Internet DRAFT - draft-iab-privsec-confidentiality-mitigations
draft-iab-privsec-confidentiality-mitigations
IAB T. Hardie, Ed.
Internet-Draft October 27, 2016
Intended status: Informational
Expires: April 30, 2017
Confidentiality in the Face of Pervasive Surveillance
draft-iab-privsec-confidentiality-mitigations-08
Abstract
The IAB has published [RFC7624] in response to several revelations of
pervasive attack on Internet communications. This document surveys
the most plausible mitigations to those threats currently available
to the designers of Internet protocols.
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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 30, 2017.
Copyright Notice
Copyright (c) 2016 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
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.
Hardie Expires April 30, 2017 [Page 1]
Internet-Draft privsec-mitigations October 2016
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Available Mitigations . . . . . . . . . . . . . . . . . . . . 3
3.1. Encryption . . . . . . . . . . . . . . . . . . . . . . . 4
3.1.1. Forward Secrecy . . . . . . . . . . . . . . . . . . . 4
3.1.2. Covert Channel Reduction . . . . . . . . . . . . . . 5
3.2. Auditing Authentication . . . . . . . . . . . . . . . . . 5
3.3. Metadata Minimization . . . . . . . . . . . . . . . . . . 6
3.3.1. Length Hiding . . . . . . . . . . . . . . . . . . . . 6
3.4. Anonymization . . . . . . . . . . . . . . . . . . . . . . 6
3.5. End-to-End Protection . . . . . . . . . . . . . . . . . . 7
4. Interplay among Mechanisms . . . . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. Contributors {Contributors} . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Normative References . . . . . . . . . . . . . . . . . . 10
8.2. Informative References . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
To ensure that the Internet can be trusted by users, it is necessary
for the Internet technical community to address the vulnerabilities
exploited in the attacks document in [RFC7258] and the threats
described in [RFC7624]. The goal of this document is to describe
more precisely the mitigations available for those threats and to lay
out the interactions among them should they be deployed in
combination.
2. Terminology
This document makes extensive use of standard security and privacy
terminology; see [RFC4949] and [RFC6973]. Terms used from [RFC6973]
include Eavesdropper, Observer, Initiator, Intermediary, Recipient,
Attack (in a privacy context), Correlation, Fingerprint, Traffic
Analysis, and Identifiability (and related terms). In addition, we
use a few terms that are specific to the attacks discussed in
[RFC7624]. Note especially that "passive" and "active" below do not
refer to the effort used to mount the attack; a "passive attack" is
any attack that accesses a flow but does not modify it, while an
"active attack" is any attack that modifies a flow. Some passive
attacks involve active interception and modifications of devices,
rather than simple access to the medium.
Hardie Expires April 30, 2017 [Page 2]
Internet-Draft privsec-mitigations October 2016
3. Available Mitigations
Given the threat model laid out in [RFC7624], how should the Internet
technical community respond to pervasive attack? The cost and risk
considerations discussed in it provide a guide to responses. Namely,
responses to passive attack should close off avenues for those
attacks that are safe, scalable, and cheap, forcing the attacker to
mount attacks that expose it to higher cost and risk. Protocols and
security measures protecting against active attacks must also limit
the impact of compromise and malfeasance by avoiding systems which
grant universal credentials.
In this section, we discuss a collection of high-level approaches to
mitigating pervasive attacks. These approaches are not meant to be
exhaustive, but rather to provide general guidance to protocol
designers in creating protocols that are resistant to pervasive
attack.
Many of these are basic tools which already exist. As Edward Snowden
put it, "properly implemented strong crypto systems are one of the
few things you can rely on". The task for the Internet community is
to ensure that applications are able to use the strong crypto and
other mitigations already available- and that these are properly
implemented and commonly turned on. Some of this work will require
architectural changes to applications, e.g., in order to limit the
information that is exposed to servers. In many other cases,
however, the need is simply to make the best use we can of the
cryptographic tools we have.
