Opportunistic Security for HTTP
draft-ietf-httpbis-http2-encryption-01

HTTPbis Working Group                                      M. Nottingham
Internet-Draft
Intended status: Experimental                                 M. Thomson
Expires: June 18, 2015                                           Mozilla
                                                       December 15, 2014

                    Opportunistic Security for HTTP
                 draft-ietf-httpbis-http2-encryption-01

Abstract

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

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   This Internet-Draft will expire on June 18, 2015.

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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.  Requiring Use of TLS  . . . . . . . . . . . . . . . . . . . .   5
     5.1.  The HTTP-TLS Header Field . . . . . . . . . . . . . . . .   5
     5.2.  Operational Considerations  . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
     6.1.  Security Indicators . . . . . . . . . . . . . . . . . . .   7
     6.2.  Downgrade Attacks . . . . . . . . . . . . . . . . . . . .   7
     6.3.  Privacy Considerations  . . . . . . . . . . . . . . . . .   7
     6.4.  Confusion Regarding Request Scheme  . . . . . . . . . . .   8
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

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 require 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.

   Opportunistic Security [I-D.dukhovni-opportunistic-security] 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.
   It does not offer the same level of protection as afforded to "https"
   URIs, but increases the likelihood that an active attack be detected.

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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 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",
   "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" [I-D.ietf-httpbis-http2].

   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 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.  Likewise, clients
   with existing alternative services information could make such a
   request before they expire, in order minimize the delays that might
   be incurred.

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3.  Server Authentication

   By their nature, "http" URIs do not require cryptographically strong
   server authentication; that is only implied by "https" URIs.
   Furthermore, doing so (as per [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.

   When connecting to an alternative service for an "http" URI, clients
   are not 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 offered certificate if
   the server does not select an unauthenticated TLS cipher suite.  This
   document doesn't define any such checks, though clients could be
   configured with a policy that defines what is acceptable.

   As stipulated by [I-D.ietf-httpbis-alt-svc], clients MUST NOT use
   alternative services with a host other than the origin's, unless the
   alternative service itself is strongly authenticated (as the origin's
   host); for example, using TLS with a certificate that validates as
   per [RFC2818].

4.  Interaction with "https" URIs

   When using alternative services, 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.

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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.

   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 alternative 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 choice to use a
   secured protocol "sticky" in the client.

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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   GET /index.html HTTP/1.1
   Host: example.com

   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

   This header field creates a commitment from the origin [RFC6454] of
   the associated resource (in the example, "http://example.com").  For
   the duration of the commitment, clients SHOULD strongly authenticate
   the server for all subsequent requests made to that origin, though
   this creates some risks for clients Section 5.2.

   Authentication for HTTP over TLS is 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; otherwise, an attacker could create a
   persistent denial of service by falsifying a commitment.

   The commitment to use authenticated TLS persists for a period
   determined by the value of the "ma" parameter.  See Section 4.2.3 of
   [RFC7234] for details of determining response age.

   ma-parameter     = delta-seconds

   The commitment made by the "HTTP-TLS" header field applies only to
   the origin of the resource that generates the "HTTP-TLS" header
   field.  Requests for an origin that has a persisted, unexpired value
   for "HTTP-TLS" MUST fail if they cannot be made over an authenticated
   TLS connection.

   Note that the commitment is not bound to a particular alternative
   service.  Clients SHOULD use alternative services that they become
   aware of.  However, clients MUST NOT use an unauthenticated
   alternative service for an origin with this commitment.  Where there
   is an active commitment, clients MAY instead ignore advertisements
   for unsecured alternatives services.

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 lower limit might be

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   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
   origin.

6.  Security Considerations

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
   responses.

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

6.3.  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).

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6.4.  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 sent over HTTP/1.1 or earlier versions of the
   protocol.  Opportunistically secured HTTP requests MUST include an
   explicit scheme identifier.

7.  References

7.1.  Normative References

   [I-D.ietf-httpbis-alt-svc]
              Nottingham, M., McManus, P., and J. Reschke, "HTTP
              Alternative Services", draft-ietf-httpbis-alt-svc-05 (work
              in progress), December 2014.

   [I-D.ietf-httpbis-http2]
              Belshe, M., Peon, R., and M. Thomson, "Hypertext Transfer
              Protocol version 2", draft-ietf-httpbis-http2-16 (work in
              progress), November 2014.

   [I-D.ietf-websec-key-pinning]
              Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
              Extension for HTTP", draft-ietf-websec-key-pinning-21
              (work in progress), October 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.

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

   [RFC7234]  Fielding, R., Nottingham, M., and J. Reschke, "Hypertext
              Transfer Protocol (HTTP/1.1): Caching", RFC 7234, June
              2014.

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7.2.  Informative References

   [I-D.dukhovni-opportunistic-security]
              Dukhovni, V., "Opportunistic Security: Some Protection
              Most of the Time", draft-dukhovni-opportunistic-
              security-06 (work in progress), November 2014.

   [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

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

   Martin Thomson
   Mozilla

   Email: martin.thomson@gmail.com

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