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NETWORK WORKING GROUP N. Williams
Internet-Draft Sun
Expires: November 13, 2005 May 12, 2005
A PRF API extension for the GSS-API
draft-ietf-kitten-gssapi-prf-03.txt
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
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."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on November 13, 2005.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document defines a Pseudo-Random Function (PRF) extension to the
Generic Security Service Application Programming Interface (GSS-API)
for keying application protocols given an established GSS-API
security context. The primary intended use of this function is to
key secure session layers that don't or cannot use GSS-API per-
message MIC (message integrity check) and wrap tokens for session
protection.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Conventions used in this document . . . . . . . . . . . . . . 3
2. GSS_Pseudo_random() . . . . . . . . . . . . . . . . . . . . . 3
2.1 C-Bindings . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2 Java Bindings . . . . . . . . . . . . . . . . . . . . . . . . 6
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
4. Security Considerations . . . . . . . . . . . . . . . . . . . 6
5. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.1 Normative References . . . . . . . . . . . . . . . . . . . . . 7
5.2 Informative References . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 7
Intellectual Property and Copyright Statements . . . . . . . . 8
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1. Introduction
A need has arisen for users of the GSS-API to key applications'
cryptographic protocols using established GSS-API security contexts.
Such applications can use the GSS-API for authentication, but not for
transport security (for whatever reasons), and since the GSS-API does
not provide a method for obtaining keying material from established
security contexts such applications cannot make effective use of the
GSS-API.
To address this need we define a pseudo-random function (PRF)
extension to the GSS-API.
1.1 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. GSS_Pseudo_random()
Inputs:
o context CONTEXT handle,
o prf_key INTEGER,
o prf_in OCTET STRING,
o desired_output_len INTEGER
Outputs:
o major_status INTEGER,
o minor_status INTEGER,
o prf_out OCTET STRING
Return major_status codes:
o GSS_S_COMPLETE indicates no error.
o GSS_S_NO_CONTEXT indicates that a null context has been provided
as input.
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o GSS_S_CONTEXT_EXPIRED indicates that an expired context has been
provided as input.
o GSS_S_UNAVAILABLE indicates that the mechanism lacks support for
this function or, if the security context is not fully
established, that the context is not ready to compute the PRF with
the given prf_key, or that the given prf_key is not available.
o GSS_S_FAILURE indicates general failure, possibly due to the given
input data being too large or of zero length, or due to the
desired_output_len being zero; the minor status code may provide
additional information.
This function applies the established context's mechanism's keyed
pseudo-random function (PRF) to the input data ('prf_in'), keyed with
key material associated with the given security context and
identified by 'prf_key', and outputs the resulting octet string
('prf_out') of desired_output_len length.
The minimum input data length is one octet.
Mechanisms MUST be able to consume all the provided prf_in input data
that is 2^14 or fewer octets.
If a mechanism cannot consume as much input data as provided by the
caller, then GSS_Pseudo_random() MUST return GSS_S_FAILURE.
The minimum desired_output_len is one.
Mechanisms MUST be able to output at least up to 2^14 octets.
If the implementation cannot produce the desired output due to lack
of resources then it MUST output what it can and still return
GSS_S_COMPLETE.
The prf_key can take on the following values: GSS_C_PRF_KEY_FULL,
GSS_C_PRF_KEY_PARTIAL or mechanism-specific values, if any. This
parameter is intended to distinguish between the best cryptographic
keys that may be available only after full security context
establishment and keys that may be available prior to full security
context establishment. For some mechanisms, or contexts, those two
prf_key values MAY refer to the same cryptographic keys; for
mechanisms like the Kerberos V GSS-API mechanism [RFC1964] where one
peer may assert a key that may be considered better than the others
they MAY be different keys.
GSS_C_PRF_KEY_PARTIAL corresponds to a key that would be have been
used while the security context was partially established, even if it
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is fully established when GSS_Pseudo_random() is actually called.
Mechanism-specific prf_key values are intended to refer to any other
keys that may be available.
The GSS_C_PRF_KEY_FULL value corresponds to the best key available
for fully-established security contexts.
