Status of This Memo

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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/rfc8740.¶

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Table of Contents

1. Introduction

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.¶

2. Requirements Language

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. Post-Handshake Authentication in HTTP/2

HTTP/2 servers MUST NOT send post-handshake TLS 1.3 CertificateRequest messages. HTTP/2 clients MUST treat such messages as connection errors (see Section 5.4.1 of [RFC7540]) of type PROTOCOL_ERROR.¶

[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.¶

4. Other Post-Handshake TLS Messages 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.¶

5. Security Considerations

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.¶

6. IANA Considerations

This document has no IANA actions.¶

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