Network Working Group K. Burgin
Internet Draft National Security Agency
Intended Status: Informational M. Peck
Expires: October 21, 2013 The MITRE Corporation
April 19, 2013
AES Encryption with HMAC-SHA2 for Kerberos 5
draft-ietf-kitten-aes-cts-hmac-sha2-00
Abstract
This document specifies two encryption types and two corresponding
checksum types for Kerberos 5. The new types use AES in CTS mode
(CBC mode with ciphertext stealing) for confidentiality and HMAC with
a SHA-2 hash for integrity.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on October 21, 2013.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions used in this Document . . . . . . . . . . . . . . 3
3. Protocol Key Representation . . . . . . . . . . . . . . . . . 3
4. Key Generation from Pass Phrases . . . . . . . . . . . . . . . 3
5. Key Derivation Function . . . . . . . . . . . . . . . . . . . 4
6. Kerberos Algorithm Protocol Parameters . . . . . . . . . . . . 5
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . . 9
Appendix A. Test Vectors . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
This document defines two encryption types and two corresponding
checksum types for Kerberos 5 using AES with 128-bit or 256-bit keys.
The new types conform to the framework specified in [RFC3961], but do
not use the simplified profile.
The new encryption types use AES in CTS mode (CBC mode with
ciphertext stealing) similar to [RFC3962] but with several
variations.
The new types use the PBKDF2 algorithm for key generation from
strings, with a modification to the use in [RFC3962] that the
pseudorandom function used by PBKDF2 is HMAC-SHA-256 or HMAC-SHA-384
instead of HMAC-SHA-1.
The new types use key derivation to produce keys for encryption,
integrity protection, and checksum operations as in [RFC3962].
However, a key derivation function from [SP800-108] which uses the
SHA-256 or SHA-384 hash algorithm is used in place of the DK key
derivation function used in [RFC3961].
The new types use the HMAC algorithm with a hash from the SHA-2
family for integrity protection and checksum operations.
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 [RFC2119].
3. Protocol Key Representation
The AES key space is dense, so we can use random or pseudorandom
octet strings directly as keys. The byte representation for the key
is described in [FIPS197], where the first bit of the bit string is
the high bit of the first byte of the byte string (octet string).
4. Key Generation from Pass Phrases
We use a variation on the key generation algorithm specified in
Section 4 of [RFC3962] with the following changes:
* The pseudorandom function used by PBKDF2 will be the SHA-256 or
SHA-384 HMAC of the passphrase and salt, instead of the SHA-1 HMAC
of the passphrase and salt. If the enctype is "aes128-cts-hmac-
sha256-128", then HMAC-SHA-256 is used as the PRF. If the enctype
is "aes256-cts-hmac-sha384-192", then HMAC-SHA-384 is used as the
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PRF.
* The salt MUST contain at least 128 random bits as required in
Section 5.1 of [SP800-132]. It MAY also contain other information
such as the principal's realm and name components.
* The final key derivation step uses the algorithm KDF-HMAC-SHA2
defined below in Section 5 instead of the DK function.
* If no string-to-key parameters are specified, the default number
of iterations is raised to 32,768.
To ensure that different long-term keys are used with different
enctypes, we prepend the enctype name to the salt string, separated
by a null byte. The enctype name is "aes128-cts-hmac-sha256-128" or
"aes256-cts-hmac-sha384-192" (without the quotes). The user's long-
term key is derived as follows
saltp = enctype-name | 0x00 | salt
tkey = random-to-key(PBKDF2(passphrase, saltp,
iter_count, keylength))
key = KDF-HMAC-SHA2(tkey, "kerberos") where "kerberos" is the
byte string {0x6b65726265726f73}.
where the pseudorandom function used by PBKDF2 is HMAC-SHA-256 when
the enctype is "aes128-cts-hmac-sha256-128" and HMAC-SHA-384 when the
enctype is "aes256-cts-hmac-sha384-192", the value for keylength is
the AES key length, and the algorithm KDF-HMAC-SHA2 is defined in
Section 5.
