Internet Engineering Task Force (IETF) G. Hudson
Request for Comments: 6803 MIT Kerberos Consortium
Category: Informational November 2012
ISSN: 2070-1721
Camellia Encryption for Kerberos 5
Abstract
This document specifies two encryption types and two corresponding
checksum types for the Kerberos cryptosystem framework defined in RFC
3961. The new types use the Camellia block cipher in CBC mode with
ciphertext stealing and the CMAC algorithm for integrity protection.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6803.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Provisions Relating to IETF Documents
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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RFC 6803 Camellia Encryption for Kerberos 5 November 2012
1. Introduction
The Camellia block cipher, described in [RFC3713], has a 128-bit
block size and a 128-bit, 192-bit, or 256-bit key size, similar to
AES. This document specifies Kerberos encryption and checksum types
for Camellia using 128-bit or 256-bit keys. The new types conform to
the framework specified in [RFC3961] but do not use the simplified
profile.
Like the simplified profile, the new types use key derivation to
produce keys for encryption, integrity protection, and checksum
operations. Instead of the key derivation function described in
[RFC3961], Section 5.1, the new types use a key derivation function
from the family specified in [SP800-108].
The new types use the CMAC algorithm for integrity protection and
checksum operations. As a consequence, they do not rely on a hash
algorithm except when generating keys from strings.
Like the AES encryption types [RFC3962], the new encryption types use
CBC mode with ciphertext stealing [RFC3962] to avoid the need for
padding. They also use the same PBKDF2 algorithm for key generation
from strings, with a modification to the salt string to ensure that
different keys are generated for Camellia and AES encryption types.
2. Protocol Key Representation
The Camellia key space is dense, so we use random octet strings
directly as keys. The first bit of the Camellia bit string is the
high bit of the first byte of the random octet string.
3. Key Derivation
We use a key derivation function from the family specified in
[SP800-108], Section 5.2, "KDF in Feedback Mode". The PRF parameter
of the key derivation function is CMAC with Camellia-128 or
Camellia-256 as the underlying block cipher; this PRF has an output
size of 128 bits. A block counter is used with a length of 4 bytes,
represented in big-endian order. The length of the output key in
bits (denoted as k) is also represented as a 4-byte string in big-
endian order. The label input to the KDF is the usage constant
supplied to the key derivation function, and the context is unused.
In the following summary, | indicates concatenation. The random-to-
key function is the identity function, as defined in Section 6. The
k-truncate function is defined in [RFC3961], Section 5.1.
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n = ceiling(k / 128)
K(0) = zeros
K(i) = CMAC(key, K(i-1) | i | constant | 0x00 | k)
DR(key, constant) = k-truncate(K(1) | K(2) | ... | K(n))
KDF-FEEDBACK-CMAC(key, constant) = random-to-key(DR(key, constant))
The constants used for key derivation are the same as those used in
the simplified profile.
4. Key Generation from Strings
We use a variation on the key generation algorithm specified in
[RFC3962], Section 4.
First, to ensure that different long-term keys are used with Camellia
and AES, we prepend the enctype name to the salt string, separated by
a null byte. The enctype name is "camellia128-cts-cmac" or
"camellia256-cts-cmac" (without the quotes).
Second, the final key derivation step uses the algorithm described in
Section 3 instead of the key derivation algorithm used by the
simplified profile.
Third, if no string-to-key parameters are specified, the default
number of iterations is raised to 32768.
saltp = enctype-name | 0x00 | salt
tkey = random-to-key(PBKDF2-HMAC-SHA1(passphrase, saltp,
iter_count, keylength))
key = KDF-FEEDBACK-CMAC(tkey, "kerberos")
5. CMAC Checksum Algorithm
For integrity protection and checksums, we use the CMAC function
defined in [SP800-38B], with Camellia-128 or Camellia-256 as the
underlying block cipher. The output length (Tlen) is 128 bits for
both key sizes.
