Proxy Mutual Authentication in SIP
draft-dotson-sip-mutual-auth-03

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Abstract

This document defines the Proxy-Authentication-Info header field for the Session Initiation Protocol (SIP). When a UA is required to authenticate to a proxy using digest authentication specified in SIP this header field allows for the UA to authenticate the proxy, enabling mutual authentication. This header field can also provide integrity checks over the bodies.

Table of Contents

1.  Introduction
2.  Terminology
3.  Motivation
4.  Overview
5.  User Agent Client (UAC) Behavior
6.  User Agent Server Behavior
7.  Proxy Behavior
8.  Extensibility Considerations
9.  Header Field Definition
10.  Security Considerations
11.  IANA Considerations
12.  Acknowledgements
13.  References
    13.1.  Normative References
    13.2.  Informative References
§  Authors' Addresses
§  Intellectual Property and Copyright Statements

1.  Introduction

The Session Initiation Protocol (SIP, [RFC3261] (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.)) provides a stateless, challenge-response based mechanism for authentication that is based on authentication in HTTP [RFC2617] (Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., Leach, P., Luotonen, A., and L. Stewart, “HTTP Authentication: Basic and Digest Access Authentication,” June 1999.). A proxy or a user receiving a request can challenge the initiator of the request to obtain assurance of the originator's identity. A UAS, registrar, or redirect server can use 401 (Unauthorized), where as proxies use 407 (Proxy Authentication Required), for authentication challenges. Challenges result in a resend of the requests with the digest authentication information that can be used to verify the authenticity of the originator. The two parties share a username and password to support this authentication mechanism. Refer to [RFC3261] (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) for more information on Digest authentication.

The SIP Digest mechanism parallels the HTTP Digest mechanism specified in [RFC2617] (Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., Leach, P., Luotonen, A., and L. Stewart, “HTTP Authentication: Basic and Digest Access Authentication,” June 1999.). HTTP Digest [RFC2617] (Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., Leach, P., Luotonen, A., and L. Stewart, “HTTP Authentication: Basic and Digest Access Authentication,” June 1999.) also allows for mutual authentication by allowing the client to authenticate the challenging entity, such as a proxy. Mutual authentication is facilitated via two headers: Authentication-Info for mutual authentication with a server, and Proxy-Authentication-Info for authentication with a proxy. These headers may be used by the challenging entities, server or proxy, to send challenge responses for authentication by the client. SIP specifies and allows for the usage of the Authentication-Info header by a server, but does not mention the Proxy-Authentication-Info header. This document presents an extension to allow for the use of the Proxy-Authentication-Info header. The header can be sent along with 2xx responses from the proxy to the client during digest authentication. The response digest in the "response-auth" directive allows the client to authenticate the proxy, i.e., it ensures that the proxy has knowledge of the password. This provides for mutual authentication when proxies challenge clients, and provides for limited integrity protection. It also allows for the Proxy to provide additional information such as the nonce value to use for a future authentication response.

2.  Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [RFC2119].

3.  Motivation

SIP ([RFC3261] (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.)) addresses User-to-User authentication and Proxy-to-User authentication. For the UA to authenticate to a server or a proxy, [RFC3261] (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) specifies the Authentication and Proxy-Authentication headers, respectively. For the UA to authenticate a server [RFC3261] (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) specifies the Authentication-Info header, which allows for mutual authentication. For the UA to authenticate a proxy [RFC3261] (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) does not specify an equivalent header. TLS can be used in such cases if the UA wishes to authenticate the next-hop proxy. However, in deployments where multiple proxies are involved in the messaging path (e.g., 3GPP IMS) the UA will not be able to use TLS to authenticate proxies located beyond the first hop.

To allow for deployments where there is a need for the UA to mutually authenticate with proxies other than the next-hop, this document specifies the Proxy-Authentication-Info header. In addition to mutual authentication, the header also allows for the optimization of digest authentication procedures by allowing the proxy to indicate the nonce to be used by the UA for future authentication responses.

4.  Overview

      +--------+                  +--------+               +--------+
      |  UAC   |                  |  Proxy |               | Server |
      +--------+                  +--------+               +--------+
          |                            |                        |
          |   SIP REQ (e.g., INVITE)   |                        |
          |--------------------------> |                        |
          |                            |                        |
          | 407 (Proxy Auth. Required) |                        |
          |<-------------------------- |                        |
          |                            |                        |
          |                            |                        |
          |   SIP REQ (with creds)     |                        |
          |--------------------------> |                        |
          |                            | SIP REQ (without creds)|
          |                            |----------------------> |
          |                            |    SIP RESPONSE        |
          | SIP RESPONSE (e.g. 200 OK) |<---------------------- |
          |<-------------------------- |                        |
          |

5.  User Agent Client (UAC) Behavior

When this header field is included by a Proxy within the 2xx response, the requirements are the same as those of a client receiving an Authentication-Info header field from a Server, as specified in [RFC3261] (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.).

