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Versions: 00 01 02 03 04 05 06 rfc7435                     Informational
Network Working Group                                        V. Dukhovni
Internet-Draft                                                 Two Sigma
Intended status: Informational                           August 25, 2014
Expires: February 26, 2015

        Opportunistic Security: Some Protection Most of the Time


   This document defines the concept "Opportunistic Security" in the
   context of communications protocols.  Protocol designs based on
   Opportunistic Security remove barriers to the widespread use of
   encryption on the Internet by using encryption even when
   authentication is not available, and using authentication when

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on February 26, 2015.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Background  . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  A New Perspective . . . . . . . . . . . . . . . . . . . .   5
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Opportunistic Security Design Principles  . . . . . . . . . .   6
   4.  Example: Opportunistic TLS in SMTP  . . . . . . . . . . . . .   8
   5.  Operational Considerations  . . . . . . . . . . . . . . . . .   9
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   Broadly speaking, Opportunistic Security (OS) is a pragmatic risk
   management approach.  With Opportunistic Security, one applies the
   tools at hand to mitigate the risks that can reasonably be addressed,
   and accepts the rest.

   Definition:  In the context of communications protocols,
      "Opportunistic Security" is defined as the use of encryption when
      possible, with authentication when possible.  In the above, the
      phrase "when possible" means when support for the corresponding
      capability is advertised by the peer, ideally in a downgrade-
      resistant manner.

   Encryption is used to mitigate the risk of passive monitoring
   attacks, while authentication is used to mitigate the risk of active
   man-in-the-middle (MiTM) attacks.  When encryption capability is
   advertised over an insecure channel, MiTM downgrade attacks to
   cleartext may be possible.  Since encryption alone mitigates only
   passive attacks, this risk is consistent with the expected level of
   protection.  For authentication based on peer capabilities to protect
   against MiTM attacks, capability advertisements need to be over an
   out-of-band authenticated channel that is itself resistant to MiTM

   To achieve widespread adoption, OS must support incremental
   deployment.  Incremental deployment implies that security
   capabilities will vary from peer to peer, perhaps for a very long
   time.  OS protocols will attempt to establish encrypted communication
   whenever both parties are capable of such, and authenticated
   communication if that is also possible.  Thus, use of an OS protocol
   may yield communication that is authenticated and encrypted,

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   unauthenticated but encrypted, or cleartext.  This last outcome will
   occur if not all parties to a communication support encryption (or if
   an active attack makes it appear that this is the case).

   For a particular protocol or application, if and when all but a
   negligible fraction of peers support encryption, the baseline
   security may be raised from cleartext to always require encryption.
   Similarly, once support for authentication is near-universal, the
   baseline may be raised to always require authentication.

   OS is not intended as a substitute for authenticated, encrypted
   communication when such communication is already mandated by policy
   or is otherwise required to access a particular resource.  In
   essence, OS is employed when one might otherwise settle for cleartext
   (or the minimum protection possible if the protocol is always
   encrypted).  OS protocols never preempt local security policies.  A
   security administrator may specify security policies that override
   OS.  For example, a policy might require authenticated, encrypted
   communication, in contrast to the default OS security policy.

1.1.  Background

   Historically, Internet security protocols have emphasized
   comprehensive "all or nothing" cryptographic protection against both
   passive and active attacks.  With each peer, such a protocol achieves
   either full protection or else total failure to communicate (hard
   fail).  As a result, operators often disable these security protocols
   when users have difficulty connecting, thereby degrading all
   communications to cleartext transmission.

   Protection against active attacks requires authentication.  The
   ability to authenticate any potential peer on the Internet requires
   an authentication mechanism that encompasses all such peers.  No IETF
   standards for authentication meet this requirement.

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   The Public Key Infrastructure (PKI) model employed by browsers to
   authenticate web servers (often called the "Web PKI") imposes cost
   and management burdens that have limited its use.  With so many
   certification authorities, which not everyone is willing to trust,
   the communicating parties don't always agree on a mutually trusted
   CA.  Without a mutually trusted CA, authentication fails, leading to
   communications failure in protocols that mandate authentication.
   These issues are compounded by operational difficulties.  For
   example, a common problem is for site operators to forget to perform
   timely renewal of expiring certificates.  In interactive
   applications, security warnings are all too frequent, and end-users
   learn to actively ignore security problems, or site administrators
   decide that the maintenance cost is not worth the benefit so they
   provide a cleartext-only service to their users.

