Internet Engineering Task Force (IETF)
Request for Comments: 7802
Obsoletes: 4402
Category: Standards Track
ISSN: 2070-1721
S. Emery
N. Williams
March 2016

A Pseudo-Random Function (PRF) for the Kerberos V Generic Security

Service Application Program Interface (GSS-API) Mechanism


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.

This document obsoletes RFC 4402 and reclassifies that document as Historic. RFC 4402 starts the PRF+ counter at 1; however, a number of implementations start the counter at 0. As a result, the original specification would not be interoperable with existing implementations.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at

Copyright Notice

Copyright © 2016 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. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   2
   3.  Kerberos V GSS Mechanism PRF  . . . . . . . . . . . . . . . .   3
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   3
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   6.  Normative References  . . . . . . . . . . . . . . . . . . . .   4
   Appendix A.  Test Vectors . . . . . . . . . . . . . . . . . . . .   6
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

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].

This document obsoletes RFC 4402 and reclassifies that document as Historic. RFC 4402 starts the PRF+ counter at 1; however, a number of implementations start the counter at 0. As a result, the original specification would not be interoperable with existing implementations.

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. 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:

  • GSS_C_PRF_KEY_FULL -- use the sub-session key asserted by the acceptor (if any exists), or the sub-session asserted by the initiator (if any exists), or the Ticket's session key.
  • GSS_C_PRF_KEY_PARTIAL -- use the sub-session key asserted by the initiator (if any exists) 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, T0 || T1 || .. || Tn)

         Tn = pseudo-random(K, n || S)

where K is the key indicated by the 'prf_key' parameter, '||' 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.

4. 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.

5. 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).

6. 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,
   [RFC2743]  Linn, J., "Generic Security Service Application Program
              Interface Version 2, Update 1", RFC 2743,
              DOI 10.17487/RFC2743, January 2000,
   [RFC2744]  Wray, J., "Generic Security Service API Version 2 :
              C-bindings", RFC 2744, DOI 10.17487/RFC2744, January 2000,
   [RFC3961]  Raeburn, K., "Encryption and Checksum Specifications for
              Kerberos 5", RFC 3961, DOI 10.17487/RFC3961, 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,
              DOI 10.17487/RFC4121, July 2005,
   [RFC4401]  Williams, N., "A Pseudo-Random Function (PRF) API
              Extension for the Generic Security Service Application
              Program Interface (GSS-API)", RFC 4401,
              DOI 10.17487/RFC4401, February 2006,

Appendix A. Test Vectors

Here are some test vectors from the MIT implementation provided by Greg Hudson. Test cases used include input string lengths of 0 and 61 bytes, and an output length of 44 bytes. 61 bytes of input is just enough to produce a partial second MD5 or SHA1 hash block with the four-byte counter prefix. 44 bytes of output requires two full and one partial RFC 3961 PRF output for all existing enctypes. All keys were randomly generated.

   Enctype: des-cbc-crc
   Key: E607FE9DABB57AE0
   Input: (empty string)
   Output: 803C4121379FC4B87CE413B67707C4632EBED2C6D6B7
   Enctype: des-cbc-crc
   Key: 54758316B6257A75
   Input: ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz123456789
   Output: 279E4105F7ADC9BD6EF28ABE31D89B442FE0058388BA
   Enctype: des3-cbc-sha1
   Key: 70378A19CD64134580C27C0115D6B34A1CF2FEECEF9886A2
   Input: (empty string)
   Output: 9F8D127C520BB826BFF3E0FE5EF352389C17E0C073D9
   Enctype: des3-cbc-sha1
   Key: 3452A167DF1094BA1089E0A20E9E51ABEF1525922558B69E
   Input: ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz123456789
   Output: 6BF24FABC858F8DD9752E4FCD331BB831F238B5BE190
   Enctype: rc4-hmac
   Key: 3BB3AE288C12B3B9D06B208A4151B3B6
   Input: (empty string)
   Output: 9AEA11A3BCF3C53F1F91F5A0BA2132E2501ADF5F3C28
   Enctype: rc4-hmac
   Key: 6DB7B33A01BD2B72F7655CB7B3D5FA0B
   Input: ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz123456789
   Output: CDA9A544869FC84873B692663A82AFDA101C8611498B
   Enctype: aes128-cts-hmac-sha1-96
   Key: 6C742096EB896230312B73972FA28B5D
   Input: (empty string)
   Output: 94208D982FC1BB7778128BDD77904420B45C9DA699F3
   Enctype: aes128-cts-hmac-sha1-96
   Key: FA61138C109D834A477D24C7311BE6DA
   Input: ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz123456789
   Output: 0FAEDF0F842CC834FEE750487E1B622739286B975FE5
   Enctype: aes256-cts-hmac-sha1-96
   Key: 08FCDAFD5832611B73BA7B497FEBFF8C954B4B58031CAD9B977C3B8C25192FD6
   Input: (empty string)
   Output: E627EFC14EF5B6D629F830C7109DEA0D3D7D36E8CD57
   Enctype: aes256-cts-hmac-sha1-96
   Key: F5B68B7823D8944F33F41541B4E4D38C9B2934F8D16334A796645B066152B4BE
   Input: ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz123456789
   Output: 112F2B2D878590653CCC7DE278E9F0AA46FA5A380B62
   Enctype: camellia128-cts-cmac
   Key: 866E0466A178279A32AC0BDA92B72AEB
   Input: (empty string)
   Output: 97FBB354BF341C3A160DCC86A7A910FDA824601DF677
   Enctype: camellia128-cts-cmac
   Key: D4893FD37DA1A211E12DD1E03E0F03B7
   Input: ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz123456789
   Output: 1DEE2FF126CA563A2A2326B9DD3F0095013257414C83
   Enctype: camellia256-cts-cmac
   Key: 203071B1AE77BD3D6FCE70174AF95C225B1CED46B35CF52B6479EFEB47E6B063
   Input: (empty string)
   Output: 9B30020634C10FDA28420CEE7B96B70A90A771CED43A
   Enctype: camellia256-cts-cmac
   Key: A171AD582C1AFBBAD52ABD622EE6B6A14D19BF95C6914B2BA40FFD99A88EC660
   Input: ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz123456789
   Output: A47CBB6E104DCC77E4DB48A7A474B977F2FB6A7A1AB6


This document is an update to RFC 4402, which was authored by Nico Williams. Greg Hudson has provided the test vectors based on MIT's implementation.

Authors' Addresses

   Shawn Emery
   Oracle Corporation
   500 Eldorado Blvd Bldg 1
   Broomfield, CO  78727
   United States

Nicolas Williams
Cryptonector, LLC