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RFC 7435

Opportunistic Security: Some Protection Most of the Time

Pages: 11

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Internet Engineering Task Force (IETF)                       V. Dukhovni
Request for Comments: 7435                                     Two Sigma
Category: Informational                                    December 2014
ISSN: 2070-1721

        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 use encryption even when authentication is not available, and use authentication when possible, thereby removing barriers to the widespread use of encryption on the Internet. Status of This Memo This document is not an Internet Standards Track specification; it is published for informational purposes. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at 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 Provisions Relating to IETF Documents ( in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Background . . . . . . . . . . . . . . . . . . . . . . . 2 1.2. A New Perspective . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. Opportunistic Security Design Principles . . . . . . . . . . 5 4. Example: Opportunistic TLS in SMTP . . . . . . . . . . . . . 8 5. Operational Considerations . . . . . . . . . . . . . . . . . 8 6. Security Considerations . . . . . . . . . . . . . . . . . . . 9 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 7.1. Normative References . . . . . . . . . . . . . . . . . . 10 7.2. Informative References . . . . . . . . . . . . . . . . . 10 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 11 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11

1. Introduction

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 standard for authentication scales as needed and has been deployed widely enough to meet this requirement. 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 (CAs), not all of which 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 Web PKI 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.
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   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 key management from a malicious

   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 authenticate all
   potential peers.  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.  Even unauthenticated, encrypted
   communication (defined below) is preferable to cleartext.

1.2. A New Perspective

This document describes 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". The new viewpoint is that without specific knowledge of peer capabilities (or explicit configuration or direct request of the application), the default protection is no protection, and anything more than that is an improvement. "Opportunistic Security" (OS) is defined as the use of cleartext as the baseline communication security policy, with encryption and authentication negotiated and applied to the communication when available.
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   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
   aim to use the maximum protection that is available.  (At some point
   in time for a particular application or protocol all but a negligible
   fraction of peers might support encryption.  At that time, the
   baseline security might be raised from cleartext to always require
   encryption, and only authentication would have to be done

   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,
   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).

   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".

   OS is not intended as a substitute for authenticated, encrypted
   communication when such communication is already mandated by policy
   (that is, by configuration or direct request of the application) or
   is otherwise required to access a particular resource.  In essence,
   OS is employed when one might otherwise settle for cleartext.  OS
   protocols never preempt explicit 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.

   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.
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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 TOFU. The phrase "leap of faith" [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]. OS protocol: A protocol that follows the opportunistic approach to security described herein.

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 explicit policy: Explicit security policies preempt OS. Opportunistic security never displaces or preempts explicit policy. Many applications and types of data are too sensitive to use OS, and more traditional security designs are appropriate in such cases.
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   Prioritize communication:  The primary goal of OS is to not impede
      communication while maximizing the deployment of usable security.
      OS protocols need to be deployable incrementally, with each peer
      configured independently by its administrator or user.  With OS,
      communication is still possible even when some peers support
      encryption or authentication and others do not.

   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.  In general, communication
      should be at least encrypted.  OS should employ PFS wherever
      possible in order to protect previously recorded encrypted
      communication from decryption even after a compromise of long-term

   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

   Encryption is used to mitigate the risk of passive monitoring
   attacks, while authentication is used to mitigate the risk of active
   MiTM attacks.  When encryption capability is advertised over an
   insecure channel, MiTM downgrade attacks to cleartext may be
   possible.  Since encryption without authentication only mitigates
   passive attacks, this risk is consistent with the expected level of
   protection.  For authentication to protect against MiTM attacks, the
   peer capability advertisements that convey support for authentication
   need to be over an out-of-band authenticated channel that is itself
   resistant to MiTM attack.

   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
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   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".

   With unauthenticated, encrypted communication, OS protocols may
   employ more liberal settings than would be best practice when
   security is mandated by policy.  Some legacy systems support
   encryption, but implement only outdated algorithms or protocol
   versions.  Compatibility with these systems avoids the need to resort
   to cleartext fallback.

   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 exclude deprecated
   algorithms that would be tolerated with unauthenticated, encrypted

   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
   "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.

   The support of cleartext and the use of outdated algorithms, and
   especially broken algorithms, is for backwards compatibility with
   systems already deployed.  Protocol designs based on Opportunistic
   Security prefer to encrypt and prefer to use the best available
   encryption algorithms available.  OS protocols employ cleartext or
   broken encryption algorithms only with peers that do not appear to be
   capable of doing otherwise.  The eventual desire is to transition
   away from cleartext and broken algorithms, and particularly for
   broken algorithms, it is highly desirable to remove such
   functionality from implementations.
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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 attacks. 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 [SMTP-DANE]. That document introduces the terms "Opportunistic TLS" and "Opportunistic DANE TLS", and is consistent 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.
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   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.  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 explicit 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 explicit policy, such a non-OS policy can be counter-productive when it demands more than many peers can in fact deliver. A non-OS policy should be used with care, lest users find it too restrictive and act to disable security entirely. When protocols follow the OS approach, attackers engaged in large- scale passive monitoring can no longer just collect everything, and have to be more selective and/or mount more active attacks. In addition, OS means active attacks on everyone all the time are much more likely to be noticed. Specific techniques for detection and mitigation of active attacks in the absence of authentication are out of scope for this document. Some existing protocols that could support OS may be vulnerable to relatively low-cost downgrade attacks for attackers on the path. However, when such attacks are employed pervasively in order to facilitate, for example, surveillance, this is often detectable; hence, even in such scenarios, OS protocols provide a positive benefit. Protocols following the OS approach may need to define additional measures to make systematic downgrades less likely to succeed or more likely to be detected. When we have more experience in this space,
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   future revisions of this or related documents may be able to make
   more generally applicable recommendations.

7. References

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

7.2. Informative References

[RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, July 2009, <>. [RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an Attack", BCP 188, RFC 7258, May 2014, <>.
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              Dukhovni, V. and W. Hardaker, "SMTP security via
              opportunistic DANE TLS", Work in Progress, draft-ietf-
              dane-smtp-with-dane-13, October 2014.

              Facebook, "The Current State of SMTP STARTTLS Deployment",
              May 2014, <

              Google, "Safer email - Transparency Report - Google", June
              2014, <


I would like to thank Dave Crocker, Peter Duchovni, Paul Hoffman, Benjamin Kaduk, Steve Kent, Scott Kitterman, Pete Resnick, Martin Thomson, Nico Williams, Paul Wouters, and Stephen Farrell for their many helpful suggestions and support.

Author's Address

Viktor Dukhovni Two Sigma EMail: