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

An Architecture for Secure Content Delegation using HTTP


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.

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Table of Contents

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.

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:

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].

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.
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.

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

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:

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.

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.

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.

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 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.

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", Internet-Draft draft-reschke-http-oob-encoding-04, March 2016.
[I-D.thomson-http-mice] Thomson, M., "Merkle Integrity Content Encoding", Internet-Draft draft-thomson-http-mice-00, 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.
[RFC7230] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230, DOI 10.17487/RFC7230, June 2014.
[RFC7540] Belshe, M., Peon, R. and M. Thomson, "Hypertext Transfer Protocol Version 2 (HTTP/2)", RFC 7540, DOI 10.17487/RFC7540, May 2015.

9.2. Informative References

[CORS] van Kesteren, A., "Cross-Origin Resource Sharing", January 2014.
[CSP] West, M., Barth, A. and D. Veditz, "Content Security Policy Level 2", August 2015.
[I-D.ietf-httpbis-encryption-encoding] Thomson, M., "Encrypted Content-Encoding for HTTP", Internet-Draft draft-ietf-httpbis-encryption-encoding-01, March 2016.
[I-D.thomson-http-content-signature] Thomson, M., "Content-Signature Header Field for HTTP", Internet-Draft draft-thomson-http-content-signature-00, July 2015.
[RFC2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, DOI 10.17487/RFC2104, February 1997.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, DOI 10.17487/RFC2818, May 2000.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454, DOI 10.17487/RFC6454, December 2011.
[SRI] Akhawe, D., Braun, F., Marier, F. and J. Weinberger, "Subresource Integrity", November 2015.

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:
Göran AP Eriksson Ericsson EMail:
Christer Holmberg Ericsson EMail: