HTTP M. Bishop
Internet-Draft Microsoft
Intended status: Standards Track March 15, 2016
Expires: September 16, 2016

Secondary Server-Certificate Authentication in HTTP/2


Many HTTP servers host content from several origins. HTTP/2 [RFC7540] permits clients to reuse an existing HTTP connection to a server provided that certain conditions are satisfied. One of these conditions is the inclusion of the secondary origin in the certificate provided during the TLS [I-D.ietf-tls-tls13] handshake.

In many cases, origins will wish to maintain separate certificates for different origins but still desire the benefits of a shared HTTP connection. This draft describes how frames which were defined to transfer client certificates might be used to provide additional server certificates as well.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on September 16, 2016.

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Table of Contents

1. Introduction

Section 9.1.1 of [RFC7540] describes how connections may be reused as long as the server is authoritative for both origins. A server is considered authoritative for both origins if DNS resolves both origins to the IP address of the server and (for TLS) if the certificate presented by the server contains both origins, either as the Subject or contained in the Subject Alternative Names field.

[I-D.ietf-httpbis-alt-svc] enables a step of abstraction from the DNS resolution. If both hosts have provided an Alternative Service at hostnames which resolve to the IP address of the server, they are considered authoritative just as if DNS resolved the origin itself to that address.

The ORIGIN extension frame, defined in [I-D.nottingham-httpbis-origin-frame], provides a negative coalescing feature – a way for a server to request that a client not reuse a connection for an origin, even when it might otherwise appear to be supported. However, the ORIGIN frame does not currently permit a server to advertise the availability of origins which do not appear in the server’s certificate as presented in the TLS handshake.

Servers which host many origins often would prefer to have separate certificates for some sets of origins. This may be for ease of certificate management (the ability to separately revoke or renew them), for legal reasons (a CDN acting on behalf of multiple origins), or any other factor which might drive this administrative decision. Clients connecting to such origins cannot currently reuse connections, even if both client and server would be willing to do so.

[I-D.thomson-http2-client-certs] defines certificate-related HTTP/2 frames, permitting a sender to offer a certificate chain along with proof that it possesses the corresponding private key to the end certificate. These frames are bound to the underlying TLS session, so that the certificates are as reliable as those provided at the TLS layer.

In this document, a mechanism for using these frames for secondary server authentication via HTTP/2 frames is defined. This mechanism can be implemented at the HTTP layer without requiring new TLS stack behavior and without breaking the existing interface between HTTP and applications above it. It primarily relaxes the one-way nature of the frames defined in [I-D.thomson-http2-client-certs], defining the processing of these frames in the reverse direction.

1.1. Origin Discovery

1.1.1. Client-driven discovery

As defined in [RFC7540], when a client finds that a https:// origin (or Alternative Service [I-D.ietf-httpbis-alt-svc]) to which it needs to make a request has the same IP address as a server to which it is already connected, it MAY check whether the TLS certificate provided contains the new origin as well, and if so, reuse the connection.

If not, but the server has advertised support for HTTP-layer certificates, the client MAY also send a CERTIFICATE_REQUEST frame Section 2.1 on stream zero requesting a certificate for the desired origin. The server responds with a series of CERTIFICATE frames containing the relevant certificate chain, if it possesses such a certificate. If not, the server responds with an empty CERTIFICATE frame.

1.1.2. Server-driven discovery

Because the approach in Section 1.1.1 requires an extra round-trip to the server before the client can determine whether a new TCP connection will be required, some origins will wish to proactively alert clients to certificates they possess. Servers might also wish to proactively prove their authority for an origin for which it intends to deliver pushed resources.

The server MAY send an ORIGIN frame including origins which are not in its TLS certificate. This represents an explicit claim by the server to possess the appropriate certificate – a claim the client MUST verify using the procedures in Section 1.1.1 before relying on the server’s authority for the claimed origin.

The server might push resources from an origin for which it is authoritative but for which the client has not received the certificate. In this case, the client SHOULD verify the server’s possession of an appropriate certificate by sending a CERTIFICATE_REQUIRED frame on the pushed stream and a CERTIFICATE_REQUEST on stream zero. The client MUST NOT use the pushed resource until an appropriate certificate has been received and validated.

2. Presenting Server Certificates at the HTTP/2 Framing Layer


When a client wishes to obtain additional certificates from a server that has signaled support for HTTP certificate authentication (see Section 4), it does this by sending at least one CERTIFICATE_REQUEST frame (see Section 2.1) on stream zero. A client MAY send multiple concurrent CERTIFICATE_REQUEST frames. If server-initiated streams are blocked until the CERTIFICATE_REQUEST has been answered, the client SHOULD send CERTIFICATE_REQUIRED frames on those streams to inform the server.

Servers respond to certificate authentication requests by sending one or more CERTIFICATE frames (see Section 2.3) followed by a CERTIFICATE_PROOF frame, on stream zero.


When the server has advertised support for HTTP certificate authentication (see Section 4), clients MAY send the CERTIFICATE_REQUEST frame. A server that has advertised support MUST NOT treat receipt of such a frame as a session error of type PROTOCOL_ERROR.

The CERTIFICATE_REQUEST frame MUST be sent on stream zero. A CERTIFICATE_REQUEST frame received on any other stream MUST be rejected with a stream error of type PROTOCOL_ERROR.

When sent from client to server, the CERTIFICATE_REQUEST frame has the same layout, with one change to the field definitions. The CA-Count and Certificate-Authorities fields are replaced by Origin-Count and Origins fields, with the same length and format. Origins is the distinguished name of the origin for which the client wishes to obtain a certificate, represented in DER-encoded [X690] format. The number of such structures is given by the 16-bit Origin-Count field, which MUST be one (0x01).


