Network Working Group Johansson Internet-Draft SU Intended status: Standards Track November 18, 2007 Expires: May 21, 2008 Channel bindings for HTTP+TLS transport draft-johansson-http-tls-cb-02 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on May 21, 2008. Copyright Notice Copyright (C) The IETF Trust (2007). Johansson Expires May 21, 2008 [Page 1] Internet-Draft http-cbindings November 2007 Abstract This document specifies a channel concept for HTTP with TLS and a representation of that channel which can be used by protocols which use channel bindings to delegate session protection to lower layers. Table of Contents 1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Introduction and motivation . . . . . . . . . . . . . . . . . 4 3. The HTTP+TLS Channel . . . . . . . . . . . . . . . . . . . . . 5 4. Discovery of the channel-bindings-proxy . . . . . . . . . . . 6 5. The Channel-Bindings-Proxy header . . . . . . . . . . . . . . 7 6. The Channel-Identifier header . . . . . . . . . . . . . . . . 8 7. Security Considerations . . . . . . . . . . . . . . . . . . . 10 8. Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 10. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 10.1. 01 to 02 . . . . . . . . . . . . . . . . . . . . . . . . 13 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14 11.1. Normative References . . . . . . . . . . . . . . . . . . 14 11.2. Informative References . . . . . . . . . . . . . . . . . 14 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15 Intellectual Property and Copyright Statements . . . . . . . . . . 16 Johansson Expires May 21, 2008 [Page 2] Internet-Draft http-cbindings November 2007 1. Terminology The keywords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT" and "MAY" that appear in this document are to be interpreted as described in [RFC2119] Johansson Expires May 21, 2008 [Page 3] Internet-Draft http-cbindings November 2007 2. Introduction and motivation For good or bad, several protocols use HTTP (or close approximations as in the case of [RFC2910]) as a transport mechanisms and often rely on [RFC2817] or [RFC2818] to provide security. Common security requirements include authentication of the service, of the client and protection of the session. It is not always appropriate to meet these requirements using the same technology. For instance, the use of certificates to authenticate users may be inefficient in an enviroment where [RFC4120] is widely deployed while at the same time certificates may be suitable for authenticating the service. Channel bindings described in [RFC5056] provide a mechanism for delegating session protection to a secure channel at a lower layer. In this case the lower layer is HTTP with TLS either using [RFC2817] or [RFC2818]. This document specifies this channel concept in a form which makes it usable to upper layer protocols which can consume channel bindings. Johansson Expires May 21, 2008 [Page 4] Internet-Draft http-cbindings November 2007 3. The HTTP+TLS Channel In general an HTTP connection consists of a series of (non-proxied) HTTP connections between a client, through a series of client-proxies with a server. The server may also use proxies (for instance concentrators) but that fact does not affect this specification. Each client/proxy in the chain either knows that the next hop is another proxy (by virtue of configuration or discovery) or has no knowledge of another proxy in which case the next hop in the chain by definition is the target endpoint or an imposter impersonating the target endpoint. The following figure illustrates the general case: C --> P0 --> P1 ===> SP --> S Here C is the client and S is the server. In between, P0 and P1 represents client proxies and SP represents a server SSL concentrator (i.e. a server proxy). Both protected (TLS) and unprotected connections may occur. In this figure the connection between P1 and SP has been emphasized for reasons which will be explained below. An HTTP+TLS channel is defined as an HTTP connection where at least the connection from the last client proxy to the target endpoint uses TLS either through [RFC2818] or [RFC2817]. The last client proxy is called the channel-binding-proxy and is given a special role in this specification. Note that in the non-proxied case an HTTP+TLS channel is just a normal TLS-protected HTTP connection. This document specifies a way to identify such channels for the purpouse of channel bindings. In the example case the connection representing the channel to be identified is the one illustrated by the second (===>) arrow from the left.. This is the link which leaves the client proxy chain and hence leaves the client trust domain. The channel-binding-proxy in this case would be