Internet DRAFT - draft-arkko-farrell-arch-model-t-3552-additions
draft-arkko-farrell-arch-model-t-3552-additions
Network Working Group J. Arkko
Internet-Draft Ericsson
Intended status: Informational S. Farrell
Expires: August 26, 2021 Trinity College Dublin
February 22, 2021
RFC 3552 additions due to evolving Internet thread model
draft-arkko-farrell-arch-model-t-3552-additions-01
Abstract
Communications security has been at the center of many security
improvements in the Internet. The goal has been to ensure that
communications are protected against outside observers and attackers.
This memo suggests additions to the RFC 3552 threat model to cater
for the evolving security and privacy issues seen on the Internet
today. For instance, it is often also necessary to protect against
endpoints that are compromised, malicious, or whose interests simply
do not align with the interests of users. While such protection is
difficult, there are some measures that can be taken and we argue
that investigation of these issues is warranted.
Status of This Memo
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This Internet-Draft will expire on August 26, 2021.
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. The range of potential changes in BCP 72/RFC 3552 . . . . . . 3
3. Simple change . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Change to add discussion of compromises . . . . . . . . . . . 4
5. Change to add guidance with regards to communications
security . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5.1. Limiting time scope of compromise . . . . . . . . . . . . 5
5.2. Forcing active attack . . . . . . . . . . . . . . . . . . 6
5.3. Traffic analysis . . . . . . . . . . . . . . . . . . . . 6
5.4. Containing compromise of trust points . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . 8
Appendix A. Contributors . . . . . . . . . . . . . . . . . . . . 9
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
Communications security has been at the center of many security
improvements in the Internet. The goal has been to ensure that
communications are protected against outside observers and attackers.
At the IETF, this approach has been formalized in BCP 72 [RFC3552],
which defined the Internet threat model in 2003.
The purpose of a threat model is to outline what threats exist in
order to assist the protocol designer. But RFC 3552 also ruled some
threats to be in scope and of primary interest, and some threats out
of scope [RFC3552]:
The Internet environment has a fairly well understood threat
model. In general, we assume that the end-systems engaging in a
protocol exchange have not themselves been compromised.
Protecting against an attack when one of the end-systems has been
compromised is extraordinarily difficult. It is, however,
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possible to design protocols which minimize the extent of the
damage done under these circumstances.
By contrast, we assume that the attacker has nearly complete
control of the communications channel over which the end-systems
communicate. This means that the attacker can read any PDU
(Protocol Data Unit) on the network and undetectably remove,
change, or inject forged packets onto the wire.
However, the communications-security -only threat model may not
entirely match today's reality, as discussed in
[I-D.arkko-farrell-arch-model-t-redux].
The rest of this document tentatively suggests some changes to the
BCP. Section 2 discussses the range of potential changes, and
Section 3, Section 4, and Section 5 present the suggested alternative
changes, in increasing amount of detail.
Comments are solicited on this document. The best place for
discussion is on the model-t list.
(https://www.ietf.org/mailman/listinfo/model-t)
2. The range of potential changes in BCP 72/RFC 3552
BCP 72/RFC 3553 [RFC3552] defines an "Internet Threat Model" and
provides guidance on writing Security Considerations sections in
other RFCs.
[RFC3552] also provided a description of classic issues for the
development of communications security protocols. However, in the
nearly 20 years since the publication of RFC 3552, the practice of
protocol design has moved on to a fair extent.
It is important to note that BCP 72 is (or should be:-) used by all
IETF participants when developing protocols. Potential changes to
RFC 3552 therefore need to be brief - IETF participants cannot in
general be expected to devote huge amounts of time to developing
their security considerations text. Potential changes also need to
be easily understood as IETF participants from all backgrounds need
to be able to use BCP 72.
In this section we provide a few initial suggested changes to BCP 72
that will need to be further developed as part of this work.
There are a range of possible updates. We could propose adding a
simple observation (Section 3), or additionally propose further
discussion about endpoint compromises and the need for system-level
security analysis (Section 4).
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Another possibility would be to add more guidance covering areas of
concern, and recommendations of broadly-applicable techniques to use.
One suggestion (due to others) for such material is provided in
Section 5.
The authors of this memo believe that any updates to RFC 3552 should
be relatively high-level and short. Additional documents may be
needed to provide further detail.
3. Simple change
This is the simple addition we are suggesting. As evidenced in the
OAuth quote in [I-D.arkko-farrell-arch-model-t] Section 4 - it can be
useful to consider the effect of compromised endpoints on those that
are not compromised. It may therefore be interesting to consider the
consequences that would follow from a change to [RFC3552] that
recognises how the landscape has changed since 2003.
One initial, draft proposal for such a change could be:
OLD:
In general, we assume that the end-systems engaging in a protocol
exchange have not themselves been compromised. Protecting against
an attack when one of the end-systems has been compromised is
extraordinarily difficult. It is, however, possible to design
protocols which minimize the extent of the damage done under these
circumstances.
