draft-ietf-kitten-aes-cts-hmac-sha2-00.txt   draft-ietf-kitten-aes-cts-hmac-sha2-01.txt 
Network Working Group K. Burgin Network Working Group K. Burgin
Internet Draft National Security Agency Internet Draft National Security Agency
Intended Status: Informational M. Peck Intended Status: Informational M. Peck
Expires: October 21, 2013 The MITRE Corporation Expires: December 30, 2013 The MITRE Corporation
April 19, 2013 June 28, 2013
AES Encryption with HMAC-SHA2 for Kerberos 5 AES Encryption with HMAC-SHA2 for Kerberos 5
draft-ietf-kitten-aes-cts-hmac-sha2-00 draft-ietf-kitten-aes-cts-hmac-sha2-01
Abstract Abstract
This document specifies two encryption types and two corresponding This document specifies two encryption types and two corresponding
checksum types for Kerberos 5. The new types use AES in CTS mode checksum types for Kerberos 5. The new types use AES in CTS mode
(CBC mode with ciphertext stealing) for confidentiality and HMAC with (CBC mode with ciphertext stealing) for confidentiality and HMAC with
a SHA-2 hash for integrity. a SHA-2 hash for integrity.
Status of this Memo Status of this Memo
skipping to change at page 1, line 34 skipping to change at page 1, line 34
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions used in this Document . . . . . . . . . . . . . . 3 2. Protocol Key Representation . . . . . . . . . . . . . . . . . 3
3. Protocol Key Representation . . . . . . . . . . . . . . . . . 3 3. Key Generation from Pass Phrases . . . . . . . . . . . . . . . 3
4. Key Generation from Pass Phrases . . . . . . . . . . . . . . . 3 4. Key Derivation Function . . . . . . . . . . . . . . . . . . . 4
5. Key Derivation Function . . . . . . . . . . . . . . . . . . . 4 5. Kerberos Algorithm Protocol Parameters . . . . . . . . . . . . 5
6. Kerberos Algorithm Protocol Parameters . . . . . . . . . . . . 5 6. Checksum Parameters . . . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8 8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9 8.1. Random Values in Salt Strings . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . . 9 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . . 9 9.1. Normative References . . . . . . . . . . . . . . . . . . . 10
Appendix A. Test Vectors . . . . . . . . . . . . . . . . . . . . 10 9.2. Informative References . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12 Appendix A. Test Vectors . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
1. Introduction 1. Introduction
This document defines two encryption types and two corresponding This document defines two encryption types and two corresponding
checksum types for Kerberos 5 using AES with 128-bit or 256-bit keys. 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 To avoid ciphertext expansion, we use the CBC-CS3 variant to CBC mode
not use the simplified profile. defined in [SP800-38A+] (this mode is also referred to as CTS). 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 Note that [SP800-38A+] requires the plaintext length to be greater
ciphertext stealing) similar to [RFC3962] but with several than the block size, so the encryption types have two cases.
variations.
The new types use the PBKDF2 algorithm for key generation from The encryption and checksum types defined in this document are
strings, with a modification to the use in [RFC3962] that the intended to support NSA's Suite B Profile for Kerberos [suiteb-
pseudorandom function used by PBKDF2 is HMAC-SHA-256 or HMAC-SHA-384 kerberos] which requires the use of SHA-256 or SHA-384 as the hash
instead of HMAC-SHA-1. algorithm. Differences between the encryption and checksum types
defined in this document and existing Kerberos encryption and
checksum types are:
The new types use key derivation to produce keys for encryption, * The pseudorandom function used by PBKDF2 is HMAC-SHA-256 or HMAC-
integrity protection, and checksum operations as in [RFC3962]. SHA-384.
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 * A key derivation function from [SP800-108] which uses the SHA-256
family for integrity protection and checksum operations. or SHA-384 hash algorithm is used to produce keys for encryption,
integrity protection, and checksum operations.
2. Conventions used in this Document * The IV used during content encryption is sent as part of the
ciphertext, instead of using a confounder. This saves one
encryption and decryption operation per message.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", * The HMAC is calculated over the AES output, instead of being
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this calculated over the plaintext. This allows the message receiver
document are to be interpreted as described in RFC 2119 [RFC2119]. to verify the integrity of the message before decrypting the
message.
3. Protocol Key Representation * The HMAC algorithm uses the SHA-256 or SHA-384 hash algorithm for
integrity protection and checksum operations.
2. Protocol Key Representation
The AES key space is dense, so we can use random or pseudorandom The AES key space is dense, so we can use random or pseudorandom
octet strings directly as keys. The byte representation for the key octet strings directly as keys. The byte representation for the key
is described in [FIPS197], where the first bit of the bit string is 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). the high bit of the first byte of the byte string (octet string).
