Network Working Group K.W. Burgin
Internet Draft National Security Agency
Intended Status: Standards Track M.A. Peck
Expires: January 12, 2012 The MITRE Corporation
July 11, 2011
AES-CBC Mode with HMAC-SHA2 For Kerberos 5
draft-burgin-kerberos-aes-cbc-hmac-sha2-01
Abstract
This document specifies two encryption types and two corresponding
checksum types for Kerberos 5. The new types use AES in CBC mode
with PKCS#5 padding for confidentiality and HMAC with a SHA-2 hash
for integrity.
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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 . . . . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
9 References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1 Normative References . . . . . . . . . . . . . . . . . . . 8
9.2 Informative References . . . . . . . . . . . . . . . . . . 9
Appendix A. AES-CBC Test Vectors . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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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 CBC mode, but do not use
ciphertext stealing as in [RFC3962]. Instead, the messages are
padded to a multiple of the AES block size as described in Section
6.3 of [RFC5652].
The new types use the PBKDF2 algorithm for key generation from
strings, with a modification to the use in [RFC3962] that the hash
algorithm in the pseudorandom function used by PBKDF2 will be SHA-256
instead of 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 HMAC
of the passphrase and salt, instead of the SHA-1 HMAC of the
passphrase and salt. The salt SHOULD contain at least 128 random
bits as recommended in [SP800-132]. It MAY also contain other
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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-cbc-hmac-sha256-128" or
"aes256-cbc-hmac-sha384-192" (without the quotes). The user's long-
term key is derived as follows
saltp = enctype-name | 0x00 | salt
tkey = random2key(PBKDF2(passphrase, saltp,
iter_count, keylength))
key = KDF-HMAC-SHA2(tkey, "kerberos") where "kerberos" is the
byte string {0x6b 0x65 0x72 0x62 0x65 0x72 0x6f 0x73}.
where the pseudorandom function used by PBKDF2 is the SHA-256 HMAC of
the passphrase and salt, 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.
When the encryption type is aes128-cbc-hmac-sha256-128:
n = 1
K1 = HMAC-SHA-256(key, 00 00 00 01 | constant | 0x00 | 00 00 00 80)
DR(key, constant) = First 128 bits of K1
KDF-HMAC-SHA2(key, constant) = random-to-key(DR(key, constant))
When the encryption type is aes256-cbc-hmac-sha384-192:
n = 1
K1 = HMAC-SHA-384(key, 00 00 00 01 | constant | 0x00 | 00 00 01 00)
DR(key, constant) = First 256 bits of K1
KDF-HMAC-SHA2(key, constant) = random-to-key(DR(key, constant))
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6. Kerberos Algorithm Protocol Parameters
The following parameters apply to the encryption types aes128-cbc-
hmac-sha256-128 and aes256-cbc-hmac-sha384-192.
The key-derivation function described in the previous section is used
to produce the three intermediate keys. CBC mode [SP800-38A]
requires the input be padded to a multiple of the encryption
algorithm block size, which is 128 bits for AES. The input will be
padded as described in Section 6.3 of [RFC5652] in which the value of
each added octet equals the number of octets that are added.
Each encryption will use a freshly generated 16-octet initialization
vector generated at random by the message originator.
The ciphertext is the concatenation of the initialization vector, the
output of AES in CBC mode, and the HMAC of the plaintext and padding
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, decrypting the
remainder, verifying the HMAC, and verifying and removing the
padding.
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-CBC-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 Initial vector of length c (128 bits)
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initial cipher state All bits zero
encryption function N = random nonce of length c (128 bits)
pad = Shortest string of non-zero length to
bring plaintext to a length that is a
multiple of m. The value of each added
octet equals the number of octets that
are added.
N1 = N + cipherState (+ denotes XOR)
C = E(Ke, plaintext | pad, N1)
H = HMAC(Ki, N | plaintext | pad)
ciphertext = N | C | H[1..h]
cipherState = N
decryption function (N,C,H) = ciphertext
(P, pad) = D(Ke, C, N + cipherState)
if (H != HMAC(Ki, N | P | pad)[1..h]
or pad is bad)
report error
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 = random2key(PBKDF2(passphrase, saltp,
iter_count,
keylength))
base-key = KDF-HMAC-SHA2(tkey, "kerberos")
where the pseudorandom function used by PBKDF2
is the SHA-256 HMAC of the passphrase and salt
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);
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Checksum Mechanism for AES-CBC-HMAC-SHA2
------------------------------------------------------------------------
associated cryptosystem AES-128-CBC or AES-256-CBC 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.
+--------------------------------------------------------------------+
| encryption types |
+--------------------------------------------------------------------+
| type name etype value key size |
+--------------------------------------------------------------------+
| aes128-cbc-hmac-sha256-128 TBD1 128 |
| aes256-cbc-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-cbc-hmac-sha256-128 and
aes256-cbc-hmac-sha384-192 in the Kerberos Encryption Type
Numbers registry.
Etype encryption type Reference
----- --------------- ---------
TBD1 aes128-cbc-hmac-sha256-128 [I.D.burgin-kerberos-aes-
cbc-hmac-sha2]
TBD2 aes256-cbc-hmac-sha384-192 [I.D.burgin-kerberos-aes-
cbc-hmac-sha2]
2. Checksum type numbers for hmac-sha256-128-aes128 and hmac-sha384-
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192-aes256 in the Kerberos Checksum Type Numbers registry.
Sumtype Checksum type Size Reference
------- ------------- ---- ---------
TBD3 hmac-sha256-128-aes128 16 [I.D.burgin-kerberos-
aes-cbc-hmac-sha2]
TBD4 hmac-sha384-192-aes256 24 [I.D.burgin-kerberos-
aes-cbc-hmac-sha2]
8. Security Considerations
Padding oracle attacks were introduced by Vaudenay in [POA]. The
attack relies on an oracle that decrypts messages that were encrypted
using CBC mode with PKCS#5 padding and returns an answer to the
sender about whether the padding is correct. This information allows
an attacker to recover the plaintext from an encrypted message
through repeated inquiries to the oracle even though the encryption
key is never learned by the attacker. The attack can be mitigated by
returning a single error type when decryption fails and not
distinguishing between failed MAC check and failed padding check.
This specification requires implementations to generate random
values. The use of inadequate pseudo-random number generators
(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
[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.
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[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD
70, RFC 5652, September 2009.
[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.
[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.
[POA] Vaudenay, Serge, "Security Flaws Induced by CBC Padding
Applications to SSL, IPSEC, WTLS...", EUROCRYPT 2002.
Appendix A. AES-CBC Test Vectors
TBD
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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