HTTP Working GroupM. Nottingham
Internet-DraftM. Thomson
Intended status: ExperimentalMozilla
Expires: September 18, 2016March 17, 2016

Opportunistic Security for HTTP


This document describes how http URIs can be accessed using Transport Layer Security (TLS) to mitigate pervasive monitoring attacks.

Note to Readers

Discussion of this draft takes place on the HTTP working group mailing list (, which is archived at

Working Group information can be found at; source code and issues list for this draft can be found at

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

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 September 18, 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 Provisions Relating to IETF Documents ( 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 Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

1. Introduction

This document describes a use of HTTP Alternative Services [I-D.ietf-httpbis-alt-svc] to decouple the URI scheme from the use and configuration of underlying encryption, allowing a http URI [RFC7230] to be accessed using TLS [RFC5246] opportunistically.

Serving https URIs require acquiring and configuring a valid certificate, which means that some deployments find supporting TLS difficult. This document describes a usage model whereby sites can serve http URIs over TLS without being required to support strong server authentication.

Opportunistic Security [RFC7435] does not provide the same guarantees as using TLS with https URIs; it is vulnerable to active attacks, and does not change the security context of the connection. Normally, users will not be able to tell that it is in use (i.e., there will be no “lock icon”).

By its nature, this technique is vulnerable to active attacks. A mechanism for partially mitigating them is described in Section 5.

1.1. Goals and Non-Goals

The immediate goal is to make the use of HTTP more robust in the face of pervasive passive monitoring [RFC7258].

A secondary goal is to limit the potential for active attacks. It is not intended to offer the same level of protection as afforded to https URIs, but instead to increase the likelihood that an active attack can be detected.

A final (but significant) goal is to provide for ease of implementation, deployment and operation. This mechanism is expected to have a minimal impact upon performance, and require a trivial administrative effort to configure.

1.2. Notational Conventions

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in [RFC2119].

2. Using HTTP URIs over TLS

An origin server that supports the resolution of http URIs can indicate support for this specification by providing an alternative service advertisement [I-D.ietf-httpbis-alt-svc] for a protocol identifier that uses TLS, such as h2 [RFC7540].

A client that receives such an advertisement MAY make future requests intended for the associated origin ([RFC6454]) to the identified service (as specified by [I-D.ietf-httpbis-alt-svc]).

A client that places the importance of protection against passive attacks over performance might choose to withhold requests until an encrypted connection is available. However, if such a connection cannot be successfully established, the client can resume its use of the cleartext connection.

A client can also explicitly probe for an alternative service advertisement by sending a request that bears little or no sensitive information, such as one with the OPTIONS method. Likewise, clients with existing alternative services information could make such a request before they expire, in order minimize the delays that might be incurred.

3. Server Authentication

[I-D.ietf-httpbis-alt-svc] requires that an alternative service only be used when there are “reasonable assurances” that it is under control of and valid for the whole origin.

As defined in that specification, one way of establishing this is using a TLS-based protocol with the certificate checks defined in [RFC2818]. Clients MAY impose additional criteria for establishing reasonable assurances.

For the purposes of this specification, an additional way of establishing reasonable assurances is available when the alternative is on the same host as the origin, using the “http-opportunistic” well-known URI defined in Section 6.

This allows deployment without the use of valid certificates, to encourage deployment of opportunistic security. When it is in use, the alternative service can provide any certificate, or even select TLS cipher suites that do not include authentication.

When the client has a valid http-opportunistic response for an origin, it MAY consider there to be reasonable assurances when:

For example, this request/response pair would constitute reasonable assurances for the origin “” for any alternative service also on “”:

GET /.well-known/http-opportunistic HTTP/1.1

HTTP/1.1 200 OK
Content-Type: application/json
Connection: close

  "origins": ["", ""]

Note that this mechanism is only defined to establish reasonable assurances for the purposes of this specification; it does not apply to other uses of alternative services unless they explicitly invoke it.