6. Encryption Algorithm Parameters
The following parameters, required by [RFC3961], Section 3, apply to
the encryption types camellia128-cts-cmac, which uses a 128-bit
protocol key, and camellia256-cts-cmac, which uses a 256-bit protocol
key.
Protocol key format: as defined in Section 2.
Specific key structure: three protocol format keys: { Kc, Ke, Ki }.
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Required checksum mechanism: as defined in Section 7.
Key generation seed length: the key size (128 or 256 bits).
String-to-key function: as defined in Section 4.
Random-to-key function: identity function.
Key-derivation function: as indicated below, with usage represented
as 4 octets in big-endian order.
String-to-key parameter format: 4 octets indicating a 32-bit
iteration count in big-endian order. Implementations may limit the
count as specified in [RFC3962], Section 4.
Default string-to-key parameters: 00 00 80 00.
Kc = KDF-FEEDBACK-CMAC(base-key, usage | 0x99)
Ke = KDF-FEEDBACK-CMAC(base-key, usage | 0xAA)
Ki = KDF-FEEDBACK-CMAC(base-key, usage | 0x55)
Cipher state: a 128-bit CBC initialization vector.
Initial cipher state: all bits zero.
Encryption function: as follows, where E() is Camellia encryption in
CBC-CTS mode, with the next-to-last block used as the CBC-style ivec,
or the last block if there is only one.
conf = Random string of 128 bits
(C, newstate) = E(Ke, conf | plaintext, oldstate)
M = CMAC(Ki, conf | plaintext)
ciphertext = C | M
Decryption function: as follows, where D() is Camellia decryption in
CBC-CTS mode, with the ivec treated as in E(). To separate the
ciphertext into C and M components, use the final 16 bytes for M and
all of the preceding bytes for C.
(C, M) = ciphertext
(P, newIV) = D(Ke, C, oldstate)
if (M != CMAC(Ki, P)) report error
newstate = newIV
Pseudo-random function: as follows.
Kp = KDF-FEEDBACK-CMAC(protocol-key, "prf")
PRF = CMAC(Kp, octet-string)
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7. Checksum Parameters
The following parameters, required by [RFC3961], Section 4, apply to
the checksum types cmac-camellia128 and cmac-camellia256, which are
the associated checksum for camellia128-cts-cmac and camellia256-cts-
cmac, respectively.
Associated cryptosystem: Camellia-128 or Camellia-256 as appropriate
for the checksum type.
get_mic: CMAC(Kc, message).
verify_mic: get_mic and compare.
8. Security Considerations
Chapter 4 of [CRYPTOENG] discusses weaknesses of the CBC cipher mode.
An attacker who can observe enough messages generated with the same
key to encounter a collision in ciphertext blocks could recover the
XOR of the plaintexts of the previous blocks. Observing such a
collision becomes likely as the number of blocks observed approaches
2^64. This consideration applies to all previously standardized
Kerberos encryption types and all uses of CBC encryption with 128-bit
block ciphers in other protocols. Kerberos deployments can mitigate
this concern by rolling over keys often enough to make observing 2^64
messages unlikely.
Because the new checksum types are deterministic, an attacker could
pre-compute checksums for a known plain-text message in 2^64 randomly
chosen protocol keys. The attacker could then observe checksums
legitimately computed in different keys until a collision with one of
the pre-computed keys is observed; this becomes likely after the
number of observed checksums approaches 2^64. Observing such a
collision allows the attacker to recover the protocol key. This
consideration applies to most previously standardized Kerberos
checksum types and most uses of 128-bit checksums in other protocols.
Kerberos deployments should not migrate to the Camellia encryption
types simply because they are newer, but should use them only if
business needs require the use of Camellia, or if a serious flaw is
discovered in AES which does not apply to Camellia.
The security considerations described in [RFC3962], Section 8,
regarding the string-to-key algorithm also apply to the Camellia
encryption types.
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At the time of writing this document, there are no known weak keys
for Camellia, and no security problem has been found on Camellia (see
[NESSIE], [CRYPTREC], and [LNCS5867]).