Hardie Expires April 30, 2017 [Page 3]
Internet-Draft privsec-mitigations October 2016
+--------------------------+----------------------------------------+
| Attack Class | High-level mitigations |
+--------------------------+----------------------------------------+
| Passive observation | Encryption for confidentiality |
| | |
| Passive inference | Path differentiation |
| | |
| Active | Authentication, monitoring |
| | |
| Metadata Analysis | Data Minimization |
| | |
| Static key exfiltration | Encryption with per-session state |
| | (PFS) |
| | |
| Dynamic key exfiltration | Transparency, validation of end |
| | systems |
| | |
| Content exfiltration | Object encryption, distributed systems |
+--------------------------+----------------------------------------+
Table 1: Table of Mitigations
3.1. Encryption
The traditional mitigation to passive attack is to render content
unintelligible to the attacker by applying encryption, for example,
by using TLS or IPsec [RFC5246][RFC4301]. Even without
authentication, encryption will prevent a passive attacker from being
able to read the encrypted content. Exploiting unauthenticated
encryption requires an active attack (man in the middle); with
authentication, a key exfiltration attack is required. For
cryptographic systems providing forward secrecy, even exfiltration of
long-term keys will not compromise data captured under session keys
used before the exfiltration.
3.1.1. Forward Secrecy
An encrypted, authenticated session is safe from content-monitoring
attacks in which neither end collaborates with the attacker, but can
still be subverted by the endpoints. The most common ciphersuites
used for HTTPS today, for example, are based on using RSA encryption
in such a way that if an attacker has the private key, the attacker
can derive the session keys from passive observation of a session.
These ciphersuites are thus vulnerable to a static key exfiltration
attack - if the attacker obtains the server's private key once, then
they can decrypt all past and future sessions for that server.
Hardie Expires April 30, 2017 [Page 4]
Internet-Draft privsec-mitigations October 2016
Static key exfiltration attacks are prevented by including ephemeral,
per-session secret information in the keys used for a session. Most
IETF security protocols include modes of operation that have this
property. These modes are known in the literature under the heading
"perfect forward secrecy" (PFS) because even if an adversary has all
of the secrets for one session, the next session will use new,
different secrets and the attacker will not be able to decrypt it.
The Internet Key Exchange (IKE) protocol used by IPsec supports PFS
by default [RFC4306], and TLS supports PFS via the use of specific
ciphersuites [RFC5246].
3.1.2. Covert Channel Reduction
Dynamic key exfiltration cannot be prevented by protocol means. By
definition, any secrets that are used in the protocol will be
transmitted to the attacker and used to decrypt what the protocol
encrypts. Likewise, no technical means will stop a willing
collaborator from sharing keys with an attacker. However, this
attack model also covers "unwitting collaborators", whose technical
resources are collaborating with the attacker without their owners'
knowledge. This could happen, for example, if flaws are built into
products or if malware is injected later on.
Standards can also define protocols that provide greater or lesser
opportunity for dynamic key exfiltration. Collaborators engaging in
key exfiltration through a standard protocol will need to use covert
channels in the protocol to leak information that can be used by the
attacker to recover the key. Such use of covert channels has been
demonstrated for SSL, TLS, and SSH. Any protocol bits that can be
freely set by the collaborator can be used as a covert channel,
including, for example, TCP options or unencrypted traffic sent
before a STARTTLS message in SMTP or XMPP. Protocol designers should
consider what covert channels their protocols expose, and how those
channels can be exploited to exfiltrate key information.
3.2. Auditing Authentication
As with traditional, limited active attacks, a basic mitigation to
pervasive active attack is to enable the endpoints of a communication
to authenticate each other over the encrypted channel. However,
attackers that can mount pervasive active attacks can often subvert
the authorities on which authentication systems rely.
Thus, in order to make authentication systems more resilient to
pervasive attack, it is beneficial to monitor these authorities to
detect misbehavior that could enable active attack. For example,
DANE and Certificate Transparency both provide mechanisms for
detecting when a CA has issued a certificate for a domain name
Hardie Expires April 30, 2017 [Page 5]
Internet-Draft privsec-mitigations October 2016
without the authorization of the holder of that domain name
[RFC6962][RFC6698]. Other systems may use external notaries to
detect certificate authority mismatch (e.g. Convergence
[Convergence]).