4. Key Generation from Pass Phrases 3. 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
PRF.
* The salt MUST contain at least 128 random bits as required in The pseudorandom function used by PBKDF2 will be the SHA-256 or SHA-
Section 5.1 of [SP800-132]. It MAY also contain other information 384 HMAC of the passphrase and salt. If the enctype is "aes128-cts-
such as the principal's realm and name components. 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 PRF.
* The final key derivation step uses the algorithm KDF-HMAC-SHA2 The final key derivation step uses the algorithm KDF-HMAC-SHA2
defined below in Section 5 instead of the DK function. defined below in Section 4.
* If no string-to-key parameters are specified, the default number If no string-to-key parameters are specified, the default number of
of iterations is raised to 32,768. iterations is raised to 32,768.
To ensure that different long-term keys are used with different To ensure that different long-term keys are used with different
enctypes, we prepend the enctype name to the salt string, separated 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 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- "aes256-cts-hmac-sha384-192" (without the quotes). The user's long-
term key is derived as follows term key is derived as follows
saltp = enctype-name | 0x00 | salt saltp = enctype-name | 0x00 | salt
tkey = random-to-key(PBKDF2(passphrase, saltp, tkey = random-to-key(PBKDF2(passphrase, saltp,
iter_count, keylength)) iter_count, keylength))
key = KDF-HMAC-SHA2(tkey, "kerberos") where "kerberos" is the key = KDF-HMAC-SHA2(tkey, "kerberos") where "kerberos" is the
byte string {0x6b65726265726f73}. byte string {0x6b65726265726f73}.
where the pseudorandom function used by PBKDF2 is HMAC-SHA-256 when 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 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 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 the AES key length, and the algorithm KDF-HMAC-SHA2 is defined in
Section 5. Section 4.
5. Key Derivation Function 4. Key Derivation Function
We use a key derivation function from Section 5.1 of [SP800-108] 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 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 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 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 order. The "Label" input to the KDF is the usage constant supplied
to the key derivation function, and the "Context" input is null. In to the key derivation function, and the "Context" input is null.
the following summary, | indicates concatenation. The random-to-key Each application of the KDF only requires a single iteration of the
function is the identity function, as defined in Section 6. The k- PRF, so n = 1 in the notation of [SP800-108].
truncate function is defined in [RFC3961], Section 5.1.
In the following summary, | indicates concatenation. The random-to-
key function is the identity function, as defined in Section 3. The
k-truncate function is defined in [RFC3961], Section 5.1.
When the encryption type is aes128-cts-hmac-sha256-128, the output 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, key length k is 128 bits for all applications of KDF-HMAC-SHA2(key,
constant) which is computed as follows: constant) which is computed as follows:
n = 1
K1 = HMAC-SHA-256(key, 00 00 00 01 | constant | 0x00 | 00 00 00 80) 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(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 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 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- output key length k is 192 bits when deriving Kc and Ki. KDF-HMAC-
SHA2(key, constant) is computed as follows: SHA2(key, constant) is computed as follows:
If deriving Kc or Ki (the constant ends with 0x99 or 0x55): If deriving Kc or Ki (the constant ends with 0x99 or 0x55):
k = 192 k = 192
n = 1
K1 = HMAC-SHA-384(key, 00 00 00 01 | constant | 0x00 | 00 00 00 C0) 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(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 Otherwise (if deriving Ke or deriving the base-key from a
passphrase as described in Section 4): passphrase as described in Section 3):
k = 256 k = 256
n = 1
K1 = HMAC-SHA-384(key, 00 00 00 01 | constant | 0x00 | 00 00 01 00) 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(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 constants used for key derivation are the same as those used in
the simplified profile. the simplified profile.
6. Kerberos Algorithm Protocol Parameters 5. Kerberos Algorithm Protocol Parameters
The following parameters apply to the encryption types aes128-cts- In cases where the plaintext length is greater than the block size:
hmac-sha256-128 and aes256-cts-hmac-sha384-192.
The key-derivation function described in the previous section is used Each encryption will use a 16-octet nonce generated at random by
to produce the three intermediate keys. Typically, CBC mode [SP800- the message originator. The initialization vector (IV) used by
38A] requires the input be padded to a multiple of the encryption AES is obtained by xoring the random nonce with the cipherstate.
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 The ciphertext is the concatenation of the random nonce, the
at random by the message originator. The initialization vector (IV) output of AES in CBC-CS3 mode, and the HMAC of the nonce
used by AES is obtained by xoring the random nonce with the concatenated with the AES output. The HMAC is computed using
cipherstate. 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.
