draft-ietf-kitten-aes-cts-hmac-sha2-11.txt   rfc8009.txt 
Network Working Group M. Jenkins Internet Engineering Task Force (IETF) M. Jenkins
Internet Draft National Security Agency Request for Comments: 8009 National Security Agency
Intended Status: Informational M. Peck Category: Informational M. Peck
Expires: February 27, 2017 The MITRE Corporation ISSN: 2070-1721 The MITRE Corporation
K. Burgin K. Burgin
August 26, 2016 October 2016
AES Encryption with HMAC-SHA2 for Kerberos 5 AES Encryption with HMAC-SHA2 for Kerberos 5
draft-ietf-kitten-aes-cts-hmac-sha2-11
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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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working documents as Internet-Drafts. The list of current Internet-
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Internet-Drafts are draft documents valid for a maximum of six months This document is a product of the Internet Engineering Task Force
and may be updated, replaced, or obsoleted by other documents at any (IETF). It represents the consensus of the IETF community. It has
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approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 7841.
This Internet-Draft will expire on February 27, 2017. 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/rfc8009.
Copyright and License Notice Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the Copyright (c) 2016 IETF Trust and the persons identified as the
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Protocol Key Representation . . . . . . . . . . . . . . . . . 3 2. Protocol Key Representation . . . . . . . . . . . . . . . . . 3
3. Key Derivation Function . . . . . . . . . . . . . . . . . . . 3 3. Key Derivation Function . . . . . . . . . . . . . . . . . . . 3
4. Key Generation from Pass Phrases . . . . . . . . . . . . . . . 5 4. Key Generation from Pass Phrases . . . . . . . . . . . . . . . 4
5. Kerberos Algorithm Protocol Parameters . . . . . . . . . . . . 5 5. Kerberos Algorithm Protocol Parameters . . . . . . . . . . . . 5
6. Checksum Parameters . . . . . . . . . . . . . . . . . . . . . 8 6. Checksum Parameters . . . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8 8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8.1. Random Values in Salt Strings . . . . . . . . . . . . . . 9 8.1. Random Values in Salt Strings . . . . . . . . . . . . . . 9
8.2. Algorithm Rationale . . . . . . . . . . . . . . . . . . . 9 8.2. Algorithm Rationale . . . . . . . . . . . . . . . . . . . 9
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 9.1. Normative References . . . . . . . . . . . . . . . . . . . 10
10.1. Normative References . . . . . . . . . . . . . . . . . . 10 9.2. Informative References . . . . . . . . . . . . . . . . . . 11
10.2. Informative References . . . . . . . . . . . . . . . . . 10 Appendix A. Test Vectors . . . . . . . . . . . . . . . . . . . . 12
Appendix A. Test Vectors . . . . . . . . . . . . . . . . . . . . 11 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19
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.
To avoid ciphertext expansion, we use a variation of the CBC-CS3 mode To avoid ciphertext expansion, we use a variation of the CBC-CS3 mode
defined in [SP800-38A+], also referred to as ciphertext stealing or defined in [SP800-38A+], also referred to as ciphertext stealing or
CTS mode. The new types conform to the framework specified in CTS mode. The new types conform to the framework specified in
[RFC3961], but do not use the simplified profile, as the simplified [RFC3961], but do not use the simplified profile, as the simplified
profile is not compliant with modern cryptographic best practices profile is not compliant with modern cryptographic best practices
such as calculating MACs over ciphertext rather than plaintext. such as calculating Message Authentication Codes (MACs) over
ciphertext rather than plaintext.
The encryption and checksum types defined in this document are The encryption and checksum types defined in this document are
intended to support environments that desire to use SHA-256 or SHA- intended to support environments that desire to use SHA-256 or
384 (defined in [FIPS180]) as the hash algorithm. Differences SHA-384 (defined in [FIPS180]) as the hash algorithm. Differences
between the encryption and checksum types defined in this document between the encryption and checksum types defined in this document
and the pre-existing Kerberos AES encryption and checksum types and the pre-existing Kerberos AES encryption and checksum types
specified in [RFC3962] are: specified in [RFC3962] are:
* The pseudorandom function used by PBKDF2 is HMAC-SHA-256 or HMAC- * The pseudorandom function (PRF) used by PBKDF2 is HMAC-SHA-256 or
SHA-384 (HMAC is defined in [RFC2104]). HMAC-SHA-384. (HMAC is defined in [RFC2104].)
* A key derivation function from [SP800-108] using the SHA-256 or * A key derivation function from [SP800-108] using the SHA-256 or
SHA-384 hash algorithm is used to produce keys for encryption, SHA-384 hash algorithm is used to produce keys for encryption,
integrity protection, and checksum operations. integrity protection, and checksum operations.
