Internet DRAFT - draft-ietf-kitten-channel-bound-flag
draft-ietf-kitten-channel-bound-flag
KITTEN R. Harwood
Internet-Draft Red Hat
Updates: 2743, 2744 (if approved) N. Williams
Intended status: Standards Track Cryptonector
Expires: August 9, 2019 February 5, 2019
Channel Binding Signalling for the Generic Security Services Application
Programming Interface
draft-ietf-kitten-channel-bound-flag-04
Abstract
Channel binding is a technique that allows applications to use a
secure channel at a lower layer without having to use authentication
at that lower layer. The concept of channel binding comes from the
Generic Security Services Application Programming Interface (GSS-
API). It turns out that the semantics commonly implemented are
different than those specified in the base GSS-API RFC (RFC2743), and
that that specification has a serious bug. This document addresses
both the inconsistency as-implemented and the specification bug.
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
reflecting existing implementation deployments, and a behavior that
is only safe when the application specifically tells the GSS-API that
it (the application) supports the new behavior. Additional API
elements related to this are also added, including a new security
context establishment API.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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."
This Internet-Draft will expire on August 9, 2019.
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Copyright Notice
Copyright (c) 2019 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
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Design and Future directions . . . . . . . . . . . . . . . 3
1.2. Conventions used in this document . . . . . . . . . . . . . 3
2. Channel Binding State Extension . . . . . . . . . . . . . . . 4
2.1. GSS_Create_sec_context() . . . . . . . . . . . . . . . . . 4
2.1.1. C-Bindings . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. GSS_Set_context_flags() . . . . . . . . . . . . . . . . . . 5
2.2.1. C-Bindings . . . . . . . . . . . . . . . . . . . . . . . 5
2.3. Return Flag for Channel Binding State Signalling . . . . . 6
2.3.1. C-Bindings . . . . . . . . . . . . . . . . . . . . . . . 6
2.4. New Mechanism Attribute . . . . . . . . . . . . . . . . . . 6
2.5. Request Flag for Acceptor Confirmation of Channel Binding . 6
2.5.1. C-Bindings . . . . . . . . . . . . . . . . . . . . . . . 6
2.6. Handling Empty Contexts in Other GSS-API Functions . . . . 6
3. Modified Channel Binding Semantics . . . . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.1. Normative References . . . . . . . . . . . . . . . . . . . 8
6.2. Informative References . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
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.
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However, GSS-APIv2u1 [RFC2743] does not specify channel binding
behavior when only one party provides provides none. In practice,
some mechanisms (such as Kerberos [RFC4121]) 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! Since the
GSS-APIv2u1 does not provide any such indication, this document
corrects that flaw.
Allowing the connection to succeed when an initiator provides
bindings but an acceptor does not has helped deployment of channel
binding in existing applications: first fix all the initiators, then
fix all the acceptors. But even this technique is insufficient when
there are many clients to fix, such that fixing them all will take a
long time. Additionally, it limits the usefulness of channel
bindings, while allowing the acceptor to provide but not enforce
would protect against man in the middle attacks (for channel binding
aware initiators).
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.
Additionally, there may be applications where it is important for
initiators to know that acceptors did use channel binding, and even
to know whether a mechanism is capable of indicating as much. We add
a request flag and a mechanism attribute for such applications.
1.1. Design and Future directions
The design for signalling application flag support and empty contexts
is based on the Java Bindings of the GSS-API [RFC5653]. This
document begins introduction of additional context inquiry and
mutation functions, which eventually will also allow for simplified
stepping to replace the GSS_Init/Accept_sec_context() loop.
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].
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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 a)
creating "empty" security context handles, b) requesting flags and
indicating which flags the application understands. We also add a
new mechanism attribute for supporting channel binding confirmation.
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:
o <none>
Outputs:
o major_status INTEGER
o minor_status INTEGER -- note: mostly useless, but we should keep
it
o context SECURITY CONTEXT -- "empty" security context
Return major status codes:
o GSS_S_COMPLETE indicates success.
o GSS_S_UNAVAILABLE indicates that memory is not available, for
example.
o 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 GSS_C_NO_CONTEXT.
2.1.1. C-Bindings
OM_uint32
gss_create_sec_context(OM_uint32 *minor_status,
gss_ctx_id_t *context);
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2.2. GSS_Set_context_flags()
Inputs:
context CONTEXT HANDLE
req_flags FLAGS Requested flags. Applicable to acceptors and
initiators.
ret_flags_understood FLAGS The set of return flags understood by the
caller.
Outputs:
o major_status INTEGER
o minor_status INTEGER
Return major status codes:
o GSS_S_COMPLETE indicates success.
o GSS_S_FAILURE indicates a general failure.
This function tells the mechanism (when one is eventually chosen and
invoked) that the application requests the given req_flags and is
prepared to check the flags in the given ret_flags_understood.
Mechanisms SHOULD NOT limit flags returned to those in
ret_flags_understood, but MAY alter behavior accordingly. Initiators
can override the req_flags in their GSS_Init_sec_context() call, but
if no flags are requested there then the req_flags set on the empty
context will be used. GSS_Accept_sec_context() is not required to
perform any action based on req_flags at this time.
NOTE: The abstract GSS-API [RFC2743] uses individual elements--one
per-flag--instead of a "FLAGS" type. This is unwieldy, therefore we
introduce an abstract type named "FLAGS" to act as a set of all the
request/return flags defined for the abstract GSS-API.
