HTTP Working Group                                         M. Nottingham
Intended status: Experimental                                 M. Thomson
Expires: December 2, 2016                                        Mozilla
                                                            May 31, 2016

                    Opportunistic Security for HTTP


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

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 .

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   This Internet-Draft will expire on December 2, 2016.

Copyright Notice

   Copyright (c) 2016 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
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Goals and Non-Goals . . . . . . . . . . . . . . . . . . .   3
     1.2.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  Using HTTP URIs over TLS  . . . . . . . . . . . . . . . . . .   3
   3.  Server Authentication . . . . . . . . . . . . . . . . . . . .   4
   4.  Interaction with "https" URIs . . . . . . . . . . . . . . . .   5
   5.  Requiring Use of TLS  . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Opportunistic Commitment  . . . . . . . . . . . . . . . .   6
     5.2.  Client Handling of A Commitment . . . . . . . . . . . . .   6
     5.3.  Operational Considerations  . . . . . . . . . . . . . . .   7
   6.  The "http-opportunistic" well-known URI . . . . . . . . . . .   7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
     8.1.  Security Indicators . . . . . . . . . . . . . . . . . . .   8
     8.2.  Downgrade Attacks . . . . . . . . . . . . . . . . . . . .   8
     8.3.  Privacy Considerations  . . . . . . . . . . . . . . . . .   9
     8.4.  Confusion Regarding Request Scheme  . . . . . . . . . . .   9
     8.5.  Server Controls . . . . . . . . . . . . . . . . . . . . .   9
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   This document describes a use of HTTP Alternative Services [RFC7838]
   to decouple the URI scheme from the use and configuration of
   underlying encryption, allowing a "http" URI [RFC7230] to be accessed
   using Transport Layer Security (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.

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   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").

   A mechanism for partially mitigating active attacks 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",
   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 [RFC7838] 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 [RFC7838]).

   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.

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   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

   [RFC7838] 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, a client can establish reasonable
   assurances when using a TLS-based protocol with the certificate
   checks defined in [RFC2818].

   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 a client has a valid http-opportunistic response for an origin
   (as per Section 6), it MAY consider there to be reasonable assurances
   as long as:

   o  The origin and alternative service's hostnames are the same when
      compared in a case-insensitive fashion, and

   o  The origin object of the http-opportunistic response has a `tls-
      ports' member, whose value is an array of numbers, one of which
      matches the port of the alternative service in question, and

   o  The chosen alternative service returns the same representation as
      the origin did for the http-opportunistic resource.

   For example, this request/response pair would constitute reasonable
   assurances for the origin "" for an alternative
   service on port 443 or 8000 of the host "":

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

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

     "": {
       "tls-ports": [443, 8000]

   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

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
   Section 2.1 of [RFC7838] 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:

   o  Because the original HTTP connection is in cleartext, it is
      vulnerable to man-in-the-middle attacks, and

   o  By default, if clients cannot reach the alternative service, they
      will fall back to using the original cleartext origin.

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   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 a secured, authenticated
   alternative service.  This is done by including the optional "tls-
   commit" member in the origin object of the http-opportunistic well-
   known response (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 "tls-commit" member is a number ([RFC7159],
   Section 6) indicating the duration of the commitment interval in

     "": {
       "tls-ports": [443,8080],
       "tls-commit": 3600

   Including "tls-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 "tls-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

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   [RFC7838] and Section 3.1 of [RFC2818].  As noted in [RFC7838],
   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
   "tls-commit" member, less the current age of the http-opportunistic
   response (as defined in Section 4.2.3 of [RFC7234]).  Note that the
   commitment interval MAY exceed the freshness lifetime of the "http-
   opportunistic" resource.

   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 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].  A client is said to have a valid http-opportunistic
   response for a given origin when:

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   o  The client has obtained a 200 (OK) response for the well-known URI
      from the origin, and it is fresh [RFC7234] (potentially through
      revalidation [RFC7232]), and

   o  That response has the media type "application/json", and

   o  That response's payload, when parsed as JSON [RFC7159], contains
      an object as the root.

   o  The root object contains a member whose name is a case-insensitive
      character-for-character match for the origin in question,
      serialised into Unicode as per Section 6.1 of [RFC6454], and whose
      value is an object (hereafter, the "origin object").

7.  IANA Considerations

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

   o  URI Suffix: http-opportunistic

   o  Change Controller: IETF

   o  Specification Document(s): Section 6 of [this specification]

   o  Related Information:

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 (see 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 [RFC7838] 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.

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   Downgrade attacks can be partially mitigated using the "tls-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

   HTTP implementations and applications sometimes use ambient signals
   to determine if a request is for an "https" resource; for example,
   they might look for TLS on the stack, or a server port number of 443.

   This might be due to limitations in the protocol (the most common
   HTTP/1.1 request form does not carry an explicit indication of the
   URI scheme), or it may be because how the server and application are
   implemented (often, they are two separate entities, with a variety of
   possible interfaces between them).

   Any security decisions based upon this information could be misled by
   the deployment of this specification, because it violates the
   assumption that the use of TLS (or port 443) means that the client is
   accessing a HTTPS URI, and operating in the security context implied
   by HTTPS.

   Therefore, servers need to carefully examine the use of such signals
   before deploying this specification.

8.5.  Server Controls

   Because this specification allows "reasonable assurances" to be
   established by the content of a well-known URI, servers SHOULD take
   suitable measures to assure that its content remains under their
   control.  Likewise, because the Alt-Svc header field is used to
   describe policies across an entire origin, servers SHOULD NOT permit
   user content to set or modify the value of this header.

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9.  References

9.1.  Normative References

   [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,

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,

   [RFC5785]  Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
              Uniform Resource Identifiers (URIs)", RFC 5785,
              DOI 10.17487/RFC5785, April 2010,

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
              DOI 10.17487/RFC6454, December 2011,

   [RFC7159]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
              2014, <>.

   [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,

   [RFC7232]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Conditional Requests", RFC 7232,
              DOI 10.17487/RFC7232, 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,

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

   [RFC7838]  Nottingham, M., McManus, P., and J. Reschke, "HTTP
              Alternative Services", RFC 7838, DOI 10.17487/RFC7838,
              April 2016, <>.

9.2.  Informative References

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <>.

   [RFC7435]  Dukhovni, V., "Opportunistic Security: Some Protection
              Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
              December 2014, <>.

   [RFC7469]  Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
              Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April
              2015, <>.

Appendix A.  Acknowledgements

   Mike Bishop contributed significant text to this document.

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

Authors' Addresses

   Mark Nottingham


   Martin Thomson


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