   The trust-on-first-use (TOFU) authentication approach assumes that an
   unauthenticated public key obtained on first contact (and retained
   for future use) will be good enough to secure future communication.
   TOFU-based protocols do not protect against an attacker who can
   hijack the first contact communication and require more care from the
   end-user when systems update their cryptographic keys.  TOFU can make
   it difficult to distinguish routine system administration from a
   malicious attack.

   DNS-Based Authentication of Named Entities (DANE [RFC6698]) defines a
   way to distribute public keys bound to DNS names.  It can provide an
   alternative to the Web PKI.  DANE needs to be used in conjunction
   with DNSSEC [RFC4033].  At the time of writing, DNSSEC is not
   sufficiently widely deployed to allow DANE to satisfy the Internet-
   wide, any-to-any authentication requirement noted above.  Protocols
   that mandate authenticated communication cannot yet generally do so
   via DANE (at the time of writing).

   The lack of a global key management system means that for many
   protocols, only a minority of communications sessions can be
   predictably authenticated.  When protocols only offer a choice
   between authenticated-and-encrypted communication, or no protection,
   the result is that most traffic is sent in cleartext.  The fact that
   most traffic is not encrypted makes pervasive monitoring easier by
   making it cost-effective, or at least not cost-prohibitive (see
   [RFC7258] for more detail).

   For encryption to be used more broadly, authentication needs to be
   optional.  The use of encryption defends against pervasive monitoring
   and other passive attacks (which are employed not only by nation
   states).  Even unauthenticated, encrypted communication (defined
   below) is preferable to cleartext.

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   For some applications or protocols the set of potential peers
   includes a long tail of implementations that support only cleartext.
   Such applications or protocols cannot set a baseline security policy
   that requires encryption without losing the ability to communicate
   with the cleartext-only peers.

1.2.  A New Perspective

   This document proposes a change of perspective.  Until now, the
   protocol designer has viewed protection against both passive and
   active attacks as the default, and anything short of that as
   "degraded security" or a "fallback".  Now, with OS, the new viewpoint
   is that without specific knowledge of peer capabilities (or
   applicable local policy), the default protection is no protection,
   and anything more than that is an improvement.

   Cleartext, not comprehensive protection, is the default baseline.  An
   OS protocol is not falling back from comprehensive protection when
   that protection is not supported by all peers; rather, OS protocols
   employ the maximum protection possible.  OS protocols work
   transparently behind the scenes, without disrupting communication.

   When less than complete protection is negotiated, there is no need to
   prompt the user with "your security may be degraded, please click OK"
   dialogs.  The negotiated protection is as good as can be expected.
   Even if not comprehensive, it is often better than the traditional
   outcome of either "no protection" or "communications failure".

   In this document, the word "opportunistic" carries a positive
   connotation.  Based on advertised peer capabilities, an OS protocol
   uses as much protection as possible.  The adjective "opportunistic"
   applies to the adaptive choice of security mechanisms peer by peer.
   Once that choice is made for a given peer, OS looks rather similar to
   other designs that happen to use the same set of mechanisms.

   The remainder of this document provides definitions of important
   terms, sets out the OS design principles, and provides an example of
   an OS design in the context of communication between mail relays.

2.  Terminology

   Trust on First Use (TOFU):  In a protocol, TOFU calls for accepting
      and storing a public key or credential associated with an asserted
      identity, without authenticating that assertion.  Subsequent
      communication that is authenticated using the cached key or
      credential is secure against an MiTM attack, if such an attack did
      not succeed during the vulnerable initial communication.  The SSH
      protocol [RFC4251] in its commonly deployed form makes use of

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      TOFU.  The phrase "leap of faith" (LoF, [RFC4949]) is sometimes
      used as a synonym.

   Authenticated, encrypted communication:  Encrypted communication
      using a session establishment method in which at least the
      initiator (or client) authenticates the identity of the acceptor
      (or server).  This is required to protect against both passive and
      active attacks.  Mutual authentication, in which the server also
      authenticates the client, plays a role in mitigating active
      attacks when the client and server roles change in the course of a
      single session.

   Unauthenticated, encrypted communication:  Encrypted communication
      using a session establishment method that does not authenticate
      the identities of the peers.  In typical usage, this means that
      the initiator (client) has not verified the identity of the target
      (server), making MiTM attacks possible.

