Kerberos Working Group K. Raeburn
Updates: Kerberos-revisions MIT
Document: draft-raeburn-krb-rijndael-krb-00.txt November 17, 2000
Rijndael, Twofish, and Serpent Cryptosystems
for Kerberos 5
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026 [RFC2026]. Internet-Drafts
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1. Abstract
The AES competition in the US [AES] has prompted the submission and
analysis of a number of new ciphers intended to be significantly
stronger and faster than the old DES algorithm. This document
describes the addition of some of these algorithms to the Kerberos
cryptosystem suite.
Comments should be sent to the author, or to the IETF Kerberos
working group (ietf-krb-wg@anl.gov).
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 RFC 2119.
3. New Encryption and Checksum Types
This document defines encryption key and checksum types for Kerberos
5 to be used with the Rijndael (chosen by NIST as the AES cipher),
Twofish and Serpent encryption algorithms. The other AES finalists
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appear more problematic from an intellectual property perspective
(involving licenses or patents), and so are not being addressed by
the author.
Each of these algorithms, as required by the AES specifications,
supports 128-bit block encryption. Longer block sizes are also
supported by some of these algorithms, but will not be used in
Kerberos.
Each of these algorithms permits 128-, 192- and 256-bit keys. Their
use in Kerberos will permit all of these key sizes.
The Twofish specification also describes a means for handling other
key sizes in between. Keys of these other sizes are effectively
converted into 192- or 256-bit keys. In order to avoid having
multiple representations of a single key causing potential confusion,
and to simplify the key derivation specification, Twofish keys in
Kerberos will always be 128, 192, or 256 bits long.
The EncryptedData objects are generated as described in [Kerb] for
des3-cbc-hmac-sha1, using one of the above encryption algorithms in
CBC mode, and a checksum algorithm of HMAC-SHA256. Unless otherwise
specified, a zero initial vector must be used for CBC mode.
(Q: Will NIST's new modes of operation include anything we might
prefer over CBC-encrypt plus checksum? Should we go ahead with this
anyways?)
These new cryptosystems will use key derivation as described in
[Kerb], with derived keys having the same length as the original
keys. The new keys will be the byte sequences generated from the key
derivation algorithm; no adjustments (such as creating parity bits
for triple-DES) are needed. Thus the number of bits required as
output are the same as the key size.
(Open question: Should we drop key derivation? The author is
somewhat but not overwhelmingly or, he likes to think, blindly in
favor of keeping it. Should we revive the argument the author
completely missed when it came up in regard to triple-DES? Perhaps
not.)
The confounder is one block, prepended to the data. The input data
is padded with zero to fifteen trailing zero-valued octets to make it
a multiple of the block size.
Since the Kerberos protocol always passes around the key type and
length as part of the EncryptionKey data, we can take advantage of
this when defining checksum types, such that a checksum algorithm can
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accept any length of key, and in the case of key derivation, use the
encryption algorithm specified by the key type when deriving a new
key. Thus we define only one new value for the sumtype field, for an
HMAC using the SHA-256 algorithm.
assigned numbers (Cliff?):
+--------------------------------------------------------------------+
| encryption types |
+--------------------------------------------------------------------+
| type name etype value key sizes |
+--------------------------------------------------------------------+
| rijndael-hmac-sha256-kd TBD 128, 192, 256 |
| twofish-hmac-sha256-kd TBD 128, 192, 256 |
| serpent-hmac-sha256-kd TBD 128, 192, 256 |
+--------------------------------------------------------------------+
The alias "aes-hmac-sha256-kd" may be used for whichever of the above
types uses the algorithm chosen as the AES, if any. Currently,
Rijndael has been chosen, and the final AES will probably be Rijndael
in its current form, but the AES FIPS is not completed. We recommend
not using this alias until the final AES FIPS is published. (Q: Or,
is it definite that there will be no changes?)
+--------------------------------------------------------------------+
| checksum types |
+--------------------------------------------------------------------+
| type name sumtype value checksum length |
+--------------------------------------------------------------------+
| hmac-sha256-kd TBD 256 |
+--------------------------------------------------------------------+
(Q: Better to just define hmac-sha256, and say that it uses key
derivation when the specified key type demands it?)
The checksum type hmac-sha256-kd will be used with the encryption
types defined above.
(Similarly, the hmac-sha1-des3 and hmac-sha1-des3-kd checksum types
in [Kerb] could be extended to be generic hmac-sha1 and hmac-sha1-kd
checksums, making use of as much key data as is supplied, and the
specified encryption algorithm. Since this document isn't making use
of SHA-1, such changes are outside its scope.)