* The HMAC is calculated over the cipherstate concatenated with the * The HMAC is calculated over the cipher state concatenated with the
AES output, instead of being calculated over the confounder and AES output, instead of being calculated over the confounder and
plaintext. This allows the message receiver to verify the plaintext. This allows the message receiver to verify the
integrity of the message before decrypting the message. integrity of the message before decrypting the message.
* The HMAC algorithm uses the SHA-256 or SHA-384 hash algorithm for * The HMAC algorithm uses the SHA-256 or SHA-384 hash algorithm for
integrity protection and checksum operations. integrity protection and checksum operations.
2. Protocol Key Representation 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).
3. Key Derivation Function 3. 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. which uses the HMAC algorithm as the PRF.
function KDF-HMAC-SHA2(key, label, [context,] k): function KDF-HMAC-SHA2(key, label, [context,] k):
k-truncate(K1) k-truncate(K1)
where the value of K1 is computed as below. where the value of K1 is computed as below.
key: The source of entropy from which subsequent keys are derived key: The source of entropy from which subsequent keys are derived.
(this is known as Ki in [SP800-108]). (This is known as "Ki" in [SP800-108].)
label: An octet string describing the intended usage of the derived label: An octet string describing the intended usage of the derived
key. key.
context: This parameter is optional. An octet string containing the context: This parameter is optional. An octet string containing the
information related to the derived keying material. This information related to the derived keying material. This
specification does not dictate a specific format for the context specification does not dictate a specific format for the context
field. The context field is only used by the pseudo-random function field. The context field is only used by the pseudorandom function
defined in section 5, where it is set to the pseudo-random function's defined in Section 5, where it is set to the pseudorandom function's
octet-string input parameter. The content of the octet-string input octet-string input parameter. The content of the octet-string input
parameter is defined by the application that uses it. parameter is defined by the application that uses it.
k: Length in bits of the key to be outputted, expressed in big-endian k: Length in bits of the key to be outputted, expressed in big-endian
binary representation in 4 bytes (this is called L in [SP800-108]). binary representation in 4 bytes. (This is called "L" in
Specifically, k=128 is represented as 0x00000080, 192 as 0x000000C0, [SP800-108].) Specifically, k=128 is represented as 0x00000080, 192
256 as 0x00000100, and 384 as 0x00000180. as 0x000000C0, 256 as 0x00000100, and 384 as 0x00000180.
When the encryption type is aes128-cts-hmac-sha256-128, k must be no When the encryption type is aes128-cts-hmac-sha256-128, k must be no
greater than 256 bits. When the encryption type is aes256-cts-hmac- greater than 256 bits. When the encryption type is
sha384-192, k must be no greater than 384 bits. aes256-cts-hmac-sha384-192, k must be no greater than 384 bits.
The k-truncate function is defined in [RFC3961], Section 5.1. It The k-truncate function is defined in Section 5.1 of [RFC3961]. It
returns the 'k' leftmost bits of the bitstring input. returns the 'k' leftmost bits of the bit-string input.
In all computations in this document, | indicates concatenation. In all computations in this document, "|" indicates concatenation.
When the encryption type is aes128-cts-hmac-sha256-128, then K1 is When the encryption type is aes128-cts-hmac-sha256-128, then K1 is
computed as follows: computed as follows:
If the context parameter is not present: If the context parameter is not present:
K1 = HMAC-SHA-256(key, 0x00000001 | label | 0x00 | k) K1 = HMAC-SHA-256(key, 0x00000001 | label | 0x00 | k)
If the context parameter is present: If the context parameter is present:
K1 = HMAC-SHA-256(key, 0x00000001 | label | 0x00 | context | k) K1 = HMAC-SHA-256(key, 0x00000001 | label | 0x00 | context | k)
skipping to change at page 5, line 12 skipping to change at page 4, line 35
If the context parameter is present: If the context parameter is present:
K1 = HMAC-SHA-384(key, 0x00000001 | label | 0x00 | context | k) K1 = HMAC-SHA-384(key, 0x00000001 | label | 0x00 | context | k)
In the definitions of K1 above, '0x00000001' is the i parameter (the In the definitions of K1 above, '0x00000001' is the i parameter (the
iteration counter) from Section 5.1 of [SP800-108]. iteration counter) from Section 5.1 of [SP800-108].
4. Key Generation from Pass Phrases 4. Key Generation from Pass Phrases
As defined below, the string-to-key function uses PBKDF2 [RFC2898] As defined below, the string-to-key function uses PBKDF2 [RFC2898]
and KDF-HMAC-SHA2 to derive the base-key from a passphrase and salt. and KDF-HMAC-SHA2 to derive the base-key from a passphrase and salt.