GSS_Pseudo_random() has the following properties:
o its output string MUST be a pseudo-random function [GGM1] [GGM2]
of the input keyed with key material from the given security
context -- the chances of getting the same output given different
input parameters should be exponentially small.
o when successfully applied to the same inputs by an initiator and
acceptor using the same security context, it MUST produce the
_same results_ for both, the initiator and acceptor, even if
called multiple times (as long as the security context is not
expired).
o upon full establishment of a security context all cryptographic
keys and/or negotiations used for computing the PRF with any
prf_key MUST be authenticated (mutually, if mutual authentication
is in effect for the given security context).
o the outputs of the mechanism's GSS_Pseudo_random() (for different
inputs) and its per-message tokens for the given security context
MUST be "cryptographically separate;" in other words, it must not
be feasible to recover key material for one mechanism operation or
transform its tokens and PRF outputs from one to the other given
only said tokens and PRF outputs. [This is a fancy way of saying
that key derivation and strong cryptographic operations and
constructions must be used.]
o as implied by the above requirement, it MUST NOT be possible to
access any raw keys of a security context through
GSS_Pseudo_random(), no matter what inputs are given.
Mechanisms MAY limit the output of the PRF, possibly in ways related
to the types of cryptographic keys available for the PRF function,
thus the prf_out output of GSS_Pseudo_random() MAY be smaller than
requested.
2.1 C-Bindings
#define GSS_C_PRF_KEY_FULL 0
#define GSS_C_PRF_KEY_PARTIAL 1
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OM_uint32 gss_pseudo_random(
OM_uint32 *minor_status,
gss_ctx_id_t context,
int prf_key,
const gss_buffer_t prf_in,
ssize_t desired_output_len,
gss_buffer_t prf_out
);
Additional major status codes for the C-bindings:
o GSS_S_CALL_INACCESSIBLE_READ
o GSS_S_CALL_INACCESSIBLE_WRITE
See [RFC2744].
2.2 Java Bindings
For Java GSS_Pseudo_random() maps to a GSSContext method, 'prf':
public static final int GSS_C_PRF_KEY_FULL = 0
public static final int GSS_C_PRF_KEY_PARTIAL = 1
public byte[] prf(int prf_key, byte inBuf[], int outlen)
throws GSSException
See [RFC2853].
3. IANA Considerations
This document has no IANA considerations currently. If and when a
relevant IANA registry of GSS-API symbols is created then the generic
and language-specific function names, constant names and constant
values described above should be added to such a registry.
4. Security Considerations
Care should be taken in properly designing a mechanism's PRF
function.
GSS mechanisms' PRF functions should use a key derived from contexts'
authenticated session keys and should preserve the forward security
properties of the mechanisms' key exchanges.
Some mechanisms may support the GSS PRF function with security
contexts that are not fully established, but applications MUST assume
that authentication, mutual or otherwise, has not completed until the
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security context is fully established.
Callers of GSS_Pseudo_random() should avoid accidentally calling it
with the same inputs. One useful technique is to prepend to the
prf_in input string, by convention, a string indicating the intended
purpose of the PRF output in such a way that unique contexts in which
the function is called yield unique inputs to it.
5. References
5.1 Normative References
[GGM1] Goldreich, O., Goldwasser, S., and S. Micali, "How to
Construct Random Functions", October 1986.
[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.
[RFC2853] Kabat, J. and M. Upadhyay, "Generic Security Service API
Version 2 : Java Bindings", RFC 2853, June 2000.
5.2 Informative References
[GGM2] Goldreich, O., Goldwasser, S., and S. Micali, "On the
Cryptographic Applications of Random Functions", 1985.
[RFC1750] Eastlake, D., Crocker, S., and J. Schiller, "Randomness
Recommendations for Security", RFC 1750, December 1994.
[RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
RFC 1964, June 1996.
Author's Address
Nicolas Williams
Sun Microsystems
5300 Riata Trace Ct
Austin, TX 78727
US
Email: Nicolas.Williams@sun.com
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Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Disclaimer of Validity
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2005). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
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NETWORK WORKING GROUP N. Williams
Internet-Draft Sun
Expires: November 13, 2005 May 12, 2005
A PRF for the Kerberos V GSS-API Mechanism
draft-ietf-kitten-krb5-gssapi-prf-03.txt
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
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."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on November 13, 2005.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document defines the Pseudo-Random Function (PRF) for the
Kerberos V mechanism for the Generic Security Service Application
Programming Interface (GSS-API), based on the PRF defined for the
Kerberos V cryptographic framework, for keying application protocols
given an established Kerberos V GSS-API security context.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Conventions used in this document . . . . . . . . . . . . . . 3
2. Kerberos V GSS Mechanism PRF . . . . . . . . . . . . . . . . . 3
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
4. Security Considerations . . . . . . . . . . . . . . . . . . . 4
5. Normative References . . . . . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 5
Intellectual Property and Copyright Statements . . . . . . . . 6
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1. Introduction
This document specifies the Kerberos V GSS-API mechanism's pseudo-
random function corresponding to [GSS-PRF]. The function is a "PRF+"
style construction.
