Kerberos GSS-API PRF: A Pseudo-Random Function (PRF) for the Kerberos V Generic Security Service Application Program Interface (GSS-API) Mechanism
Generating fresh secret keys from an existing Kerberos security context.
Williams, N., "A Pseudo-Random Function (PRF) for the Kerberos V Generic Security Service Application Program Interface (GSS-API) Mechanism", RFC 4402, DOI 10.17487/RFC4402, February 2006, <https://www.rfc-editor.org/info/rfc4402>.
@misc{rfc4402,
series = {Request for Comments},
number = 4402,
howpublished = {RFC 4402},
publisher = {RFC Editor},
doi = {10.17487/RFC4402},
url = {https://www.rfc-editor.org/info/rfc4402},
author = {N. Williams},
title = {{A Pseudo-Random Function (PRF) for the Kerberos V Generic Security Service Application Program Interface (GSS-API) Mechanism}},
pagetotal = 5,
year = 2006,
month = feb,
} RFC text
Contents
Network Working Group N. Williams
Request for Comments: 4402 Sun
Category: Standards Track February 2006
A Pseudo-Random Function (PRF) for the Kerberos V Generic Security
Service Application Program Interface (GSS-API) Mechanism
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) The Internet Society (2006).
Abstract
This document defines the Pseudo-Random Function (PRF) for the
Kerberos V mechanism for the Generic Security Service Application
Program 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.
Table of Contents
1. Introduction ....................................................2
1.1. Conventions Used in This Document ..........................2
2. Kerberos V GSS Mechanism PRF ....................................2
3. IANA Considerations .............................................3
4. Security Considerations .........................................3
5. Normative References ............................................4
1. Introduction
This document specifies the Kerberos V GSS-API mechanism's [RFC4121]
pseudo-random function corresponding to [RFC4401]. The function is a
"PRF+" style construction. For more information see [RFC4401],
[RFC2743], [RFC2744], and [RFC4121].
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 [RFC4401] function for the Kerberos V mechanism
[RFC4121] 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 [RFC4401] and
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 [RFC4401] for generic security considerations of
GSS_Pseudo_random().
See also [RFC3961] for generic security considerations of the
Kerberos V cryptographic framework.
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
octet strings received from their peers without integrity protection.
5. 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.
[RFC3961] Raeburn, K., "Encryption and Checksum Specifications for
Kerberos 5", RFC 3961, February 2005.
[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.
[RFC4401] Williams, N., "A Pseudo-Random Function (PRF) API
Extension for the Generic Security Service Application
Program Interface (GSS-API)", RFC 4401, February 2006.
Author's Address
Nicolas Williams
Sun Microsystems
5300 Riata Trace Ct
Austin, TX 78727
US
EMail: Nicolas.Williams@sun.com
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On this page
§1Purpose
This RFC defines the pseudo-random function (PRF) for the Kerberos V mechanism of GSS-API. GSS-API is a generic interface that applications use for security operations like encryption and authentication. Once two applications have established a Kerberos security context—proving their identity to each other—they need a way to generate fresh secret keys for protecting messages with different protocols.
The PRF defined here is based on the one in RFC 3961 (the Kerberos V cryptographic framework). It allows applications to derive keys of arbitrary length from the session key established during Kerberos authentication.
When two programs have proven they trust each other, they need new secret numbers for different jobs. This explains the method for making them.
§2The PRF+ Function
The function is called PRF+ and is a counter-based extension of the basic Kerberos V PRF. It takes three inputs: a key (K), a length in octets (L), and a source string (S). The output is the first L octets of a concatenated sequence of values.
Each value is computed by applying the Kerberos pseudo-random function to a counter (starting at 1) concatenated with the source string. The counter increments for each block, allowing generation of output longer than the PRF's native output size. The maximum output size is 2^32 times the basic PRF output for a given encryption type.
The implementation must handle two key types: GSS_C_PRF_KEY_FULL (preferring sub-session keys from the acceptor, then initiator, or the ticket session key) and GSS_C_PRF_KEY_PARTIAL (using the initiator's sub-session key or the ticket session key).
The function counts up from 1, mixing each number with a secret string and running a scrambler. This makes a long stream of random-looking bits that programs can use as secret keys.
§3Security Concerns
The PRF values depend on the encryption type in use. Legacy Kerberos encryption types, particularly single-DES, may be weak. Applications should ensure that initiators and acceptors exchange sub-session keys; using only the ticket session key creates a risk of key reuse across different contexts.
The computational cost of PRF+ can be significant for large inputs and outputs. Applications should limit the size of input strings accepted from untrusted peers to prevent denial-of-service attacks. Despite the counter-based construction allowing parallelization, an attacker could request very long output to consume resources.
Old encryption types can be weak. If programs reuse the same master secret for many jobs, a thief who cracks one job might crack them all. Also, asking for a huge stream of secret bits all at once could slow down the program.