The ciphertext is the concatenation of the random nonce, the output Decryption is performed by removing the HMAC, verifying the HMAC
of AES in CBC-CS3 mode, and the HMAC of the nonce concatenated with against the remainder, and then decrypting the remainder if the
the AES output. The HMAC is computed using either SHA-256 or SHA- HMAC is correct.
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 In cases where the plaintext length is less than or equal to the
against the remainder, and then decrypting the remainder if the HMAC block size, a different algorithm is specified.
is correct.
The encryption and checksum mechanisms below use the following Each encryption will use a 16-octet nonce generated at random by
notation from [RFC3961]. the message originator. The initialization vector (IV) used by
AES is obtained by xoring the random nonce with the cipherstate.
HMAC output size, h The plaintext is padded with zeros so the length of the result is
message block size, m one block length (no zeros are added if the plaintext length
encryption/decryption functions, E and D equals the block length). The padded plaintext is xored with the
cipher block size, c IV, then encrypted using AES in ECB mode. The output of AES is
split into two parts, so that the length of the first part equals
the length of the unpadded plaintext. The nonce is also split
into two parts, so that the length of the first part equals the
length of the unpadded plaintext.
Encryption Mechanism for AES-CTS-HMAC-SHA2 The ciphertext is the concatenation of the first part of the
random nonce, the second part of the AES output followed by the
first part of the AES output, and the HMAC of the concatenation of
the first part of the random nonce, the second part of the AES
output followed by the first part of 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.
protocol key format 128- or 256-bit string Decryption is performed by first removing the HMAC, and verifying
the HMAC against the remainder. If the HMAC is correct, separate
the remainder into N' and C' by taking the first 16 bytes as N',
and the following bytes as C'. Split N' into two parts, so that
the length of the first part equals the length of C'. Decrypt the
concatenation of C' with the second part of N' using ECB mode to
get a value P' whose length is one block length. The nonce is
recovered by taking the concatenation of the first part of N' with
the second part of P' xored with the cipherState (where again, the
length of the first part equals the length of C'). The IV is
recovered as the nonce xored with cipherState, and the plaintext
is recovered as the first part of P' xored with the IV.
specific key structure Three protocol-format keys: { Kc, Ke, Ki }. The following parameters apply to the encryption types aes128-cts-
hmac-sha256-128 and aes256-cts-hmac-sha384-192.
required checksum As defined below. protocol key format: as defined in Section 2.
mechanism
key-generation seed key size (128 or 256 bits) specific key structure: three protocol-format keys: { Kc, Ke, Ki }.
length
cipher state Random nonce of length c (128 bits) required checksum mechanism: as defined in Section 6.
initial cipher state All bits zero key-generation seed length: key size (128 or 256 bits).
encryption function N = random nonce of length c (128 bits) string-to-key function: as defined in Section 3.
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
decryption function (N, C, H) = ciphertext default string-to-key parameters: 00 00 80 00.
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") random-to-key function: identity function.
PRF = HMAC(Kp, octet-string)
key generation functions: key-derivation function: KDF-HMAC-SHA2 as defined in Section 4. The
key usage number is expressed as four octets in big-endian order.
string-to-key function tkey = random-to-key(PBKDF2(passphrase, saltp, Kc = KDF-HMAC-SHA2(base-key, usage | 0x99)
iter_count, Ke = KDF-HMAC-SHA2(base-key, usage | 0xAA)
keylength)) Ki = KDF-HMAC-SHA2(base-key, usage | 0x55)
base-key = KDF-HMAC-SHA2(tkey, "kerberos")
where the pseudorandom function used by PBKDF2 cipherState: a 128-bit random nonce.
is HMAC-SHA-256 or HMAC-SHA-384 as described
in Section 4.
default string-to-key 00 00 80 00 initial cipherState: all bits zero.
parameters
random-to-key function identity function encryption function: as follows. When the plaintext length is
greater than the block size, CTS mode is used. When the plaintext
is less than or equal to the block size, ECB mode is used.