6.  User Agent Server Behavior

UAS behavior is unaffected by this specification.

7.  Proxy Behavior

A Proxy MAY include this header field in a 2xx response to a request that was successfully authenticated using digest based on the Authorization header field.

Syntax and semantics follow those specified in [RFC2617] (Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., Leach, P., Luotonen, A., and L. Stewart, “HTTP Authentication: Basic and Digest Access Authentication,” June 1999.), which also defines mechanisms for backwards compatibility using the qop attribute in the request. These mechanisms MUST be used by a proxy to determine if the client supports the new mechanisms in [RFC2617] (Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., Leach, P., Luotonen, A., and L. Stewart, “HTTP Authentication: Basic and Digest Access Authentication,” June 1999.) that were not specified in [RFC2069] (Franks, J., Hallam-Baker, P., Hostetler, J., Leach, P., Luotonen, A., Sink, E., and L. Stewart, “An Extension to HTTP : Digest Access Authentication,” January 1997.).

Example:

Proxy-Authentication-Info: nextnonce="47364c23432d2e131a5fb210812c



The proxy SHOULD at least include the 'qop', 'cnonce', 'nc', and 'rspauth' parameters in the Proxy-Authentication-Info header field.

When forwarding a response from downstream that contains one or more Proxy-Authentication-Info header fields, a proxy MUST include those fields in a Proxy-Authentication-Info header in the forwarded response.

8.  Extensibility Considerations

This document introduces the Proxy-Authentication-Info header that may be sent from a proxy to a client during authentication. If present, it provides an opportunity for the client to authenticate the proxy, enabling mutual authentication. A proxy that is not compliant with this specification will not include the header. However, implementors need to understand that without the specified header mutual authentication may not be possible within Proxy-to-User authentication as specified by SIP. Additionally, the presence of this header allows for the proxy to indicate the nonce to be used by the client during a future authentication response. If the nextnonce field is present the client SHOULD use it when constructing the Proxy-Authorization header for its next request. This document does not alter this requirement. However, implementers need to understand that the failure of the client to act on the nextnonce field may result in a request to re-authenticate from the proxy with the "stale=TRUE". This behavior is specified in [RFC2617] (Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., Leach, P., Luotonen, A., and L. Stewart, “HTTP Authentication: Basic and Digest Access Authentication,” June 1999.), and is not altered by this document.

9.  Header Field Definition

The grammar for the Proxy-Authentication-Info header is defined as follows:

Proxy-Authentication-Info = "Proxy-Authentication-Info" HCOLON painfo
                             *(COMMA painfo)
painfo                    =  nextnonce / message-qop
                             / response-auth / cnonce
                             / nonce-count
nextnonce                 =  "nextnonce" EQUAL nonce-value
response-auth             =  "rspauth" EQUAL response-digest
response-digest           =  LDQUOT *LHEX RDQUOT

Figure 2 (Extension to Table 3) is an extension to Table 3 of [RFC3261] (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) for the Proxy-Authentication-Info header:

  Header field                 where    proxy ACK BYE CAN INV OPT REG

Proxy-Authentication-Info       2xx       o    -   o   -   o   o   -

10.  Security Considerations

This document defines a SIP message header that provides mutual authentication during proxy authentication of a UA. When challenged by a proxy or server to perform authentication (e.g., after sending an INVITE or SUBSCRIBE request), the Proxy-Authorization header provides the proxy with proof the UA knows the correct credentials for the identity being used. By adding support for the Proxy-Authentication-Info header, proxies may provide UAs with a challenge response to prove to the UA it also knows the correct credentials. The use case most affected is where the proxy/server performing the challenge is not the next-hop proxy/server of the UA.

When the proxy/server is the next-hop proxy/server for the UA, TLS should be relied upon instead of this mechanism, as a malicious next-hop proxy or Man-in-The-Middle (MITM) could merely not challenge the UA, or simply not use the optional Proxy-Authorization-Info header. This header is most meaningful in environments where the UA is expecting (i.e., is configured) to perform mutual authenitication - malicious entities would be forced to prove knowledge of the UAs credentials, adding an additional layer of defense.

11.  IANA Considerations

This document defines a new SIP header field "Proxy-Authentication-Info".

Name of header: Proxy-Authentication-Info

Short form: none

Registrant: Sumanth Channabasappa, sumanth@cablelabs.com

Normative description: RFCXXXX

Note to RFC Editor: Please replace XXXX with the RFC number for this document.

12.  Acknowledgements

The authors would like to thank Scott Lawrence from Pingtel for his feedback that lead to the support of multiple Proxy-Authentication-Info header field values. Thanks also to Wolf Dietrich Moeller from Nokia Siemens Networks and Francois Audet from Nortel. The authors are also appreciative of the assistance provided by Dean Willis and Keith Drage.

13.  References

13.1. Normative References

13.2. Informative References

Authors' Addresses

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