Key Exchange Without Forward Secrecy is NOT RECOMMENDED
draft-mattsson-tls-psk-ke-dont-dont-dont-03
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| Author | John Preuß Mattsson | ||
| Last updated | 2023-01-08 | ||
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draft-mattsson-tls-psk-ke-dont-dont-dont-03
Network Working Group J. Preuß Mattsson
Internet-Draft Ericsson
Intended status: Informational 9 January 2023
Expires: 13 July 2023
Key Exchange Without Forward Secrecy is NOT RECOMMENDED
draft-mattsson-tls-psk-ke-dont-dont-dont-03
Abstract
Massive pervasive monitoring attacks using key exfiltration and made
possible by key exchange without forward secrecy have been reported.
If key exchange without Diffie-Hellman is used, static exfiltration
of the long-term authentication keys enables passive attackers to
compromise all past and future connections. Malicious actors can get
access to long-term keys in different ways: physical attacks,
hacking, social engineering attacks, espionage, or by simply
demanding access to keying material with or without a court order.
Exfiltration attacks are a major cybersecurity threat. The use of
psk_ke is not following zero trust principles of minimizing the
impact of breach and governments have already made deadlines for its
deprecation. This document updates the IANA PskKeyExchangeMode
registry by setting the "Recommended" value for psk_ke to "N".
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-mattsson-tls-psk-ke-dont-dont-
dont/.
Discussion of this document takes place on the Transport Layer
Security (tls) Working Group mailing list (mailto:tls@ietf.org),
which is archived at https://mailarchive.ietf.org/arch/browse/tls/.
Subscribe at https://www.ietf.org/mailman/listinfo/tls/.
Source for this draft and an issue tracker can be found at
https://github.com/emanjon/draft-mattsson-tls-psk-ke-dont-dont-dont.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 13 July 2023.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
2. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
3. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Normative References . . . . . . . . . . . . . . . . . . 6
3.2. Informative References . . . . . . . . . . . . . . . . . 6
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
Key exchange without forward secrecy enables passive monitoring
[RFC7258]. Massive pervasive monitoring attacks using key
exfiltration and made possible by key exchange without forward
secrecy have been reported [Heist], and many more have likely
happened without ever being reported. If key exchange without
Diffie-Hellman is used, access to the long-term authentication keys
enables passive attackers to compromise all past and future
connections. Malicious actors can get access to long-term keys in
different ways: physical attacks, hacking, social engineering
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attacks, espionage, or by simply demanding access to keying material
with or without a court order. Exfiltration attacks are a major
cybersecurity threat [Exfiltration].
All cipher suites without forward secrecy have been marked as NOT
RECOMMENDED in TLS 1.2 [RFC8447], and static RSA and DH are forbidden
in TLS 1.3 [RFC8446]. A large number of TLS profiles and
implementations forbid the use of key exchange without Diffie-
Hellman.
* ANSSI states that for all versions of TLS: "The perfect forward
secrecy property must be ensured" [ANSSI-TLS].
* The general 3GPP TLS 1.2 profile follows [RFC9113] and states:
"Only cipher suites with AEAD (e.g., GCM) and PFS (e.g. ECDHE,
DHE) shall be supported" [T3GPP.33.210].
* BoringSSL does not implement psk_ke, so TLS 1.3 resumption always
incorporates fresh key material [BoringSSL].
Unfortunately, TLS 1.3 allows key exchange without forward secrecy in
both full handshakes and resumption handshakes with the psk_ke. As
stated in [RFC8446], psk_ke does not fulfill one of the fundamental
TLS 1.3 security properties, namely "Forward secret with respect to
long-term keys". When the PSK is a group key, which is now formally
allowed in TLS 1.3 [RFC9257], psk_ke fails yet another one of the
fundamental TLS 1.3 security properties, namely "Secrecy of the
session keys" [Akhmetzyanova] [RFC9257]. PSK authentication has yet
another big inherent weakness as it often does not provide
"Protection of endpoint identities". It could be argued that PSK
authentication should be not recommended solely based on this
significant privacy weakness. The 3GPP radio access network that to
a large degree relies on PSK are fixing the vulnerabilities by
augmenting PSK with ECIES and ECDHE, see Annex C of [T3GPP.33.501]
and [I-D.ietf-emu-aka-pfs].
Together with rsa_pkcs1, psk_ke is one of the bad apples in the TLS
1.3 fruit basket. Organizations like BSI [BSI] has already produced
recommendations regarding its deprecation.
* BSI states regarding psk_ke that "This mode should only be used in
special applications after consultation of an expert." and has set
a deadline that use is only allowed until 2026.
Two essential zero trust principles are to assume that breach is
inevitable or has likely already occurred [NSA-ZT], and to minimize
impact when breach occur [NIST-ZT]. One type of breach is key
compromise or key exfiltration. Different types of exfiltration are
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defined and discussed in [RFC7624]. Static exfiltration where the
keys are transferred once has a lower risk profile than dynamic
exfiltration where keying material or content is transferred to the
attacker frequently. Forcing an attacker to do dynamic exfiltration
should be considered best practice. This significantly increases the
risk of discovery for the attacker.
