Internet Engineering Task Force (IETF) V. Dukhovni

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

Abstract

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
http://www.rfc-editor.org/info/rfc7435.

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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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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
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 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
opportunistically.)

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
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
communication.

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
communication.

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,
<http://www.rfc-editor.org/info/rfc3207>.

[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", RFC
4033, March 2005,
<http://www.rfc-editor.org/info/rfc4033>.

[RFC4251] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Protocol Architecture", RFC 4251, January 2006,
<http://www.rfc-editor.org/info/rfc4251>.

[RFC4949] Shirey, R., "Internet Security Glossary, Version 2", RFC
4949, August 2007,
<http://www.rfc-editor.org/info/rfc4949>.

[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.

[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008, <http://www.rfc-editor.org/info/rfc5321>.

[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, August 2012,
<http://www.rfc-editor.org/info/rfc6698>.

7.2. Informative References

[RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, July
2009, <http://www.rfc-editor.org/info/rfc5598>.

[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, May 2014,
<http://www.rfc-editor.org/info/rfc7258>.

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[SMTP-DANE]
Dukhovni, V. and W. Hardaker, "SMTP security via
opportunistic DANE TLS", Work in Progress, draft-ietf-
dane-smtp-with-dane-13, October 2014.

[fb-starttls]
Facebook, "The Current State of SMTP STARTTLS Deployment",
May 2014, <https://www.facebook.com/notes/protect-the-
graph/the-current-state-of-smtp-starttls-deployment/
1453015901605223>.

[goog-starttls]
Google, "Safer email - Transparency Report - Google", June
2014, <https://www.google.com/transparencyreport/
saferemail/>.

Acknowledgements

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: [email protected]

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