key-derivation function KDF-HMAC-SHA2 as defined in Section 5. The h = size of truncated HMAC
key usage number is expressed as four octets E() = encryption function
in big-endian order. D() = decryption function
c = block size of the encryption algorithm
L(x) = length of x
< = less-than operator; true == 1, false == 0
zeroblock = one block (length c) of zeros
o[start:len] = sub-string operation returning the substring of
length len of string o starting at byte start
(zero-based)
Kc = KDF-HMAC-SHA2(base-key, usage | 0x99) encryption function:
Ke = KDF-HMAC-SHA2(base-key, usage | 0xAA) N = random nonce of length 128 bits
Ki = KDF-HMAC-SHA2(base-key, usage | 0x55); IV = N XOR cipherState
if (L(P) > c)
PC = 0
P' = P
C = E(Ke, P', IV)
// using CBC-CS3-Encrypt defined
// in [SP800-38A+]
N' = N
C' = C
else
PC = c - L(P)
P' = P | zeroblock[0:PC]
C = E(Ke, P' XOR IV)
// using ECB mode
N' = N[0:c - PC] | C[c - PC:PC]
C' = C[0:c - PC]
H = HMAC(Ki, N' | C')
ciphertext = N' | C' | H[1..h]
cipherState = N
Checksum Mechanism for AES-CTS-HMAC-SHA2 decryption function:
associated cryptosystem AES-128-CTS or AES-256-CTS as appropriate (N', C', H) = ciphertext
if (H != HMAC(Ki, N' | C')[1..h])
stop, report error
get_mic HMAC(Kc, message)[1..h] if (L(C') > c)
// Not short-plaintext
IV = N' XOR cipherState
P = D(Ke, C', IV)
// using CBC-CS3-Decrypt defined
// in [SP800-38A+]
cipherState = N'
stop, output P, success
else
// Short plaintext
PC = c - L(C')
C = C' | N'[c - PC:PC]
P' = D(Ke, C)
// using ECB mode
verify_mic get_mic and compare // P' here == (P | zeroblock[0:PC]) XOR IV
// so IV[c - PC:PC] == P'[c - PC:PC]
// In the non-short-pt case we'd recover
// IV as N XOR cipherState, but here we only know
// a head of N and tail of IV.
Using this profile with each key size gives us two each of encryption N = N'[0:c -PC] | (P' XOR cipherState)[c - PC:PC]
and checksum algorithm definitions. IV = N XOR cipherState
P = (P' XOR IV)[0:PC]
cipherState = N
stop, output P, success
+--------------------------------------------------------------------+ pseudo-random function:
| encryption types | Kp = KDF-HMAC-SHA2(protocol-key, "prf")
+--------------------------------------------------------------------+ PRF = HMAC(Kp, octet-string)
| type name etype value key size |
+--------------------------------------------------------------------+
| aes128-cts-hmac-sha256-128 TBD1 128 |
| aes256-cts-hmac-sha384-192 TBD2 256 |
+--------------------------------------------------------------------+
+--------------------------------------------------------------------+ 6. Checksum Parameters
| 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 The following parameters apply to the checksum types hmac-sha256-128-
types defined above. aes128 and hmac-sha384-192-aes256, which are the associated checksums
for aes128-cts-hmac-sha256-128 and aes256-cts-hmac-sha384-192,
respectively.
associated cryptosystem: AES-128-CTS or AES-256-CTS as appropriate
get_mic: HMAC(Kc, message)[1..h]
verify_mic: get_mic and compare
7. IANA Considerations 7. IANA Considerations
IANA is requested to assign: IANA is requested to assign:
1. Encryption type numbers for aes128-cts-hmac-sha256-128 and Encryption type numbers for aes128-cts-hmac-sha256-128 and
aes256-cts-hmac-sha384-192 in the Kerberos Encryption Type aes256-cts-hmac-sha384-192 in the Kerberos Encryption Type Numbers
Numbers registry. registry.
Etype encryption type Reference Etype encryption type Reference
----- --------------- --------- ----- --------------- ---------
TBD1 aes128-cts-hmac-sha256-128 [this document] TBD1 aes128-cts-hmac-sha256-128 [this document]
TBD2 aes256-cts-hmac-sha384-192 [this document] TBD2 aes256-cts-hmac-sha384-192 [this document]
2. Checksum type numbers for hmac-sha256-128-aes128 and Checksum type numbers for hmac-sha256-128-aes128 and hmac-sha384-192-
hmac-sha384-192-aes256 in the Kerberos Checksum Type Numbers aes256 in the Kerberos Checksum Type Numbers registry.
registry.
Sumtype Checksum type Size Reference Sumtype Checksum type Size Reference
------- ------------- ---- --------- ------- ------------- ---- ---------
TBD3 hmac-sha256-128-aes128 16 [this document] TBD3 hmac-sha256-128-aes128 16 [this document]
TBD4 hmac-sha384-192-aes256 24 [this document] TBD4 hmac-sha384-192-aes256 24 [this document]
8. Security Considerations 8. Security Considerations
This specification requires implementations to generate random This specification requires implementations to generate random
values. The use of inadequate pseudo-random number generators values. The use of inadequate pseudo-random number generators
(PRNGs) can result in little or no security. The generation of (PRNGs) can result in little or no security. The generation of
quality random numbers is difficult. NIST Special Publication 800-90 quality random numbers is difficult. NIST Special Publication 800-90
[SP800-90] and [RFC4086] offer random number generation guidance. [SP800-90] and [RFC4086] offer random number generation guidance.
This document specifies a mechanism for generating keys from pass This document specifies a mechanism for generating keys from pass
phrases or passwords. The salt and iteration count resist brute phrases or passwords. The salt and iteration count resist brute
force and dictionary attacks, however, it is still important to force and dictionary attacks, however, it is still important to
choose or generate strong passphrases. choose or generate strong passphrases.
9. References 8.1. Random Values in Salt Strings
9.1. Normative References NIST guidance in Section 5.1 of [SP800-132] requires the salt used as
input to the PBKDF to contain at least 128 bits of random. Some
known issues with including random values in Kerberos encryption type
salt strings are:
[SP800-38A+] National Institute of Standards and Technology, * Cross-realm TGTs are currently managed by entering the same
"Recommendation for Block Cipher Modes of Operation: password at two KDCs to get the same keys. If each KDC uses a
Three Variants of Ciphertext Stealing for CBC Mode", random salt, they won't have the same keys.
Addendum to NIST Special Publication 800-38A, October
2010.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate * The string-to-key function as defined in [RFC3961] requires the
Requirement Levels", BCP 14, RFC 2119, March 1997. salt to be valid UTF-8 strings. Not every 128-bit random string
will be valid UTF-8.
* Current implementations of password history checking will not
work.
* ktutil's add_entry command assumes the default salt.
9. References
9.1. Normative References
[RFC3961] Raeburn, K., "Encryption and Checksum Specifications for [RFC3961] Raeburn, K., "Encryption and Checksum Specifications for
Kerberos 5", RFC 3961, February 2005. 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, [RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, "Randomness Requirements for Security", BCP 106,
RFC 4086, June 2005. RFC 4086, June 2005.
[FIPS197] National Institute of Standards and Technology, [FIPS197] National Institute of Standards and Technology,
"Advanced Encryption Standard (AES)", FIPS PUB 197, "Advanced Encryption Standard (AES)", FIPS PUB 197,
November 2001. November 2001.
9.2. Informative References 9.2. Informative References
[SP800-38A] National Institute of Standards and Technology, [SP800-38A+] National Institute of Standards and Technology,
"Recommendation for Block Cipher Modes of Operation - "Recommendation for Block Cipher Modes of Operation:
Methods and Techniques", NIST Special Publication 800- Three Variants of Ciphertext Stealing for CBC Mode",
38A, February 2001. Addendum to NIST Special Publication 800-38A, October
2010.
[SP800-90] National Institute of Standards and Technology, [SP800-90] National Institute of Standards and Technology,
Recommendation for Random Number Generation Using Recommendation for Random Number Generation Using
Deterministic Random Bit Generators (Revised), NIST Deterministic Random Bit Generators (Revised), NIST
Special Publication 800-90, March 2007. Special Publication 800-90, March 2007.
[SP800-108] National Institute of Standards and Technology, [SP800-108] National Institute of Standards and Technology,
"Recommendation for Key Derivation Using Pseudorandom "Recommendation for Key Derivation Using Pseudorandom
Functions", NIST Special Publication 800-108, October Functions", NIST Special Publication 800-108, October
2009. 2009.
[SP800-132] National Institute of Standards and Technology, [SP800-132] National Institute of Standards and Technology,
"Recommendation for Password-Based Key Derivation, Part "Recommendation for Password-Based Key Derivation, Part
1: Storage Applications", NIST Special Publication 800- 1: Storage Applications", NIST Special Publication 800-
132, June 2010. 132, June 2010.
[suiteb-kerberos]
Burgin, K. and K. Igoe, "Suite B Profile for
Kerberos 5", internet-draft draft-burgin-kerberos-
suiteb-01, 2012.