The string-to-key parameter string is four octets indicating an The string-to-key parameter string is 4 octets indicating an unsigned
unsigned number in big-endian order, consistent with [RFC3962], number in big-endian order, consistent with [RFC3962], except that
except that the default is decimal 32768 if the parameter is not the default is decimal 32768 if the parameter is not specified.
specified.
To ensure that different long-term base-keys are used with different To ensure that different long-term base-keys are used with different
enctypes, we prepend the enctype name to the salt, separated by a enctypes, we prepend the enctype name to the salt, separated by a
null byte. The enctype-name is "aes128-cts-hmac-sha256-128" or null byte. The enctype-name is "aes128-cts-hmac-sha256-128" or
"aes256-cts-hmac-sha384-192" (without the quotes). "aes256-cts-hmac-sha384-192" (without the quotes).
The user's long-term base-key is derived as follows: The user's long-term base-key is derived as follows:
iter_count = string-to-key parameter, default is decimal 32768 iter_count = string-to-key parameter, default is decimal 32768
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))
base-key = random-to-key(KDF-HMAC-SHA2(tkey, "kerberos", base-key = random-to-key(KDF-HMAC-SHA2(tkey, "kerberos",
keylength)) keylength))
where "kerberos" is the octet-string 0x6B65726265726F73. where "kerberos" is the octet-string 0x6B65726265726F73.
where PBKDF2 is the function of that name from RFC 2898, the where PBKDF2 is the function of that name from RFC 2898, the
pseudorandom function used by PBKDF2 is HMAC-SHA-256 when the enctype 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 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 "aes256-cts-hmac-sha384-192", the value for keylength is the AES key
length (128 or 256 bits), and the algorithm KDF-HMAC-SHA2 is defined length (128 or 256 bits), and the algorithm KDF-HMAC-SHA2 is defined
in Section 3. in Section 3.
5. Kerberos Algorithm Protocol Parameters 5. Kerberos Algorithm Protocol Parameters
The RFC 3961 cipher state that maintains cryptographic state across The cipher state defined in RFC 3961 that maintains cryptographic
different encryption operations using the same key is used as the state across different encryption operations using the same key is
formal initialization vector (IV) input into CBC-CS3. The plaintext used as the formal initialization vector (IV) input into CBC-CS3.
is prepended with a 16-octet random value generated by the message The plaintext is prepended with a 16-octet random value generated by
originator, known as a confounder. the message originator, known as a confounder.
The ciphertext is a concatenation of the output of AES in CBC-CS3 The ciphertext is a concatenation of the output of AES in CBC-CS3
mode and the HMAC of the cipher state concatenated with the AES mode and the HMAC of the cipher state concatenated with the AES
output. The HMAC is computed using either SHA-256 or SHA-384 output. The HMAC is computed using either SHA-256 or SHA-384
depending on the encryption type. The output of HMAC-SHA-256 is depending on the encryption type. The output of HMAC-SHA-256 is
truncated to 128 bits and the output of HMAC-SHA-384 is truncated to truncated to 128 bits, and the output of HMAC-SHA-384 is truncated to
192 bits. Sample test vectors are given in Appendix A. 192 bits. Sample test vectors are given in Appendix A.
Decryption is performed by removing the HMAC, verifying the HMAC Decryption is performed by removing the HMAC, verifying the HMAC
against the cipher state concatenated with the ciphertext, and then against the cipher state concatenated with the ciphertext, and then
decrypting the ciphertext if the HMAC is correct. Finally, the first decrypting the ciphertext if the HMAC is correct. Finally, the first
16 octets of the decryption output (the confounder) is discarded, and 16 octets of the decryption output (the confounder) is discarded, and
the remainder is returned as the plaintext decryption output. the remainder is returned as the plaintext decryption output.
The following parameters apply to the encryption types aes128-cts- The following parameters apply to the encryption types
hmac-sha256-128 and aes256-cts-hmac-sha384-192. aes128-cts-hmac-sha256-128 and aes256-cts-hmac-sha384-192.
protocol key format: as defined in Section 2. protocol key format: as defined in Section 2.
specific key structure: three derived keys: { Kc, Ke, Ki }. specific key structure: three derived keys: { Kc, Ke, Ki }.
Kc: the checksum key, inputted into HMAC to provide the checksum Kc: the checksum key, inputted into HMAC to provide the checksum
mechanism defined in Section 6. mechanism defined in Section 6.
Ke: the encryption key, inputted into AES encryption and decryption Ke: the encryption key, inputted into AES encryption and decryption
as defined in "encryption function" and "decryption function" below. as defined in "encryption function" and "decryption function" below.
skipping to change at page 6, line 40 skipping to change at page 6, line 23
key-generation seed length: key size (128 or 256 bits). key-generation seed length: key size (128 or 256 bits).
string-to-key function: as defined in Section 4. string-to-key function: as defined in Section 4.
default string-to-key parameters: iteration count of decimal 32768. default string-to-key parameters: iteration count of decimal 32768.
random-to-key function: identity function. random-to-key function: identity function.
key-derivation function: KDF-HMAC-SHA2 as defined in Section 3. The key-derivation function: KDF-HMAC-SHA2 as defined in Section 3. The
key usage number is expressed as four octets in big-endian order. key usage number is expressed as 4 octets in big-endian order.