5. Key Derivation Function
We use a key derivation function from Section 5.1 of [SP800-108]
which uses the HMAC algorithm as the PRF. The counter i is expressed
as four octets in big-endian order. The length of the output key in
bits (denoted as k) is also represented as four octets in big-endian
order. The "Label" input to the KDF is the usage constant supplied
to the key derivation function, and the "Context" input is null. 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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When the encryption type is aes128-cts-hmac-sha256-128, the output
key length k is 128 bits for all applications of KDF-HMAC-SHA2(key,
constant) which is computed as follows:
n = 1
K1 = HMAC-SHA-256(key, 00 00 00 01 | constant | 0x00 | 00 00 00 80)
DR(key, constant) = k-truncate(K1)
KDF-HMAC-SHA2(key, constant) = random-to-key(DR(key, constant))
When the encryption type is aes256-cts-hmac-sha384-192, the output
key length k is 256 bits when computing the base-key and Ke, and the
output key length k is 192 bits when deriving Kc and Ki. KDF-HMAC-
SHA2(key, constant) is computed as follows:
If deriving Kc or Ki (the constant ends with 0x99 or 0x55):
k = 192
n = 1
K1 = HMAC-SHA-384(key, 00 00 00 01 | constant | 0x00 | 00 00 00 C0)
DR(key, constant) = k-truncate(K1)
KDF-HMAC-SHA2(key, constant) = random-to-key(DR(key, constant))
Otherwise (if deriving Ke or deriving the base-key from a
passphrase as described in Section 4):
k = 256
n = 1
K1 = HMAC-SHA-384(key, 00 00 00 01 | constant | 0x00 | 00 00 01 00)
DR(key, constant) = k-truncate(K1)
KDF-HMAC-SHA2(key, constant) = random-to-key(DR(key, constant))
The constants used for key derivation are the same as those used in
the simplified profile.
6. Kerberos Algorithm Protocol Parameters
The following parameters apply to the encryption types aes128-cts-
hmac-sha256-128 and aes256-cts-hmac-sha384-192.
The key-derivation function described in the previous section is used
to produce the three intermediate keys. Typically, CBC mode [SP800-
38A] requires the input be padded to a multiple of the encryption
algorithm block size, which is 128 bits for AES. However, to avoid
ciphertext expansion, we use the CBC-CS3 variant to CBC mode defined
in [SP800-38A+] (this mode is also referred to as CTS). Note that
[SP800-38A+] requires the plaintext length to be greater than or
equal to the block size.
Each encryption will use a freshly generated 16-octet nonce generated
at random by the message originator. The initialization vector (IV)
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used by AES is obtained by xoring the random nonce with the
cipherstate.
The ciphertext is the concatenation of the random nonce, the output
of AES in CBC-CS3 mode, and the HMAC of the nonce concatenated with
the AES output. The HMAC is computed using either SHA-256 or SHA-
384. The output of SHA-256 is truncated to 128 bits and the output
of SHA-384 is truncated to 192 bits. Sample test vectors are given in
Appendix A.
Decryption is performed by removing the HMAC, verifying the HMAC
against the remainder, and then decrypting the remainder if the HMAC
is correct.
The encryption and checksum mechanisms below use the following
notation from [RFC3961].
