draft-raeburn-krb-rijndael-krb-02.txt [plain text]
Kerberos Working Group K. Raeburn
Document: draft-raeburn-krb-rijndael-krb-02.txt MIT
November 1, 2002
expires May 1, 2003
AES Encryption for Kerberos 5
Abstract
Recently the US National Institute of Standards and Technology chose
a new Advanced Encryption Standard [AES], which is significantly
faster and (it is believed) more secure than the old DES algorithm.
This document is a specification for the addition of this algorithm
to the Kerberos cryptosystem suite [KCRYPTO].
Comments should be sent to the author, or to the IETF Kerberos
working group (ietf-krb-wg@anl.gov).
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
are working documents of the Internet Engineering Task Force (IETF),
its areas, and its working groups. Note that other groups may also
distribute working documents as Internet-Drafts. Internet-Drafts are
draft documents valid for a maximum of six months and may be updated,
replaced, or obsoleted by other documents at any time. It is
inappropriate to use Internet-Drafts as reference material or to cite
them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
1. Introduction
This document defines encryption key and checksum types for Kerberos
5 using the AES algorithm recently chosen by NIST. These new types
support 128-bit block encryption, and key sizes of 128 or 256 bits.
Using the "simplified profile" of [KCRYPTO], we can define a pair of
encryption and checksum schemes. AES is used with cipher text
stealing to avoid message expansion, and SHA-1 [SHA1] is the
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associated checksum function.
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. Protocol Key Representation
The profile in [KCRYPTO] treats keys and random octet strings as
conceptually different. But since the AES key space is dense, we can
use any bit string as a key. We use the byte representation for the
key described in [AES], where the first bit of the bit string is the
high bit of the first byte of the byte string (octet string)
representation.
4. Key Generation From Pass Phrases or Random Data
Given the above format for keys, we can generate keys from the
appropriate amounts of random data (128 or 256 bits) by simply
copying the input string.
To generate an encryption key from a pass phrase and salt string, we
use the PBKDF2 function from PKCS #5 v2.0 ([PKCS5]), with parameters
indicated below, to generate an intermediate key (of the same length
as the desired final key), which is then passed into the DK function
with the 8-octet ASCII string "kerberos" as is done for des3-cbc-
hmac-sha1-kd in [KCRYPTO]. (In [KCRYPTO] terms, the PBKDF2 function
produces a "random octet string", hence the application of the
random-to-key function even though it's effectively a simple identity
operation.) The resulting key is the user's long-term key for use
with the encryption algorithm in question.
tkey = random2key(PBKDF2(passphrase, salt, iter_count, keylength))
key = DK(tkey, "kerberos")
The pseudorandom function used by PBKDF2 will be a SHA-1 HMAC of the
passphrase and salt, as described in Appendix B.1 to PKCS#5.
The number of iterations is specified by the string-to-key parameters
supplied. The parameter string is four octets indicating an unsigned
number in big-endian order. This is the number of iterations to be
performed. If the value is 00 00 00 00, the number of iterations to
be performed is 4294967296 (2**32). (Thus the minimum expressable
iteration count is 1.)
For environments where slower hardware is the norm, implementations
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may wish to limit the number of iterations to prevent a spoofed
response from consuming lots of client-side CPU time; it is
recommended that this bound be no less than 50000. Even for
environments with fast hardware, 4 billion iterations is likely to
take a fairly long time; much larger bounds might still be enforced,
and it might be wise for implementations to permit interruption of
this operation by the user if the environment allows for it.
If the string-to-key parameters are not supplied, the default value
to be used is 00 00 b0 00 (decimal 45056, indicating 45056
iterations, which takes slightly under 1 second on a 300MHz Pentium
II in tests run by the author).
Sample test vectors are given in the appendix.
5. Cipher Text Stealing
Cipher block chaining is used to encrypt messages. Unlike previous
Kerberos cryptosystems, we use cipher text stealing to handle the
possibly partial final block of the message.
Cipher text stealing is described on pages 195-196 of [AC], and
section 8 of [RC5]; it has the advantage that no message expansion is
done during encryption of messages of arbitrary sizes as is typically
done in CBC mode with padding.
Cipher text stealing, as defined in [RC5], assumes that more than one
block of plain text is available. Since a one-block confounder is
added in the simplified profile of [KCRYPTO], and [KCRYPTO] requires
that the message to be encrypted cannot be empty, the minimum length
to be encrypted is one block plus one byte. Thus we do not need to
do anything special to meet this constraint.
For consistency, cipher text stealing is always used for the last two
blocks of the data to be encrypted, as in [RC5]. If the data length
is a multiple of the block size, this is equivalent to plain CBC mode
with the last two cipher text blocks swapped.
A test vector is given in the appendix.
