Channel Binding Signalling for the Generic Security Services Application Programming Interface
draft-williams-kitten-channel-bound-flag-00

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 5, 2013.

Copyright Notice

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Table of Contents

• 1. Introduction
• 1.1. Error in RFC2743
• 1.2. Conventions used in this document
• 2. Channel Binding State Extension
• 2.1. GSS_Set_cred_option()
• 2.1.1. C-Bindings
• 2.2. GSS_Set_cred_option_critical()
• 2.2.1. C-Bindings
• 2.3. channel_bound_flag
• 2.3.1. C-Bindings
• 3. References
• 3.1. Normative References
• 3.2. 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.

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. 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 options on credential handles, along with an option for signalling understanding of the new flag in the acceptor applications.

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

2.1. GSS_Set_cred_option()

Inputs:

cred_handle CREDENTIAL HANDLE

If no credential handle is given then the option MAY be applied globally to the default credential handle, but the implementation MAY return an error instead.

desired_object OBJECT IDENTIFIER

Desired option; MUST NOT be GSS_C_NO_OID.

value OCTET STRING

Value for the option.

Outputs:

• major_status INTEGER
• minor_status INTEGER

Return major status codes:

• GSS_S_COMPLETE indicates success.
• GSS_S_UNAVAILABLE indicates that the the given option is not supported by any mechanism.
• GSS_S_FAILURE indicates a general failure.

This function sets the given value to a credential option named by desired_object on the given cred_handle.

2.1.1. C-Bindings

   OM_uint32
   gss_set_cred_option(OM_uint32 *minor_status,
                       gss_cred_id_t *cred_handle,
                       const gss_OID desired_object,
                       const gss_buffer_t value);

   OM_uint32
   gss_set_cred_option_critical(OM_uint32 *minor_status,
                                gss_const_cred_id_t input_cred_handle,
                                gss_cred_id_t *output_cred_handle,
                                const gss_OID desired_object,
                                const gss_buffer_t value);

   #define GSS_C_CHANNEL_BOUND 4096 /* 0x1000
   gss_const_OID GSS_C_CHANNEL_BOUND_CRED_OPT_OID; /* OID TBD */