If the enctype is aes128-cts-hmac-sha256-128: If the enctype is aes128-cts-hmac-sha256-128:
Kc = KDF-HMAC-SHA2(base-key, usage | 0x99, 128) Kc = KDF-HMAC-SHA2(base-key, usage | 0x99, 128)
Ke = KDF-HMAC-SHA2(base-key, usage | 0xAA, 128) Ke = KDF-HMAC-SHA2(base-key, usage | 0xAA, 128)
Ki = KDF-HMAC-SHA2(base-key, usage | 0x55, 128) Ki = KDF-HMAC-SHA2(base-key, usage | 0x55, 128)
If the enctype is aes256-cts-hmac-sha384-192: If the enctype is aes256-cts-hmac-sha384-192:
Kc = KDF-HMAC-SHA2(base-key, usage | 0x99, 192) Kc = KDF-HMAC-SHA2(base-key, usage | 0x99, 192)
Ke = KDF-HMAC-SHA2(base-key, usage | 0xAA, 256) Ke = KDF-HMAC-SHA2(base-key, usage | 0xAA, 256)
Ki = KDF-HMAC-SHA2(base-key, usage | 0x55, 192) Ki = KDF-HMAC-SHA2(base-key, usage | 0x55, 192)
skipping to change at page 7, line 4 skipping to change at page 6, line 34
If the enctype is aes128-cts-hmac-sha256-128: If the enctype is aes128-cts-hmac-sha256-128:
Kc = KDF-HMAC-SHA2(base-key, usage | 0x99, 128) Kc = KDF-HMAC-SHA2(base-key, usage | 0x99, 128)
Ke = KDF-HMAC-SHA2(base-key, usage | 0xAA, 128) Ke = KDF-HMAC-SHA2(base-key, usage | 0xAA, 128)
Ki = KDF-HMAC-SHA2(base-key, usage | 0x55, 128) Ki = KDF-HMAC-SHA2(base-key, usage | 0x55, 128)
If the enctype is aes256-cts-hmac-sha384-192: If the enctype is aes256-cts-hmac-sha384-192:
Kc = KDF-HMAC-SHA2(base-key, usage | 0x99, 192) Kc = KDF-HMAC-SHA2(base-key, usage | 0x99, 192)
Ke = KDF-HMAC-SHA2(base-key, usage | 0xAA, 256) Ke = KDF-HMAC-SHA2(base-key, usage | 0xAA, 256)
Ki = KDF-HMAC-SHA2(base-key, usage | 0x55, 192) Ki = KDF-HMAC-SHA2(base-key, usage | 0x55, 192)
cipher state: a 128-bit CBC initialization vector derived from a cipher state: a 128-bit CBC initialization vector derived from a
previous (if any) ciphertext using the same encryption key, as previous ciphertext (if any) using the same encryption key, as
specified below. specified below.
initial cipher state: all bits zero. initial cipher state: all bits zero.
encryption function: as follows, where E() is AES encryption in encryption function: as follows, where E() is AES encryption in
CBC-CS3 mode, and h is the size of truncated HMAC (128 bits or CBC-CS3 mode, and h is the size of truncated HMAC (128 bits or 192
192 bits as described above). bits as described above).
N = random value of length 128 bits (the AES block size) N = random value of length 128 bits (the AES block size)
IV = cipher state IV = cipher state
C = E(Ke, N | plaintext, IV) C = E(Ke, N | plaintext, IV)
H = HMAC(Ki, IV | C) H = HMAC(Ki, IV | C)
ciphertext = C | H[1..h] ciphertext = C | H[1..h]
Steps to compute the 128-bit cipher state: Steps to compute the 128-bit cipher state:
L = length of C in bits L = length of C in bits
portion C into 128-bit blocks, placing any remainder portion C into 128-bit blocks, placing any remainder of less
of less than 128 bits into a final block than 128 bits into a final block
if L == 128: cipher state = C if L == 128: cipher state = C
else if L mod 128 > 0: cipher state = last full (128-bit) else if L mod 128 > 0: cipher state = last full (128-bit) block
block of C (the of C (the next-to-last
next-to-last block) block)
else if L mod 128 == 0: cipher state = next-to-last block else if L mod 128 == 0: cipher state = next-to-last block of C
of C
(note that L will never be less than 128 because of the (Note that L will never be less than 128 because of the
presence of N in the encryption input) presence of N in the encryption input.)
decryption function: as follows, where D() is AES decryption in decryption function: as follows, where D() is AES decryption in
CBC-CS3 mode, and h is the size of truncated HMAC. CBC-CS3 mode, and h is the size of truncated HMAC.