4. Interaction with “https” URIs

When using alternative services, requests for resources identified by both http and https URIs might use the same connection, because HTTP/2 permits requests for multiple origins on the same connection.

Since https URIs rely on server authentication, a connection that is initially created for http URIs without authenticating the server cannot be used for https URIs until the server certificate is successfully authenticated. Section 3.1 of [RFC2818] describes the basic mechanism, though the authentication considerations in [I-D.ietf-httpbis-alt-svc] also apply.

Connections that are established without any means of server authentication (for instance, the purely anonymous TLS cipher suites), cannot be used for https URIs.

5. Requiring Use of TLS

Even when the alternative service is strongly authenticated, opportunistically upgrading cleartext HTTP connections to use TLS is subject to active attacks. In particular:

Given that the primary goal of this specification is to prevent passive attacks, these are not critical failings (especially considering the alternative - HTTP over cleartext). However, a modest form of protection against active attacks can be provided for clients on subsequent connections.

When an origin is able to commit to providing service for a particular origin over TLS for a bounded period of time, clients can choose to rely upon its availability, failing when it cannot be contacted. Effectively, this makes the choice to use a secured protocol “sticky”.

5.1. Opportunistic Commitment

An origin can reduce the risk of attacks on opportunistically secured connections by committing to provide an secured, authenticated alternative service. This is done by including the optional commit member in the http-opportunistic well-known resource (see Section 6). This feature is optional due to the requirement for server authentication and the potential risk entailed (see Section 5.3).

The value of the commit member is a number ([RFC7159], Section 6) indicating the duration of the commitment interval in seconds.

  "origins": ["", ""],
  "commit": 86400

Including commit creates a commitment to provide a secured alternative service for the advertised period. Clients that receive this commitment can assume that a secured alternative service will be available for the indicated period. Clients might however choose to limit this time (see Section 5.3).

5.2. Client Handling of A Commitment

The value of the commit member MUST be ignored unless the alternative service can be strongly authenticated. The same authentication requirements that apply to https:// resources SHOULD be applied to authenticating the alternative. Minimum authentication requirements for HTTP over TLS are described in Section 2.1 of [I-D.ietf-httpbis-alt-svc] and Section 3.1 of [RFC2818]. As noted in [I-D.ietf-httpbis-alt-svc], clients can impose other checks in addition to this minimum set. For instance, a client might choose to apply key pinning [RFC7469].

A client that receives a commitment and that successfully authenticates the alternative service can assume that a secured alternative will remain available for the commitment interval. The commitment interval starts when the commitment is received and authenticated and runs for a number of seconds equal to value of the commit member, less the current age of the http-opportunistic response (as defined in Section 4.2.3 of [RFC7234]). A client SHOULD avoid sending requests via cleartext protocols or to unauthenticated alternative services for the duration of the commitment interval, except to discover new potential alternatives.

A commitment only applies to the origin of the http-opportunistic well-known resource that was retrieved; other origins listed in the origins member MUST be independently discovered and authenticated.

A commitment is not bound to a particular alternative service. Clients are able to use alternative services that they become aware of. However, once a valid and authenticated commitment has been received, clients SHOULD NOT use an unauthenticated alternative service. Where there is an active commitment, clients SHOULD ignore advertisements for unsecured alternative services. A client MAY send requests to an unauthenticated origin in an attempt to discover potential alternative services, but these requests SHOULD be entirely generic and avoid including credentials.

5.3. Operational Considerations

Errors in configuration of commitments has the potential to render even the unsecured origin inaccessible for the duration of a commitment. Initial deployments are encouraged to use short duration commitments so that errors can be detected without causing the origin to become inaccessible to clients for extended periods.

To avoid situations where a commitment causes errors, clients MAY limit the time over which a commitment is respected for a given origin. A lower limit might be appropriate for initial commitments; the certainty that a site has set a correct value - and the corresponding limit on persistence - might increase as a commitment is renewed multiple times.

