Channel Binding Signalling for the Generic Security Services Application Programming Interface
draft-williams-kitten-channel-bound-flag-02

Abstract

This Internet-Draft proposes the addition of a “channel bound” return flag for the GSS_Init_sec_context() and GSS_Accept_sec_context() functions. Two behaviors are specified: a default, safe behavior, and a behavior that is only safe when the application specifically tells the Generic Security Services Application Programming Interface (GSS-API) that it (the applicaiton) supports the new behavior.

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 August 05, 2013.

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Table of Contents

• 1. Introduction
• 1.1. Error in RFC2743
• 1.2. Design
• 1.3. Alternative Design
• 1.4. Future Directions
• 1.5. Conventions used in this document
• 2. Channel Binding State Extension
• 2.1. GSS_Create_sec_context()
• 2.1.1. C-Bindings
• 2.2. GSS_Set_context_flags()
• 2.2.1. C-Bindings
• 2.3. Return Flag for Channel Binding State Signalling
• 2.3.1. C-Bindings
• 3. Modified Channel Binding Semantics
• 4. Security Considerations
• 5. IANA Considerations
• 6. References
• 6.1. Normative References
• 6.2. Informative References
• Author's Address

1. Introduction

The GSS-API [RFC2743] supports “channel binding” [RFC5056], a technique for detection of man-in-the-middle (MITM) attacks in secure channels at lower network layers. This facility is meant to be all-or-nothing: either both the initiator and acceptor use it and it succeeds, or both must not use it. This has created a negotiation problem when retrofitting the use of channel binding into existing application protocols.

Many implementations of the Kerberos V5 GSS-API mechanism [RFC4121] cause the acceptor to succeed when the initiator used channel binding but the acceptor application did not. This has helped deployment of channel binding in existing applications: first fix all the initiators, then fix all the acceptors. But even this is insufficient when there are many clients to fix, such that fixing them all will take a long time.

This document proposes a new method for deployment of channel binding that allows the feature to be enabled on the acceptor side before fixing all initiators. If the GSS-API had always had a return flag by which to indicate channel binding state then we could have had a simpler method of deploying channel binding: applications check that return flag and act accordingly (e.g., fail when channel binding is required). We cannot safely introduce this behavior now without an indication of support by the application.

It is worth noting that at least one implementor of GSS-API mechanisms (but not of the GSS-API itself) has similar semantics in its API to those proposed herein. [XXX add references to the relevant SSPI docs? -Nico]

1.1. Error in RFC2743

The GSS-APIv2u1 [RFC2743] seems to indicate that mechanisms must ignore channel bindings when one party provided none. In practice some mechanisms ignore channel bindings when the acceptor provides none, but not when the initiator provides none. Note that it would be useless to allow security context establishment to succeed when the initiator does not provide channel bindings but the acceptor does, at least as long as there's no outward indication of whether channel binding was used! And indeed, the GSS-APIv2u1 does not provide any such indication. We correct this flaw in this document.

1.2. Design

After some discussion on the mailing list of various designs for signalling application support for the new flag we've settled on copying an aspect of the Java Bindings of the GSS-API [RFC5653], specifically the notion of creating an “empty” SECURITY CONTEXT handle that can then be passed to GSS_Init_sec_context() and GSS_Accept_sec_context() where they normally expect a NULL handle. This empty security context handle can then be used to set options relating to security context token establishment.

1.3. Alternative Design

The previous design was based on an existing, non-standard extension for carrying security context establishment options in CREDENTIAL HANDLEs. Note that a notion of CREDENTIAL HANDLE options might still be useful for options that are really specific to credentials rather than security context tokens (for example: setting an acceptable cryptographic security profile on a CREDENTIAL HANDLE and receiving a new handle with possibly fewer elements, reflecting that some credentials cannot meet the requirement).

1.4. Future Directions

We're likely to introduce additional mutator functions of empty contexts, with mutators corresponding to many of the existing input arguments of GSS_Init_sec_context() and GSS_Accept_sec_context(), as well as a few additional security context inquiry functions. We're also likely to then introduce new variants of GSS_Init_sec_context() and GSS_Accept_sec_context() with all of those input and output parameters removed that could be set or retrieved with the other new functions. The only inputs that the new GSS_Init/Accept_sec_context() must have are: a security context handle (never NULL), and an input context token, and the only outputs should be the status indicators and an output token -- in fact, we may want to have just one new function called, perhaps, GSS_Step_sec_context(), with the role of initiator or acceptor set as a context option.

1.5. Conventions used in this document

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 [RFC2119].

2. Channel Binding State Extension

We propose a new return flag for GSS_Init_sec_context() and GSS_Accept_sec_context(), as well as a pair of functions for setting a) creating “empty” security context handles, b) setting req_flags and indicating which ret_flags the application understands.

C bindings of these extensions are provided along the lines of [RFC2744] and [RFC5587].

In the future we might move more of the many input (and output) arguments to GSS_Init_sec_context() and GSS_Accept_sec_context() into mutators on empty security context handles.

2.1. GSS_Create_sec_context()

Inputs:

• <none>

Outputs:

• major_status INTEGER
• minor_status INTEGER -- note: mostly useless, but we should keep it
• context SECURITY CONTEXT

Return major status codes:

• GSS_S_COMPLETE indicates success.
• GSS_S_UNAVAILABLE indicates that memory is not available, for example.
• GSS_S_FAILURE indicates a general failure.

This function creates an “empty” security context handle that can be passed to GSS_Init_sec_context() or GSS_Accept_sec_context() where they expect a NULL context.

2.1.1. C-Bindings

 OM_uint32
 gss_create_sec_context(OM_uint32 *minor_status,
                        gss_ctx_id_t *context);

 OM_uint32
 gss_set_context_flags(OM_uint32 *minor_status,
                       gss_ctx_id_t context,
                       uint64_t req_flags,
                       uint64_t ret_flags);

 #define GSS_C_CHANNEL_BOUND_FLAG 2048 /* 0x00000800 */