HMAC output size, h
message block size, m
encryption/decryption functions, E and D
cipher block size, c
Encryption Mechanism for AES-CTS-HMAC-SHA2
------------------------------------------------------------------------
protocol key format 128- or 256-bit string
specific key structure Three protocol-format keys: { Kc, Ke, Ki }.
required checksum As defined below.
mechanism
key-generation seed key size (128 or 256 bits)
length
cipher state Random nonce of length c (128 bits)
initial cipher state All bits zero
encryption function N = random nonce of length c (128 bits)
IV = N + cipherState (+ denotes XOR)
C = E(Ke, plaintext, IV)
using CBC-CS3-Encrypt defined
in [SP800-38A+]
H = HMAC(Ki, N | C)
ciphertext = N | C | H[1..h]
cipherState = N
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decryption function (N, C, H) = ciphertext
if (H != HMAC(Ki, N | C)[1..h])
stop, report error
IV = N + cipherState (+ denotes XOR)
P = D(Ke, C, IV)
using CBC-CS3-Decrypt defined
in [SP800-38A+]
cipherState = N
pseudo-random function Kp = KDF-HMAC-SHA2(protocol-key, "prf")
PRF = HMAC(Kp, octet-string)
key generation functions:
string-to-key function tkey = random-to-key(PBKDF2(passphrase, saltp,
iter_count,
keylength))
base-key = KDF-HMAC-SHA2(tkey, "kerberos")
where the pseudorandom function used by PBKDF2
is HMAC-SHA-256 or HMAC-SHA-384 as described
in Section 4.
default string-to-key 00 00 80 00
parameters
random-to-key function identity function
key-derivation function KDF-HMAC-SHA2 as defined in Section 5. The
key usage number is expressed as four octets
in big-endian order.
Kc = KDF-HMAC-SHA2(base-key, usage | 0x99)
Ke = KDF-HMAC-SHA2(base-key, usage | 0xAA)
Ki = KDF-HMAC-SHA2(base-key, usage | 0x55);
Checksum Mechanism for AES-CTS-HMAC-SHA2
------------------------------------------------------------------------
associated cryptosystem AES-128-CTS or AES-256-CTS as appropriate
get_mic HMAC(Kc, message)[1..h]
verify_mic get_mic and compare
Using this profile with each key size gives us two each of encryption
and checksum algorithm definitions.
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+--------------------------------------------------------------------+
| encryption types |
+--------------------------------------------------------------------+
| type name etype value key size |
+--------------------------------------------------------------------+
| aes128-cts-hmac-sha256-128 TBD1 128 |
| aes256-cts-hmac-sha384-192 TBD2 256 |
+--------------------------------------------------------------------+
+--------------------------------------------------------------------+
| checksum types |
+--------------------------------------------------------------------+
| type name sumtype value length |
+--------------------------------------------------------------------+
| hmac-sha256-128-aes128 TBD3 128 |
| hmac-sha384-192-aes256 TBD4 192 |
+--------------------------------------------------------------------+
These checksum types will be used with the corresponding encryption
types defined above.
7. IANA Considerations
IANA is requested to assign:
1. Encryption type numbers for aes128-cts-hmac-sha256-128 and
aes256-cts-hmac-sha384-192 in the Kerberos Encryption Type
Numbers registry.
Etype encryption type Reference
----- --------------- ---------
TBD1 aes128-cts-hmac-sha256-128 [this document]
TBD2 aes256-cts-hmac-sha384-192 [this document]
2. Checksum type numbers for hmac-sha256-128-aes128 and
hmac-sha384-192-aes256 in the Kerberos Checksum Type Numbers
registry.
Sumtype Checksum type Size Reference
------- ------------- ---- ---------
TBD3 hmac-sha256-128-aes128 16 [this document]
TBD4 hmac-sha384-192-aes256 24 [this document]
8. Security Considerations
This specification requires implementations to generate random
values. The use of inadequate pseudo-random number generators
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(PRNGs) can result in little or no security. The generation of
quality random numbers is difficult. NIST Special Publication 800-90
[SP800-90] and [RFC4086] offer random number generation guidance.
This document specifies a mechanism for generating keys from pass
phrases or passwords. The salt and iteration count resist brute
force and dictionary attacks, however, it is still important to
choose or generate strong passphrases.
