This document updates RFC 7540 by forbidding TLS 1.3 post-handshake authentication, as an analog to the existing TLS 1.2 renegotiation restriction.
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TLS 1.2 [RFC5246] and earlier versions of TLS support renegotiation, a mechanism for changing parameters and keys partway through a connection. This was sometimes used to implement reactive client authentication in HTTP/1.1 [RFC7230], where the server decides whether or not to request a client certificate based on the HTTP request.
HTTP/2 [RFC7540] multiplexes multiple HTTP requests over a single connection, which is incompatible with the mechanism above. Clients cannot correlate the certificate request with the HTTP request that triggered it. Thus, Section 9.2.1 of [RFC7540] forbids renegotiation.
TLS 1.3 [RFC8446] removes renegotiation and replaces it with separate post-handshake authentication and key update mechanisms. Post-handshake authentication has the same problems with multiplexed protocols as TLS 1.2 renegotiation, but the prohibition in [RFC7540] only applies to renegotiation.
This document updates HTTP/2 [RFC7540] to similarly forbid TLS 1.3 post-handshake authentication.
[RFC7540] permitted renegotiation before the HTTP/2 connection preface to provide confidentiality of the client certificate. TLS 1.3 encrypts the client certificate in the initial handshake, so this is no longer necessary. HTTP/2 servers MUST NOT send post-handshake TLS 1.3 CertificateRequest messages before the connection preface.
The above applies even if the client offered the post_handshake_auth TLS extension. This extension is advertised independently of the selected Application-Layer Protocol Negotiation (ALPN) protocol [RFC7301], so it is not sufficient to resolve the conflict with HTTP/2. HTTP/2 clients that also offer other ALPN protocols, notably HTTP/1.1, in a TLS ClientHello MAY include the post_handshake_auth extension to support those other protocols. This does not indicate support in HTTP/2.
[RFC8446] defines two other messages that are exchanged after the handshake is complete: KeyUpdate and NewSessionTicket.
KeyUpdate messages only affect TLS itself and do not require any interaction with the application protocol. HTTP/2 implementations MUST support key updates when TLS 1.3 is negotiated.
NewSessionTicket messages are also permitted. Though these interact with HTTP when early data is enabled, these interactions are defined in [RFC8470] and are allowed for in the design of HTTP/2.
Unless the use of a new type of TLS message depends on an interaction with the application-layer protocol, that TLS message can be sent after the handshake completes.
This document resolves a compatibility concern between HTTP/2 and TLS 1.3 when supporting post-handshake authentication with HTTP/1.1. This lowers the barrier for deploying TLS 1.3, a major security improvement over TLS 1.2.
This document has no IANA actions.
7.1. Normative References
- Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels”, BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <http://www.rfc-editor.org/info/rfc2119>.
- Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2”, RFC 5246, DOI 10.17487/RFC5246, August 2008, <http://www.rfc-editor.org/info/rfc5246>.
- Fielding, R., Ed. and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing”, RFC 7230, DOI 10.17487/RFC7230, June 2014, <http://www.rfc-editor.org/info/rfc7230>.
- Friedl, S., Popov, A., Langley, A., and E. Stephan, “Transport Layer Security (TLS) Application-Layer Protocol Negotiation Extension”, RFC 7301, DOI 10.17487/RFC7301, July 2014, <http://www.rfc-editor.org/info/rfc7301>.
- Belshe, M., Peon, R., and M. Thomson, Ed., “Hypertext Transfer Protocol Version 2 (HTTP/2)”, RFC 7540, DOI 10.17487/RFC7540, May 2015, <http://www.rfc-editor.org/info/rfc7540>.
- Leiba, B., “Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words”, BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <http://www.rfc-editor.org/info/rfc8174>.
- Rescorla, E., “The Transport Layer Security (TLS) Protocol Version 1.3”, RFC 8446, DOI 10.17487/RFC8446, August 2018, <http://www.rfc-editor.org/info/rfc8446>.