HTTP Working Group                                         M. Nottingham
Intended status: Experimental                                 M. Thomson
Expires: June 19, September 18, 2016                                      Mozilla
                                                          March 17, 2015 2016

                    Opportunistic Security for HTTP


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

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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 . . . . . . . . . . . . . . . .   4   5
   5.  Requiring Use of TLS  . . . . . . . . . . . . . . . . . . . .   5
     5.1.  The HTTP-TLS Header Field  Opportunistic Commitment  . . . . . . . . . . . . . . . .   5   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 . . . . . . . . . . . . . . . . . . .   7
     6.1.   8
     8.1.  Security Indicators . . . . . . . . . . . . . . . . . . .   7
     6.2.   8
     8.2.  Downgrade Attacks . . . . . . . . . . . . . . . . . . . .   7
     6.3.   8
     8.3.  Privacy Considerations  . . . . . . . . . . . . . . . . .   8
     6.4.   9
     8.4.  Confusion Regarding Request Scheme  . . . . . . . . . . .   8
   7.   9
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.   9
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     7.2.   9
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   9  10
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   9  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9  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.

   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",
   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

   By their nature, "http" URIs do not require cryptographically strong
   server authentication;

   [I-D.ietf-httpbis-alt-svc] requires that is an alternative service only implied by "https" URIs.
   Furthermore, doing so (as per [RFC2818]) creates a number of
   operational challenges.  For these reasons, server authentication
   be used when there are "reasonable assurances" that it is
   not mandatory under
   control of and valid for "http" URIs when using the mechanism described whole origin.

   As defined in that specification, one way of establishing this specification.

   When connecting to an alternative service for an "http" URI, clients
   are not required to perform is
   using a TLS-based protocol with the server authentication procedure
   described certificate checks defined in Section 3.1 of
   [RFC2818].  The server certificate, if
   one  Clients MAY impose additional criteria for establishing
   reasonable assurances.

   For the purposes of this specification, an additional way of
   establishing reasonable assurances is proffered by available when the alternative service,
   is not necessarily
   checked for validity, expiration, issuance by a trusted certificate
   authority or matched against on the same host as the origin, using the name "http-opportunistic"
   well-known URI defined in Section 6.

   This allows deployment without the URI.  Therefore, 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 perform additional checks on the offered certificate if
   the server does not select an unauthenticated TLS cipher suite.  This
   document doesn't define any such checks, though clients could consider there to be
   configured with a policy that defines what is acceptable.

   As stipulated by [I-D.ietf-httpbis-alt-svc], clients MUST NOT use reasonable assurances when:

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

   o  The chosen alternative service returns the origin's, unless same response as above.

   For example, this request/response pair would constitute reasonable
   assurances for the origin "" for any
   alternative service itself 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 strongly authenticated (as the origin's
   host); only defined to establish reasonable
   assurances for example, using TLS with a certificate that validates as
   per [RFC2818]. 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
   [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

   Editors' Note: this is a very rough take on an approach that would
   provide a limited form of protection against downgrade attack.  It's
   unclear at this point whether

   Even when the additional effort (and modest
   operational cost) alternative service is worthwhile.

   The mechanism described in this specification strongly authenticated,
   opportunistically upgrading cleartext HTTP connections to use TLS is trivial
   subject to mount an active attack against, for two reasons:

   o  A client that doesn't perform authentication is an easy victim of
      server impersonation, through man-in-the-middle attacks.  In particular:

   o  A client that is willing to use  Because the original HTTP over cleartext connection is in cleartext, it is
      vulnerable to resolve man-in-the-middle attacks, and

   o  By default, if clients cannot reach the
      resource alternative service, they
      will do so if access fall back to any TLS-enabled alternative
      services is blocked at using the network layer. 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
   modest form of protection against active attacks can be provided for
   clients on subsequent connections.

   When an alternative service 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" in the client. "sticky".

5.1.  The HTTP-TLS Header Field

   A alternative service  Opportunistic Commitment

   An origin can make this commitment by sending a "HTTP-
   TLS" header field, described here using reduce the '#' ABNF extension
   defined in Section 7 risk of [RFC7230]:

   HTTP-TLS     = 1#parameter

   When it appears in a HTTP response from a strongly attacks on opportunistically secured
   connections by committing to provide an secured, authenticated
   alternative service, this header field indicates that service.  This is done by including the
   availability of optional "commit"
   member in the origin through TLS-protected alternative services http-opportunistic well-known resource (see Section 6).
   This feature is "sticky", and that the client MUST NOT fall back optional due to cleartext
   protocols while this information is considered fresh.

