NETWORK WORKING GROUP                                           S. Emery
Internet-Draft                                                    Oracle
Obsoletes: 4402 (if approved)                                N. Williams
Intended status: Standards Track                            Cryptonector
Expires: October 30, 2014                                 April 28, 2014

   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 was underspecified, leading to interoperability

Status of This Memo

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   This Internet-Draft will expire on October 30, 2014.

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   the Trust Legal Provisions and are provided without warranty as
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Table of Contents

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

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.  Kerberos V GSS Mechanism PRF

   The GSS-API PRF [RFC4401] 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, T0 || T1 || .. || 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 counter 'n' MUST start at zero (0) and MUST be incremented by one

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   for each corresponding Tn.

   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

   This document obsoletes RFC 4402 and reclassifies that document as
   historic.  RFC 4402 starts the PRF+ counter at 1, however a number
   implementations starts the counter at 0.  As a result, the original
   specification would not be interoperable with existing
   implementations.  The figure showing the PRF+ construction was also
   modified to show the Tn sequence starting at T0.  These are the only
   two material changes since RFC 4402.

1.1.1.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  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.

3.  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

   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

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   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.

4.  Acknowledgements

   This document is an update to Nico Williams' RFC.  Greg Hudson has
   provided the test vectors based on MIT's implementation.

5.  Normative References

   [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.

   [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

   [RFC4401]  Williams, N., "A Pseudo-Random Function (PRF) API
              Extension for the Generic Security Service Application
              Program Interface (GSS-API)", RFC 4401, 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

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   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

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   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

Authors' Addresses

   Shawn Emery
   500 Eldorado Blvd Bldg 1
   Broomfield, CO  78727


   Nicolas             Williams
   Cryptonector, LLC


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