P1. In order to be used by protocols consuming channel bindings, an HTTP+ TLS channel must be identified by a value which can be observed by the client or the channel-binding-proxy on behalf of the client. The identifier for the channel is defined to be the hash of the endpoint certificate using the digest algorithm from the certificate, also known as a certificate fingerprint. In the non-proxied case the client has direct access to the endpoint certificate and can compute the fingerprint. In the general case the channel-binding-proxy has access to the certifiate and is able to communicate it to the client. Johansson Expires May 21, 2008 [Page 5] Internet-Draft http-cbindings November 2007 4. Discovery of the channel-bindings-proxy It is desirable for a client to be able to discover the existence of a channel-bindings-proxy somewhere in its proxy-chain. In order to accomplish this the client needs a method to inquire the the first proxy in the chain if it knows if there is a channel-binding-proxy in the chain. By either forwarding the request or (if the proxy is a channel-bindings-proxy or has no knowledge about an upstream proxy) answering it the proxy can inform the client about the existence or absense of a channel-bindings-proxy in the chain. Unfortunately this client-request cannot be implemented using a request-header since there are no widely implemented request-headers which are handled by proxies except for the Proxy-Authentication and Proxy-Authorization headers which are not possible to reuse for this application. Instead the OPTIONS verb is used in the following way: The client sends an OPTIONS request for the Request-URI "*" as per [RFC2616] section 9.2 with Max-Forwards set to "0". As specified in [RFC2616] a proxy is required to respond to this message itself rather than forward it upstream. A proxy implementing this specification that is configured as a channel-bindings-proxy MUST include a Channel- Bindings-Proxy header (cf below) in the response. Each proxy sitting between the client and the channel-bindings-proxy MUST re-issue any OPTIONS request received this awy and include any Channel-Identifier header received in the response-message to the client/proxy. Upon receipt of the (final) response-message the client may (if the message contains a Channel-Bindings-Proxy header) assume that the proxy-chain contains a channel-bindings-proxy implementing this specification. Note well that this mechanism does not violate the intent of the treatment of the Max-Forwards since the semantics of the Channel- Bindings-Proxy is that the proxy has knowledge about a channel- bindings-proxy somewhere in the proxy-chain (including the proxy itself). Johansson Expires May 21, 2008 [Page 6] Internet-Draft http-cbindings November 2007 5. The Channel-Bindings-Proxy header Information about the channel-bindings-proxy used in the discovery process described above is represented by new the HTTP/1.1 header 'Channel-Bindings-Proxy' which contains the hostname of the channel- bindings-proxy. Johansson Expires May 21, 2008 [Page 7] Internet-Draft http-cbindings November 2007 6. The Channel-Identifier header An HTTP+TLS channel is represented as a new HTTP/1.1 header 'Channel- Identifier' which contains the representation of the HTTP+TLS channel. The 'Channel-Identifier' header has the following syntax in Augmented BNF defined in [RFC2616]: channel-identifier = hash-function *(SP channel-value) hash-function = token channel-value = 2HEX *(":" 2HEX) ; a hex-encoded sequence of bytes ; separated by colons of length > 0 The definitions of HEX, token and SP are taken from [RFC2616]. An HTTP proxy implementing this specification MUST NOT modify the Channel-Identifier header unless explicitly configured to act as a channel-bindings-proxy. A proxy configured as a channel-bindings- proxy MUST for each TLS-protected HTTP connection add the Channel- Identifier header to each received response-message. A client implementing this specification MUST NOT accept or use the Channel-Identifier header unless it has established that the proxy- chain ends in a channel-bindings-proxy. Clients MUST support manual configuration for the existence of a channel-bindings-proxy and SHOULD support discovery as specified in the previous section. Other mechanisms for discovering the existance of a channel-bindings-proxiy may be specified in the future. The client MUST NOT assume knowledge about which proxy is acting as the channel-bindings-proxy. If the client does not have a proxy the client may compute the Channel-Identifier header value given direct access to the endpoint certificate. The client MUST NOT compute or use the Channel-Identifier header value if it has a known proxy-chain which does