6. The “http-opportunistic” well-known URI

This specification defines the “http-opportunistic” well-known URI [RFC5785]. An origin is said to have a valid http-opportunistic resource when:

This specification defines one additional, optional member of the root object, “commit” in Section 5. Unrecognised members MUST be ignored.

7. IANA Considerations

This specification registers a Well-known URI [RFC5785]:

8. Security Considerations

8.1. Security Indicators

User Agents MUST NOT provide any special security indicia when an http resource is acquired using TLS. In particular, indicators that might suggest the same level of security as https MUST NOT be used (e.g., a “lock device”).

8.2. Downgrade Attacks

A downgrade attack against the negotiation for TLS is possible. With commitment Section 5, this is limited to occasions where clients have no prior information (see Section 8.3), or when persisted commitments have expired.

For example, because the Alt-Svc header field [I-D.ietf-httpbis-alt-svc] likely appears in an unauthenticated and unencrypted channel, it is subject to downgrade by network attackers. In its simplest form, an attacker that wants the connection to remain in the clear need only strip the Alt-Svc header field from responses.

Downgrade attacks can be partially mitigated using the commit member of the http-opportunistic well-known resource, because when it is used, a client can avoid using cleartext to contact a supporting server. However, this only works when a previous connection has been established without an active attacker present; a continuously present active attacker can either prevent the client from ever using TLS, or offer its own certificate.

8.3. Privacy Considerations

Cached alternative services can be used to track clients over time; e.g., using a user-specific hostname. Clearing the cache reduces the ability of servers to track clients; therefore clients MUST clear cached alternative service information when clearing other origin-based state (i.e., cookies).

8.4. Confusion Regarding Request Scheme

Many existing HTTP/1.1 implementations use the presence or absence of TLS in the stack to determine whether requests are for http or https resources. This is necessary in many cases because the most common form of an HTTP/1.1 request does not carry an explicit indication of the URI scheme.

HTTP/1.1 MUST NOT be used for opportunistically secured requests.

Some HTTP/1.1 implementations use ambient signals to determine if a request is for an https resource. For example, implementations might look for TLS on the stack or a port number of 443. An implementation that supports opportunistically secured requests SHOULD suppress these signals if there is any potential for confusion.

9. References

9.1. Normative References

mnot, m., McManus, P., and J. Reschke, “HTTP Alternative Services”, Internet-Draft draft-ietf-httpbis-alt-svc-14 (work in progress), March 2016.
Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels”, BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <>.
Rescorla, E., “HTTP Over TLS”, RFC 2818, DOI 10.17487/RFC2818, May 2000, <>.
Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2”, RFC 5246, DOI 10.17487/RFC5246, August 2008, <>.
Nottingham, M. and E. Hammer-Lahav, “Defining Well-Known Uniform Resource Identifiers (URIs)”, RFC 5785, DOI 10.17487/RFC5785, April 2010, <>.
Barth, A., “The Web Origin Concept”, RFC 6454, DOI 10.17487/RFC6454, December 2011, <>.
Bray, T., Ed., “The JavaScript Object Notation (JSON) Data Interchange Format”, RFC 7159, DOI 10.17487/RFC7159, March 2014, <>.
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, <>.
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, <>.
Belshe, M., Peon, R., and M. Thomson, Ed., “Hypertext Transfer Protocol Version 2 (HTTP/2)”, RFC 7540, DOI 10.17487/RFC7540, May 2015, <>.

Appendix A. Acknowledgements

Thanks to Patrick McManus, Eliot Lear, Stephen Farrell, Guy Podjarny, Stephen Ludin, Erik Nygren, Paul Hoffman, Adam Langley, Eric Rescorla and Richard Barnes for their feedback and suggestions.

Authors' Addresses

Mark Nottingham
Martin Thomson