GSS-API channel bindings: Clarifications and Extensions to the Generic Security Service Application Program Interface (GSS-API) for the Use of Channel Bindings
Defines how to tie authentication to a specific secure channel
Williams, N., "Clarifications and Extensions to the Generic Security Service Application Program Interface (GSS-API) for the Use of Channel Bindings", RFC 5554, DOI 10.17487/RFC5554, May 2009, <https://www.rfc-editor.org/info/rfc5554>.
@misc{rfc5554,
series = {Request for Comments},
number = 5554,
howpublished = {RFC 5554},
publisher = {RFC Editor},
doi = {10.17487/RFC5554},
url = {https://www.rfc-editor.org/info/rfc5554},
author = {N. Williams},
title = {{Clarifications and Extensions to the Generic Security Service Application Program Interface (GSS-API) for the Use of Channel Bindings}},
pagetotal = 4,
year = 2009,
month = may,
} RFC text
Contents
Network Working Group N. Williams
Request for Comments: 5554 Sun
Updates: 2743 May 2009
Category: Standards Track
Clarifications and Extensions to
the Generic Security Service Application Program Interface (GSS-API)
for the Use of Channel Bindings
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Abstract
This document clarifies and generalizes the Generic Security Service
Application Programming Interface (GSS-API) "channel bindings"
facility, and imposes requirements on future GSS-API mechanisms and
programming language bindings of the GSS-API.
Table of Contents
1. Introduction ....................................................2
2. Conventions Used in This Document ...............................2
3. New Requirements for GSS-API Mechanisms .........................2
4. Generic Structure for GSS-API Channel Bindings ..................2
5. Security Considerations .........................................3
6. References ......................................................4
6.1. Normative References .......................................4
6.2. Informative References .....................................4
1. Introduction
The base GSS-API version 2, update 1 specification [RFC2743] provides
a facility for channel binding (see also [RFC5056]), but its
treatment is incomplete. The GSS-API C-bindings specification
[RFC2744] expands somewhat on this facility in what should be a
generic way, but is instead a C-specific way, thus leaving the
treatment of this facility incomplete.
This document clarifies the GSS-API's channel binding facility and
generalizes the parts of it that are specified in the C-bindings
document but that should have been generic from the start.
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].
3. New Requirements for GSS-API Mechanisms
Given the publication of RFC 5056, we now assert that all new GSS-API
mechanisms that support channel binding MUST conform to [RFC5056].
4. Generic Structure for GSS-API Channel Bindings
The base GSS-API version 2, update 1 specification [RFC2743] provides
a facility for channel binding. It models channel bindings as an
OCTET STRING and leaves it to the GSS-API version 2, update 1
C-bindings specification to specify the structure of the contents of
the channel bindings OCTET STRINGs. The C-bindings specification
[RFC2744] then defines, in terms of C, what should have been a
generic structure for channel bindings. The Kerberos V GSS mechanism
[RFC4121] also defines a method for encoding GSS channel bindings in
a way that is independent of the C-bindings -- otherwise, the
mechanism's channel binding facility would not be useable with other
language bindings.
In other words, the structure of GSS channel bindings given in
[RFC2744] is actually generic in spite of being specified in terms of
C concepts and syntax.
We generalize it as shown below, using the same pseudo-ASN.1 as is
used in RFC 2743. Although the figure below is, indeed, a valid
ASN.1 [CCITT.X680] type, we do not provide a full ASN.1 module as
none is needed because no standard encoding of this structure is
needed -- the definition below is part of an abstract API, not part
of a protocol defining bits on the wire. GSS-API mechanisms do need
to encode the contents of this structure, but that encoding will be
mechanism specific (see below).
GSS-CHANNEL-BINDINGS ::= SEQUENCE {
initiator-address-type INTEGER, -- See RFC2744
initiator-address OCTET STRING, -- See RFC2744
acceptor-address-type INTEGER, -- See RFC2744
acceptor-address OCTET STRING, -- See RFC2744
application-data OCTET STRING -- See RFC5056
}
Abstract GSS-API Channel Bindings Structure
The values for the address fields are described in [RFC2744].
New language-specific bindings of the GSS-API SHOULD specify a
language-specific formulation of this structure.
Where a language binding of the GSS-API models channel bindings as
OCTET STRINGs (or the language's equivalent), then the implementation
MUST assume that the given bindings correspond only to the
application-data field of GSS-CHANNEL-BINDINGS as shown above, rather
than some encoding of GSS-CHANNEL-BINDINGS.
As mentioned above, [RFC4121] describes an encoding of the above GSS-
CHANNEL-BINDINGS structure and then hashes that encoding. Other GSS-
API mechanisms are free to use that encoding.
5. Security Considerations
For general security considerations relating to channel bindings, see
[RFC5056].
Language bindings that use OCTET STRING (or equivalent) for channel
bindings will not support the use of network addresses as channel
bindings. This should not cause any security problems, as the use of
network addresses as channel bindings is not generally secure.
However, it is important that "end-point channel bindings" not be
modeled as network addresses; otherwise, such channel bindings may
not be useable with all language bindings of the GSS-API.
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
[RFC2744] Wray, J., "Generic Security Service API Version 2 :
C-bindings", RFC 2744, January 2000.
[RFC4121] Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos
Version 5 Generic Security Service Application Program
Interface (GSS-API) Mechanism: Version 2", RFC 4121,
July 2005.
[RFC5056] Williams, N., "On the Use of Channel Bindings to Secure
Channels", RFC 5056, November 2007.
6.2. Informative References
[CCITT.X680] International Telephone and Telegraph Consultative
Committee, "Abstract Syntax Notation One (ASN.1):
Specification of basic notation", CCITT Recommendation
X.680, July 2002.
Author's Address
Nicolas Williams
Sun Microsystems
5300 Riata Trace Ct
Austin, TX 78727
US
EMail: Nicolas.Williams@sun.com
AI ELI5
On this page
§1What Are Channel Bindings
GSS-API is a security programming interface that applications use to authenticate users and encrypt communications. Channel bindings are a security mechanism that ties authentication to a specific secure channel.
Without channel bindings, an attacker could intercept communications and potentially forge authentication. Channel bindings ensure that the authentication credentials match the specific encrypted channel being used, making attacks harder. However, the original GSS-API specification in RFC 2743 did not fully describe how channel bindings should work across different programming languages and authentication mechanisms.
Proving you are really you is important, but also proving that your secret code only works with this specific private tunnel makes it even harder for bad actors to trick the system.
§2Generic Structure for Channel Bindings
This RFC defines a standard structure for channel bindings that works regardless of what programming language or authentication mechanism is in use. The structure includes initiator and acceptor addresses (representing the two parties), an address type for each (such as IP address format), and application-specific data.
The actual format of the address fields is taken from the earlier C-language bindings specification, but this RFC generalizes them so they apply everywhere. Different mechanisms may encode these bindings differently internally, but they all follow this common structure.
Think of it like a standard envelope that describes both you and the recipient, plus special information the application needs, so that every security system can understand it the same way.
§3Requirements for New Mechanisms
The RFC also sets a requirement that any new GSS-API mechanism supporting channel bindings must follow RFC 5056, which describes the security considerations for using channel bindings in practice.
For language bindings that treat channel bindings as simple byte strings rather than structured data, the RFC specifies that these bytes should represent only the application-specific data part, not the full structure including addresses.
New security systems that support channel bindings must follow the rules for keeping them secure, and simple bindings should only include the application's special data, not the addresses.