6. Kerberos Algorithm Profile Parameters
This is a summary of the parameters to be used with the simplified
algorithm profile described in [KCRYPTO]:
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+--------------------------------------------------------------------+
| protocol key format 128- or 256-bit string |
| |
| string-to-key function PBKDF2+DK with variable |
| iteration count (see |
| above) |
| |
| default string-to-key parameters 00 09 |
| |
| key-generation seed length key size |
| |
| random-to-key function identity function |
| |
| hash function, H SHA-1 |
| |
| HMAC output size, h 12 octets (96 bits) |
| |
| confounder size, c 16 octets |
| |
| message block size, m 1 octet |
| |
| encryption/decryption functions, AES in CBC-CTS mode with |
| E and D zero ivec |
+--------------------------------------------------------------------+
Using this profile with each key size gives us two each of encryption
and checksum algorithm definitions.
7. Assigned Numbers
The following encryption type numbers are assigned:
+--------------------------------------------------------------------+
| encryption types |
+--------------------------------------------------------------------+
| type name etype value key size |
+--------------------------------------------------------------------+
| aes128-cts-hmac-sha1-96 17 128 |
| aes256-cts-hmac-sha1-96 18 256 |
+--------------------------------------------------------------------+
The following checksum type numbers are assigned:
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+--------------------------------------------------------------------+
| checksum types |
+--------------------------------------------------------------------+
| type name sumtype value length |
+--------------------------------------------------------------------+
| hmac-sha1-96-aes128 10 96 |
| hmac-sha1-96-aes256 11 96 |
+--------------------------------------------------------------------+
These checksum types will be used with the corresponding encryption
types defined above.
8. Recommendations
Both new cryptosystems are RECOMMENDED. They should be more secure
than DES cryptosystems, and much faster than triple-DES.
9. Security Considerations
This new algorithm has not been around long enough to receive the
decades of intense analysis that DES has received. It is possible
that some weakness exists that has not been found by the
cryptographers analyzing these algorithms before and during the AES
selection process.
The use of the HMAC function has drawbacks for certain pass phrase
lengths. For example, a pass phrase longer than the hash function
block size (64 bytes, for SHA-1) is hashed to a smaller size (20
bytes) before applying the main HMAC algorithm. However, entropy is
generally sparse in pass phrases, especially in long ones, so this
may not be a problem in the rare cases of users with long pass
phrases.
Also, generating a 256-bit key from a pass phrase of any length may
be deceptive, since the effective entropy in pass-phrase-derived key
cannot be nearly that large.
The iteration count in PBKDF2 appears to be useful primarily as a
constant multiplier for the amount of work required for an attacker
using brute-force methods. Unfortunately, it also multiplies, by the
same amount, the work needed by a legitimate user with a valid
password. Thus the work factor imposed on an attacker (who may have
many powerful workstations at his disposal) must be balanced against
the work factor imposed on the legitimate user (who may have a PDA or
cell phone); the available computing power on either side increases
as time goes on, as well. A better way to deal with the brute-force
attack is through preauthentication mechanisms that provide better
protection of, the user's long-term key. Use of such mechanisms is
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out of scope for this document.
Any benefit against other attacks specific to the HMAC or SHA-1
algorithms is probably achieved with a fairly small number of
iterations.
Cipher text stealing mode, since it requires no additional padding,
will reveal the exact length of each message being encrypted, rather
than merely bounding it to a small range of possible lengths as in
CBC mode. Such obfuscation should not be relied upon at higher
levels in any case; if the length must be obscured from an outside
observer, it should be done by intentionally varying the length of
the message to be encrypted.
The author is not a cryptographer. Caveat emptor.
10. IANA Considerations
None.
11. Acknowledgements
Thanks to John Brezak, Gerardo Diaz Cuellar and Marcus Watts for
feedback on earlier versions of this document.
12. Normative References
[AC] Schneier, B., "Applied Cryptography", second edition, John Wiley
and Sons, New York, 1996.
[AES] National Institute of Standards and Technology, U.S. Department
of Commerce, "Advanced Encryption Standard", Federal Information
Processing Standards Publication 197, Washington, DC, November 2001.
[KCRYPTO] Raeburn, K., "Encryption and Checksum Specifications for
Kerberos 5", draft-ietf-krb-wg-crypto-01.txt, May, 2002. Work in
progress.
[PKCS5] Kaliski, B., "PKCS #5: Password-Based Cryptography
Specification Version 2.0", RFC 2898, September 2000.
[RC5] Baldwin, R, and R. Rivest, "The RC5, RC5-CBC, RC5-CBC-Pad, and
RC5-CTS Algorithms", RFC 2040, October 1996.
[RFC2026] Bradner, S., "The Internet Standards Process -- Revision
3", RFC 2026, October 1996.
