Network Working Group                                      M. Nottingham
Intended status: Standards Track                              M. Thomson
Expires: November 21, 2014                                       Mozilla
                                                            May 20, 2014

                 Opportunistic Encryption for HTTP URIs


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

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   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on November 21, 2014.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Goals and Non-Goals . . . . . . . . . . . . . . . . . . .   2
     1.2.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  Using HTTP URIs over TLS  . . . . . . . . . . . . . . . . . .   3
   3.  Server Authentication . . . . . . . . . . . . . . . . . . . .   3
   4.  Interaction with "https" URIs . . . . . . . . . . . . . . . .   4
   5.  Requiring Use of TLS  . . . . . . . . . . . . . . . . . . . .   4
     5.1.  The HTTP-TLS Header Field . . . . . . . . . . . . . . . .   5
     5.2.  Operational Considerations  . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
     6.1.  Security Indicators . . . . . . . . . . . . . . . . . . .   7
     6.2.  Downgrade Attacks . . . . . . . . . . . . . . . . . . . .   7
     6.3.  Privacy Considerations  . . . . . . . . . . . . . . . . .   7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

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 to
   be accessed using TLS [RFC5246] opportunistically.

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

   A mechanism for limiting the potential for active attacks is
   described in Section 5.  This provides clients with additional
   protection against them for a period after successfully connecting to
   a server using TLS.  This does not offer the same level of protection
   as afforded to "https" URIs, but increases the likelihood that an
   active attack be detected.

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

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   "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 should have
   a minimal impact upon performance, and should not require extensive
   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" [I-D.ietf-httpbis-http2].

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

   A client that places the importance of passive protections over
   performance might choose to withold requests until an encrypted
   connection is available.  However, if such a connection cannot be
   successfully established, the client MAY 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.  Clients with
   expired alternative services information could make a similar request
   in parallel to an attempt to contact an alternative service, to
   minimize the delays that might be incurred by failing to contact the
   alternative service.

3.  Server Authentication

   There are no existing expectations with respect to cryptographically
   strong server authentication when it comes to resolving HTTP URIs.
   Establishing it, as described in [RFC2818], creates a number of
   operational challenges.  For these reasons, server authentication is
   not mandatory for HTTP URIs when using the mechanism described in
   this specification.

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   When connecting to an alternative service for an "http" URI, clients
   are required to perform the server authentication procedure described
   in Section 3.1 of [RFC2818].  The server certificate, if one is
   proffered by the alternative service, is not necessarily checked for
   validity, expiration, issuance by a trusted certificate authority or
   matched against the name in the URI.  Therefore, the alternative
   service MAY provide any certificate, or even select TLS cipher suites
   that do not include authentication.

   A client MAY perform additional checks on the certificate that it is
   offered (if the server does not select an unauthenticated TLS cipher
   suite).  For instance, a client could examine the certificate to see
   if it has changed over time.

   In order to retain the authority properties of "http" URIs, and as
   stipulated by [I-D.ietf-httpbis-alt-svc], clients MUST NOT use
   alternative services that identify a host other than that of the
   origin, unless the alternative service indication itself is strongly
   authenticated.  This is not currently possible for "http" URIs on
   cleartext transports.

4.  Interaction with "https" URIs

   An alternative service that is discovered to support "http" URIs
   might concurrently support "https" URIs, because HTTP/2 permits the
   sending of requests for multiple origins (see [RFC6454]) on the one
   connection.  Therefore, when using alternative services, both HTTP
   and HTTPS URIs might be sent on the same connection.

   "https" URIs rely on server authentication.  Therefore, if a
   connection is initially created without authenticating the server,
   requests for "https" resources cannot be sent over that connection
   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] could
   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 the additional effort (and modest
   operational cost) is worthwhile.

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   The mechanism described in this specification is trival to mount an
   active attack against, for two reasons:

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

   o  A client that is willing to use cleartext to resolve the resource
      will do so if access to any TLS-enabled alternative services is
      blocked at the network layer.