(C, H) = ciphertext (Note: H is the last h bits of the ciphertext) (C, H) = ciphertext
(Note: H is the last h bits of the ciphertext.)
IV = cipher state IV = cipher state
if H != HMAC(Ki, IV | C)[1..h] if H != HMAC(Ki, IV | C)[1..h]
stop, report error stop, report error
(N, P) = D(Ke, C, IV) (N, P) = D(Ke, C, IV)
Note: N is set to the first block of the decryption output,
P is set to the rest of the output. (Note: N is set to the first block of the decryption output; P is
set to the rest of the output.)
cipher state = same as described above in encryption function cipher state = same as described above in encryption function
pseudo-random function: pseudorandom function:
If the enctype is aes128-cts-hmac-sha256-128: If the enctype is aes128-cts-hmac-sha256-128:
PRF = KDF-HMAC-SHA2(input-key, "prf", octet-string, 256) PRF = KDF-HMAC-SHA2(input-key, "prf", octet-string, 256)
If the enctype is aes256-cts-hmac-sha384-192: If the enctype is aes256-cts-hmac-sha384-192:
PRF = KDF-HMAC-SHA2(input-key, "prf", octet-string, 384) PRF = KDF-HMAC-SHA2(input-key, "prf", octet-string, 384)
where "prf" is the octet-string 0x707266 where "prf" is the octet-string 0x707266
6. Checksum Parameters 6. Checksum Parameters
The following parameters apply to the checksum types hmac-sha256-128- The following parameters apply to the checksum types
aes128 and hmac-sha384-192-aes256, which are the associated checksums hmac-sha256-128-aes128 and hmac-sha384-192-aes256, which are the
for aes128-cts-hmac-sha256-128 and aes256-cts-hmac-sha384-192, associated checksums for aes128-cts-hmac-sha256-128 and
respectively. aes256-cts-hmac-sha384-192, respectively.
associated cryptosystem: aes128-cts-hmac-sha256-128 or aes256-cts- associated cryptosystem: aes128-cts-hmac-sha256-128 or
hmac-sha384-192 as appropriate. aes256-cts-hmac-sha384-192 as appropriate.
get_mic: HMAC(Kc, message)[1..h]. get_mic: HMAC(Kc, message)[1..h].
where h is 128 bits for checksum type hmac-sha256-128-aes128 where h is 128 bits for checksum type hmac-sha256-128-aes128 and
and 192 bits for checksum type hmac-sha384-192-aes256 192 bits for checksum type hmac-sha384-192-aes256
verify_mic: get_mic and compare. verify_mic: get_mic and compare.
7. IANA Considerations 7. IANA Considerations
IANA is requested to assign: IANA has assigned encryption type numbers as follows in the "Kerberos
Encryption Type Numbers" registry.
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 etype encryption type Reference
----- --------------- --------- ----- --------------- ---------
TBD1 aes128-cts-hmac-sha256-128 [this document] 19 aes128-cts-hmac-sha256-128 RFC 8009
TBD2 aes256-cts-hmac-sha384-192 [this document] 20 aes256-cts-hmac-sha384-192 RFC 8009
Checksum type numbers for hmac-sha256-128-aes128 and hmac-sha384-192- IANA has assigned checksum type numbers as follows in the "Kerberos
aes256 in the Kerberos Checksum Type Numbers registry. Checksum Type Numbers" registry.
Sumtype Checksum type Size Reference sumtype Checksum type checksum Reference
------- ------------- ---- --------- value size
TBD3 hmac-sha256-128-aes128 16 [this document] ------- ------------- -------- ---------
TBD4 hmac-sha384-192-aes256 24 [this document] 19 hmac-sha256-128-aes128 16 RFC 8009
20 hmac-sha384-192-aes256 24 RFC 8009
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 pseudorandom number generators (PRNGs)
(PRNGs) can result in little or no security. The generation of can result in little or no security. The generation of quality
quality random numbers is difficult. [RFC4086] offers random number random numbers is difficult. [RFC4086] offers guidance on random
generation guidance. number generation.
This document specifies a mechanism for generating keys from This document specifies a mechanism for generating keys from
passphrases or passwords. The use of PBKDF2, a salt, and a large passphrases or passwords. The use of PBKDF2, a salt, and a large
iteration count adds some resistance to off-line dictionary attacks iteration count adds some resistance to offline dictionary attacks by
by passive eavesdroppers. Salting prevents rainbow table attacks, passive eavesdroppers. Salting prevents "rainbow table" attacks,
while large iteration counts slow password guess attempts. while large iteration counts slow password-guess attempts.
