draft-ietf-kitten-krb5-gssapi-prf-00.txt   draft-ietf-kitten-krb5-gssapi-prf-01.txt 
NETWORK WORKING GROUP N. Williams NETWORK WORKING GROUP N. Williams
Internet-Draft Sun Internet-Draft Sun
Expires: December 30, 2004 July 2004 Expires: December 30, 2004 July 2004
A PRF for the Kerberos V GSS-API Mechanism A PRF for the Kerberos V GSS-API Mechanism
draft-ietf-kitten-krb5-gssapi-prf-00.txt draft-ietf-kitten-krb5-gssapi-prf-01.txt
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and any of which I become aware will be disclosed, in accordance with and any of which I become aware will be disclosed, in accordance with
RFC 3668. RFC 3668.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
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This Internet-Draft will expire on December 30, 2004. This Internet-Draft will expire on December 30, 2004.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved. Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract Abstract
This document defines the Pseudo-Random Function (PRF) for the This document defines the Pseudo-Random Function (PRF) for the
Kerberos V GSS-API mechanism, based on the PRF defined 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 Kerberos V cryptographic framework, for keying application protocols
given an established Kerberos V GSS-API security context. given an established Kerberos V GSS-API security context.
Table of Contents Table of Contents
1. Conventions used in this document . . . . . . . . . . . . . . . 3 1. Conventions used in this document . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Kerberos V GSS Mechanism PRF . . . . . . . . . . . . . . . . . . 4
3. Security Considerations . . . . . . . . . . . . . . . . . . . . 5 3. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
4. Normative . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 4. Normative References . . . . . . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5
Intellectual Property and Copyright Statements . . . . . . . . . 6 Intellectual Property and Copyright Statements . . . . . . . . . 7
1. Conventions used in this document 1. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
2. Introduction 2. Kerberos V GSS Mechanism PRF
The GSS-API PRF [GSS-PRF] function for the Kerberos V mechanism shall The GSS-API PRF [GSS-PRF] function for the Kerberos V mechanism [CFX]
be the output of a PRF+ function based on the enctype's PRF function shall be the output of a PRF+ function based on the enctype's PRF
keyed with the negotiated session key of the security context (e.g., function keyed with the negotiated session key of the security
the acceptor's subkey) and key usage X (TBD). context and key usage X (TBD).
The PRF+ function is a simple counter-based extension of the The security context MUST be fully established, else the mechanism
enctype's prf using an 16-bit network byte order unsigned binary MUST fail with GSS_S_FAILURE as the major status code and
counter: PRF+(k, input) = random-to-key( k-truncate( prf(k, 0 || GSS_KRB5_S_KG_CTX_INCOMPLETE as the minor status code.
input) || prf(k, 1 || input), .. prf(k, n || input))). The maximum
output length for this PRF+ then is 65536 times the output length of
the Kerberos V cryptographic framework PRF for the enctype of the
input key.
If the desired_output_len input parameter exceeds the maximum output This PRF+ MUST be keyed with a key derived, with key usage (TBD),
of this function then the maximum will be output instead. from the session used by the initiator and acceptor, after the
security context is fully established, to derive keys for per-message
tokens. For the current Kerberos V mechanism [CFX] this means that
the PRF+ MUST be keyed with the acceptor-asserted subkey, if it did
assert such a key, or the initiator's sub-session key otherwise.
The PRF+ function is a simple counter-based extension of the Kerberos
V pseudo-random function [KRB5-CRYPTO] for the enctype of the
security context's keys:
PRF+(K, L, S) = truncate(L, T1 || T2 || .. || Tn)
Tn = pseudo-random-function(K, n || S)
where '||' is the concatenation operator, 'n' is encoded as a
network byte order 32-bit unsigned binary number, and where
truncate(L, S) truncates the input octet string S to length L.
The maximum output size of the Kerberos V mechanism's GSS-API PRF
then is, necessarily, 2^32 octets.
Implementations MUST support output size of up to 2^14 octets at
least.
If the implementation cannot produce the desired output then it MUST
output what it can.
The minimum input octet string length that implementations MUST
support is also 2^14 octets. If the input octet string is longer
than the maximum that an implementation can process then the
implementation MUST fail with GSS_S_FAILURE as the major status code
and GSS_KRB5_S_KG_INPUT_TOO_LONG as the minor status code.
3. Security Considerations 3. Security Considerations
Kerberos V enctypes' PRF functions use a key derived from contexts' Kerberos V enctypes' PRF functions use a key derived from contexts'
session keys and should preserve the forward security properties of session keys and should preserve the forward security properties of
the mechanisms' key exchanges. the mechanisms' key exchanges.
Legacy Kerberos V enctypes may be weak, particularly the single-DES
enctypes.
See also [GSS-PRF] for generic security considerations of See also [GSS-PRF] for generic security considerations of
GSS_Pseudo_random(). GSS_Pseudo_random().
4 Normative The computational cost of computing this PRF+ may vary depending on
the Kerberos V enctypes 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 constitue a
denial of service attack on the application; therefore applications
SHOULD place appropriate limits on the size of any input octet
strings received from their peers without integrity protection.
4 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". [GSS-PRF] Williams, N., "A PRF API extension for the GSS-API".
[KRB5-CRYPTO]
Raeburn, K., "Encryption and Checksum Specifications for
Kerberos 5".
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2743] Linn, J., "Generic Security Service Application Program [RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000. Interface Version 2, Update 1", RFC 2743, January 2000.
[RFC2744] Wray, J., "Generic Security Service API Version 2 : [RFC2744] Wray, J., "Generic Security Service API Version 2 :
C-bindings", RFC 2744, January 2000. C-bindings", RFC 2744, January 2000.
 End of changes. 

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