9. IANA Considerations
IANA has assigned the following numbers from the Encryption Type
Numbers and Checksum Type Numbers registries defined in [RFC3961],
Section 11.
Encryption types
+-------+----------------------+-----------+
| etype | encryption type | Reference |
+-------+----------------------+-----------+
| 25 | camellia128-cts-cmac | [RFC6803] |
| 26 | camellia256-cts-cmac | [RFC6803] |
+-------+----------------------+-----------+
Checksum types
+---------------+------------------+---------------+-----------+
| sumtype value | Checksum type | checksum size | Reference |
+---------------+------------------+---------------+-----------+
| 17 | cmac-camellia128 | 16 | [RFC6803] |
| 18 | cmac-camellia256 | 16 | [RFC6803] |
+---------------+------------------+---------------+-----------+
10. Test Vectors
Sample results for string-to-key conversion:
Iteration count = 1
Pass phrase = "password"
Salt = "ATHENA.MIT.EDUraeburn"
128-bit Camellia key:
57 D0 29 72 98 FF D9 D3 5D E5 A4 7F B4 BD E2 4B
256-bit Camellia key:
B9 D6 82 8B 20 56 B7 BE 65 6D 88 A1 23 B1 FA C6
82 14 AC 2B 72 7E CF 5F 69 AF E0 C4 DF 2A 6D 2C
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Iteration count = 2
Pass phrase = "password"
Salt = "ATHENA.MIT.EDUraeburn"
128-bit Camellia key:
73 F1 B5 3A A0 F3 10 F9 3B 1D E8 CC AA 0C B1 52
256-bit Camellia key:
83 FC 58 66 E5 F8 F4 C6 F3 86 63 C6 5C 87 54 9F
34 2B C4 7E D3 94 DC 9D 3C D4 D1 63 AD E3 75 E3
Iteration count = 1200
Pass phrase = "password"
Salt = "ATHENA.MIT.EDUraeburn"
128-bit Camellia key:
8E 57 11 45 45 28 55 57 5F D9 16 E7 B0 44 87 AA
256-bit Camellia key:
77 F4 21 A6 F2 5E 13 83 95 E8 37 E5 D8 5D 38 5B
4C 1B FD 77 2E 11 2C D9 20 8C E7 2A 53 0B 15 E6
Iteration count = 5
Pass phrase = "password"
Salt=0x1234567878563412
128-bit Camellia key:
00 49 8F D9 16 BF C1 C2 B1 03 1C 17 08 01 B3 81
256-bit Camellia key:
11 08 3A 00 BD FE 6A 41 B2 F1 97 16 D6 20 2F 0A
FA 94 28 9A FE 8B 27 A0 49 BD 28 B1 D7 6C 38 9A
Iteration count = 1200
Pass phrase = (64 characters)
"XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"
Salt="pass phrase equals block size"
128-bit Camellia key:
8B F6 C3 EF 70 9B 98 1D BB 58 5D 08 68 43 BE 05
256-bit Camellia key:
11 9F E2 A1 CB 0B 1B E0 10 B9 06 7A 73 DB 63 ED
46 65 B4 E5 3A 98 D1 78 03 5D CF E8 43 A6 B9 B0
Iteration count = 1200
Pass phrase = (65 characters)
"XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"
Salt = "pass phrase exceeds block size"
128-bit Camellia key:
57 52 AC 8D 6A D1 CC FE 84 30 B3 12 87 1C 2F 74
256-bit Camellia key:
61 4D 5D FC 0B A6 D3 90 B4 12 B8 9A E4 D5 B0 88
