RFC 9662: Updates to the Cipher Suites in Secure Syslog

Status of This Memo

This is an Internet Standards Track document.¶

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.¶

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc9662.¶

Copyright Notice

Copyright (c) 2024 IETF Trust and the persons identified as the document authors. All rights reserved.¶

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2. 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.¶

3. Support for Updating

[RFC8447bis] generally reminds us that cryptographic algorithms and parameters will be broken or weakened over time. Blindly implementing the cryptographic algorithms listed in any specification is not advised. Implementers and users need to check that the cryptographic algorithms specified continue to provide the expected level of security.¶

As the Syslog Working Group determined, syslog clients and servers MUST use certificates as defined in [RFC5280]. Since both [RFC5425] and [RFC6012] REQUIRED the use of TLS_RSA_WITH_AES_128_CBC_SHA, it is very likely that RSA certificates have been implemented in devices adhering to those specifications. RFC 9325 [BCP195] notes that ECDHE cipher suites exist for both RSA and ECDSA certificates, so moving to an ECDHE cipher suite will not require replacing or moving away from any currently installed RSA-based certificates.¶

[DEPRECATE-KEX] documents that the cipher suite TLS_RSA_WITH_AES_128_CBC_SHA, along with some other cipher suites, may require mitigation techniques to achieve expected security, which may be difficult to effectively implement. Along those lines, RFC 9325 [BCP195] notes that TLS_RSA_WITH_AES_128_CBC_SHA does not provide forward secrecy, a feature that is highly desirable in securing event messages. That document also goes on to recommend TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 as a cipher suite that does provide forward secrecy.¶

As such, the community is moving away from algorithms that do not provide forward secrecy. For example, the International Electrotechnical Commission (IEC) has selected more robust suites such as TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256, which is also listed as a currently RECOMMENDED algorithm in [RFC8447bis] for their deployments of secure syslog.¶

Additionally, RFC 8996 [BCP195] deprecates the use of DTLS 1.0 [RFC4347], which is the mandatory-to-implement transport protocol per [RFC6012]. Therefore, that transport protocol must be updated.¶

Finally, RFC 9325 [BCP195] provides guidance on the support of TLS 1.3 [RFC8446] and DTLS 1.3 [RFC9147].¶

Therefore, to maintain interoperability across implementations, the mandatory-to-implement cipher suites listed in [RFC5425] and [RFC6012] should be updated so that implementations of secure syslog will still interoperate and provide an acceptable and expected level of security.¶

However, since there are many implementations of syslog using the cipher suites mandated by [RFC6012], a sudden change is not desirable. To accommodate a migration path, TLS_RSA_WITH_AES_128_CBC_SHA or TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 may be used, but it is REQUIRED that TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 be preferred.¶

4. Updates to RFC 5425

The mandatory-to-implement cipher suites are REQUIRED to be TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 and TLS_RSA_WITH_AES_128_CBC_SHA.¶

Implementations of [RFC5425] SHOULD offer TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 but MAY offer TLS_RSA_WITH_AES_128_CBC_SHA.¶

Implementations of [RFC5425] MUST continue to use TLS 1.2 [RFC5246] as the mandatory-to-implement transport protocol.¶

As per RFC 9325 [BCP195], implementations of [RFC5425] SHOULD support TLS 1.3 [RFC8446] and, if implemented, MUST prefer to negotiate TLS 1.3 over earlier versions of TLS.¶

5. Updates to RFC 6012

The mandatory-to-implement cipher suites are REQUIRED to be TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 and TLS_RSA_WITH_AES_128_CBC_SHA.¶

Implementations of [RFC6012] SHOULD offer TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 but MAY offer TLS_RSA_WITH_AES_128_CBC_SHA.¶

As specified in RFCs 8996 and 9325 [BCP195], implementations of [RFC6012] MUST NOT use DTLS 1.0 [RFC4347]. Implementations MUST use DTLS 1.2 [RFC6347].¶

DTLS 1.2 [RFC6347] implementations SHOULD support and prefer the mandatory-to-implement cipher suite TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256.¶

As per RFC 9325 [BCP195], implementations of [RFC6012] SHOULD support DTLS 1.3 [RFC9147] and, if implemented, MUST prefer to negotiate DTLS version 1.3 over earlier versions of DTLS.¶

6. Early Data

Early data (aka 0-RTT data) is a mechanism defined in TLS 1.3 [RFC8446] that allows a client to send data ("early data") as part of the first flight of messages to a server. Early data is permitted by TLS 1.3 when the client and server share a PSK, either obtained externally or via a previous handshake. The client uses the PSK to authenticate the server and to encrypt the early data.¶

As noted in Section 2.3 of [RFC8446bis], the security properties for early data are weaker than those for subsequent TLS-protected data. In particular, early data is not forward secret, and there are no protections against the replay of early data between connections. Appendix E.5 of [RFC8446bis] requires that applications not use early data without a profile that defines its use. Because syslog does not support replay protection (see Section 8.4 of [RFC5424]) and most implementations establish a long-lived connection, this document specifies that implementations MUST NOT use early data.¶

7. IANA Considerations

This document has no IANA actions.¶

8. Security Considerations

RFCs 8996 and 9325 [BCP195] deprecate an insecure DTLS transport protocol from [RFC6012] and deprecate insecure cipher suites from [RFC5425] and [RFC6012]. However, the installed base of syslog implementations is not easily updated to immediately adhere to those changes.¶

This document updates the mandatory-to-implement cipher suites to allow for a migration from TLS_RSA_WITH_AES_128_CBC_SHA to TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 without deprecating the former. Implementations should prefer to use TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256.¶

If a device currently only has TLS_RSA_WITH_AES_128_CBC_SHA, an administrator of the network should evaluate the conditions and determine if TLS_RSA_WITH_AES_128_CBC_SHA should be allowed so that syslog messages may continue to be delivered until the device is updated to have TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256.¶

Acknowledgments

The authors would like to thank Arijit Kumar Bose, Steffen Fries, and the members of IEC TC57 WG15 for their review, comments, and suggestions. The authors would also like to thank Tom Petch, Juergen Schoenwaelder, Hannes Tschofenig, Viktor Dukhovni, and the IESG members for their comments and constructive feedback.¶

Authors' Addresses