Network Working Group                                        V. Dukhovni
Internet-Draft                                                 Two Sigma
Intended status: Informational                              July 6, 2014
Expires: January 7, 2015

        Opportunistic Security: some protection most of the time


   This memo defines the term "opportunistic security".  In contrast to
   the established approach of delivering strong protection some of the
   time, opportunistic security strives to deliver at least some
   protection most of the time.  The primary goal is therefore broad
   interoperability, with security policy tailored to the capabilities
   of peer systems.

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   This Internet-Draft will expire on January 7, 2015.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Opportunistic Security Design Philosophy  . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   Historically, Internet security protocols have prioritized strong
   protection for peers capable and motivated to absorb the associated
   costs.  Since strong protection is not universally applicable, while
   communications traffic was sometimes strongly secured, more typically
   it was not protected at all.  The fact that most traffic is
   unprotected facilitates nation-state pervasive monitoring (PM
   [RFC7258]) by making it cost-effective (or at least not cost-
   prohibitive).  Indiscriminate collection of communications traffic
   would be substantially less attractive if security protocols were
   designed to operate at a range of protection levels with encrypted
   transmission accessible to most if not all peers, and stronger
   security still available where required by policy or
   opportunistically negotiated.

   Encryption is easy, but key management is difficult.  Key management
   at Internet scale remains an incompletely solved problem.  The PKIX
   ([RFC5280]) key management model introduces costs that not all peers
   are willing to bear and is also not sufficient to secure
   communications when the peer reference identity is obtained
   indirectly over an insecure channel or communicating parties don't
   agree on a mutually trusted certification authority (CA).  DNSSEC is
   not at this time sufficiently widely adopted to make DANE a viable
   alternative at scale.  Trust on first use (TOFU) key management
   models (as with saved SSH fingerprints and various certificate
   pinning approaches) don't protect initial contact and require user
   intervention when key continuity fails.

   Without Internet-scale key management, authentication is often not
   possible.  When protocols only offer the options of strongly-
   authenticated secure channels or else no security, most traffic gets
   no security protection.  Therefore, in order to make encryption more
   ubiquitous, authentication needs to be optional.  When strongly
   authenticated communication is not possible, unauthenticated
   encryption is still substantially stronger than cleartext.
   Opportunistic security encourages peers to employ as much security as
   possible, without falling back to unnecessarily weak options.  In

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   particular, opportunistic security encourages unauthenticated
   encryption when authentication is not an option.

2.  Opportunistic Security Design Philosophy

   Interoperate to maximize deployment:  The primary goal of designs
      that feature opportunistic security is to be able to communicate
      with any reasonably configured peer.  If many peers are only
      capable of cleartext, then it is acceptable to fall back to
      cleartext when encryption is not possible.  If authentication is
      only possible for some peers, then it is acceptable to
      authenticate only those peers and not the rest.  Interoperability
      must be possible without bilateral coordination.  Applications
      employing opportunistic security need to be deployable at Internet
      scale, with each peer independently configured to meet its own
      security needs (within the practical bounds of the application
      protocol).  Opportunistic security must not get in the way of the
      peers communicating if neither end is misconfigured.

   Maximize security peer by peer:  Subject to the above large-scale
      interoperability goal, opportunistic security strives to maximize
      security based on the capabilities of the peer (or peers).  For
      some opportunistic security protocols the maximal protection
      possible may be just unauthenticated encryption.  For others,
      greater security may be an option, and opportunistic security may
      at times (in partial conflict with the interoperability goal)
      refuse to continue with peers where higher security is expected,
      but for some reason not achieved.  The conditions under which
      connections fail should generally be limited to operational errors
      at one or the other peer or an active attack, so that well-
      maintained systems rarely encounter problems in normal use of
      opportunistic security.

   Encrypt by default:  An opportunistic security protocol MUST
      interoperably achieve at least unauthenticated encryption between
      peer systems that don't explicitly disable this capability.  Over
      time, as peer software is updated to support opportunistic
      security, only legacy systems or a minority of systems where
      encryption is disabled should be communicating in cleartext.
      Whenever possible, opportunistic security should employ Perfect
      Forward Secrecy (PFS) to make recovery of previously sent keys and
      plaintext computationally expensive even after disclosure of long-
      term keys.

   No misrepresentation of security:  Unauthenticated communication or
      use of authentication that is vulnerable to MiTM attacks is not
      represented as strong security.  Where strong security is

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      required, opportunistic security is not a substitute, though the
      underlying mechanisms may in some cases be very similar.

   In summary, opportunistic security is an umbrella term that
   encompasses protocol designs that remove barriers to the widespread
   use of encryption in the Internet.  The actual protection provided by
   opportunistic security depends on the capabilities of the
   communicating peers; opportunistic security MUST attempt to at least
   encrypt network traffic, while allowing fallback to cleartext with
   peers that do not appear to be encryption capable.

   It is important to note that opportunistic security is not limited to
   unauthenticated encryption.  When possible, opportunistic security
   SHOULD provide stronger security on a peer-by-peer basis.  For
   example, some peers may be authenticated via DANE, TOFU or other
   means.  Though authentication failure MAY be a reason to abort a
   connection to a peer that is expected to be authenticated, it MUST
   NOT instead lead to communication in cleartext when encryption is an
   option.  Some sending MTAs employing STARTTLS have been observed to
   abort TLS transmission when the receiving MTA fails authentication,
   only to immediately deliver the same message over a cleartext
   connection.  This design blunder MUST be avoided.

3.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in

   The following definitions are derived from the Internet Security
   Glossary [RFC4949], where applicable.

   Perfect Forward Secrecy (PFS):  For a key management protocol, the
      property that compromise of long-term keys does not compromise
      session/traffic/content keys that are derived from or distributed
      using the long-term keys.

   Man-in-the-Middle attack (MiTM):  A form of active wiretapping attack
      in which an attacker intercepts and may selectively modify
      communicated data to masquerade as one of the entities involved in
      a communication.  Masquerading enables the MiTM to violate the
      confidentiality and/or the integrity of communicated data passing
      through it.

   Trust on First Use (TOFU):  In a protocol, TOFU typically consists of
      accepting an asserted identity, without authenticating that
      assertion, and caching a key or credential associated with the

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      identity.  Subsequent communication using the cached key/
      credential is secure against a MiTM attack, if such an attack did
      not succeed during the (vulnerable) initial communication or if
      the MiTM is not present for all subsequent communications.  The
      SSH protocol makes use of TOFU.  The phrase "leap of faith" (LoF)
      is sometimes used as a synonym.

   Unauthenticated Encryption:  Encryption using a key management
      technique that enables unauthenticated communication between
      parties.  The communication may be 1-way or 2-way unauthenticated.
      If 1-way, the initiator (client) or the target (server) may be

4.  Security Considerations

   Though opportunistic security potentially supports transmission in
   cleartext, unauthenticated encryption, or other protection levels
   short of the strongest potentially applicable, the effective security
   for users is increased, not reduced.  Provided strong security is not
   required by policy or securely negotiated, nothing is lost by
   allowing weaker protection levels, indeed opportunistic security is
   strictly stronger than the alternative of providing no security
   services when maximal security is not applicable.

5.  Acknowledgements

   I would like to thank Steve Kent.  Some of the text in this document
   is based on his earlier draft.

6.  References

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

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

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, May 2008.

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

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Author's Address

   Viktor Dukhovni
   Two Sigma


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