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
draft-dukhovni-opportunistic-security-04
Abstract
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
possible.
Status of This Memo
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This Internet-Draft will expire on February 26, 2015.
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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
attack.
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
cases.
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
communication.
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
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
[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
entirely.
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
[I-D.ietf-dane-smtp-with-dane]
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
2009.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, May 2014.
[fb-starttls]
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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/>.
Author's Address
Viktor Dukhovni
Two Sigma
Email: ietf-dane@dukhovni.org
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