3.3. Metadata Minimization
The additional capabilities of a pervasive passive attacker, however,
require some changes in how protocol designers evaluate what
information is encrypted. In addition to directly collecting
unencrypted data, a pervasive passive attacker can also make
inferences about the content of encrypted messages based on what is
observable. For example, if a user typically visits a particular set
of web sites, then a pervasive passive attacker observing all of the
user's behavior can track the user based on the hosts the user
communicates with, even if the user changes IP addresses, and even if
all of the connections are encrypted.
Thus, in designing protocols to be resistant to pervasive passive
attacks, protocol designers should consider what information is left
unencrypted in the protocol, and how that information might be
correlated with other traffic. Some of the data left unencrypted may
be considered "metadata" within the context of a single protocol, as
it provides adjunct information used for delivery or display, rather
than the data directly created or consumed by protocol users. This
does not mean it is not useful to attackers, however, and when this
metadata is not protected by encryption it may leak substantial
amounts of information. Data minimization strategies should thus be
applied to any data left unencrypted, whether it be payload or
metadata. Information that cannot be encrypted or omited should be
be dissociated from other information. For example, the TOR overlay
routing network [TOR] anonymizes IP addresses by using multi-hop
onion routing.
3.3.1. Length Hiding
One fundamental limitation of encryption is that it exposes the size
of the plaintext that protects. A passive attacker can use this to
obtain information about the plaintext [CLINIC]. Protocols that use
encryption can provide the ability to pad plaintext. This enables
control over the size of ciphertext by endpoints, which can be used
to reduce the information available to passive attackers.
3.4. Anonymization
While encryption and authentication protect the security of
individual sessions, these sessions may still leak information, such
as IP addresses or server names, that a pervasive attacker can use to
Hardie Expires April 30, 2017 [Page 6]
Internet-Draft privsec-mitigations October 2016
correlate sessions and derive additional information about the
target. Thus, pervasive attack highlights the need for anonymization
technologies, which make correlation more difficult. Typical
approaches to anonymization against traffic analysis include:
o Aggregation: Routing sessions for many endpoints through a common
mid-point (e.g, an HTTP proxy). The midpoint appears as the
origin of the communication when traffic analysis is conducted
from points after it, so individual sources cannot be
distinguished. If traffic analysis is being conducted prior to
the mid-point, all flows appear to be destined to the same point,
which leaks very little information. Even when traffic analysis
is being performed both before and after the mid-point,
simultaneous connections may make it difficult to correlate the
traffic going into and out of the mid-point. For this to be
effective as a mitigation, traffic to the mid-point must be
encrypted and traffic from the mid-point should be.
o Onion routing: Routing a session through several mid-points,
rather than directly end-to-end, with encryption that guarantees
that each node can only see the previous and next hops. This
ensures that the source and destination of a communication are
never revealed simultaneously. Note, however, that onion routing
anonymity guarantees depend on an attacker being unable to control
many of the routing nodes [TorPaper].
o Multi-path: Routing different sessions via different paths (even
if they originate from the same endpoint). This reduces the
probability that the same attacker will be able to collect many
sessions or associate them with the same individual. If, for
example, a device has both a cellular and 802.11 interface,
routing some traffic across the cellular network and other traffic
over the 802.11 interface means that traffic analysis conducted
only with one network will be incomplete. Even if conducted in
both, it may be more difficult for the attacker to associate the
traffic in each network with the other. For this to be effective
as a mitigation, signalling protocols which gather and transmit
data about multiple interfaces (such as SIP) must be encrypted to
avoid the information being used in cross-correlation.
3.5. End-to-End Protection
Content exfiltration has some similarity to the dynamic exfiltration
case, in that nothing can prevent a collaborator from revealing what
they know, and the mitigations against becoming an unwitting
collaborator apply. In this case, however, applications can limit
what the collaborator is able to reveal. For example, the S/MIME and
PGP systems for secure email both deny intermediate servers access to
Hardie Expires April 30, 2017 [Page 7]
Internet-Draft privsec-mitigations October 2016
certain parts of the message [RFC5750][RFC2015]. Even if a server
were to provide an attacker with full access, the attacker would
still not be able to read the protected parts of the message.