2.2.1. C-Bindings
OM_uint32
gss_set_context_flags(OM_uint32 *minor_status,
gss_ctx_id_t context,
uint64_t req_flags,
uint64_t ret_flags_understood);
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2.3. Return Flag for Channel Binding State Signalling
Whenever both the initiator and the acceptor provide matching channel
bindings to GSS_Init_sec_context() and GSS_Accept_sec_context(),
respectively, then the mechanism SHALL indicate that the context is
channel bound via an output flag, ret_channel_bound_flag, for the
established context. Note that some mechanisms have no way for the
acceptor to signal CB success to the initiator, in which case
GSS_Init_sec_context() MUST NOT output the ret_channel_bound_flag.
2.3.1. C-Bindings
#define GSS_C_CHANNEL_BOUND_FLAG 2048 /* 0x00000800 */
2.4. New Mechanism Attribute
o We add a new mechanism attribute, GSS_C_MA_CBINDING_CONFIRM, to
indicate that the initiator can and always does learn whether the
acceptor application supplied channel bindings (assuming mutual
auth has been requested). Mechanisms advertising this attribute
MUST always indicate acceptor channel bound state to the
initiator.
OID assignments TBD.
2.5. Request Flag for Acceptor Confirmation of Channel Binding
We add a new request flag for GSS_Init_sec_context(),
req_cb_confirmation_flag, to be used by initiators that insist on
acceptors providing channel bindings. If set, the mechanism MUST
prefer establishment of contexts which provide channel binding
confirmation. It SHOULD NOT fail to negotiate just because it cannot
provide the GSS_C_MA_CBINDING_CONFIRM attribute.
2.5.1. C-Bindings
Because GSS_C_CHANNEL_BOUND_FLAG is a return flag only, and this flag
is a request flag only, and to save on precious flag bits, we use the
same flag bit assignment for both flags:
#define GSS_C_CB_CONFIRM_FLAG 2048 /* 0x00000800 */
2.6. Handling Empty Contexts in Other GSS-API Functions
GSS_Init_sec_context() and GSS_Accept_sec_context() operate on empty
security contexts as specified above (i.e., examining flags).
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All other GSS-API functions MUST treat empty contexts as they would
GSS_C_NO_CONTEXT as well. For most functions, this will result in
returning GSS status code GSS_S_NO_CONTEXT.
GSS_Delete_sec_context() MUST NOT output a context deletion token
when applied to empty security contexts.
3. Modified Channel Binding Semantics
The channel binding semantics of the base GSS-API are modified as
follows:
o Whenever both the initiator and acceptor have provided
input_channel_bindings to GSS_Init/Accept_sec_context() and the
channel bindings do not match, then the mechanism MUST fail to
establish a security context token. (This is a restatement of an
existing requirement in the base specification.)
o Whenever the acceptor application has a) provided channel bindings
to GSS_Accept_sec_context(), and b) not indicated support for the
ret_channel_bound_flag flag, then the mechanism MUST fail to
establish a security context if the initiator did not provide
channel bindings data. This requirement is critical for security
purposes, to make applications predating this document secure, and
this requirement reflects actual implementations as deployed.
o Whenever the initiator application has a) provided channel
bindings to GSS_Init_sec_context(), and b) not indicated support
for the ret_channel_bound_flag flag, then the mechanism SHOULD NOT
fail to establish a security context just because the acceptor
failed to provide channel bindings data. This strong sugestion is
for interoperability purposes, and reflects actual implementations
that have been deployed.
o Whenever the application has a) provided channel bindings to
GSS_Init_sec_context() or GSS_Accept_sec_context(), and b)
indicated support for the ret_channel_bound_flag flag, then the
mechanism SHOULD NOT fail to establish a security context just
because the peer did not provide channel bindings data. The
mechanism MUST output the ret_channel_bound_flag if both peers
provided the same input_channel_bindings to GSS_Init_sec_context()
and GSS_Accept_sec_context(). The mechanism MUST NOT output the
ret_channel_bound_flag if either (or both) peer did not provide
input_channel_bindings to GSS_Init/Accept_sec_context(). This
requirement restores the original base GSS-API specified behavior,
with the addition of the ret_channel_bound_flag flag.
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4. Security Considerations
This document deals with security. There are no security
considerations that should be documented separately in this section.
To recap, this document fixes a significant flaw in the base GSS-API
[RFC2743] specification that fortunately has not been implemented,
and it adds a feature (that should have been in the base
specification) for improved negotiation of use of channel binding
[RFC5056].
5. IANA Considerations
The GSS-API mechanism attribute is to be added to the "SMI Security
for Mechanism gsscma Codes" registry established by RFC5587
[RFC5587]. See Section 2.4.
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743,
DOI 10.17487/RFC2743, January 2000,
<https://www.rfc-editor.org/info/rfc2743>.
[RFC2744] Wray, J., "Generic Security Service API Version 2 :
C-bindings", RFC 2744, DOI 10.17487/RFC2744, January 2000,
<https://www.rfc-editor.org/info/rfc2744>.
[RFC5056] Williams, N., "On the Use of Channel Bindings to Secure
Channels", RFC 5056, DOI 10.17487/RFC5056, November 2007,
<https://www.rfc-editor.org/info/rfc5056>.
[RFC5587] Williams, N., "Extended Generic Security Service Mechanism
Inquiry APIs", RFC 5587, DOI 10.17487/RFC5587, July 2009,
<https://www.rfc-editor.org/info/rfc5587>.
6.2. Informative References
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[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,
DOI 10.17487/RFC4121, July 2005,
<https://www.rfc-editor.org/info/rfc4121>.
[RFC5653] Upadhyay, M. and S. Malkani, "Generic Security Service API
Version 2: Java Bindings Update", RFC 5653,
DOI 10.17487/RFC5653, August 2009,
<https://www.rfc-editor.org/info/rfc5653>.
Authors' Addresses
Robbie Harwood
Red Hat, Inc.
Email: rharwood@redhat.com
Nicolas Williams
Cryptonector, LLC
Email: nico@cryptonector.com
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