One way to force an attacker to do dynamic exfiltration is to
frequently rerun ephemeral Diffie-Hellman. For IPsec, ANSSI
[ANSSI-PFS] recommends enforcing periodic rekeying with ephemeral
Diffie-Hellman, e.g., every hour and every 100 GB of data, in order
to limit the impact of a key compromise. This should be considered
best practice for all protocols and systems. The Double Ratchet
Algorithm in the Signal protocol [Signal] enables very frequent use
of ephemeral Diffie-Hellman. The practice of frequently rerunning
ephemeral Diffie-Hellman follows directly from the two zero trust
principles mentioned above.
In TLS 1.3, the application_traffic_secret can be rekeyed using
key_update, a resumption handshake, or a full handshake. The term
forward secrecy is not very specific, and it is often better to talk
about the property that compromise of key A does not lead to
compromise of key B. Figure 1 illustrates the impact of some
examples of static key exfiltration when psk_ke, key_update, and
(ec)dhe are used for rekeying. Each time period Tᵢ uses a single
application_traffic_secret. ✘ means that the attacker has access to
the application_traffic_secret in that time period and can passively
eavesdrop on the communication. ✔ means that the attacker does not
have access to the application_traffic_secret. Exfiltration and
frequently rerunning EC(DHE) is discussed in Appendix F of
[I-D.ietf-tls-rfc8446bis].
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rekeying with psk_ke
static exfiltration of psk in T₃:
+-----+-----+-----+-----+-----+-----+-----+-----+ +-----+-----+
| ✘ | ✘ | ✘ | ✘ | ✘ | ✘ | ✘ | ✘ | ... | ✘ | ✘ |
+-----+-----+-----+-----+-----+-----+-----+-----+ +-----+-----+
T₀ T₁ T₂ T₃ T₄ T₅ T₆ T₇ ... Tₙ₋₁ Tₙ
<--------------------------------------------------------------->
rekeying with key_update
static exfiltration of application_traffic_secret in T₃:
+-----+-----+-----+-----+-----+-----+-----+-----+ +-----+-----+
| ✔ | ✔ | ✔ | ✘ | ✘ | ✘ | ✘ | ✘ | ... | ✘ | ✘ |
+-----+-----+-----+-----+-----+-----+-----+-----+ +-----+-----+
T₀ T₁ T₂ T₃ T₄ T₅ T₆ T₇ ... Tₙ₋₁ Tₙ
<--------------------------------------------->
rekeying with (ec)dhe
static exfiltration of all keys in T₃:
+-----+-----+-----+-----+-----+-----+-----+-----+ +-----+-----+
| ✔ | ✔ | ✔ | ✘ | ✔ | ✔ | ✔ | ✔ | ... | ✔ | ✔ |
+-----+-----+-----+-----+-----+-----+-----+-----+ +-----+-----+
T₀ T₁ T₂ T₃ T₄ T₅ T₆ T₇ ... Tₙ₋₁ Tₙ
<--->
Figure 1: Impact of static key exfiltration in time period T3
when psk_ke, key_update, and (ec)dhe are used.
Modern ephemeral key exchange algorithms like x25519 [RFC7748] are
very fast and have small message overhead. The public keys are 32
bytes long and the cryptographic operations take 53 microseconds per
endpoint on a single core AMD Ryzen 5 5560U [eBACS-DH]. Ephemeral
key exchange with the quantum-resistant algorithm Kyber that NIST
will standardize is even faster. For the current non-standardized
version of Kyber512 the cryptographic operations take 12 microseconds
for the client and 8 microseconds for the server [eBACS-KEM].
Unfortunately, psk_ke is marked as "Recommended" in the IANA
PskKeyExchangeMode registry. This may severely weaken security in
deployments following the "Recommended" column. Introducing TLS 1.3
in 3GPP had the unfortunate and surprising effect of drastically
lowering the minimum security when TLS is used with PSK
authentication. Some companies in 3GPP have been unwilling to mark
psk_ke as not recommended as it is so clearly marked as "Recommended"
by the IETF. By labeling psk_ke as "Recommended", IETF is
legitimizing and implicitly promoting bad security practice.
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This document updates the PskKeyExchangeMode registry under the
Transport Layer Security (TLS) Parameters heading. For psk_ke the
"Recommended" value has been set to "N".
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. IANA Considerations
IANA is requested to update the PskKeyExchangeMode registry under the
Transport Layer Security (TLS) Parameters heading. For psk_ke the
"Recommended" value has been set to "N".
3. References
3.1. 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,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[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>.
[RFC8447] Salowey, J. and S. Turner, "IANA Registry Updates for TLS
and DTLS", RFC 8447, DOI 10.17487/RFC8447, August 2018,
<https://www.rfc-editor.org/info/rfc8447>.
3.2. Informative References
[Akhmetzyanova]
Akhmetzyanova, L., Alekseev, E., Smyshlyaeva, E., and A.