B6 12 B3 16 51 09 94 67 9D DB 43 83 C7 12 6D DF
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Iteration count = 50
Pass phrase = g-clef (0xf09d849e)
Salt = "EXAMPLE.COMpianist"
128-bit Camellia key:
CC 75 C7 FD 26 0F 1C 16 58 01 1F CC 0D 56 06 16
256-bit Camellia key:
16 3B 76 8C 6D B1 48 B4 EE C7 16 3D F5 AE D7 0E
20 6B 68 CE C0 78 BC 06 9E D6 8A 7E D3 6B 1E CC
Sample results for key derivation:
128-bit Camellia key:
57 D0 29 72 98 FF D9 D3 5D E5 A4 7F B4 BD E2 4B
Kc value for key usage 2 (constant = 0x0000000299):
D1 55 77 5A 20 9D 05 F0 2B 38 D4 2A 38 9E 5A 56
Ke value for key usage 2 (constant = 0x00000002AA):
64 DF 83 F8 5A 53 2F 17 57 7D 8C 37 03 57 96 AB
Ki value for key usage 2 (constant = 0x0000000255):
3E 4F BD F3 0F B8 25 9C 42 5C B6 C9 6F 1F 46 35
256-bit Camellia key:
B9 D6 82 8B 20 56 B7 BE 65 6D 88 A1 23 B1 FA C6
82 14 AC 2B 72 7E CF 5F 69 AF E0 C4 DF 2A 6D 2C
Kc value for key usage 2 (constant = 0x0000000299):
E4 67 F9 A9 55 2B C7 D3 15 5A 62 20 AF 9C 19 22
0E EE D4 FF 78 B0 D1 E6 A1 54 49 91 46 1A 9E 50
Ke value for key usage 2 (constant = 0x00000002AA):
41 2A EF C3 62 A7 28 5F C3 96 6C 6A 51 81 E7 60
5A E6 75 23 5B 6D 54 9F BF C9 AB 66 30 A4 C6 04
Ki value for key usage 2 (constant = 0x0000000255):
FA 62 4F A0 E5 23 99 3F A3 88 AE FD C6 7E 67 EB
CD 8C 08 E8 A0 24 6B 1D 73 B0 D1 DD 9F C5 82 B0
Sample encryptions (all using the default cipher state):
Plaintext: (empty)
128-bit Camellia key:
1D C4 6A 8D 76 3F 4F 93 74 2B CB A3 38 75 76 C3
Random confounder:
B6 98 22 A1 9A 6B 09 C0 EB C8 55 7D 1F 1B 6C 0A
Ciphertext:
C4 66 F1 87 10 69 92 1E DB 7C 6F DE 24 4A 52 DB
0B A1 0E DC 19 7B DB 80 06 65 8C A3 CC CE 6E B8
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RFC 6803 Camellia Encryption for Kerberos 5 November 2012
Plaintext: 1
Random confounder:
6F 2F C3 C2 A1 66 FD 88 98 96 7A 83 DE 95 96 D9
128-bit Camellia key:
50 27 BC 23 1D 0F 3A 9D 23 33 3F 1C A6 FD BE 7C
Ciphertext:
84 2D 21 FD 95 03 11 C0 DD 46 4A 3F 4B E8 D6 DA
88 A5 6D 55 9C 9B 47 D3 F9 A8 50 67 AF 66 15 59
B8
Plaintext: 9 bytesss
Random confounder:
A5 B4 A7 1E 07 7A EE F9 3C 87 63 C1 8F DB 1F 10
128-bit Camellia key:
A1 BB 61 E8 05 F9 BA 6D DE 8F DB DD C0 5C DE A0
Ciphertext:
61 9F F0 72 E3 62 86 FF 0A 28 DE B3 A3 52 EC 0D
0E DF 5C 51 60 D6 63 C9 01 75 8C CF 9D 1E D3 3D
71 DB 8F 23 AA BF 83 48 A0
Plaintext: 13 bytes byte
Random confounder:
19 FE E4 0D 81 0C 52 4B 5B 22 F0 18 74 C6 93 DA
128-bit Camellia key:
2C A2 7A 5F AF 55 32 24 45 06 43 4E 1C EF 66 76
Ciphertext:
B8 EC A3 16 7A E6 31 55 12 E5 9F 98 A7 C5 00 20