[SHA1] National Institute of Standards and Technology, U.S.
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Department of Commerce, "Secure Hash Standard", Federal Information
Processing Standards Publication 180-1, Washington, DC, April 1995.
13. Informative References
[PECMS] Gutmann, P., "Password-based Encryption for CMS", RFC 3211,
December 2001.
14. Author's Address
Kenneth Raeburn
Massachusetts Institute of Technology
77 Massachusetts Avenue
Cambridge, MA 02139
raeburn@mit.edu
15. Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
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English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
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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
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."
A. Sample test vectors
Sample values for the string-to-key function are included below.
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Iteration count = 1
Pass phrase = "password"
Salt = "ATHENA.MIT.EDUraeburn"
128-bit PBKDF2 output:
cd ed b5 28 1b b2 f8 01 56 5a 11 22 b2 56 35 15
128-bit AES key:
42 26 3c 6e 89 f4 fc 28 b8 df 68 ee 09 79 9f 15
256-bit PBKDF2 output:
cd ed b5 28 1b b2 f8 01 56 5a 11 22 b2 56 35 15
0a d1 f7 a0 4b b9 f3 a3 33 ec c0 e2 e1 f7 08 37
256-bit AES key:
fe 69 7b 52 bc 0d 3c e1 44 32 ba 03 6a 92 e6 5b
bb 52 28 09 90 a2 fa 27 88 39 98 d7 2a f3 01 61
Iteration count = 2
Pass phrase = "password"
Salt="ATHENA.MIT.EDUraeburn"
128-bit PBKDF2 output:
01 db ee 7f 4a 9e 24 3e 98 8b 62 c7 3c da 93 5d
128-bit AES key:
c6 51 bf 29 e2 30 0a c2 7f a4 69 d6 93 bd da 13
256-bit PBKDF2 output:
01 db ee 7f 4a 9e 24 3e 98 8b 62 c7 3c da 93 5d
a0 53 78 b9 32 44 ec 8f 48 a9 9e 61 ad 79 9d 86
256-bit AES key:
a2 e1 6d 16 b3 60 69 c1 35 d5 e9 d2 e2 5f 89 61
02 68 56 18 b9 59 14 b4 67 c6 76 22 22 58 24 ff
Iteration count = 1200
Pass phrase = "password"
Salt = "ATHENA.MIT.EDUraeburn"
128-bit PBKDF2 output:
5c 08 eb 61 fd f7 1e 4e 4e c3 cf 6b a1 f5 51 2b
128-bit AES key:
4c 01 cd 46 d6 32 d0 1e 6d be 23 0a 01 ed 64 2a
256-bit PBKDF2 output:
5c 08 eb 61 fd f7 1e 4e 4e c3 cf 6b a1 f5 51 2b
a7 e5 2d db c5 e5 14 2f 70 8a 31 e2 e6 2b 1e 13
256-bit AES key:
55 a6 ac 74 0a d1 7b 48 46 94 10 51 e1 e8 b0 a7
54 8d 93 b0 ab 30 a8 bc 3f f1 62 80 38 2b 8c 2a
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Iteration count = 5
Pass phrase = "password"
Salt=0x1234567878563412
128-bit PBKDF2 output:
d1 da a7 86 15 f2 87 e6 a1 c8 b1 20 d7 06 2a 49
128-bit AES key:
e9 b2 3d 52 27 37 47 dd 5c 35 cb 55 be 61 9d 8e
256-bit PBKDF2 output:
d1 da a7 86 15 f2 87 e6 a1 c8 b1 20 d7 06 2a 49
3f 98 d2 03 e6 be 49 a6 ad f4 fa 57 4b 6e 64 ee
256-bit AES key:
97 a4 e7 86 be 20 d8 1a 38 2d 5e bc 96 d5 90 9c
ab cd ad c8 7c a4 8f 57 45 04 15 9f 16 c3 6e 31
(This test is based on values given in [PECMS].)