Nonetheless, computing power continues to rapidly improve, including Nonetheless, computing power continues to rapidly improve, including
the potential for use of graphics processing units (GPUs) in password the potential for use of graphics processing units (GPUs) in
guess attempts. It is important to choose strong passphrases. Use of password-guess attempts. It is important to choose strong
Kerberos extensions that protect against off-line dictionary attacks passphrases. Use of Kerberos extensions that protect against offline
should also be considered, as should the use of public key dictionary attacks should also be considered, as should the use of
cryptography for initial Kerberos authentication [RFC4556] to public key cryptography for initial Kerberos authentication [RFC4556]
eliminate the use of passwords or passphrases within the Kerberos to eliminate the use of passwords or passphrases within the Kerberos
protocol. protocol.
The NIST guidance in section 5.3 of [SP800-38A], requiring that CBC The NIST guidance in Section 5.3 of [SP800-38A], requiring that CBC
initialization vectors be unpredictable, is satisfied by the use of a initialization vectors be unpredictable, is satisfied by the use of a
random confounder as the first block of plaintext. The confounder random confounder as the first block of plaintext. The confounder
fills the cryptographic role typically played by an initialization fills the cryptographic role typically played by an initialization
vector. This approach was chosen to align with other Kerberos vector. This approach was chosen to align with other Kerberos
cryptosystem approaches. cryptosystem approaches.
8.1. Random Values in Salt Strings 8.1. Random Values in Salt Strings
NIST guidance in Section 5.1 of [SP800-132] requires at least 128 The NIST guidance in Section 5.1 of [SP800-132] requires at least 128
bits of the salt to be randomly generated. The string-to-key function bits of the salt to be randomly generated. The string-to-key
as defined in [RFC3961] requires the salt to be valid UTF-8 strings function as defined in [RFC3961] requires the salt to be valid UTF-8
[RFC3629]. Not every 128-bit random string will be valid UTF-8, so a strings [RFC3629]. Not every 128-bit random string will be valid
UTF-8 compatible encoding would be needed to encapsulate the random UTF-8, so a UTF-8-compatible encoding would be needed to encapsulate
bits. However, using a salt containing a random portion may have the the random bits. However, using a salt containing a random portion
following issues with some implementations: may have the following issues with some implementations:
* Cross-realm krbtgt keys are typically managed by entering the * Keys for cross-realm krbtgt services [RFC4120] are typically
same password at two KDCs to get the same keys. If each KDC uses managed by entering the same password at two Key Distribution
a random salt, they won't have the same keys. Centers (KDCs) to get the same keys. If each KDC uses a random
salt, they won't have the same keys.
* Random salts may interfere with password history checking. * Random salts may interfere with checking of password history.
8.2. Algorithm Rationale 8.2. Algorithm Rationale
This document has been written to be consistent with common This document has been written to be consistent with common
implementations of AES and SHA-2. The encryption and hash algorithm implementations of AES and SHA-2. The encryption and hash algorithm
sizes have been chosen to create a consistent level of protection, sizes have been chosen to create a consistent level of protection,
with consideration to implementation efficiencies. So, for instance, with consideration to implementation efficiencies. So, for instance,
SHA-384, which would normally be matched to AES-192, is instead SHA-384, which would normally be matched to AES-192, is instead
matched to AES-256 to leverage the fact that there are efficient matched to AES-256 to leverage the fact that there are efficient
hardware implementations of AES-256. Note that, as indicated by the hardware implementations of AES-256. Note that, as indicated by the
enc-type name "aes256-cts-hmac-sha384-192", the truncation of the enc-type name "aes256-cts-hmac-sha384-192", the truncation of the
HMAC-SHA-384 output to 192-bits results in an overall 192-bit level HMAC-SHA-384 output to 192 bits results in an overall 192-bit level
of security. of security.
9. Acknowledgements 9. References
Kelley Burgin was employed at the National Security Agency during 9.1. Normative References
much of the work on this document.
10. References [FIPS180] National Institute of Standards and Technology, "Secure
Hash Standard", FIPS PUB 180-4,
DOI 10.6028/NIST.FIPS.180-4, August 2015.
10.1. Normative References [FIPS197] National Institute of Standards and Technology,
"Advanced Encryption Standard (AES)", FIPS PUB 197,
November 2001.
[RFC2104] Krawczyk, H. et al., "HMAC: Keyed-Hashing for Message [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Authentication", RFC 2104, February 1997. Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997,
<http://www.rfc-editor.org/info/rfc2104>.