   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 alternate service 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 avilability, failing when it cannot be
   contacted.  Effectively, this makes the alternative service "sticky"
   in the client.

   One drawback with this approach is that clients need to strongly
   authenticate the alternative service to act upon such a commitment;
   otherwise, an attacker could create a persistent denial of service.

5.1.  The HTTP-TLS Header Field

   A alternative service can make this commitment by sending a "HTTP-
   TLS" header field:

   HTTP-TLS     = 1#parameter

   When it appears in a HTTP response from a strongly authenticated
   alternative service, this header field indicates that the
   availability of the origin through TLS-protected alternative services
   is "sticky", and that the client MUST NOT fall back to cleartext
   protocols while this information is considered fresh.

   For example:

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

   Note that the commitment is not bound to a particular alternative
   service; clients SHOULD use other alternative services that they
   become aware of, as long as the requirements regarding authentication
   and avoidance of cleartext protocols are met.

   When this header field appears in a response, clients MUST strongly
   authenticate the alternative service, as described in Section 3.1 of
   [RFC2818], noting the additional requirements in
   [I-D.ietf-httpbis-alt-svc].  The header field MUST be ignored if
   strong authentication fails.

   Persisted information expires after a period determined by the value
   of the "ma" parameter.  See Section 4.2.3 of
   [I-D.ietf-httpbis-p6-cache] for details of determining response age.

   ma-parameter     = delta-seconds

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

5.2.  Operational Considerations

   To avoid situations where a persisted value of "HTTP-TLS" causes a
   client to be unable to contact a site, clients SHOULD limit the time
   that a value is persisted for a given origin.  A hard limit might be
   set to a month.  A lower limit might be appropriate for initial
   observations of "HTTP-TLS"; the certainty that a site has set a
   correct value - and the corresponding limit on persistence - can
   increase as the value is seen more over time.

   Once a server has indicated that it will support authenticated TLS, a
   client MAY use key pinning [I-D.ietf-websec-key-pinning] or any other
   mechanism that would otherwise be restricted to use with HTTPS URIs,
   provided that the mechanism can be restricted to a single HTTP

6.  Security Considerations

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6.1.  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").

6.2.  Downgrade Attacks

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

   As long as a client is willing to use cleartext TCP to contact a
   server, these attacks are possible.  The "HTTP-TLS" header field
   provides an imperfect mechanism for establishing a commitment.  The
   advantage is that this only works if a previous connection is
   established where an active attacker was not present.  A continuously
   present active attacker can either prevent the client from ever using
   TLS, or offer a self-signed certificate.  This would prevent the
   client from ever seeing the "HTTP-TLS" header field, or if the header
   field is seen, from successfully validating and persisting it.

6.3.  Privacy Considerations

   Clients that persist state for origins can be tracked over time based
   on their use of this information.  Persisted information can be
   cleared to reduce the ability of servers to track clients.  A browser
   client MUST clear persisted all alternative service information when
   clearing other origin-based state (i.e., cookies).

7.  References

7.1.  Normative References

              Nottingham, M., McManus, P., and J. Reschke, "HTTP
              Alternative Services", draft-ietf-httpbis-alt-svc-01 (work
              in progress), April 2014.

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              Belshe, M., Peon, R., and M. Thomson, "Hypertext Transfer
              Protocol version 2", draft-ietf-httpbis-http2-12 (work in
              progress), April 2014.

              Fielding, R., Nottingham, M., and J. Reschke, "Hypertext
              Transfer Protocol (HTTP/1.1): Caching", draft-ietf-
              httpbis-p6-cache-26 (work in progress), February 2014.

              Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
              Extension for HTTP", draft-ietf-websec-key-pinning-13
              (work in progress), May 2014.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

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

7.2.  Informative References

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454, December

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

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


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   Martin Thomson


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