Opportunistic Security for HTTP
draft-ietf-httpbis-http2-encryption-04

HTTP Working Group                                         M. Nottingham
Internet-Draft
Intended status: Experimental                                 M. Thomson
Expires: September 18, 2016                                      Mozilla
                                                          March 17, 2016

                    Opportunistic Security for HTTP
                 draft-ietf-httpbis-http2-encryption-04

Abstract

   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 (ietf-http-wg@w3.org), which is archived at
   https://lists.w3.org/Archives/Public/ietf-http-wg/ .

   Working Group information can be found at http://httpwg.github.io/ ;
   source code and issues list for this draft can be found at
   https://github.com/httpwg/http-extensions/labels/opp-sec .

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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Copyright Notice

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

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   (http://trustee.ietf.org/license-info) in effect on the date of
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   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
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

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.

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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").

   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.

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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

   [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:

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

   o  The chosen alternative service returns the same response as above.

   For example, this request/response pair would constitute reasonable
   assurances for the origin "http://www.example.com" for any
   alternative service also on "www.example.com":

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   GET /.well-known/http-opportunistic HTTP/1.1
   Host: www.example.com

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

   {
     "origins": ["http://example.com", "http://www.example.com:81"]
   }

   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:

   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.

   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

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   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": ["http://example.com", "http://www.example.com:81"],
     "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

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   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:

   o  The client has obtained a 200 (OK) response for the well-known URI
      from the origin, or refreshed one in cache [RFC7234], and

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

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

   o  The "origins" member of the root object has a value of an array of
      strings, one of which is a case-insensitive character-for-
      character match for the origin in question, serialised into
      Unicode as per [RFC6454], Section 6.1, and

   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]:

   o  URI Suffix: http-opportunistic

   o  Change Controller: IETF

   o  Specification Document(s): [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 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.

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   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

   [I-D.ietf-httpbis-alt-svc]
              mnot, m., McManus, P., and J. Reschke, "HTTP Alternative
              Services", draft-ietf-httpbis-alt-svc-14 (work in
              progress), March 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>.

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

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <http://www.rfc-editor.org/info/rfc5246>.

   [RFC5785]  Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
              Uniform Resource Identifiers (URIs)", RFC 5785,
              DOI 10.17487/RFC5785, April 2010,
              <http://www.rfc-editor.org/info/rfc5785>.

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

   [RFC7159]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
              2014, <http://www.rfc-editor.org/info/rfc7159>.

   [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>.

   [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,
              <http://www.rfc-editor.org/info/rfc7234>.

   [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

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <http://www.rfc-editor.org/info/rfc7258>.

   [RFC7435]  Dukhovni, V., "Opportunistic Security: Some Protection
              Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
              December 2014, <http://www.rfc-editor.org/info/rfc7435>.

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   [RFC7469]  Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
              Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April
              2015, <http://www.rfc-editor.org/info/rfc7469>.

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

   Email: mnot@mnot.net
   URI:   http://www.mnot.net/

   Martin Thomson
   Mozilla

   Email: martin.thomson@gmail.com

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