Mechanisms like S/MIME and PGP are often referred to as "end-to-end"
security mechanisms, as opposed to "hop-by-hop" or "end-to-middle"
mechanisms like the use of SMTP over TLS. These two different
mechanisms address different types of attackers: Hop-by-hop
mechanisms protect from attackers on the wire (passive or active),
while end-to-end mechanisms protect against attackers within
intermediate nodes as well as those on the wire. Even end-to-end
mechanisms are not complete protection in themselves, as intermediate
nodes can still access some metadata. For example:
o Two users messaging via Facebook over HTTPS are protected against
passive and active attackers in the network between the users and
Facebook. However, if Facebook is a collaborator in an
exfiltration attack, their communications can still be monitored.
They would need to encrypt their messages end-to-end in order to
protect themselves against this risk.
o Two users exchanging PGP-protected email have protected the
content of their exchange from network attackers and intermediate
servers, but the header information (e.g., To and From addresses)
is unnecessarily exposed to passive and active attackers that can
see communications among the mail agents handling the email
messages. These mail agents need to use hop-by-hop encryption and
traffic analysis mitigation to address this risk.
Mechanisms such as S/MIME and PGP are also known as "object-based"
security mechanisms (as opposed to "communications security"
mechanisms), since they operate at the level of objects, rather than
communications sessions. Such secure object can be safely handled by
intermediaries in order to realize, for example, store and forward
messaging. In the examples above, the encrypted instant messages or
email messages would be the secure objects. Hop-to-hop security
mechanisms may be susceptible to downgrade attacks (e.g., STARTTLS-
secured SMTP has been downgraded by intermediate network nodes
[WaPo-STARTTLS]) in which case end-to-end mechanisms are advised.
The mitigations to the content exfiltration case regard participants
in the protocol as potential passive attackers themselves, and apply
the mitigations discussed above with regard to passive attack.
Information that is not necessary for these participants to fulfill
their role in the protocol can be encrypted, and other information
can be anonymized.
Hardie Expires April 30, 2017 [Page 8]
Internet-Draft privsec-mitigations October 2016
4. Interplay among Mechanisms
One of the key considerations in selecting mitigations is how to
manage the interplay among different mechanisms. Care must be taken
to avoid situations where a mitigation is rendered fruitless because
of mechanisms which working at a different time scale or with a
different aim.
The tools that we currently have have not generally been designed
with all of these mitigations in mind, so they may need elaboration
or adjustment to be completely suitable. Thus, managing the
integration of one mitigation with the environment in which it is
deployed is critical.
As an example, there is work in progress in IEEE 802 to standardize a
method for the randomization of MAC Addresses. This work aims to
enable the MAC address to vary as the device connects to different
networks, or connects at different times. In theory, the
randomization will mitigate tracking by MAC address. However, the
randomization will be defeated if the adversary can link the
randomized MAC address to other identifiers such as the interface
identifier used in IPv6 addresses, the unique identifiers used in
DHCP or DHCPv6, or unique identifiers used in various link-local
discovery protocols.
For mitigations which rely on aggregation to separate the origin of
traffic from its destination, care must be taken that the protocol
mechanics do not expose origin IP through secondary means.
[I-D.ietf-dnsop-edns-client-subnet] for example, documents a method
to carry the IP address or subnet of a querying party through a
recursive resolver to an authoritative resolver. Even with a
truncated IP address, this mechanism increases the likelihood that a
pervasive monitor would be able to associate query traffic and
responses.
If a client wished to ensure that its traffic did not expose this
data, it would need to require that its stub resolver emit any
privacy-sensitive queries with a source NETMASK set to 0, as detailed
in Section 5.1 of [I-D.ietf-dnsop-edns-client-subnet]. Given that
setting this only occasionally might also be used a signal to
observers, any client wishing to have any privacy sensitive traffic
would, in essence have to emit this for every query. While this
would succeed at providing the required privacy, given the mechanism
proposed, it would also mean no split-DNS adjustments in response
would be possible for the privacy sensitive client.