9. References
9.1. Normative References
[SP800-38A+] National Institute of Standards and Technology,
"Recommendation for Block Cipher Modes of Operation:
Three Variants of Ciphertext Stealing for CBC Mode",
Addendum to NIST Special Publication 800-38A, October
2010.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106,
RFC 4086, June 2005.
[FIPS197] National Institute of Standards and Technology,
"Advanced Encryption Standard (AES)", FIPS PUB 197,
November 2001.
9.2. Informative References
[SP800-38A] National Institute of Standards and Technology,
"Recommendation for Block Cipher Modes of Operation -
Methods and Techniques", NIST Special Publication 800-
38A, February 2001.
[SP800-90] National Institute of Standards and Technology,
Recommendation for Random Number Generation Using
Deterministic Random Bit Generators (Revised), NIST
Special Publication 800-90, March 2007.
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[SP800-108] National Institute of Standards and Technology,
"Recommendation for Key Derivation Using Pseudorandom
Functions", NIST Special Publication 800-108, October
2009.
[SP800-132] National Institute of Standards and Technology,
"Recommendation for Password-Based Key Derivation, Part
1: Storage Applications", NIST Special Publication 800-
132, June 2010.
Appendix A. Test Vectors
Sample results for string-to-key conversion:
Iteration count = 32768
Pass phrase = "password"
Saltp for creating 128-bit master key:
61 65 73 31 32 38 2D 63 74 73 2D 68 6D 61 63 2D
73 68 61 32 35 36 2D 31 32 38 00 F3 60 61 DC E2
E1 B3 59 00 83 87 46 B8 78 2F 1D 41 54 48 45 4E
41 2E 4D 49 54 2E 45 44 55 72 61 65 62 75 72 6E
(The saltp is "aes128-cts-hmac-sha256-128" | 0x00 |
16 random bytes | "ATHENA.MIT.EDUraeburn")
128-bit master key:
37 05 D9 60 80 C1 77 28 A0 E8 00 EA B6 E0 D2 3C
Saltp for creating 256-bit master key:
61 65 73 32 35 36 2D 63 74 73 2D 68 6D 61 63 2D
73 68 61 33 38 34 2D 31 39 32 00 F3 60 61 DC E2
E1 B3 59 00 83 87 46 B8 78 2F 1D 41 54 48 45 4E
41 2E 4D 49 54 2E 45 44 55 72 61 65 62 75 72 6E
(The saltp is "aes256-cts-hmac-sha384-192" | 0x00 |
16 random bytes | "ATHENA.MIT.EDUraeburn")
256-bit master key:
6D 40 4D 37 FA F7 9F 9D F0 D3 35 68 D3 20 66 98
00 EB 48 36 47 2E A8 A0 26 D1 6B 71 82 46 0C 52
Sample results for key derivation:
enctype aes128-cts-hmac-sha256-128:
128-bit master key:
37 05 D9 60 80 C1 77 28 A0 E8 00 EA B6 E0 D2 3C
Kc value for key usage 2 (constant = 0x0000000299):
B3 1A 01 8A 48 F5 47 76 F4 03 E9 A3 96 32 5D C3
Ke value for key usage 2 (constant = 0x00000002AA):
9B 19 7D D1 E8 C5 60 9D 6E 67 C3 E3 7C 62 C7 2E
Ki value for key usage 2 (constant = 0x0000000255):
9F DA 0E 56 AB 2D 85 E1 56 9A 68 86 96 C2 6A 6C
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enctype aes256-cts-hmac-sha384-192:
256-bit master key:
6D 40 4D 37 FA F7 9F 9D F0 D3 35 68 D3 20 66 98
00 EB 48 36 47 2E A8 A0 26 D1 6B 71 82 46 0C 52
Kc value for key usage 2 (constant = 0x0000000299):
EF 57 18 BE 86 CC 84 96 3D 8B BB 50 31 E9 F5 C4
BA 41 F2 8F AF 69 E7 3D
Ke value for key usage 2 (constant = 0x00000002AA):
56 AB 22 BE E6 3D 82 D7 BC 52 27 F6 77 3F 8E A7
A5 EB 1C 82 51 60 C3 83 12 98 0C 44 2E 5C 7E 49
Ki value for key usage 2 (constant = 0x0000000255):
69 B1 65 14 E3 CD 8E 56 B8 20 10 D5 C7 30 12 B6
22 C4 D0 0F FC 23 ED 1F
Sample encryptions (using the default cipher state):
128-bit master key:
37 05 D9 60 80 C1 77 28 A0 E8 00 EA B6 E0 D2 3C
128-bit AES key (Ke, key usage 2):
9B 19 7D D1 E8 C5 60 9D 6E 67 C3 E3 7C 62 C7 2E
128-bit HMAC key (Ki, key usage 2):
9F DA 0E 56 AB 2D 85 E1 56 9A 68 86 96 C2 6A 6C
Plaintext:
00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
10 11 12 13 14
IV | Ciphertext | Authentication Tag:
8D 32 50 F6 36 AB 81 02 BE 6F AB 1E 57 D8 F8 17
13 64 FB 39 DC C0 E3 D9 83 A7 DB 5B 4B 9F FB CA
42 F6 65 88 29 F2 1F C8 95 75 AE 93 C7 57 18 AB
3C 7C FB 28 E1
256-bit master key:
6D 40 4D 37 FA F7 9F 9D F0 D3 35 68 D3 20 66 98
00 EB 48 36 47 2E A8 A0 26 D1 6B 71 82 46 0C 52
256-bit AES key (Ke, key usage 2):
56 AB 22 BE E6 3D 82 D7 BC 52 27 F6 77 3F 8E A7
A5 EB 1C 82 51 60 C3 83 12 98 0C 44 2E 5C 7E 49
192-bit HMAC key (Ki, key usage 2):
69 B1 65 14 E3 CD 8E 56 B8 20 10 D5 C7 30 12 B6
22 C4 D0 0F FC 23 ED 1F
Plaintext:
00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
10 11 12 13 14
IV | Ciphertext | Authentication Tag:
8D 32 50 F6 36 AB 81 02 BE 6F AB 1E 57 D8 F8 17
50 CB FF DC DF 38 69 D7 0B EA FF C3 2C 47 0B C6
5B 72 C3 37 2D 6E D7 B3 47 E9 0B BD 8F 31 F5 79
58 F9 69 50 BA A1 41 64 6E 65 6C F6 7C
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Sample checksums:
Checksum type: hmac-sha256-128-aes128
128-bit master key:
37 05 D9 60 80 C1 77 28 A0 E8 00 EA B6 E0 D2 3C
128-bit HMAC key (Kc, key usage 2):
B3 1A 01 8A 48 F5 47 76 F4 03 E9 A3 96 32 5D C3
Plaintext:
00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
10 11 12 13 14
Checksum:
D7 83 67 18 66 43 D6 7B 41 1C BA 91 39 FC 1D EE
Checksum type: hmac-sha384-192-aes256
256-bit master key:
6D 40 4D 37 FA F7 9F 9D F0 D3 35 68 D3 20 66 98
00 EB 48 36 47 2E A8 A0 26 D1 6B 71 82 46 0C 52
192-bit HMAC key (Kc, key usage 2):
EF 57 18 BE 86 CC 84 96 3D 8B BB 50 31 E9 F5 C4
BA 41 F2 8F AF 69 E7 3D
Plaintext:
00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
10 11 12 13 14
Checksum:
45 EE 79 15 67 EE FC A3 7F 4A C1 E0 22 2D E8 0D
43 C3 BF A0 66 99 67 2A
Authors' Addresses
Kelley W. Burgin
National Security Agency
EMail: kwburgi@tycho.ncsc.mil
Michael A. Peck
The MITRE Corporation
EMail: mpeck@mitre.org
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