NEW:
In general, we assume that the end-system engaging in a protocol
exchange has not itself been compromised. Protecting against an
attack of a protocol implementation itself is extraordinarily
difficult. It is, however, possible to design protocols which
minimize the extent of the damage done when the other parties in a
protocol become compromised or do not act in the best interests
the end-system implementing a protocol.
4. Change to add discussion of compromises
The following new section could be added to discuss the capabilities
required to mount an attack:
NEW:
3.x. Other endpoint compromise
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In this attack, the other endpoints in the protocol become
compromised. As a result, they can, for instance, misuse any
information that the end-system implementing a protocol has sent
to the compromised endpoint.
System and architecture aspects definitely also need more attention
from Internet technology developers and standards organizations.
Here is one possible addition:
NEW:
The design of any Internet technology should start from an
understanding of the participants in a system, their roles, and
the extent to which they should have access to information and
ability to control other participants.
5. Change to add guidance with regards to communications security
Finally, the following new text could be added to cover some of the
aspects that should be considered when designing a communications
security protocol that are not covered in detail in RFC 3552.
5.1. Limiting time scope of compromise
[RFC3552] Section 3 says:
The Internet environment has a fairly well understood threat
model. In general, we assume that the end-systems engaging in a
protocol exchange have not themselves been compromised.
Protecting against an attack when one of the end-systems has been
compromised is extraordinarily difficult. It is, however,
possible to design protocols which minimize the extent of the
damage done under these circumstances.
Although this text is technically correct, modern protocol designs
such as TLS 1.3 and MLS often try to provide a fair amount of defense
against various kinds of temporary compromise. Specifically:
NEW:
Forward Security: Many protocols are designed so that compromise
of an endpoint at time T does not lead to compromise of data
transmitted prior to some time T' < T. For instance, if a
protocol is based on Diffie-Hellman key establishment, then
compromise of the long-term keys does not lead to compromise of
traffic sent prior to compromise if the DH ephemerals and traffic
keys have been deleted.
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Post-Compromise Security: Conversely, if an endpoint is
compromised at time T, it is often desirable to have the protocol
"self-heal" so that a purely passive adversary cannot access
traffic after a certain time T' > T. MLS, for instance, is
designed with this property.
Containing Partial Authentication Key Compromise: If an endpoint
is stolen and its authentication secret is stolen, then an
attacker can impersonate that endpoint. However, there a number
of scenarios in which an attacker can obtain use of an
authentication key but not the secret itself (see, for instance
[Jager2015]). It is often desirable to limit the impact of such
compromises (for instance, by avoiding unlimited delegation from
such keys).
Short-lived keys: Typical TLS certificates last for months or
years. There is a trend towards shorter certificate lifetimes so
as to minimize risk of exposure in the event of key compromise.
Relatedly, delegated credentials are short-lived keys the
certificate's owner has delegated for use in TLS. These help
reduce private key lifetimes without compromising or sacrificing
reliability.
5.2. Forcing active attack
[RFC3552] Section 3.2 notes that it is important to consider passive
attacks. This is still valid, but needs further elaboration:
NEW:
In general, it is much harder to mount an active attack, both in
terms of the capabilities required and the chance of being
detected. A theme in recent IETF protocols design is to build
systems which might have limited defense against active attackers
but are strong against passive attackers, thus forcing the
attacker to go active.
Examples include DTLS-SRTP and the trend towards opportunistic
security. However, ideally protocols are built with strong defenses
against active attackers. One prominent example is QUIC, which takes
steps to ensure that off-path connection resets are intractable in
practice.
5.3. Traffic analysis
[RFC3552] Section 3.2.1 describes how the absence of TLS or other
transport-layer encryption may lead to obvious confidentiality
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violations against passive attackers. This too is still valid, but
does not take into account additional aspects:
NEW:
However, recent trends in traffic analysis indicate encryption
alone may be insufficient protection for some types of application
data [I-D.wood-pearg-website-fingerprinting]. Encrypted traffic
metadata, especially message size, can leak information about the
underlying plaintext. DNS queries and responses are particularly
at risk given their size distributions. Recent protocols account
for this leakage by supporting padding.
Some examples of recent work in this area include support for padding
either generically in the transport protocol (QUIC
[I-D.ietf-quic-transport] and TLS [RFC8446]), or specifically in the
application protocol (EDNS(0) padding option for DNS messages
[RFC7830]).
5.4. Containing compromise of trust points
Many protocols are designed to depend on trusted third parties (the
WebPKI is perhaps the canonical example); if those trust points
misbehave, the security of the protocol can be completely
compromised.
Some additional guidance in RFC 3552 might be needed to remind
protocol readers of this.