5E 5F 63 FF 3B B3 89 AF 1C 41 A2 1D 64 0D 86 15
C9 ED 3F BE B0 5A B6 AC B6 76 89 B5 EA
Plaintext: 30 bytes bytes bytes bytes byt
Random confounder:
CA 7A 7A B4 BE 19 2D AB D6 03 50 6D B1 9C 39 E2
128-bit Camellia key:
78 24 F8 C1 6F 83 FF 35 4C 6B F7 51 5B 97 3F 43
Ciphertext:
A2 6A 39 05 A4 FF D5 81 6B 7B 1E 27 38 0D 08 09
0C 8E C1 F3 04 49 6E 1A BD CD 2B DC D1 DF FC 66
09 89 E1 17 A7 13 DD BB 57 A4 14 6C 15 87 CB A4
35 66 65 59 1D 22 40 28 2F 58 42 B1 05 A5
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RFC 6803 Camellia Encryption for Kerberos 5 November 2012
Plaintext: (empty)
Random confounder:
3C BB D2 B4 59 17 94 10 67 F9 65 99 BB 98 92 6C
256-bit Camellia key:
B6 1C 86 CC 4E 5D 27 57 54 5A D4 23 39 9F B7 03
1E CA B9 13 CB B9 00 BD 7A 3C 6D D8 BF 92 01 5B
Ciphertext:
03 88 6D 03 31 0B 47 A6 D8 F0 6D 7B 94 D1 DD 83
7E CC E3 15 EF 65 2A FF 62 08 59 D9 4A 25 92 66
Plaintext: 1
Random confounder:
DE F4 87 FC EB E6 DE 63 46 D4 DA 45 21 BB A2 D2
256-bit Camellia key:
1B 97 FE 0A 19 0E 20 21 EB 30 75 3E 1B 6E 1E 77
B0 75 4B 1D 68 46 10 35 58 64 10 49 63 46 38 33
Ciphertext:
2C 9C 15 70 13 3C 99 BF 6A 34 BC 1B 02 12 00 2F
D1 94 33 87 49 DB 41 35 49 7A 34 7C FC D9 D1 8A
12
Plaintext: 9 bytesss
Random confounder:
AD 4F F9 04 D3 4E 55 53 84 B1 41 00 FC 46 5F 88
256-bit Camellia key:
32 16 4C 5B 43 4D 1D 15 38 E4 CF D9 BE 80 40 FE
8C 4A C7 AC C4 B9 3D 33 14 D2 13 36 68 14 7A 05
Ciphertext:
9C 6D E7 5F 81 2D E7 ED 0D 28 B2 96 35 57 A1 15
64 09 98 27 5B 0A F5 15 27 09 91 3F F5 2A 2A 9C
8E 63 B8 72 F9 2E 64 C8 39
Plaintext: 13 bytes byte
Random confounder:
CF 9B CA 6D F1 14 4E 0C 0A F9 B8 F3 4C 90 D5 14
256-bit Camellia key:
B0 38 B1 32 CD 8E 06 61 22 67 FA B7 17 00 66 D8
8A EC CB A0 B7 44 BF C6 0D C8 9B CA 18 2D 07 15
Ciphertext:
EE EC 85 A9 81 3C DC 53 67 72 AB 9B 42 DE FC 57
06 F7 26 E9 75 DD E0 5A 87 EB 54 06 EA 32 4C A1
85 C9 98 6B 42 AA BE 79 4B 84 82 1B EE
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RFC 6803 Camellia Encryption for Kerberos 5 November 2012
Plaintext: 30 bytes bytes bytes bytes byt
Random confounder:
64 4D EF 38 DA 35 00 72 75 87 8D 21 68 55 E2 28
256-bit Camellia key:
CC FC D3 49 BF 4C 66 77 E8 6E 4B 02 B8 EA B9 24
A5 46 AC 73 1C F9 BF 69 89 B9 96 E7 D6 BF BB A7
Ciphertext:
0E 44 68 09 85 85 5F 2D 1F 18 12 52 9C A8 3B FD
8E 34 9D E6 FD 9A DA 0B AA A0 48 D6 8E 26 5F EB
F3 4A D1 25 5A 34 49 99 AD 37 14 68 87 A6 C6 84
57 31 AC 7F 46 37 6A 05 04 CD 06 57 14 74
Sample checksums:
Plaintext: abcdefghijk
Checksum type: cmac-camellia128
128-bit Camellia key:
1D C4 6A 8D 76 3F 4F 93 74 2B CB A3 38 75 76 C3
Key usage: 7
Checksum:
11 78 E6 C5 C4 7A 8C 1A E0 C4 B9 C7 D4 EB 7B 6B
Plaintext: ABCDEFGHIJKLMNOPQRSTUVWXYZ
Checksum type: cmac-camellia128
128-bit Camellia key:
50 27 BC 23 1D 0F 3A 9D 23 33 3F 1C A6 FD BE 7C
Key usage: 8
Checksum:
D1 B3 4F 70 04 A7 31 F2 3A 0C 00 BF 6C 3F 75 3A
Plaintext: 123456789
Checksum type: cmac-camellia256
256-bit Camellia key:
B6 1C 86 CC 4E 5D 27 57 54 5A D4 23 39 9F B7 03
1E CA B9 13 CB B9 00 BD 7A 3C 6D D8 BF 92 01 5B
Key usage: 9
Checksum:
87 A1 2C FD 2B 96 21 48 10 F0 1C 82 6E 77 44 B1
Plaintext: !@#$%^&*()!@#$%^&*()!@#$%^&*()
Checksum type: cmac-camellia256
256-bit Camellia key:
32 16 4C 5B 43 4D 1D 15 38 E4 CF D9 BE 80 40 FE
8C 4A C7 AC C4 B9 3D 33 14 D2 13 36 68 14 7A 05
Key usage: 10
Checksum:
3F A0 B4 23 55 E5 2B 18 91 87 29 4A A2 52 AB 64
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11. References
11.1. Normative References
[RFC3713] Matsui, M., Nakajima, J., and S. Moriai, "A Description
of the Camellia Encryption Algorithm", RFC 3713,
April 2004.
[RFC3961] Raeburn, K., "Encryption and Checksum Specifications for
Kerberos 5", RFC 3961, February 2005.
[RFC3962] Raeburn, K., "Advanced Encryption Standard (AES)
Encryption for Kerberos 5", RFC 3962, February 2005.
[SP800-108] Chen, L., "Recommendation for Key Derivation Using
Pseudorandom Functions", NIST Special Publication 800&
nhby;108, October 2009.
[SP800-38B] Dworkin, M., "Recommendation for Block Cipher Modes of
Operation: The CMAC Mode for Authentication", NIST
Special Publication 800-38B, October 2009.
11.2. Informative References
[CRYPTOENG] Schneier, B., "Cryptography Engineering", March 2010.
[CRYPTREC] Information-technology Promotion Agency (IPA), Japan,
"Cryptography Research and Evaluation Committees",
<http://www.ipa.go.jp/security/enc/CRYPTREC/
index-e.html>.
[LNCS5867] Mala, H., Shakiba, M., Dakhilalian, M., and G.
Bagherikaram, "New Results on Impossible Different
Cryptanalysis of Reduced-Round Camellia-128", Lecture
Notes in Computer Science, Vol. 5867, November 2009,
<http://www.springerlink.com/content/e55783u422436g77/>.
[NESSIE] The NESSIE Project, "New European Schemes for
Signatures, Integrity, and Encryption",
<http://www.cosic.esat.kuleuven.be/nessie/>.
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Appendix A. Acknowledgements
The author would like to thank Ken Raeburn, Satoru Kanno, Jeffrey
Hutzelman, Nico Williams, Sam Hartman, and Tom Yu for their help in
reviewing and providing feedback on this document.
Author's Address
Greg Hudson
MIT Kerberos Consortium
EMail: ghudson@mit.edu
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