Appendix A. Test Vectors Appendix A. Test Vectors
Sample results for string-to-key conversion: Sample results for string-to-key conversion:
--------------------------------------------
Iteration count = 32768 Iteration count = 32768
Pass phrase = "password" Pass phrase = "password"
Saltp for creating 128-bit master key: Saltp for creating 128-bit master key:
61 65 73 31 32 38 2D 63 74 73 2D 68 6D 61 63 2D 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 73 68 61 32 35 36 2D 31 32 38 00 10 DF 9D D7 83
E1 B3 59 00 83 87 46 B8 78 2F 1D 41 54 48 45 4E E5 BC 8A CE A1 73 0E 74 35 5F 61 41 54 48 45 4E
41 2E 4D 49 54 2E 45 44 55 72 61 65 62 75 72 6E 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 | (The saltp is "aes128-cts-hmac-sha256-128" | 0x00 |
16 random bytes | "ATHENA.MIT.EDUraeburn") random 16 byte valid UTF-8 sequence | "ATHENA.MIT.EDUraeburn")
128-bit master key: 128-bit master key:
37 05 D9 60 80 C1 77 28 A0 E8 00 EA B6 E0 D2 3C 3C 44 03 85 28 06 BF 5C EE E6 36 48 6C 29 2F D6
Saltp for creating 256-bit master key: Saltp for creating 256-bit master key:
61 65 73 32 35 36 2D 63 74 73 2D 68 6D 61 63 2D 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 73 68 61 33 38 34 2D 31 39 32 00 10 DF 9D D7 83
E1 B3 59 00 83 87 46 B8 78 2F 1D 41 54 48 45 4E E5 BC 8A CE A1 73 0E 74 35 5F 61 41 54 48 45 4E
41 2E 4D 49 54 2E 45 44 55 72 61 65 62 75 72 6E 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 | (The saltp is "aes256-cts-hmac-sha384-192" | 0x00 |
16 random bytes | "ATHENA.MIT.EDUraeburn") random 16 byte valid UTF-8 sequence | "ATHENA.MIT.EDUraeburn")
256-bit master key: 256-bit master key:
6D 40 4D 37 FA F7 9F 9D F0 D3 35 68 D3 20 66 98 53 96 0C AF 44 D5 57 4D FF 4D 44 37 38 75 22 B0
00 EB 48 36 47 2E A8 A0 26 D1 6B 71 82 46 0C 52 7F 5B 02 5C 5E 65 BF EF 29 C2 B4 28 98 3B 37 08
Sample results for key derivation: Sample results for key derivation:
----------------------------------
enctype aes128-cts-hmac-sha256-128: enctype aes128-cts-hmac-sha256-128:
128-bit master key: 128-bit master key:
37 05 D9 60 80 C1 77 28 A0 E8 00 EA B6 E0 D2 3C 37 05 D9 60 80 C1 77 28 A0 E8 00 EA B6 E0 D2 3C
Kc value for key usage 2 (constant = 0x0000000299): Kc value for key usage 2 (constant = 0x0000000299):
B3 1A 01 8A 48 F5 47 76 F4 03 E9 A3 96 32 5D C3 B3 1A 01 8A 48 F5 47 76 F4 03 E9 A3 96 32 5D C3
Ke value for key usage 2 (constant = 0x00000002AA): Ke value for key usage 2 (constant = 0x00000002AA):
9B 19 7D D1 E8 C5 60 9D 6E 67 C3 E3 7C 62 C7 2E 9B 19 7D D1 E8 C5 60 9D 6E 67 C3 E3 7C 62 C7 2E
Ki value for key usage 2 (constant = 0x0000000255): Ki value for key usage 2 (constant = 0x0000000255):
9F DA 0E 56 AB 2D 85 E1 56 9A 68 86 96 C2 6A 6C 9F DA 0E 56 AB 2D 85 E1 56 9A 68 86 96 C2 6A 6C
skipping to change at page 11, line 12 skipping to change at page 12, line 4
Ki value for key usage 2 (constant = 0x0000000255): Ki value for key usage 2 (constant = 0x0000000255):
9F DA 0E 56 AB 2D 85 E1 56 9A 68 86 96 C2 6A 6C 9F DA 0E 56 AB 2D 85 E1 56 9A 68 86 96 C2 6A 6C
enctype aes256-cts-hmac-sha384-192: enctype aes256-cts-hmac-sha384-192:
256-bit master key: 256-bit master key:
6D 40 4D 37 FA F7 9F 9D F0 D3 35 68 D3 20 66 98 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 00 EB 48 36 47 2E A8 A0 26 D1 6B 71 82 46 0C 52
Kc value for key usage 2 (constant = 0x0000000299): Kc value for key usage 2 (constant = 0x0000000299):
EF 57 18 BE 86 CC 84 96 3D 8B BB 50 31 E9 F5 C4 EF 57 18 BE 86 CC 84 96 3D 8B BB 50 31 E9 F5 C4
BA 41 F2 8F AF 69 E7 3D BA 41 F2 8F AF 69 E7 3D
Ke value for key usage 2 (constant = 0x00000002AA): Ke value for key usage 2 (constant = 0x00000002AA):
56 AB 22 BE E6 3D 82 D7 BC 52 27 F6 77 3F 8E A7 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 A5 EB 1C 82 51 60 C3 83 12 98 0C 44 2E 5C 7E 49
Ki value for key usage 2 (constant = 0x0000000255): Ki value for key usage 2 (constant = 0x0000000255):
69 B1 65 14 E3 CD 8E 56 B8 20 10 D5 C7 30 12 B6 69 B1 65 14 E3 CD 8E 56 B8 20 10 D5 C7 30 12 B6
22 C4 D0 0F FC 23 ED 1F 22 C4 D0 0F FC 23 ED 1F
Sample encryptions (using the default cipher state): Sample encryptions (using the default cipher state):
----------------------------------------------------
128-bit master key: 128-bit AES key:
37 05 D9 60 80 C1 77 28 A0 E8 00 EA B6 E0 D2 3C 2B 7E 15 16 28 AE D2 A6 AB F7 15 88 09 CF 4F 3C
128-bit AES key (Ke, key usage 2): 128-bit HMAC key:
67 C3 31 A4 D7 AB 52 EF 3A A9 73 E0 39 AD D3 32
Nonce:
7E 58 95 EA F2 67 24 35 BA D8 17 F5 45 A3 71 48
Plaintext: (length less than block size)
49 6E 63 6F 6E 63 65 69 76 61 62 6C 65
AES Output:
1C 17 3E AD FC 67 C8 BC B3 A5 93 02 98 CB FC 60
HMAC Output (truncated):
35 E8 32 B2 EB F4 6A 46 C2 E6 50 D2 50 AB 84 43
Ciphertext: (Nonce* | AES Output** | Truncated HMAC Output)
7E 58 95 EA F2 67 24 35 BA D8 17 F5 45 CB FC 60
1C 17 3E AD FC 67 C8 BC B3 A5 93 02 98 35 E8 32
B2 EB F4 6A 46 C2 E6 50 D2 50 AB 84 43
* Only the first 13 bytes of Nonce are sent.