   For example:

   GET /index.html HTTP/1.1

   HTTP/1.1 200 OK
   Content-Type: text/html
   Cache-Control: max-age=600
   Age: 30
   Date: Thu, 1 May 2014 16:20:09 GMT
   HTTP-TLS: ma=3600

   This header field creates a commitment from the origin [RFC6454] of requirement for server
   authentication and the associated resource (in potential risk entailed (see Section 5.3).

   The value of the example, "").  For "commit" member is a number ([RFC7159], Section 6)
   indicating the duration of the commitment, clients SHOULD strongly authenticate
   the server for all subsequent requests made commitment interval in seconds.

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

   Including "commit" creates a commitment to provide a secured
   alternative service for the advertised period.  Clients that origin, though receive
   this creates some risks commitment can assume that a secured alternative service will be
   available for clients the indicated period.  Clients might however choose to
   limit this time (see Section 5.2).

   Authentication 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 is are described in Section 3.1 2.1 of
   [RFC2818], noting the additional requirements in
   [I-D.ietf-httpbis-alt-svc] and Section 2.1 3.1 of
   [I-D.ietf-httpbis-alt-svc].  The header field MUST be ignored if
   strong authentication fails; otherwise, an attacker could create [RFC2818].  As noted in
   [I-D.ietf-httpbis-alt-svc], clients can impose other checks in
   addition to this minimum set.  For instance, a
   persistent denial of client might choose to
   apply key pinning [RFC7469].

   A client that receives a commitment and that successfully
   authenticates the alternative service by falsifying can assume that a commitment. secured
   alternative will remain available for the commitment interval.  The
   commitment to use interval starts when the commitment is received and
   authenticated TLS persists and runs for a period
   determined by the number of seconds equal to value of the "ma" parameter.  See
   "commit" member, less the current age of the http-opportunistic
   response (as defined in Section 4.2.3 of
   [RFC7234] [RFC7234]).  A client SHOULD
   avoid sending requests via cleartext protocols or to unauthenticated
   alternative services for details the duration of determining response age.

   ma-parameter     = delta-seconds

   The commitment made by the "HTTP-TLS" header field applies commitment interval,
   except to discover new potential alternatives.

   A commitment only applies to the origin of the http-opportunistic
   well-known resource that generates was retrieved; other origins listed in the "HTTP-TLS" header

   Requests for an origin that has a persisted, unexpired value for
   "origins" member MUST fail if they cannot be made over an authenticated TLS

   Note that the 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 alternative services short duration
   commitments so that they become
   aware of.  However, clients MUST NOT use an unauthenticated
   alternative service for an errors can be detected without causing the origin with this commitment.  Where there
   is an active commitment,
   to become inaccessible to clients MAY instead ignore advertisements for unsecured alternatives services.

5.2.  Operational Considerations extended periods.

   To avoid situations where a persisted value of "HTTP-TLS" commitment causes a
   client to be unable to contact a site, errors, clients SHOULD MAY
   limit the time
   that over which a value commitment is persisted respected for a given
   origin.  A lower limit might be appropriate for initial observations of "HTTP-TLS"; commitments;
   the certainty that a site has set a correct value - and the
   corresponding limit on persistence - can might increase as a commitment
   is renewed multiple times.

6.  The "http-opportunistic" well-known URI

   This specification defines the value "http-opportunistic" well-known URI
   [RFC5785].  An origin is seen more over time.

   Once said to have a server valid http-opportunistic
   resource when:

   o  The client has indicated that it will support authenticated TLS, obtained a
   client MAY use key pinning [RFC7469] 200 (OK) response for the well-known URI
      from the origin, or any other mechanism that
   would otherwise be restricted to use with "https" URIs, provided that refreshed one in cache [RFC7234], and

   o  That response has the mechanism can media type "application/json", and
   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 restricted to

7.  IANA Considerations

   This specification registers a single HTTP origin.

6. 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., using a "lock device").


8.2.  Downgrade Attacks

   A downgrade attack against the negotiation for TLS is possible.  With
   the "HTTP-TLS" header field,
   commitment Section 5, this is limited to occasions where clients have
   no prior information (see Section 6.3), 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

   Downgrade attacks can be partially mitigated using the "HTTP-TLS"
   header field, "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


9.  References


9.1.  Normative References

              mnot, m., McManus, P., and J. Reschke, "HTTP Alternative
              Services", draft-ietf-httpbis-alt-svc-09 draft-ietf-httpbis-alt-svc-14 (work in
              progress), November 2015. 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,

   [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,

   [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,

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

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


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

   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