[RFC2898] Kaliski, B., "PKCS #5: Password-Based Cryptography [RFC2898] Kaliski, B., "PKCS #5: Password-Based Cryptography
Specification Version 2.0", RFC 2898, September 2000. Specification Version 2.0", RFC 2898,
DOI 10.17487/RFC2898, September 2000,
<http://www.rfc-editor.org/info/rfc2898>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 3629, November 2003. 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <http://www.rfc-editor.org/info/rfc3629>.
[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, DOI 10.17487/RFC3961, February
2005, <http://www.rfc-editor.org/info/rfc3961>.
[RFC3962] Raeburn, K., "Advanced Encryption Standard (AES) [RFC3962] Raeburn, K., "Advanced Encryption Standard (AES)
Encryption for Kerberos 5", RFC 3962, February 2005. Encryption for Kerberos 5", RFC 3962,
DOI 10.17487/RFC3962, February 2005,
[FIPS180] National Institute of Standards and Technology, "Secure <http://www.rfc-editor.org/info/rfc3962>.
Hash Standard", FIPS PUB 180-4, August 2015.
[FIPS197] National Institute of Standards and Technology,
"Advanced Encryption Standard (AES)", FIPS PUB 197,
November 2001.
[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:
Three Variants of Ciphertext Stealing for CBC Mode", Three Variants of Ciphertext Stealing for CBC Mode",
NIST Special Publication 800-38A Addendum, October 2010. NIST Special Publication 800-38A Addendum, October 2010.
[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.
10.2. Informative References 9.2. Informative References
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker, [RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC "Randomness Requirements for Security", BCP 106,
4086, June 2005. RFC 4086, DOI 10.17487/RFC4086, June 2005,
<http://www.rfc-editor.org/info/rfc4086>.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
DOI 10.17487/RFC4120, July 2005,
<http://www.rfc-editor.org/info/rfc4120>.
[RFC4556] Zhu, L. and B. Tung, "Public Key Cryptography for
Initial Authentication in Kerberos (PKINIT)", RFC 4556,
DOI 10.17487/RFC4556, June 2006,
<http://www.rfc-editor.org/info/rfc4556>.
[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 Methods and Techniques", NIST Special Publication
800-38A, December 2001. 800-38A, December 2001.
[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
132, June 2010. 800-132, June 2010.
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 base-key: Saltp for creating 128-bit base-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 10 DF 9D D7 83 73 68 61 32 35 36 2D 31 32 38 00 10 DF 9D D7 83
E5 BC 8A CE A1 73 0E 74 35 5F 61 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 |
random 16 byte valid UTF-8 sequence | "ATHENA.MIT.EDUraeburn") random 16-byte valid UTF-8 sequence | "ATHENA.MIT.EDUraeburn")
128-bit base-key: 128-bit base-key:
08 9B CA 48 B1 05 EA 6E A7 7C A5 D2 F3 9D C5 E7 08 9B CA 48 B1 05 EA 6E A7 7C A5 D2 F3 9D C5 E7
Saltp for creating 256-bit base-key: Saltp for creating 256-bit base-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 10 DF 9D D7 83 73 68 61 33 38 34 2D 31 39 32 00 10 DF 9D D7 83
E5 BC 8A CE A1 73 0E 74 35 5F 61 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 |
random 16 byte valid UTF-8 sequence | "ATHENA.MIT.EDUraeburn") random 16-byte valid UTF-8 sequence | "ATHENA.MIT.EDUraeburn")
256-bit base-key: 256-bit base-key:
45 BD 80 6D BF 6A 83 3A 9C FF C1 C9 45 89 A2 22 45 BD 80 6D BF 6A 83 3A 9C FF C1 C9 45 89 A2 22
36 7A 79 BC 21 C4 13 71 89 06 E9 F5 78 A7 84 67 36 7A 79 BC 21 C4 13 71 89 06 E9 F5 78 A7 84 67
Sample results for key derivation: Sample results for key derivation:
---------------------------------- ----------------------------------
enctype aes128-cts-hmac-sha256-128: enctype aes128-cts-hmac-sha256-128:
128-bit base-key: 128-bit base-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
skipping to change at page 12, line 28 skipping to change at page 14, line 7
BA 41 F2 8F AF 69 E7 3D BA 41 F2 8F AF 69 E7 3D
Ke value for key usage 2 (label = 0x00000002AA): Ke value for key usage 2 (label = 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 (label = 0x0000000255): Ki value for key usage 2 (label = 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 (all using the default cipher state): Sample encryptions (all using the default cipher state):
-------------------------------------------------------- --------------------------------------------------------
These sample encryptions use the above sample key
derivation results, including use of the same These sample encryptions use the above sample key derivation results,
base-key and key usage values. including use of the same base-key and key usage values.
