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Kerberos Working Group K. Raeburn
Document: draft-raeburn-krb-rijndael-krb-05.txt MIT
June 20, 2003
expires December 20, 2003
AES Encryption 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
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.
Abstract
Recently the US National Institute of Standards and Technology chose
a new Advanced Encryption Standard, 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.
Comments should be sent to the author, or to the IETF Kerberos
working group (ietf-krb-wg@anl.gov).
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.
Raeburn [Page 1]
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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
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 of appropriate length 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
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iteration count is 1.)
For environments where slower hardware is the norm, implementations
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 value used is
00 00 10 00 (decimal 4096, indicating 4096 iterations).
Note that this is NOT a requirement, nor even a recommendation, for
this value to be used in "optimistic preauthentication" (e.g.,
attempting timestamp-based preauthentication using the user's long-
term key, without having first communicated with the KDC) in the
absence of additional information, nor as a default value for sites
to use for their principals' long-term keys in their Kerberos
database. It is simply the interpretation of the absence of the
string-to-key parameter field when the KDC has had an opportunity to
provide it.
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. If exactly one block is to be
encrypted, that block is simply encrypted with AES (also known as ECB
mode). Input of less than one block is padded at the end to one
block; the values of the padding bits are unspecified.
(Implementations may use all-zero padding, but protocols should not
rely on the result being deterministic. Implementations may use
random padding, but protocols should not rely on the result not being
deterministic. Note that in most cases, the Kerberos encryption
profile will add a random confounder independent of this padding.)
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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]:
+--------------------------------------------------------------------+
| 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 00 10 00 |
| |
| 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) |
| |
| message block size, m 1 octet |
| |
| encryption/decryption functions, AES in CBC-CTS mode with |
| E and D zero ivec (cipher block |
| size 16 octets) |
+--------------------------------------------------------------------+
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:
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+--------------------------------------------------------------------+
| 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:
+--------------------------------------------------------------------+
| checksum types |
+--------------------------------------------------------------------+
| type name sumtype value length |
+--------------------------------------------------------------------+
| hmac-sha1-96-aes128 15 96 |
| hmac-sha1-96-aes256 16 96 |
+--------------------------------------------------------------------+
These checksum types will be used with the corresponding encryption
types defined above.
8. 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
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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
out of scope for this document.
If a site does wish to use this means of protection against a brute-
force attack, the iteration count should be chosen based on the
facilities expected to be available to an attacker, and the amount of
work the attacker should be required to perform to acquire the key or
password.
As an example:
The author's tests on a 2GHz Pentium 4 system indicated that in
one second, nearly 90000 iterations could be done, producing a
256-bit key. This was using the SHA-1 assembly implementation
from OpenSSL, and a pre-release version of the PBKDF2 code for
MIT's Kerberos package, on a single system. No attempt was made
to do multiple hashes in parallel, so we assume an attacker doing
so can probably do at least 100000 iterations per second --
rounded up to 2**17, for ease of calculation. For simplicity, we
also assume the final AES encryption step costs nothing.
Paul Leach estimates [LEACH] that a password-cracking dictionary
may have on the order of 2**21 entries, with capitalization,
punctuation, and other variations contributing perhaps a factor of
2**11, giving a ballpark estimate of 2**32.
Thus, for a known iteration count N and a known salt string, an
attacker with some number of computers comparable to the author's
would need roughly N*2**15 CPU seconds to convert the entire
dictionary plus variations into keys.
An attacker using a dozen such computers for a month would have
roughly 2**25 CPU seconds available. So using 2**12 (4096)
iterations would mean an attacker with a dozen such computers
dedicated to a brute-force attack against a single key (actually,
any password-derived keys sharing the same salt and iteration
count) would process all the variations of the dictionary entries
in four months, and on average, would likely find the user's
password in two months.
Thus, if this form of attack is of concern, an iteration count a
few orders of magnitude higher should be chosen, and users should
be required to change their passwords every few months. Perhaps
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several orders of magnitude, since many users will tend to use the
shorter and simpler passwords (as much as they can get away with,
given a site's password quality checks) that the attacker would
likely try first.
Since this estimate is based on currently available CPU power, the
iteration counts used for this mode of defense should be increased
over time, at perhaps 40%-60% each year or so.
Note that if the attacker has a large amount of storage available,
intermediate results could be cached, saving a lot of work for the
next attack with the same salt and a greater number of iterations
than had been run at the point where the intermediate results were
saved. Thus, it would be wise to generate a new random salt
string when passwords are changed. The default salt string,
derived from the principal name, only protects against the use of
one dictionary of keys against multiple users.
If the PBKDF2 iteration count can be spoofed by an intruder on the
network, and the limit on the accepted iteration count is very high,
the intruder may be able to introduce a form of denial of service
attack against the client by sending a very high iteration count,
causing the client to spend a great deal of CPU time computing an
incorrect key.
An intruder spoofing the KDC reply, providing a low iteration count,
and reading the client's reply from the network may be able to reduce
the work needed in the brute-force attack outlined above. Thus,
implementations may wish to enforce lower bounds on the number of
iterations that will be used.
Since threat models and typical end-user equipment will vary widely
from site to site, allowing site-specific configuration of such
bounds is recommended.
Any benefit against other attacks specific to the HMAC or SHA-1
algorithms is probably achieved with a fairly small number of
iterations.
In the "optimistic preauthentication" case mentioned in section 3,
the client may attempt to produce a key without first communicating
with the KDC. If the client has no additional information, it can
only guess as to the iteration count to be used. Even such
heuristics as "iteration count X was used to acquire tickets for the
same principal only N hours ago" can be wrong. Given the
recommendation above for increasing the iteration counts used over
time, it is impossible to recommend any specific default value for
this case; allowing site-local configuration is recommended. (If the
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lower and upper bound checks described above are implemented, the
default count for optimistic preauthentication should be between
those bounds.)
Cipher text stealing mode, since it requires no additional padding in
most cases, 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.
9. IANA Considerations
None.
10. Acknowledgements
Thanks to John Brezak, Gerardo Diaz Cuellar, Ken Hornstein, Paul
Leach, Marcus Watts and others for feedback on earlier versions of
this document.
A. Sample test vectors
Sample values for the PBKDF2 HMAC-SHA1 string-to-key function are
included below.
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
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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
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].)
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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
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.
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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
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
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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 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
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.
Department of Commerce, "Secure Hash Standard", Federal Information
Processing Standards Publication 180-1, Washington, DC, April 1995.
Informative References
[LEACH] Leach, P., email to IETF Kerberos working group mailing list,
5 May 2003, ftp://ftp.ietf.org/ietf-mail-archive/krb-wg/2003-05.mail.
[PECMS] Gutmann, P., "Password-based Encryption for CMS", RFC 3211,
December 2001.
Raeburn [Page 12]
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Author's Address
Kenneth Raeburn
Massachusetts Institute of Technology
77 Massachusetts Avenue
Cambridge, MA 02139
raeburn@mit.edu
Full Copyright Statement
Copyright (C) The Internet Society (2003). 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
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"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
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."
Notes to RFC Editor
Assuming this document goes through Last Call along with the Kerberos
crypto framework draft, the reference entry for [KCRYPTO] will list
the draft name, not the RFC number. This should be replaced with the
RFC info.
Remove Kerberos working group contact info from the Abstract; it's
right for the draft, but not the final RFC.
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