4. Key Generation From Pass Phrases
As the des3-cbc-hmac-sha1-kd encryption type is specified in [Kerb],
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the recommended algorithm for generating a key from a pass phrase
(primarily for users' long-term keys, as is assumed in the
descriptive text here, but also occasionally for other purposes)
involves n-folding the pass phrase to produce an intermediate
encryption key, which is fed into the key derivation algorithm with a
well-known constant to produce the final key of the user. While the
n-fold function does cause the bits of the input string to contribute
equally to the output ([n-fold]), there are cases in which it does a
poor job of entropy preservation, and indeed entropy preservation was
never described as a property of the algorithm in the original paper.
Thus for these algorithms we use the new NIST hash function SHA-256
in generating the intermediate key. The catenation of salt and UTF-8
pass phrase is passed to the SHA-256 function. The two halves of the
hash function output are XORed together to get a 128-bit intermediate
key. This key is passed into the key derivation algorithm with the
constant string "kerberos" as in [Kerb]. The resulting 128-bit key
is the user's long-term key.
Since in general memorable pass phrases will give nowhere near one
block's worth of entropy, the author sees no need to make this
algorithm capable of generating longer keys at this time.
Sample test vectors are given in the appendix.
(Q: Any weak keys?)
5. Recommendations
Rijndael, as the proposed AES cipher, is strongly RECOMMENDED.
Twofish and Serpent, described in the AES report as weaker that
Rijndael in terms of performance or implementability in certain
environments but stronger in terms of resistance to certain types of
possible attacks, are OPTIONAL.
6. Implementation notes
Preauthentication algorithms involving smart cards or other hardware
may provide additional unpredictability that may be used to generate
longer keys, or simply be factored into a stronger new 128-bit key.
Such schemes are outside the scope of this document, but implementors
should recognize that using longer keys with these algorithms for
AS_REP messages and preauth data may be plausible.
7. Security Considerations
These new algorithms have not been around long enough to receive the
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decades of intense analysis that DES has received. It is possible
that some weakness exists that has not been found by the
cryptosystems' authors or other cryptographers analyzing these
algorithms before and during the AES competition. The AES report
does indicate that arguments were put forth relating to this in favor
of deploying multiple algorithms in case one is found to be
significantly weaker than previously believed.
The 256-bit SHA algorithm is a work in progress by the US National
Institute of Standards and Technology. To the best of the author's
knowledge, the review process has not been completed. The use of
this algorithm in this document is with the assumption that the
standardization process will go smoothly.
The author is not a cryptographer.
8. References
[AES] Nechvatal, J., Barker, E., Bassham, L., Burr, W., Dworkin, M.,
Foti, J., Roback, E., "Report on the Development of the Advanced
Encryption Standard (AES)", National Institute of Standards and
Technology, October 2, 2000.
[Kerb] Neuman, C., Kohl, J., Ts'o, T., "The Kerberos Network
Authentication Service (V5)", draft-ietf-cat-kerberos-
revisions-06.txt, July 14, 2000. Work in progress.
[Rijn] Daemen, J., Rijmen, V., "AES Proposal: Rijndael", September 3,
1999. *
[Twof] Schneier, B., Kelsey, J., Whiting, D., Wagner, D., Hall, C.,
Ferguson, N., "The Twofish Encrytion Algorithm: A 128-Bit Block
Cipher", Wiley Computer Publishing, 1999.
[Serp] Anderson, R., Biham, E., Knudsen, L., "Serpent: A Proposal for
the Advanced Encryption Standard", June 1998. *
[RFC2026] Bradner, S., "The Internet Standards Process -- Revision
3", RFC 2026, October, 1996.
[SHA256] NIST doc ... *
[n-fold] Blumenthal & Bellovin ...
* Need more substantial references (RFCs or published papers) if
possible; web-accessible copy may not be a permanent reference.
9. Author's Address
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Kenneth Raeburn
Massachusetts Institute of Technology
77 Massachusetts Avenue
Cambridge, MA 02139
10. Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved.
This document and translations of it may be copied and furnished to
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or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
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included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
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English.
The limited permissions granted above are perpetual and will not be
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"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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A. Sample test vectors
Some sample test vectors for the string-to-key algorithm:
(values to be filled in later)
Salt: none
Pass phrase: "test"
74 65 73 74
SHA-256 folded to intermediate key:
xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx
Rijndael key:
xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx
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Twofish key:
xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx
Serpent key:
xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx
Salt: "ATHENA.MIT.EDUraeburn"
Pass phrase: "password"
...
SHA-256 folded to intermediate key:
xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx
Rijndael key:
xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx
Twofish key:
xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx
Serpent key:
xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx
Salt: none
Pass phrase: something with a variety of non-ASCII characters
...
SHA-256 folded to intermediate key:
xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx
Rijndael key:
xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx
Twofish key:
xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx
Serpent key:
xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx
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