Iteration count = 1200
Pass phrase = (64 characters)
"XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"
Salt="pass phrase equals block size"
128-bit PBKDF2 output:
13 9c 30 c0 96 6b c3 2b a5 5f db f2 12 53 0a c9
128-bit AES key:
59 d1 bb 78 9a 82 8b 1a a5 4e f9 c2 88 3f 69 ed
256-bit PBKDF2 output:
13 9c 30 c0 96 6b c3 2b a5 5f db f2 12 53 0a c9
c5 ec 59 f1 a4 52 f5 cc 9a d9 40 fe a0 59 8e d1
256-bit AES key:
89 ad ee 36 08 db 8b c7 1f 1b fb fe 45 94 86 b0
56 18 b7 0c ba e2 20 92 53 4e 56 c5 53 ba 4b 34
Iteration count = 1200
Pass phrase = (65 characters)
"XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"
Salt = "pass phrase exceeds block size"
128-bit PBKDF2 output:
9c ca d6 d4 68 77 0c d5 1b 10 e6 a6 87 21 be 61
128-bit AES key:
cb 80 05 dc 5f 90 17 9a 7f 02 10 4c 00 18 75 1d
256-bit PBKDF2 output:
9c ca d6 d4 68 77 0c d5 1b 10 e6 a6 87 21 be 61
1a 8b 4d 28 26 01 db 3b 36 be 92 46 91 5e c8 2a
256-bit AES key:
d7 8c 5c 9c b8 72 a8 c9 da d4 69 7f 0b b5 b2 d2
14 96 c8 2b eb 2c ae da 21 12 fc ee a0 57 40 1b
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Iteration count = 50
Pass phrase = g-clef (0xf09d849e)
Salt = "EXAMPLE.COMpianist"
128-bit PBKDF2 output:
6b 9c f2 6d 45 45 5a 43 a5 b8 bb 27 6a 40 3b 39
128-bit AES key:
f1 49 c1 f2 e1 54 a7 34 52 d4 3e 7f e6 2a 56 e5
256-bit PBKDF2 output:
6b 9c f2 6d 45 45 5a 43 a5 b8 bb 27 6a 40 3b 39
e7 fe 37 a0 c4 1e 02 c2 81 ff 30 69 e1 e9 4f 52
256-bit AES key:
4b 6d 98 39 f8 44 06 df 1f 09 cc 16 6d b4 b8 3c
57 18 48 b7 84 a3 d6 bd c3 46 58 9a 3e 39 3f 9e
Some test vectors for CBC with cipher text stealing, using an initial
vector of all-zero.
AES 128-bit key:
63 68 69 63 6b 65 6e 20 74 65 72 69 79 61 6b 69
Input:
49 20 77 6f 75 6c 64 20 6c 69 6b 65 20 74 68 65
20
Output:
c6 35 35 68 f2 bf 8c b4 d8 a5 80 36 2d a7 ff 7f
97
Input:
49 20 77 6f 75 6c 64 20 6c 69 6b 65 20 74 68 65
20 47 65 6e 65 72 61 6c 20 47 61 75 27 73 20
Output:
fc 00 78 3e 0e fd b2 c1 d4 45 d4 c8 ef f7 ed 22
97 68 72 68 d6 ec cc c0 c0 7b 25 e2 5e cf e5
Input:
49 20 77 6f 75 6c 64 20 6c 69 6b 65 20 74 68 65
20 47 65 6e 65 72 61 6c 20 47 61 75 27 73 20 43
Output:
39 31 25 23 a7 86 62 d5 be 7f cb cc 98 eb f5 a8
97 68 72 68 d6 ec cc c0 c0 7b 25 e2 5e cf e5 84
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Input:
49 20 77 6f 75 6c 64 20 6c 69 6b 65 20 74 68 65
20 47 65 6e 65 72 61 6c 20 47 61 75 27 73 20 43
68 69 63 6b 65 6e 2c 20 70 6c 65 61 73 65 2c
Output:
97 68 72 68 d6 ec cc c0 c0 7b 25 e2 5e cf e5 84
b3 ff fd 94 0c 16 a1 8c 1b 55 49 d2 f8 38 02 9e
39 31 25 23 a7 86 62 d5 be 7f cb cc 98 eb f5
Input:
49 20 77 6f 75 6c 64 20 6c 69 6b 65 20 74 68 65
20 47 65 6e 65 72 61 6c 20 47 61 75 27 73 20 43
68 69 63 6b 65 6e 2c 20 70 6c 65 61 73 65 2c 20
Output:
97 68 72 68 d6 ec cc c0 c0 7b 25 e2 5e cf e5 84
9d ad 8b bb 96 c4 cd c0 3b c1 03 e1 a1 94 bb d8
39 31 25 23 a7 86 62 d5 be 7f cb cc 98 eb f5 a8
Input:
49 20 77 6f 75 6c 64 20 6c 69 6b 65 20 74 68 65
20 47 65 6e 65 72 61 6c 20 47 61 75 27 73 20 43
68 69 63 6b 65 6e 2c 20 70 6c 65 61 73 65 2c 20
61 6e 64 20 77 6f 6e 74 6f 6e 20 73 6f 75 70 2e
Output:
97 68 72 68 d6 ec cc c0 c0 7b 25 e2 5e cf e5 84
39 31 25 23 a7 86 62 d5 be 7f cb cc 98 eb f5 a8
48 07 ef e8 36 ee 89 a5 26 73 0d bc 2f 7b c8 40
9d ad 8b bb 96 c4 cd c0 3b c1 03 e1 a1 94 bb d8
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