Hardie Expires April 30, 2017 [Page 9]
Internet-Draft privsec-mitigations October 2016
5. IANA Considerations
This memo makes no request of IANA.
6. Security Considerations
This memorandum describes a series of mitigations to the attacks
described in [RFC7258]. No such list could possibly be
comprehensive, nor is the attack therein described the only possible
attack.
7. Contributors {Contributors}
This document is derived in part from the work initially done on the
Perpass mailing list and at the STRINT workshop. Work from Brian
Trammell, Bruce Schneier, Christian Huitema, Cullen Jennings, Daniel
Borkmann, Martin Thomson, and Richard Barnes is incorporated here, as
are ideas and commentary from Jeff Hodges, Phillip Hallam-Baker, and
Stephen Farrell.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<http://www.rfc-editor.org/info/rfc4949>.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973,
DOI 10.17487/RFC6973, July 2013,
<http://www.rfc-editor.org/info/rfc6973>.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
2014, <http://www.rfc-editor.org/info/rfc7258>.
Hardie Expires April 30, 2017 [Page 10]
Internet-Draft privsec-mitigations October 2016
[RFC7624] Barnes, R., Schneier, B., Jennings, C., Hardie, T.,
Trammell, B., Huitema, C., and D. Borkmann,
"Confidentiality in the Face of Pervasive Surveillance: A
Threat Model and Problem Statement", RFC 7624,
DOI 10.17487/RFC7624, August 2015,
<http://www.rfc-editor.org/info/rfc7624>.
8.2. Informative References
[CLINIC] Miller, B., Huang, L., Joseph, A., and J. Tygar, "I Know
Why You Went to the Clinic: Risks and Realization of HTTPS
Traffic Analysis", March 2014.
[Convergence]
M Marlinspike, ., "Convergence Project", August 2011,
<http://convergenc.io>.
[I-D.ietf-dnsop-edns-client-subnet]
Contavalli, C., Gaast, W., tale, t., and W. Kumari,
"Client Subnet in DNS Queries", draft-ietf-dnsop-edns-
client-subnet-08 (work in progress), April 2016.
[RFC2015] Elkins, M., "MIME Security with Pretty Good Privacy
(PGP)", RFC 2015, DOI 10.17487/RFC2015, October 1996,
<http://www.rfc-editor.org/info/rfc2015>.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
December 2005, <http://www.rfc-editor.org/info/rfc4301>.
[RFC4306] Kaufman, C., Ed., "Internet Key Exchange (IKEv2)
Protocol", RFC 4306, DOI 10.17487/RFC4306, December 2005,
<http://www.rfc-editor.org/info/rfc4306>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[RFC5750] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Certificate
Handling", RFC 5750, DOI 10.17487/RFC5750, January 2010,
<http://www.rfc-editor.org/info/rfc5750>.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
2012, <http://www.rfc-editor.org/info/rfc6698>.
Hardie Expires April 30, 2017 [Page 11]
Internet-Draft privsec-mitigations October 2016
[RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
<http://www.rfc-editor.org/info/rfc6962>.
[STRINT] S Farrell, ., "Strint Workshop Report", April 2014,
<https://www.w3.org/2014/strint/draft-iab-strint-
report.html>.
[TOR] The Tor Project, "Tor", 2013,
<https://www.torproject.org/>.
[TorPaper]
Dingledine, R., Mathewson, N., and P. Syverson, "Tor: The
Second-Generation Onion Router", 2004,
<http://static.usenix.org/event/sec04/tech/full_papers/
dingledine/dingledine.pdf>.
[WaPo-STARTTLS]
Scola, N. and A. Soltani, "Mobile ISP Cricket was
thwarting encrypted emails, researchers find", 2014,
<https://www.washingtonpost.com/news/the-
switch/wp/2014/10/28/mobile-isp-thwarted-customers-
attempts-to-send-encrypted-e-mails-research-finds/>.
Author's Address
Ted Hardie (editor)
Email: ted.ietf@gmail.com
Hardie Expires April 30, 2017 [Page 12]