The CERTIFICATE_REQUIRED frame is sent by clients to indicate that processing of a server-initiated stream (for example, a pushed resource) is blocked pending certificate authentication. The frame includes a request identifier which can be used to correlate the stream with a CERTIFICATE_REQUEST frame received on stream zero. The layout and fields are unmodified from [I-D.thomson-http2-client-certs].

When the server has advertised support for HTTP certificate authentication (see Section 4), clients MAY send the CERTIFICATE_REQUIRED frame. A server that has advertised support MUST NOT treat receipt of such a frame as a stream error of type PROTOCOL_ERROR.

The client MUST NOT send a CERTIFICATE_REQUIRED frame on stream zero or a client-initiated stream. A server that receives a CERTIFICATE_REQUIRED frame on an inappropriate stream SHOULD treat this as a connection error of type PROTOCOL_ERROR.

2.3. The CERTIFICATE frame

The CERTIFICATE frame allows the sender to present a certificate which should be used as authentication for previous or subsequent requests.

The payload of a CERTIFICATE frame contains elements of a certificate chain, terminating in an end certificate. The layout, fields, and processing are unmodified from [I-D.thomson-http2-client-certs].


The CERTIFICATE_PROOF frame allows the sender to prove possession of a certificate which should be used as authentication for previous or subsequent requests. The payload of a CERTIFICATE_PROOF frame contains proof of possession of the private key corresponding to an end certificate previously presented in a series of CERTIFICATE frames. The layout, fields, and processing are unmodified from [I-D.thomson-http2-client-certs].

Servers MUST set the AUTOMATIC_USE flag when sending a CERTIFICATE_PROOF frame.


The USE_CERTIFICATE frame is sent by servers to indicate that processing of a server-initiated stream should use a certificate provided in a previous series of CERTIFICATE and CERTIFICATE_PROOF frames. The frame includes a certificate identifier which can be used to correlate the stream with a certificate received on stream zero.

A USE_CERTIFICATE frame with no payload expresses the server’s choice to proceed without providing a certificate. Clients SHOULD process the request as authenticated solely by the certificate provided at the TLS layer, likely by discarding the pushed resource and terminating the stream.

Otherwise, the USE_CERTIFICATE frame contains a single octet, which is the authentication request identifier. A server that receives a USE_CERTIFICATE of any other length MUST treat this as a stream error of type PROTOCOL_ERROR. Frames with identical request identifiers refer to the same certificate chain.

The server MUST NOT send a USE_CERTIFICATE frame on stream zero or a client-initiated stream. A client that receives a USE_CERTIFICATE frame on an inappropriate stream SHOULD treat this as a connection error of type PROTOCOL_ERROR.

3. Indicating failures during Certificate Authentication

The errors defined by [I-D.thomson-http2-client-certs] MAY be used by either clients or servers, as appropriate.

4. Indicating Support for HTTP-Layer Certificate Authentication

Servers that support HTTP-layer certificate authentication indicate this using the HTTP/2 SETTINGS_HTTP_CERT_AUTH setting defined in [I-D.thomson-http2-client-certs].

The initial value for the SETTINGS_HTTP_CERT_AUTH setting is 0, indicating that the server does not support HTTP-layer certificate authentication. A server sets the SETTINGS_HTTP_CERT_AUTH setting to a value of 1 to indicate support for HTTP-layer certificate authentication as defined in this document. Any value other than 0 or 1 MUST be treated as a connection error (Section 5.4.1 of [RFC7540]) of type PROTOCOL_ERROR.

5. Security Considerations

This mechanism defines an alternate way to obtain server certificates other than the TLS handshake. While the signature of exporter values is expected to be equally secure, it is important to recognize that a vulnerability in this code path is equal to a vulnerability in the TLS handshake.

This draft defines a mechanism which could be used to probe servers for origins they support, but opens no new attack versus making repeat TLS connections with different SNI values. Servers SHOULD impose similar denial-of-service mitigations (e.g. request rate limits) to CERTIFICATE_REQUEST frames as to new TLS connections.

6. IANA Considerations

No changes are made to the registrations in [I-D.thomson-http2-client-certs].

7. Acknowledgements

8. References

8.1. Normative References

[I-D.thomson-http2-client-certs] Thomson, M. and M. Bishop, "Reactive Certificate-Based Client Authentication in HTTP/2", Internet-Draft draft-thomson-http2-client-certs-01, January 2016.
[RFC7540] Belshe, M., Peon, R. and M. Thomson, "Hypertext Transfer Protocol Version 2 (HTTP/2)", RFC 7540, DOI 10.17487/RFC7540, May 2015.
[X690] ITU-T, "Information technology - ASN.1 encoding Rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)", ISO ISO/IEC 8825-1:2002, 2002.

8.2. Informative References

[I-D.ietf-httpbis-alt-svc] mnot, m., McManus, P. and J. Reschke, "HTTP Alternative Services", Internet-Draft draft-ietf-httpbis-alt-svc-12, February 2016.
[I-D.ietf-tls-tls13] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", Internet-Draft draft-ietf-tls-tls13-11, December 2015.
[I-D.nottingham-httpbis-origin-frame] mnot, m. and E. Nygren, "The ORIGIN HTTP/2 Frame", Internet-Draft draft-nottingham-httpbis-origin-frame-01, January 2016.

Author's Address

Mike Bishop Microsoft EMail: