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Applications and Real-Time HTTP HTTP renegotiation post-handshake client authentication This document updates RFC 7540 by forbidding TLS 1.3 post-handshake authentication, as an analog to the existing TLS 1.2 renegotiation restriction.

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 .

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Table of Contents

• .  Introduction
• .  Requirements Language
• .  Post-Handshake Authentication in HTTP/2
• .  Other Post-Handshake TLS Messages in HTTP/2
• .  Security Considerations
• .  IANA Considerations
• .  References
  • .  Normative References
  • .  Informative References
• Author's Address

Introduction TLS 1.2 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 , where the server decides whether or not to request a client certificate based on the HTTP request. HTTP/2 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, forbids renegotiation. TLS 1.3 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 only applies to renegotiation. This document updates HTTP/2 to similarly forbid TLS 1.3 post-handshake authentication.

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 when, and only when, they appear in all capitals, as shown here.

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 ) of type PROTOCOL_ERROR. 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 , 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.

Other Post-Handshake TLS Messages in HTTP/2 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 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.

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.

IANA Considerations This document has no IANA actions.

References Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document specifies Version 1.2 of the Transport Layer Security (TLS) protocol. The TLS protocol provides communications security over the Internet. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. [STANDARDS-TRACK] The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document provides an overview of HTTP architecture and its associated terminology, defines the "http" and "https" Uniform Resource Identifier (URI) schemes, defines the HTTP/1.1 message syntax and parsing requirements, and describes related security concerns for implementations. This document describes a Transport Layer Security (TLS) extension for application-layer protocol negotiation within the TLS handshake. For instances in which multiple application protocols are supported on the same TCP or UDP port, this extension allows the application layer to negotiate which protocol will be used within the TLS connection. This specification describes an optimized expression of the semantics of the Hypertext Transfer Protocol (HTTP), referred to as HTTP version 2 (HTTP/2). HTTP/2 enables a more efficient use of network resources and a reduced perception of latency by introducing header field compression and allowing multiple concurrent exchanges on the same connection. It also introduces unsolicited push of representations from servers to clients. This specification is an alternative to, but does not obsolete, the HTTP/1.1 message syntax. HTTP's existing semantics remain unchanged. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. This document updates RFCs 5705 and 6066, and obsoletes RFCs 5077, 5246, and 6961. This document also specifies new requirements for TLS 1.2 implementations. Informative References Using TLS early data creates an exposure to the possibility of a replay attack. This document defines mechanisms that allow clients to communicate with servers about HTTP requests that are sent in early data. Techniques are described that use these mechanisms to mitigate the risk of replay.

Author's Address Google LLC

davidben@google.com