1.1 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. Kerberos V GSS Mechanism PRF
The GSS-API PRF [GSS-PRF] function for the Kerberos V mechanism
[RFC1964] shall be the output of a PRF+ function based on the
encryption type's PRF function keyed with the negotiated session key
of the security context corresponding to the 'prf_key' input
parameter of GSS_Pseudo_random().
This PRF+ MUST be keyed with the key indicated by the 'prf_key' input
parameter as follows:
o GSS_C_PRF_KEY_FULL -- use the sub-session key asserted by the
acceptor, if any, or the sub-session asserted by the initiator, if
any, or the Ticket's session key
o GSS_C_PRF_KEY_PARTIAL -- use the sub-session key asserted by the
initiator, if any, or the Ticket's session key
The PRF+ function is a simple counter-based extension of the Kerberos
V pseudo-random function [RFC3961] for the encryption type of the
security context's keys:
PRF+(K, L, S) = truncate(L, T1 || T2 || .. || Tn)
Tn = pseudo-random(K, n || S)
where '||' is the concatenation operator, 'n' is encoded as a network
byte order 32-bit unsigned binary number, truncate(L, S) truncates
the input octet string S to length L, and pseudo-random() is the
Kerberos V pseudo-random function [RFC3961].
The maximum output size of the Kerberos V mechanism's GSS-API PRF
then is, necessarily, 2^32 times the output size of the pseudo-
random() function for the encryption type of the given key.
When the input size is longer than 2^14 octets as per [GSS-PRF] and
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exceeds an implementation's resources then the mechanism MUST return
GSS_S_FAILURE and GSS_KRB5_S_KG_INPUT_TOO_LONG as the minor status
code.
3. IANA Considerations
This document has no IANA considerations currently. If and when a
relevant IANA registry of GSS-API symbols and constants is created
then the GSS_KRB5_S_KG_INPUT_TOO_LONG minor status code should be
added to such a registry.
4. Security Considerations
Kerberos V encryption types' PRF functions use a key derived from
contexts' session keys and should preserve the forward security
properties of the mechanisms' key exchanges.
Legacy Kerberos V encryption types may be weak, particularly the
single-DES encryption types.
See also [GSS-PRF] for generic security considerations of
GSS_Pseudo_random().
See also [RFC3961] for generic security considerations of the
Kerberos V cryptographic framework.
Care should be taken not to exceed the useful lifetime of an
established security context's session key's useful lifetime as
implementations are not required to prevent overuse of the
GSS_Pseudo_random() function. This can effectively be achieved by
limiting the number of GSS_Pseudo_random() calls to, say, a handful
of calls per-security context.
Use of Ticket session keys, rather than sub-session keys, when
initiators and acceptors fail to assert sub-session keys, is
dangerous as ticket reuse can lead to key reuse, therefore initiators
should assert sub-session keys always, and acceptors should assert
sub-session keys at least when initiators fail to do so..
The computational cost of computing this PRF+ may vary depending on
the Kerberos V encryption types being used, but generally the
computation of this PRF+ gets more expensive as the input and output
octet string lengths grow (note that the use of a counter in the PRF+
construction allows for parallelization). This means that if an
application can be tricked into providing very large input octet
strings and requesting very long output octet strings then that may
constitute a denial of service attack on the application; therefore
applications SHOULD place appropriate limits on the size of any input
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octet strings received from their peers without integrity protection.
5. Normative References
[CFX] Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos
Version 5 GSS-API Mechanism: Version 2".
[GSS-PRF] Williams, N., "A PRF API extension for the GSS-API".
[RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
RFC 1964, June 1996.
[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.
[RFC3961] Raeburn, K., "Encryption and Checksum Specifications for
Kerberos 5", RFC 3961, February 2005.
Author's Address
Nicolas Williams
Sun Microsystems
5300 Riata Trace Ct
Austin, TX 78727
US
Email: Nicolas.Williams@sun.com
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Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Disclaimer of Validity
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2005). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
Williams Expires November 13, 2005 [Page 6]