** The AES Output is split and rearranged as described in Section 5
since the plaintext length is less than the block size.
128-bit AES key:
2B 7E 15 16 28 AE D2 A6 AB F7 15 88 09 CF 4F 3C
128-bit HMAC key:
67 C3 31 A4 D7 AB 52 EF 3A A9 73 E0 39 AD D3 32
Nonce:
7E 58 95 EA F2 67 24 35 BA D8 17 F5 45 A3 71 48
Plaintext: (length equals block size)
67 61 73 74 72 6F 69 6E 74 65 73 74 69 6E 61 6C
AES Output:
F6 71 0B 75 0C 60 65 E8 2E BF F8 9D DC E0 C9 B9
HMAC Output (truncated):
7B 2C D9 70 E6 DF 18 F5 E0 3D 8B 8E 40 02 F4 C0
Ciphertext: (Nonce | AES Output | Truncated HMAC Output)
7E 58 95 EA F2 67 24 35 BA D8 17 F5 45 A3 71 48
F6 71 0B 75 0C 60 65 E8 2E BF F8 9D DC E0 C9 B9
7B 2C D9 70 E6 DF 18 F5 E0 3D 8B 8E 40 02 F4 C0
256-bit AES key:
60 3D EB 10 15 CA 71 BE 2B 73 AE F0 85 7D 77 81
1F 35 2C 07 3B 61 08 D7 2D 98 10 A3 09 14 DF F4
192-bit HMAC key:
37 16 14 EB 62 24 E1 F0 C4 72 6E E6 BE A7 A3 D2
F4 62 C6 AC 66 42 A6 AC
Nonce:
7E 58 95 EA F2 67 24 35 BA D8 17 F5 45 A3 71 48
Plaintext: (length less than block size)
49 6E 63 6F 6E 63 65 69 76 61 62 6C 65
AES Output:
BD AE EC 5C F9 C9 B6 3C 9D DB A2 B7 9D 5C 6C 0B
HMAC Output (truncated):
65 D4 C7 07 8E 14 65 8B C9 B3 C4 EA F5 F7 C2 6F
ED 36 AC 7A CD 59 19 2B
Ciphertext: (Nonce* | AES Output* | Truncated HMAC Output)
7E 58 95 EA F2 67 24 35 BA D8 17 F5 45 5C 6C 0B
BD AE EC 5C F9 C9 B6 3C 9D DB A2 B7 9D 65 D4 C7
07 8E 14 65 8B C9 B3 C4 EA F5 F7 C2 6F ED 36 AC
7A CD 59 19 2B
* Only the first 13 bytes of Nonce are sent.
** The AES Output is split and rearranged as described in Section 5
since the plaintext length is less than the block size.