not include a channel-bindings-proxy. When a client receives a reply message with a Channel-Identifier header the client may use this as input to an upper-layer authentication protocol which consumes channel-bindings. This means that if the Channel-Identifier header changes the upper-layer protocol will detect this change as a man-in-the middle attack and the client MUST terminate the connection to the server. Clients MAY cache channel-identifier/endpoint pairs. The first time a client communicates with a target endpoint it has no channel-identifier to bind authentication protocols to. In several situations the first communication between a client and a server results in an authentication mechanism negotiation challange from the Johansson Expires May 21, 2008 [Page 8] Internet-Draft http-cbindings November 2007 server at which point the channel-binding-proxy (or the client itself) will have a channel-identifier to use for the first authentication mechanism challenge. If the client has a cached channel-identifier for an endpoint the client SHOULD include the Channel-Identifier header in each outgoing request. This gives the channel-binding-proxy the oportunity to determine if response packets contain a forged channel-identifier and may decide to terminate such connections at the proxy. Johansson Expires May 21, 2008 [Page 9] Internet-Draft http-cbindings November 2007 7. Security Considerations There are several situations when the client has no knowledge of proxies intercepting traffic. Such proxies are essentially man-in- the-middle attacks but are also in many cases implicitly part of the trust-domain of the client. In the case when the hidden proxy sits between the client and the channel-bindings-proxy and does not alter the Channel-Identifier header it does not matter. In deployments where the last client proxy does not support the channel-bindings- proxy feature it may be necessary to add another proxy outside this proxy which can act as the channel-bindings-proxy. The basic principle of the Channel-Bindings header is that it contains data which is either observed and verified or computed by a trusted proxy between the client and server. By replacing this header value another proxy effectively moves the endpoint of the channel. By using the header value as channel binding data for upper layer authentication protocols the client effectively trusts the proxy setting the value. It is very imporant that a proxy configured to act as a channel- bindings-proxy actually sets the Channel-Identifier header. A rogue or non-functional channel-bindings-proxy which announces the channel- bindings-feature but fails to provide or even filter-out the header will make the client vulnerable to attack. Clients mayll wish to use proxy authentication to identify trusted proxies. Johansson Expires May 21, 2008 [Page 10] Internet-Draft http-cbindings November 2007 8. Notes What about TLS mechs besides X.509? This "only" requires finding something which corresponds to the certificate fingerprint: i.e something which can be computed by the client and the server together or observed independently by the client. Johansson Expires May 21, 2008 [Page 11] Internet-Draft http-cbindings November 2007 9. IANA Considerations Allocating the Channel-Identifier header. Johansson Expires May 21, 2008 [Page 12] Internet-Draft http-cbindings November 2007 10. Changes 10.1. 01 to 02 Changed from ABNF to Augmented BNF to align with [RFC2616]. Johansson Expires May 21, 2008 [Page 13] Internet-Draft http-cbindings November 2007 11. References 11.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. [RFC2817] Khare, R. and S. Lawrence, "Upgrading to TLS Within HTTP/1.1", RFC 2817, May 2000. [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000. [RFC4234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 4234, October 2005. [RFC5056] Williams, N., "On the Use of Channel Bindings to Secure Channels", RFC 5056, November 2007. 11.2. Informative References [RFC2910] Herriot, R., Butler, S., Moore, P., Turner, R., and J. Wenn, "Internet Printing Protocol/1.1: Encoding and Transport", RFC 2910, September 2000. [RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The Kerberos Network Authentication Service (V5)", RFC 4120, July 2005. Johansson Expires May 21, 2008 [Page 14] Internet-Draft http-cbindings November 2007 Author's Address Leif Johansson Stockholm university Email: leifj@it.su.se URI: http://www.su.se/ Johansson Expires May 21, 2008 [Page 15] Internet-Draft http-cbindings November 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 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The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Johansson Expires May 21, 2008 [Page 16]