Sokolov, "Continuing to reflect on TLS 1.3 with external
PSK", April 2019, <https://eprint.iacr.org/2019/421.pdf>.
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[ANSSI-PFS]
Agence nationale de la sécurité des systèmes
d'information, "Recommendations for securing networks with
IPsec", August 2015,
<https://www.ssi.gouv.fr/uploads/2015/09/NT_IPsec_EN.pdf>.
[ANSSI-TLS]
Agence nationale de la sécurité des systèmes
d'information, "Security Recommendations for TLS", January
2017, <https://www.ssi.gouv.fr/uploads/2017/02/security-
recommendations-for-tls_v1.1.pdf>.
[BoringSSL]
Google, "BoringSSL", January 2023,
<https://boringssl.googlesource.com/boringssl/>.
[BSI] Bundesamt für Sicherheit in der Informationstechnik,
"Technical Guideline TR-02102-2 Cryptographic Mechanisms:
Recommendations and Key Lengths Part 2 – Use of Transport
Layer Security (TLS)", February 2022, <https://www.bsi.bun
d.de/SharedDocs/Downloads/EN/BSI/Publications/
TechGuidelines/TG02102/BSI-TR-02102-2.pdf>.
[eBACS-DH] eBACS: ECRYPT Benchmarking of Cryptographic Systems,
"Measurements of public-key Diffie–Hellman secret-sharing
systems, indexed by machine", January 2023,
<https://bench.cr.yp.to/results-dh.html>.
[eBACS-KEM]
eBACS: ECRYPT Benchmarking of Cryptographic Systems,
"Measurements of key-encapsulation mechanisms, indexed by
machine", January 2023,
<https://bench.cr.yp.to/results-kem.html>.
[Exfiltration]
APNIC, "How to: Detect and prevent common data
exfiltration attacks", March 2022,
<https://blog.apnic.net/2022/03/31/how-to-detect-and-
prevent-common-data-exfiltration-attacks/>.
[Heist] The Intercept, "How Spies Stole the Keys to the Encryption
Castle", February 2015,
<https://theintercept.com/2015/02/19/great-sim-heist/>.
[I-D.ietf-emu-aka-pfs]
Arkko, J., Norrman, K., Torvinen, V., and J. P. Mattsson,
"Forward Secrecy for the Extensible Authentication
Protocol Method for Authentication and Key Agreement (EAP-
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AKA' FS)", Work in Progress, Internet-Draft, draft-ietf-
emu-aka-pfs-08, 23 October 2022,
<https://www.ietf.org/archive/id/draft-ietf-emu-aka-pfs-
08.txt>.
[I-D.ietf-tls-rfc8446bis]
Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", Work in Progress, Internet-Draft, draft-
ietf-tls-rfc8446bis-05, 24 October 2022,
<https://www.ietf.org/archive/id/draft-ietf-tls-
rfc8446bis-05.txt>.
[NIST-ZT] National Institute of Standards and Technology,
"Implementing a Zero Trust Architecture", December 2022,
<https://www.nccoe.nist.gov/sites/default/files/2022-12/
zta-nist-sp-1800-35b-preliminary-draft-2.pdf>.
[NSA-ZT] National Security Agency, "Embracing a Zero Trust Security
Model", February 2021, <https://media.defense.gov/2021/
Feb/25/2002588479/-1/-1/0/
CSI_EMBRACING_ZT_SECURITY_MODEL_UOO115131-21.PDF>.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
2014, <https://www.rfc-editor.org/info/rfc7258>.
[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,
<https://www.rfc-editor.org/info/rfc7624>.
[RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
for Security", RFC 7748, DOI 10.17487/RFC7748, January
2016, <https://www.rfc-editor.org/info/rfc7748>.
[RFC9113] Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113,
DOI 10.17487/RFC9113, June 2022,
<https://www.rfc-editor.org/info/rfc9113>.
[RFC9257] Housley, R., Hoyland, J., Sethi, M., and C. A. Wood,
"Guidance for External Pre-Shared Key (PSK) Usage in TLS",
RFC 9257, DOI 10.17487/RFC9257, July 2022,
<https://www.rfc-editor.org/info/rfc9257>.
[Signal] Signal, "The Double Ratchet Algorithm", November 2016,
<https://signal.org/docs/specifications/doubleratchet/>.
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[T3GPP.33.210]
3GPP, "TS 33.210 Network Domain Security (NDS); IP network
layer security", September 2022,
<https://portal.3gpp.org/desktopmodules/Specifications/
SpecificationDetails.aspx?specificationId=2279>.
[T3GPP.33.501]
3GPP, "TS 33.501 Security architecture and procedures for
5G System", September 2022,
<https://portal.3gpp.org/desktopmodules/Specifications/
SpecificationDetails.aspx?specificationId=3169>.
Acknowledgements
The authors want to thank Ari Keränen, Eric Rescorla, and Paul
Wouters for their valuable comments and feedback.
Author's Address
John Preuß Mattsson
Ericsson AB
SE-164 80 Stockholm
Sweden
Email: john.mattsson@ericsson.com
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