Client Authentication over New TLS Connection
draft-thomson-httpbis-cant-00

Abstract

This document defines an HTTP authentication scheme that can be added to an error response to indicate to a client that a successful response will only be provided over a new TLS connection, and only if the client has provided a certificate on that connection.

Status of This Memo

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This Internet-Draft will expire on November 09, 2014.

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Table of Contents

• 1. Introduction
• 1.1. Conventions and Terminology
• 2. Client Certificate Challenge
• 3. Security Considerations
• 4. IANA Considerations
• 5. Acknowledgements
• 6. References
• 6.1. Normative References
• 6.2. Informational References
• Author's Address

1. Introduction

Client authentication in HTTP sometimes relies on certificate-based authentication of clients in Transport Layer Security (TLS) [RFC5246]. Some uses of client authentication rely on TLS renegotiation, triggering renegotiation in response to a request for a particular resource.

HTTP/2 [I-D.ietf-httpbis-http2] forbids the use of renegotiation, except for at the very beginning of a connection. This makes addressing some client authentication use cases difficult.

This document defines a new authentication scheme, ClientCertificate, for use in HTTP authentication challenges [I-D.ietf-httpbis-p7-auth]. In combination with the 401 (Unauthorized) status code, this indicates that the resource requires client authentication at the TLS layer in order to access it.

1.1. Conventions and Terminology

At times, this document falls back on shorthands for establishing interoperability requirements on implementations: the capitalized words "MUST", "SHOULD" and "MAY". These terms are defined in [RFC2119].

2. Client Certificate Challenge

A new authentication scheme ([I-D.ietf-httpbis-p7-auth]) for the WWW-Authenticate and Proxy-Authenticate header fields is defined with the name ClientCertificate.

A challenge with this authentication scheme does not define any parameters except realm. The realm can be used to select an appropriate certificate, or if a certificate is already in use, to indicate the need for a different certificate. Other challenge parameters MAY be used to provide a client with information it can use to select an appropriate certificate. Unknown parameters MUST be ignored.

This challenge cannot be satisfied by constructing an Authorization header field [I-D.ietf-httpbis-p7-auth], it can only be satisfied by making the request on a TLS connection where an appropriate certificate has been provided by the client.

To effectively use this authentication scheme, a new connection is needed for every protection space used by a given origin server. A client can use the ClientCertificate challenge as a trigger to open a new connection and to use client authentication on that connection. The client can use the mechanism in [I-D.thomson-tls-care] to prompt the server to request a client certificate, to avoid the problem where the server doesn't know to make this request.

3. Security Considerations

Clients that support this authentication scheme will create a new connection each time that they see a challenge. This could be exploited in order to generate additional load in terms of connections on both server and client.

Using new connections for client authentication has additional processing costs to the client in proving access to the private keys associated with the client certificate; and to the server in proving access to the private keys associated with their certificate twice in the case that the client opts for confidentiality protection on the client certificate.

HTTP/2 [I-D.ietf-httpbis-http2] allows clients to use the same connection for multiple canonical root URIs. Certificate-based client authentication as defined by this specification is bound to a single origin. This could create issues whereby the security properties of a connection could become confused. Clients MUST ensure that a client-authenticated connection is only used for the origin for which it was created.

4. IANA Considerations

IANA is requested to create an entry in the HTTP Authentication Scheme Registry with the following information:

• ClientCertificate
• This document
• This scheme does not rely on the Authorization header field.

5. Acknowledgements

Eric Rescorla helped identify the problem and formulate this mechanism. Julian Reschke and Michael Koeller provided excellent feedback.

6. References

6.1. Normative References

6.2. Informational References

Author's Address