   Perfect Forward Secrecy (PFS):  As defined in [RFC4949].

   Man-in-the-Middle (MiTM) attack:  As defined in [RFC4949].

3.  Opportunistic Security Design Principles

   OS provides a near-term approach to counter passive attacks by
   removing barriers to the widespread use of encryption.  OS offers an
   incremental path to authenticated, encrypted communication in the
   future, as suitable authentication technologies are deployed.  OS
   promotes the following design principles:

   Coexist with local policy:  Local security policies preempt OS.
      Opportunistic security never displaces or preempts local policy.
      Many applications and types of data are too sensitive to use OS,
      and more traditional security designs are appropriate in such

   Emphasize enabling communication:  The primary goal of OS is to
      enable communication and maximize the deployment of usable
      security.  OS protocols need to be deployable incrementally, with
      each peer configured independently by its administrator or user.

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   Maximize security peer by peer:  OS protocols use encryption when it
      is mutually supported.  OS protocols enforce peer authentication
      when an authenticated out-of-band channel is available to provide
      the requisite keys or credentials.  Communication should generally
      be at least encrypted.  OS should employ Perfect Forward Secrecy
      (PFS) wherever possible in order to protect previously recorded
      encrypted communication from decryption even after a compromise of
      long-term keys.

   No misrepresentation of security:  Unauthenticated, encrypted
      communication must not be misrepresented to users or in
      application logs of non-interactive applications as equivalent to
      authenticated, encrypted communication.

   An OS protocol first determines the capabilities of the peer with
   which it is attempting to communicate.  Peer capabilities may be
   discovered by out-of-band or in-band means.  (Out-of-band mechanisms
   include the use of DANE records or cached keys or credentials
   acquired via TOFU.  In-band determination implies negotiation between
   peers.)  The capability determination phase may indicate that the
   peer supports authenticated, encrypted communication;
   unauthenticated, encrypted communication; or only cleartext

   Opportunistic security protocols may hard-fail with peers for which a
   security capability fails to function as advertised.  Security
   services that work reliably (when not under attack) are more likely
   to be deployed and enabled by default.  It is vital that the
   capabilities advertised for an OS-compatible peer match the deployed
   reality.  Otherwise, OS systems will detect such a broken deployment
   as an active attack and communication may fail.  This might mean that
   advertised peer capabilities are further filtered to consider only
   those capabilities that are sufficiently operationally reliable.
   Capabilities that can't be expected to work reliably should be
   treated by an OS protocol as "not present" or "undefined".

   For greater assurance of channel security, an OS protocol may enforce
   more stringent cryptographic parameters when the session is
   authenticated.  For example, the set of enabled Transport Layer
   Security (TLS [RFC5246]) cipher suites might be more restrictive for
   authenticated sessions.

   OS protocols should produce authenticated, encrypted communication
   when authentication of the peer is "expected".  Here, "expected"
   means a determination via a downgrade-resistant method that
   authentication of that peer is expected to work.  Downgrade-resistant
   methods include: validated DANE DNS records, existing TOFU identity
   information, and manual configuration.  Such use of authentication is

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   "opportunistic", in that it is performed when possible, on a per-
   session basis.

   When communicating with a peer that supports encryption but not
   authentication, any authentication checks enabled by default must be
   disabled or configured to soft-fail in order to avoid unnecessary
   communications failure or needless downgrade to cleartext.

   Cleartext is supported for backwards compatibility with systems
   already deployed.  Even when cleartext needs to be supported,
   protocol designs based on Opportunistic Security prefer to encrypt,
   employing cleartext only with peers that do not appear to be
   encryption capable.

4.  Example: Opportunistic TLS in SMTP

   Most Message Transfer Agents (MTAs, [RFC5598]) support the STARTTLS
   ([RFC3207]) ESMTP extension.  MTAs acting as SMTP ([RFC5321]) clients
   generally support cleartext transmission of email.  They negotiate
   TLS encryption when the SMTP server announces STARTTLS support.
   Since the initial ESMTP negotiation is not cryptographically
   protected, the STARTTLS advertisement is vulnerable to MiTM downgrade

   Recent reports from a number of large providers (e.g., [fb-starttls]
   and [goog-starttls]) suggest that the majority of SMTP email
   transmission on the Internet is now encrypted, and the trend is
   toward increasing adoption.

   Various MTAs that advertise STARTTLS exhibit interoperability
   problems in their implementations.  As a work-around, it is common
   for a client MTA to fall back to cleartext when the TLS handshake
   fails, or when TLS fails during message transmission.  This is a
   reasonable trade-off, since STARTTLS only protects against passive
   attacks.  In the absence of an active attack TLS failures are
   generally one of the known interoperability problems.

   Some client MTAs employing STARTTLS abandon the TLS handshake when
   the server MTA fails authentication, and immediately start a
   cleartext connection.  Other MTAs have been observed to accept
   unverified self-signed certificates, but not expired certificates;
   again falling back to cleartext.  These and similar behaviors are NOT
   consistent with OS principles, since they needlessly fall back to
   cleartext when encryption is clearly possible.

   Protection against active attacks for SMTP is described in
   [I-D.ietf-dane-smtp-with-dane].  That document introduces the terms
   "Opportunistic TLS" and "Opportunistic DANE TLS", and is consistent

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   with the OS design principles defined in this document.  With
   "Opportunistic DANE TLS", authenticated, encrypted communication is
   enforced with peers for which appropriate DANE records are present.
   For the remaining peers, "Opportunistic TLS" is employed as before.

5.  Operational Considerations

   OS protocol designs should minimize the possibility of failure of
   negotiated security mechanisms.  OS protocols may need to employ
   "fallback", to work-around a failure of a security mechanisms that is
   found in practice to encounter interoperability problems.  The choice
   to implement or enable fallback should only be made in response to
   significant operational obstacles.

   When protection only against passive attacks is negotiated over a
   channel vulnerable to active downgrade attacks, and the use of
   encryption fails, a protocol might elect non-intrusive fallback to
   cleartext.  An active attacker could equally have suppressed the use
   of encryption during negotiation, so failure to encrypt may be more
   often a symptom of an interoperability problem than an active attack.
   In such a situation occasional fallback to cleartext may serve the
   greater good.  Even though some traffic is sent in the clear, the
   alternative is to ask the administrator or user to manually work-
   around such interoperability problems.  If the incidence of such
   problems is non-negligible, the user or administrator might find it
   more expedient to just disable Opportunistic Security.

6.  Security Considerations

   OS supports communication that is authenticated and encrypted,
   unauthenticated and encrypted, or cleartext.  And yet the security
   provided to communicating peers is not reduced by the use of OS
   because the default OS policy employs the best security services
   available based on the capabilities of the peers, and because local
   security policies take precedence over the default OS policy.  OS is
   an improvement over the status quo; it provides better security than
   the alternative of providing no security services when authentication
   is not possible (and not strictly required).

   While the use of OS is preempted by a non-OS local policy, such a
   non-OS policy can be counter-productive when it demands more than
   many peers can in fact deliver.  Non-OS policy should be used with
   care, lest users find it too restrictive and act to disable security

7.  Acknowledgements

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   I would like to thank Steve Kent.  Some of the text in this document
   is based on his earlier draft.  I would like to thank Dave Crocker,
   Peter Duchovni, Paul Hoffman, Benjamin Kaduk, Steve Kent, Scott
   Kitterman, Martin Thomson, Nico Williams, Paul Wouters and Stephen
   Farrell for their many helpful suggestions and support.

8.  References

8.1.  Normative References

   [RFC3207]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
              Transport Layer Security", RFC 3207, February 2002.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements", RFC
              4033, March 2005.

   [RFC4251]  Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
              Protocol Architecture", RFC 4251, January 2006.

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

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              October 2008.

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, August 2012.

8.2.  Informative References

              Dukhovni, V. and W. Hardaker, "SMTP security via
              opportunistic DANE TLS", draft-ietf-dane-smtp-with-dane-12
              (work in progress), August 2014.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2", RFC
              4949, August 2007.

   [RFC5598]  Crocker, D., "Internet Mail Architecture", RFC 5598, July

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


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              Facebook, "The Current State of SMTP STARTTLS Deployment",
              May 2014, <https://www.facebook.com/notes/protect-the-

              Google, "Safer email - Transparency Report - Google", June
              2014, <https://www.google.com/transparencyreport/

Author's Address

   Viktor Dukhovni
   Two Sigma

   Email: ietf-dane@dukhovni.org

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