer needs to contact the origin server to
   acquire the described resources.

   This approach has some signficant deployment drawbacks, so explicit
   data formats for carrying this data might be defined.

   Note:  We need a separate draft on these alternative methods.

6.  Error Handling

   Error handling for clients is described in
   [I-D.reschke-http-oob-encoding].

   For idempotent requests, a second request might be made to the origin
   server.  This request would omit any indication of support for out-
   of-band content coding from the Accept-Encoding header field, plus a
   link relation indicating the secondary resource and the reason for
   failure.

   An origin server can use this information to inform choices about
   whether to use content delegation.

   Non-idempotent requests cannot be safely retried.  Therefore, clients
   cannot retry a a request and provide information about errors to the
   origin server.  For this reason, origin servers SHOULD NOT delegate
   content for non-idempotent methods.








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7.  Security Considerations

   This document describes a framework whereby content might be
   distributed to a CDN, without losing integrity with respect to the
   content that is distributed.

   This design relies on integrity and confidentiality for the request
   and response made to the origin server.  These requests MUST be made
   using HTTP over TLS (HTTPS) [RFC2818] only.  Though there is a lesser
   requirement for confidentiality, requests made to the CDN cache MUST
   also be secured using HTTPS.

7.1.  Confidentiality Protection Limitations

   Content that requires only integrity protection can be safely
   distributed by a third-party using this design.  Entities that make a
   decision about confidentiality for others have often been shown to be
   incorrect in the past.  An incorrect conclusion have serious
   consequences.  Thus the choice of whether confidentiality protection
   is needed is quite important.

   Some confidentiality protection against the CDN is provided, but that
   is limited to content that is not otherwise accessible to that server
   (see Section 4).  Only content that has access controls on the origin
   server that prevent access by the CDN can retain confidentiality
   protection.

   Content with different access control policies MUST use different
   keying material for encryption.  This prevents a client with access
   to one resource from acquiring keys that can be used for resources
   they are not authorized to access.

   Clients that wish to retain control over the confidentiality of
   responses can omit the out-of-band label from the Accept-Encoding
   header field on requests, thereby indicating that a direct response
   is necessary.

7.2.  Cross-Origin Access

   The content delegation creates the possibility that an origin server
   could adopt remotely hosted content.  On the web, this is normally
   limited by Cross-Origin Resource Sharing [CORS], which requires that
   a client first request permission to make a resource accessible to
   another origin.

   This document describes a method whereby content hosted on a remote
   server can be made accessible to another origin.  The content of the
   out-of-band resource is written into the content of a response from



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   the origin.  All an origin needs to make this happen is knowledge of
   the identity of the out-of-band resource, something that might be
   difficult based on the guidance in Section 2.2, but not infeasible.
   A client requests this content using any ambient authority available
   to it (such as HTTP authentication header fields and cookies).

   The simplest option for reducing the ability to steal content in this
   fashion is to require that the origin demonstrate that it knows the
   content of the resource.  Unfortunately, this demonstration is
   difficult without imposing significant performance penalties, so we
   require a lesser assurance: that the origin knows how to decrypt the
   content.

   This makes content confidentiality Section 4 mandatory and limits the
   resources that can be stolen by an origin to those that are already
   encrypted.  Most importantly, only resources for which the origin
   knows the encryption key can be stolen.

   For this protection to be effective, origins MUST use different
   encryption keys for resources with different sets of authorized
   recipients.  Otherwise, an attacker might learn the encryption key
   for one resource then use that to decrypt a resource that it is not
   authorized to read.

   Resources that rely on signature-based integrity protection are made
   only marginally more difficult to steal, since the origin needs to
   learn the signing public key.  However, this is not expected to be
   difficult, since confidentiality protection for public keys.
   Resources that rely on hash-based integrity protection require that
   the origin learn the hash of the resource.

7.3.  Traffic Analysis

   Using a CDN reveals a great deal of information to the CDN about
   resources even if confidentiality protection is effective.  The size
   of responses and the pattern of requests for resources can reveal
   information about their contents.  When used carefully, padding as
   described in [I-D.ietf-httpbis-encryption-encoding] can obscure the
   length of responses and reduce the information that the CDN is able
   to learn.

   A random or unpredictable mapping from the primary resource URL on
   the origin to the URL of the content is necessary, see Section 2.2.

   Length hiding for header fields on responses the origin server might
   be more important when an out-of-band encoding is used, since the
   body of the response becomes less variable.




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   Making requests for content to multiple different servers can improve
   the amount of content length information available to network
   observers.  HTTP/2 multiplexing might have otherwise reduced the
   exposure of length information, but using out-of-band content
   encoding could expose lengths for those resources that can be
   distributed by a CDN.  Note that this is not fundamentally worse than
   HTTP/1.1 in the absence of pipelining.  Padding in HTTP/2 or
   encrypted content encoding can be used to further obscure lengths.

8.  IANA Considerations

   This document has no IANA actions.

9.  References

9.1.  Normative References

   [I-D.reschke-http-oob-encoding]
              Reschke, J. and S. Loreto, "'Out-Of-Band' Content Coding
              for HTTP", draft-reschke-http-oob-encoding-04 (work in
              progress), March 2016.

   [I-D.thomson-http-mice]
              Thomson, M., "Merkle Integrity Content Encoding", draft-
              thomson-http-mice-00 (work in progress), January 2016.

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

   [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,
              <http://www.rfc-editor.org/info/rfc7230>.

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

9.2.  Informative References

   [CORS]     van Kesteren, A., "Cross-Origin Resource Sharing", January
              2014, <https://www.w3.org/TR/cors/>.






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   [CSP]      West, M., Barth, A., and D. Veditz, "Content Security
              Policy Level 2", August 2015, <https://w3c.github.io/
              webappsec-csp/2/>.

   [I-D.ietf-httpbis-encryption-encoding]
              Thomson, M., "Encrypted Content-Encoding for HTTP", draft-
              ietf-httpbis-encryption-encoding-01 (work in progress),
              March 2016.

   [I-D.thomson-http-content-signature]
              Thomson, M., "Content-Signature Header Field for HTTP",
              draft-thomson-http-content-signature-00 (work in
              progress), July 2015.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,
              <http://www.rfc-editor.org/info/rfc2104>.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,
              <http://www.rfc-editor.org/info/rfc2818>.

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
              DOI 10.17487/RFC6454, December 2011,
              <http://www.rfc-editor.org/info/rfc6454>.

   [SRI]      Akhawe, D., Braun, F., Marier, F., and J. Weinberger,
              "Subresource Integrity", November 2015,
              <https://w3c.github.io/webappsec-subresource-integrity>.

Appendix A.  Acknowledgements

   Magnus Westerlund noted the potential for a violation of the cross
   origin protections offered in browsers.

Authors' Addresses

   Martin Thomson
   Mozilla

   Email: martin.thomson@gmail.com


   Goeran AP Eriksson
   Ericsson

   Email: goran.ap.eriksson@ericsson.com



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   Christer Holmberg
   Ericsson

   Email: christer.holmberg@ericsson.com















































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