| Internet-Draft | psk_ke don't don't don't | May 2021 |
| Preuß Mattsson | Expires 19 November 2021 | [Page] |
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.¶
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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].¶
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 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.¶
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.¶
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.¶
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".¶
The authors want to thank Ari Keraenen for their valuable comments and feedback.¶