NEW:
A number of recent protocols have attempted to reduce the power of
trust points that the protocol or application depends on. For
instance, Certificate Transparency attempts to ensure that a CA
cannot issue valid certificates without publishing them, allowing
third parties to detect certain classes of misbehavior by those
CA. Similarly, Key Transparency attempts to ensure that (public)
keys associated with a given entity are publicly visible and
auditable in tamper-proof logs. This allows users of these keys
to check them for correctness.
In the realm of software, Reproducible Builds and Binary Transparency
are intended to allow a user to determine that they have received a
valid copy of the binary that matches the auditable source code.
Blockchain protocols such as Bitcoin and Ethereum also employ this
principle of transparency and are intended to detect misbehavior by
members of the network.
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6. Security Considerations
The entire memo covers the security considerations.
7. IANA Considerations
There are no IANA considerations in this work.
8. References
8.1. Normative References
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
DOI 10.17487/RFC3552, July 2003,
<https://www.rfc-editor.org/info/rfc3552>.
8.2. Informative References
[I-D.arkko-farrell-arch-model-t]
Arkko, J. and S. Farrell, "Challenges and Changes in the
Internet Threat Model", draft-arkko-farrell-arch-model-
t-04 (work in progress), July 2020.
[I-D.arkko-farrell-arch-model-t-redux]
Arkko, J. and S. Farrell, "Internet Threat Model
Evolution: Background and Principles", draft-arkko-
farrell-arch-model-t-redux-00 (work in progress), November
2020.
[I-D.ietf-quic-transport]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-34 (work
in progress), January 2021.
[I-D.wood-pearg-website-fingerprinting]
Goldberg, I., Wang, T., and C. Wood, "Network-Based
Website Fingerprinting", draft-wood-pearg-website-
fingerprinting-00 (work in progress), November 2019.
[Jager2015]
Jager, T., Schwenk, J., and J. Somorovsky, "On the
Security of TLS 1.3 and QUIC Against Weaknesses in PKCS#1
v1.5 Encryption", Proceedings of ACM CCS 2015, DOI
10.1145/2810103.2813657, https://www.nds.rub.de/media/nds/
veroeffentlichungen/2015/08/21/Tls13QuicAttacks.pdf ,
October 2015.
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[RFC7830] Mayrhofer, A., "The EDNS(0) Padding Option", RFC 7830,
DOI 10.17487/RFC7830, May 2016,
<https://www.rfc-editor.org/info/rfc7830>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
Appendix A. Contributors
Eric Rescorla and Chris Wood provided much of the text in Section 5.
Appendix B. Acknowledgements
The authors would like to thank the IAB:
Alissa Cooper, Wes Hardaker, Ted Hardie, Christian Huitema, Zhenbin
Li, Erik Nordmark, Mark Nottingham, Melinda Shore, Jeff Tantsura,
Martin Thomson, Brian Trammel, Mirja Kuhlewind, and Colin Perkins.
The authors would also like to thank the participants of the IETF
SAAG meeting where this topic was discussed:
Harald Alvestrand, Roman Danyliw, Daniel Kahn Gilmore, Wes Hardaker,
Bret Jordan, Ben Kaduk, Dominique Lazanski, Eliot Lear, Lawrence
Lundblade, Kathleen Moriarty, Kirsty Paine, Eric Rescorla, Ali
Rezaki, Mohit Sethi, Ben Schwartz, Dave Thaler, Paul Turner, David
Waltemire, and Jeffrey Yaskin.
The authors would also like to thank the participants of the IAB 2019
DEDR workshop:
Tuomas Aura, Vittorio Bertola, Carsten Bormann, Stephane Bortzmeyer,
Alissa Cooper, Hannu Flinck, Carl Gahnberg, Phillip Hallam-Baker, Ted
Hardie, Paul Hoffman, Christian Huitema, Geoff Huston, Konstantinos
Komaitis, Mirja Kuhlewind, Dirk Kutscher, Zhenbin Li, Julien
Maisonneuve, John Mattson, Moritz Muller, Joerg Ott, Lucas Pardue,
Jim Reid, Jan-Frederik Rieckers, Mohit Sethi, Melinda Shore, Jonne
Soininen, Andrew Sullivan, and Brian Trammell.
The authors would also like to thank the participants of the November
2016 meeting at the IETF:
Carsten Bormann, Randy Bush, Tommy C, Roman Danyliw, Ted Hardie,
Christian Huitema, Ben Kaduk, Dirk Kutscher, Dominique Lazanski, Eric
Rescorla, Ali Rezaki, Mohit Sethi, Melinda Shore, Martin Thomson, and
Robin Wilton
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Finally, the authors would like to thank numerous other people for
insightful comments and discussions in this space.
Authors' Addresses
Jari Arkko
Ericsson
Valitie 1B
Kauniainen
Finland
Email: jari.arkko@piuha.net
Stephen Farrell
Trinity College Dublin
College Green
Dublin
Ireland
Email: stephen.farrell@cs.tcd.ie
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