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 athttps://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-handshakeauthentication.¶

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 serversMUST NOT send post-handshake TLS 1.3 CertificateRequest messages.HTTP/2 clientsMUST treat such messages as connection errors (seeSection 5.4.1 of [RFC7540]) of type PROTOCOL_ERROR.¶

[RFC7540] permitted renegotiation before the HTTP/2 connection preface toprovide confidentiality of the client certificate. TLS 1.3 encrypts the clientcertificate in the initial handshake, so this is no longer necessary. HTTP/2serversMUST NOT send post-handshake TLS 1.3 CertificateRequest messages beforethe connection preface.¶

The above applies even if the client offered thepost_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 ClientHelloMAY include thepost_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 iscomplete: KeyUpdate and NewSessionTicket.¶

KeyUpdate messages only affect TLS itself and do not require any interactionwith the application protocol. HTTP/2 implementationsMUST support key updateswhen 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 whensupporting post-handshake authentication with HTTP/1.1. This lowers the barrierfor deploying TLS 1.3, a major security improvement over TLS 1.2.¶

6.IANA Considerations

This document has no IANA actions.¶

Author's Address