256-bit AES key:
60 3D EB 10 15 CA 71 BE 2B 73 AE F0 85 7D 77 81
1F 35 2C 07 3B 61 08 D7 2D 98 10 A3 09 14 DF F4
192-bit HMAC key:
37 16 14 EB 62 24 E1 F0 C4 72 6E E6 BE A7 A3 D2
F4 62 C6 AC 66 42 A6 AC
Nonce:
7E 58 95 EA F2 67 24 35 BA D8 17 F5 45 A3 71 48
Plaintext: (length equals block size)
67 61 73 74 72 6F 69 6E 74 65 73 74 69 6E 61 6C
AES Output:
5D E5 49 BE D6 50 23 18 78 8F 14 D2 E1 17 E0 5A
HMAC Output (truncated):
2C EA DF D5 B0 60 38 DE A9 22 29 2D 7C 56 50 10
C5 D6 D2 8D F6 21 E9 7A
Ciphertext: (Nonce | AES Output | Truncated HMAC Output)
7E 58 95 EA F2 67 24 35 BA D8 17 F5 45 A3 71 48
5D E5 49 BE D6 50 23 18 78 8F 14 D2 E1 17 E0 5A
2C EA DF D5 B0 60 38 DE A9 22 29 2D 7C 56 50 10
C5 D6 D2 8D F6 21 E9 7A
128-bit AES key:
9B 19 7D D1 E8 C5 60 9D 6E 67 C3 E3 7C 62 C7 2E 9B 19 7D D1 E8 C5 60 9D 6E 67 C3 E3 7C 62 C7 2E
128-bit HMAC key (Ki, key usage 2):
128-bit HMAC key:
9F DA 0E 56 AB 2D 85 E1 56 9A 68 86 96 C2 6A 6C 9F DA 0E 56 AB 2D 85 E1 56 9A 68 86 96 C2 6A 6C
Plaintext: Nonce:
8D 32 50 F6 36 AB 81 02 BE 6F AB 1E 57 D8 F8 17
Plaintext: (length greater than the block size)
00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
10 11 12 13 14 10 11 12 13 14
IV | Ciphertext | Authentication Tag: AES Output:
13 64 FB 39 DC C0 E3 D9 83 A7 DB 5B 4B 9F FB CA
42 F6 65 88 29
HMAC Output (truncated):
F2 1F C8 95 75 AE 93 C7 57 18 AB 3C 7C FB 28 E1
Ciphertext: (Nonce | AES Output | HMAC Output)
8D 32 50 F6 36 AB 81 02 BE 6F AB 1E 57 D8 F8 17 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 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 42 F6 65 88 29 F2 1F C8 95 75 AE 93 C7 57 18 AB
3C 7C FB 28 E1 3C 7C FB 28 E1
256-bit master key: 256-bit AES 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 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 A5 EB 1C 82 51 60 C3 83 12 98 0C 44 2E 5C 7E 49
192-bit HMAC key (Ki, key usage 2): 192-bit HMAC key:
69 B1 65 14 E3 CD 8E 56 B8 20 10 D5 C7 30 12 B6 69 B1 65 14 E3 CD 8E 56 B8 20 10 D5 C7 30 12 B6
22 C4 D0 0F FC 23 ED 1F 22 C4 D0 0F FC 23 ED 1F
Plaintext: Nonce:
8D 32 50 F6 36 AB 81 02 BE 6F AB 1E 57 D8 F8 17
Plaintext: (length greater than the block size)
00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
10 11 12 13 14 10 11 12 13 14
IV | Ciphertext | Authentication Tag: AES Output:
50 CB FF DC DF 38 69 D7 0B EA FF C3 2C 47 0B C6
5B 72 C3 37 2D
HMAC Output (truncated):
6E D7 B3 47 E9 0B BD 8F 31 F5 79 58 F9 69 50 BA
A1 41 64 6E 65 6C F6 7C
Ciphertext: (Nonce | AES Output | HMAC Output)
8D 32 50 F6 36 AB 81 02 BE 6F AB 1E 57 D8 F8 17 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 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 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 58 F9 69 50 BA A1 41 64 6E 65 6C F6 7C
Sample checksums: Sample checksums:
-----------------
Checksum type: hmac-sha256-128-aes128 Checksum type: hmac-sha256-128-aes128
128-bit master key: 128-bit master key:
37 05 D9 60 80 C1 77 28 A0 E8 00 EA B6 E0 D2 3C 37 05 D9 60 80 C1 77 28 A0 E8 00 EA B6 E0 D2 3C
128-bit HMAC key (Kc, key usage 2): 128-bit HMAC key (Kc, key usage 2):
B3 1A 01 8A 48 F5 47 76 F4 03 E9 A3 96 32 5D C3 B3 1A 01 8A 48 F5 47 76 F4 03 E9 A3 96 32 5D C3
Plaintext: Plaintext:
00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
10 11 12 13 14 10 11 12 13 14
Checksum: Checksum:
D7 83 67 18 66 43 D6 7B 41 1C BA 91 39 FC 1D EE D7 83 67 18 66 43 D6 7B 41 1C BA 91 39 FC 1D EE
Checksum type: hmac-sha384-192-aes256 Checksum type: hmac-sha384-192-aes256
256-bit master key: 256-bit master key:
6D 40 4D 37 FA F7 9F 9D F0 D3 35 68 D3 20 66 98 6D 40 4D 37 FA F7 9F 9D F0 D3 35 68 D3 20 66 98
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