Network Working Group J. Preuss Mattsson
Internet-Draft Ericsson
Intended status: Informational May 18, 2021
Expires: November 19, 2021
Key Exchange Without Forward Secrecy is NOT RECOMMENDED
draft-mattsson-tls-psk-ke-dont-dont-dont-01
Abstract
Key exchange without forward secrecy enables passive monitoring.
Massive pervasive monitoring attacks relying on key exchange without
forward secrecy has been reported, 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 a
passive attacker to compromise past and future sessions. Entities
can get access to long-term key material in different ways: physical
attacks, hacking, social engineering attacks, espionage, or by simply
demanding access to keying material with or without a court order.
psk_ke does not provide forward secrecy and is NOT RECOMMENDED. This
document sets the IANA registration of psk_ke to NOT RECOMMENDED.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
3. References . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Normative References . . . . . . . . . . . . . . . . . . 4
3.2. Informative References . . . . . . . . . . . . . . . . . 4
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
Key exchange without forward secrecy enables passive monitoring
[RFC7258]. Massive pervasive monitoring attacks relying on key
exchange without forward secrecy has been reported [Heist]
[I-D.ietf-emu-aka-pfs], 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 a passive
attacker to compromise past and future sessions. Entities can get
access to long-term key material in different ways: physical attacks,
hacking, social engineering attacks, espionage, or by simply
demanding access to keying material with or without a court order.
All TLS 1.2 [RFC5246] cipher suites without forward secrecy has been
marked as NOT RECOMMENDED [RFC8447], and static RSA has been
forbidden in TLS 1.3 [RFC8446]. A large number of TLS profiles
forbid use of key exchange without Diffie-Hellman for TLS 1.2
[RFC7540], [ANSSI], [TS3GPP].
o ANSSI states that for all versions of TLS: "The perfect forward
secrecy property must be ensured."
o 3GPP based their general TLS 1.2 profile on [RFC7540] states:
"Only cipher suites with AEAD (e.g. GCM) and PFS (e.g. ECDHE,
DHE) shall be supported.
In addition to the very serious weaknesses of not providing
protection against key leakage and enabling passive monitoring
[RFC7258], psk_ke has other significant security problems. As stated
in [RFC8446], psk_ke does not fulfill one of the fundamental TLS 1.3
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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 [I-D.ietf-tls-external-psk-guidance], psk_ke fails yet
another one of the fundamental TLS 1.3 security properties, namely
"Secrecy of the session keys" [Akhmetzyanova19]
[I-D.ietf-tls-external-psk-guidance].
Together with ffdhe, and 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 TLS 1.3.
o BSI states regarding psk_ke that it "This mode should only be used
in special applications after consultation of an expert." and has
set a deadline of 2026 when psk_ke should not be used at all
anymore.
Unfortunately psk_ke is marked as "Recommended" in the IANA
PskKeyExchangeMode registry. This may 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 has been unwilling to disrecommend psk_ke as IETF
has so clearly marked it as "Recommended".
PSK authentication has yet another big inherent weakness as it 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.
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".
Editor's note: The current IANA action is based on the present very
limited single column in the IANA TLS registries. If more granular
classifications were possible in the future, it would likely make
sense to difference between different use cases where psk_ke might be
useful such as very constrained IoT.
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.
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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
[Akhmetzyanova19]
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>.
[ANSSI] Agence nationale de la securite des systemes
d'information, ., "Security Recommendations for TLS",
January 2017, <https://www.ssi.gouv.fr/uploads/2017/02/
security-recommendations-for-tls_v1.1.pdf>.
[BSI] Bundesamt fuer Sicherheit in der Informationstechnik, .,
"Technical Guideline TR-02102-2 Cryptographic Mechanisms:
Recommendations and Key Lengths Part 2 - Use of Transport
Layer Security (TLS)", January 2021, <https://www.bsi.bund
.de/SharedDocs/Downloads/EN/BSI/Publications/
TechGuidelines/TG02102/BSI-TR-02102-2.pdf>.
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[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., and V. Torvinen, "Perfect-Forward
Secrecy for the Extensible Authentication Protocol Method
for Authentication and Key Agreement (EAP-AKA' PFS)",
draft-ietf-emu-aka-pfs-05 (work in progress), October
2020.
[I-D.ietf-tls-external-psk-guidance]
Housley, R., Hoyland, J., Sethi, M., and C. A. Wood,
"Guidance for External PSK Usage in TLS", draft-ietf-tls-
external-psk-guidance-02 (work in progress), February
2021.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.
[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>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>.
[TS3GPP] 3GPP, ., "TS 33.210 Network Domain Security (NDS); IP
network layer security", July 2020,
<https://portal.3gpp.org/desktopmodules/Specifications/
SpecificationDetails.aspx?specificationId=2279>.
Acknowledgements
The authors want to thank Ari Keraenen for their valuable comments
and feedback.
Author's Address
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John Preuss Mattsson
Ericsson AB
SE-164 80 Stockholm
Sweden
Email: john.mattsson@ericsson.com
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