The following test vectors are for The following test vectors are for
enctype aes128-cts-hmac-sha256-128: enctype aes128-cts-hmac-sha256-128:
Plaintext: (empty) Plaintext: (empty)
Confounder: Confounder:
7E 58 95 EA F2 67 24 35 BA D8 17 F5 45 A3 71 48 7E 58 95 EA F2 67 24 35 BA D8 17 F5 45 A3 71 48
128-bit AES key (Ke): 128-bit AES key (Ke):
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): 128-bit HMAC key (Ki):
skipping to change at page 16, line 4 skipping to change at page 18, line 7
FE F6 EC B6 47 D6 29 5F AE 07 7A 1F EB 51 75 08 FE F6 EC B6 47 D6 29 5F AE 07 7A 1F EB 51 75 08
D2 C1 6B 41 92 E0 1F 62 D2 C1 6B 41 92 E0 1F 62
Ciphertext: Ciphertext:
40 01 3E 2D F5 8E 87 51 95 7D 28 78 BC D2 D6 FE 40 01 3E 2D F5 8E 87 51 95 7D 28 78 BC D2 D6 FE
10 1C CF D5 56 CB 1E AE 79 DB 3C 3E E8 64 29 F2 10 1C CF D5 56 CB 1E AE 79 DB 3C 3E E8 64 29 F2
B2 A6 02 AC 86 FE F6 EC B6 47 D6 29 5F AE 07 7A B2 A6 02 AC 86 FE F6 EC B6 47 D6 29 5F AE 07 7A
1F EB 51 75 08 D2 C1 6B 41 92 E0 1F 62 1F EB 51 75 08 D2 C1 6B 41 92 E0 1F 62
Sample checksums: Sample checksums:
----------------- -----------------
These sample checksums use the above sample key
derivation results, including use of the same These sample checksums use the above sample key derivation results,
base-key and key usage values. including use of the same base-key and key usage values.
Checksum type: hmac-sha256-128-aes128 Checksum type: hmac-sha256-128-aes128
128-bit HMAC key (Kc): 128-bit HMAC key (Kc):
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
skipping to change at page 17, line 6 skipping to change at page 19, line 6
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
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:
45 EE 79 15 67 EE FC A3 7F 4A C1 E0 22 2D E8 0D 45 EE 79 15 67 EE FC A3 7F 4A C1 E0 22 2D E8 0D
43 C3 BF A0 66 99 67 2A 43 C3 BF A0 66 99 67 2A
Sample pseudorandom function (PRF) invocations: Sample pseudorandom function (PRF) invocations:
---------------------------------------- -----------------------------------------------
PRF input octet-string: "test" (0x74657374) PRF input octet-string: "test" (0x74657374)
enctype aes128-cts-hmac-sha256-128: enctype aes128-cts-hmac-sha256-128:
input-key value / HMAC-SHA-256 key: input-key value / HMAC-SHA-256 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
HMAC-SHA-256 input message: HMAC-SHA-256 input message:
00 00 00 01 70 72 66 00 74 65 73 74 00 00 01 00 00 00 00 01 70 72 66 00 74 65 73 74 00 00 01 00
PRF output: PRF output:
9D 18 86 16 F6 38 52 FE 86 91 5B B8 40 B4 A8 86 9D 18 86 16 F6 38 52 FE 86 91 5B B8 40 B4 A8 86
skipping to change at page 18, line 5 skipping to change at page 19, line 30
input-key value / HMAC-SHA-384 key: input-key value / HMAC-SHA-384 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
HMAC-SHA-384 input message: HMAC-SHA-384 input message:
00 00 00 01 70 72 66 00 74 65 73 74 00 00 01 80 00 00 00 01 70 72 66 00 74 65 73 74 00 00 01 80
PRF output: PRF output:
98 01 F6 9A 36 8C 2B F6 75 E5 95 21 E1 77 D9 A0 98 01 F6 9A 36 8C 2B F6 75 E5 95 21 E1 77 D9 A0
7F 67 EF E1 CF DE 8D 3C 8D 6F 6A 02 56 E3 B1 7D 7F 67 EF E1 CF DE 8D 3C 8D 6F 6A 02 56 E3 B1 7D
B3 C1 B6 2A D1 B8 55 33 60 D1 73 67 EB 15 14 D2 B3 C1 B6 2A D1 B8 55 33 60 D1 73 67 EB 15 14 D2
Acknowledgements
Kelley Burgin was employed at the National Security Agency during
much of the work on this document.
Authors' Addresses Authors' Addresses
Michael J. Jenkins Michael J. Jenkins
National Security Agency National Security Agency
EMail: mjjenki@tycho.ncsc.mil Email: mjjenki@tycho.ncsc.mil
Michael A. Peck Michael A. Peck
The MITRE Corporation The MITRE Corporation
EMail: mpeck@mitre.org Email: mpeck@mitre.org
Kelley W. Burgin Kelley W. Burgin
Email: kelley.burgin@gmail.com Email: kelley.burgin@gmail.com
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