A Pseudo-Random
Function (PRF) for the Kerberos V Generic Security Service Application
Program Interface (GSS-API) MechanismOracle500 Eldorado Blvd Bldg 1BroomfieldCO78727USshawn.emery@oracle.comCryptonector, LLCnico@cryptonector.com
Security
NETWORK WORKING GROUPInternet-DraftThis 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
problems.This document specifies the Kerberos V GSS-API
mechanism's pseudo-random function
corresponding to . The function is a
"PRF+" style construction. For more information see [RFC4401],
[RFC2743], [RFC2744] and [RFC4121].The GSS-API PRF function for the
Kerberos V mechanism 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, or the sub-session
asserted by the initiator, if any, or the Ticket's
session keyGSS_C_PRF_KEY_PARTIAL -- use the sub-session key
asserted by the initiator, if any, or the
Ticket's session keyThe PRF+ function is a simple counter-based extension of
the Kerberos V pseudo-random function 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 .The counter 'n' MUST start at zero (0) and MUST be
incremented by one 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
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.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.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 .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.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 for generic security
considerations of GSS_Pseudo_random().See also 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. This document is an update to Nico Williams' RFC.
Greg Hudson has provided the test vectors based on MIT's
implementation.
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
2A55E878836E35E21600D915D590DED5B6D77BB30A1F
Enctype: des-cbc-crc
Key: 54758316B6257A75
Input: ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz123456789
Output: 279E4105F7ADC9BD6EF28ABE31D89B442FE0058388BA
33264ACB5729562DC637950F6BD144B654BE7700B2D6
Enctype: des3-cbc-sha1
Key: 70378A19CD64134580C27C0115D6B34A1CF2FEECEF9886A2
Input: (empty string)
Output: 9F8D127C520BB826BFF3E0FE5EF352389C17E0C073D9
AC4A333D644D21BA3EF24F4A886D143F85AC9F6377FB
Enctype: des3-cbc-sha1
Key: 3452A167DF1094BA1089E0A20E9E51ABEF1525922558B69E
Input: ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz123456789
Output: 6BF24FABC858F8DD9752E4FCD331BB831F238B5BE190
4EEA42E38F7A60C588F075C5C96A67E7F8B7BD0AECF4
Enctype: rc4-hmac
Key: 3BB3AE288C12B3B9D06B208A4151B3B6
Input: (empty string)
Output: 9AEA11A3BCF3C53F1F91F5A0BA2132E2501ADF5F3C28
3C8A983AB88757CE865A22132D6100EAD63E9E291AFA
Enctype: rc4-hmac
Key: 6DB7B33A01BD2B72F7655CB7B3D5FA0B
Input: ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz123456789
Output: CDA9A544869FC84873B692663A82AFDA101C8611498B
A46138B01E927C9B95EEC953B562807434037837DDDF
Enctype: aes128-cts-hmac-sha1-96
Key: 6C742096EB896230312B73972FA28B5D
Input: (empty string)
Output: 94208D982FC1BB7778128BDD77904420B45C9DA699F3
117BCE66E39602128EF0296611A6D191A5828530F20F
Enctype: aes128-cts-hmac-sha1-96
Key: FA61138C109D834A477D24C7311BE6DA
Input: ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz123456789
Output: 0FAEDF0F842CC834FEE750487E1B622739286B975FE5
B7F45AB053143C75CA0DF5D3D4BBB80F6A616C7C9027
Enctype: aes256-cts-hmac-sha1-96
Key: 08FCDAFD5832611B73BA7B497FEBFF8C954B4B58031CAD9B977C3B8C25192FD6
Input: (empty string)
Output: E627EFC14EF5B6D629F830C7109DEA0D3D7D36E8CD57
A1F301C5452494A1928F05AFFBEE3360232209D3BE0D
Enctype: aes256-cts-hmac-sha1-96
Key: F5B68B7823D8944F33F41541B4E4D38C9B2934F8D16334A796645B066152B4BE
Input: ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz123456789
Output: 112F2B2D878590653CCC7DE278E9F0AA46FA5A380B62
59F774CB7C134FCD37F61A50FD0D9F89BF8FE1A6B593
Enctype: camellia128-cts-cmac
Key: 866E0466A178279A32AC0BDA92B72AEB
Input: (empty string)
Output: 97FBB354BF341C3A160DCC86A7A910FDA824601DF677
68797BACEEBF5D250AE929DEC9760772084267F50A54
Enctype: camellia128-cts-cmac
Key: D4893FD37DA1A211E12DD1E03E0F03B7
Input: ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz123456789
Output: 1DEE2FF126CA563A2A2326B9DD3F0095013257414C83
FAD4398901013D55F367C82681186B7B2FE62F746BA4
Enctype: camellia256-cts-cmac
Key: 203071B1AE77BD3D6FCE70174AF95C225B1CED46B35CF52B6479EFEB47E6B063
Input: (empty string)
Output: 9B30020634C10FDA28420CEE7B96B70A90A771CED43A
D8346554163E5949CBAE2FB8EF36AFB6B32CE75116A0
Enctype: camellia256-cts-cmac
Key: A171AD582C1AFBBAD52ABD622EE6B6A14D19BF95C6914B2BA40FFD99A88EC660
Input: ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz123456789
Output: A47CBB6E104DCC77E4DB48A7A474B977F2FB6A7A1AB6
52317D50508AE72B7BE2E4E4BA24164E029CBACF786B