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Versions: 00 01 02

Network Working Group                                          K. Burgin
Internet Draft                                  National Security Agency
Intended Status: Informational                                   M. Peck
Expires: April 22, 2013                            The MITRE Corporation
                                                        October 19, 2012

              AES Encryption with HMAC-SHA2 for Kerberos 5
               draft-burgin-kerberos-aes-cbc-hmac-sha2-02


Abstract

   This document specifies two encryption types and two corresponding
   checksum types for Kerberos 5.  The new types use AES in CBC mode
   with ciphertext stealing for confidentiality and HMAC with a SHA-2
   hash for integrity.

Status of this Memo

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   This Internet-Draft will expire on February 21, 2013.

Copyright and License Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
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   described in the Simplified BSD License.



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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 with ciphertext stealing
   similar to [RFC3962] but with several variations.

   The new types use the PBKDF2 algorithm for key generation from
   strings, with a modification to the use in [RFC3962] that the 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
      information such as the principal's realm and name components.




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   *  The final key derivation step uses the algorithm KDF-HMAC-SHA2
      defined below in Section 5 instead of the DK function.

   *  If no string-to-key parameters are specified, the default number
      of iterations is raised to 32,768.

   To ensure that different long-term keys are used with different
   enctypes, we prepend the enctype name to the salt string, separated
   by a null byte.  The enctype name is "aes128-cts-hmac-sha256-128" or
   "aes256-cts-hmac-sha384-192" (without the quotes). The user's long-
   term key is derived as follows

     saltp = enctype-name | 0x00 | salt
     tkey = random-to-key(PBKDF2(passphrase, saltp,
                              iter_count, keylength))
     key = KDF-HMAC-SHA2(tkey, "kerberos") where "kerberos" is the
           byte string {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-cts-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-cts-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-cts-
   hmac-sha256-128 and aes256-cts-hmac-sha384-192.

   The key-derivation function described in the previous section is used
   to produce the three intermediate keys.  Typically, CBC mode [SP800-
   38A] requires the input be padded to a multiple of the encryption
   algorithm block size, which is 128 bits for AES.  However, to avoid
   ciphertext expansion, we use the CBC-CS3 variant to CBC mode defined
   in [SP800-38A+].

   Each encryption will use a freshly generated 16-octet nonce generated
   at random by the message originator.  The initialization vector (IV)
   used by AES is obtained by xoring the random nonce with the
   cipherstate.

   The ciphertext is the concatenation of the random nonce, the output
   of AES in CBC-CS3 mode, and the HMAC of the initialization vector
   concatenated with the AES output.  The HMAC is computed using either
   SHA-256 or SHA-384.  The output of SHA-256 is truncated to 128 bits
   and the output of SHA-384 is truncated to 192 bits. Sample test
   vectors are given in Appendix A.

   Decryption is performed by removing the HMAC, verifying the HMAC
   against the remainder, and then decrypting the remainder if the HMAC
   is correct.

   The encryption and checksum mechanisms below use the following
   notation from [RFC3961].

   HMAC output size, h
   message block size, m
   encryption/decryption functions, E and D
   cipher block size, c

               Encryption Mechanism for AES-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



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cipher state              Random nonce of length c (128 bits)

initial cipher state      All bits zero

encryption function       N = random nonce of length c (128 bits)
                          IV = N + cipherState (+ denotes XOR)
                          C = E(Ke, plaintext, IV)
                              using CBC-CS3-Encrypt defined
                              in [SP800-38A+]
                          H = HMAC(Ki, N | C)
                          ciphertext =  N | C | H[1..h]
                          cipherState = N

decryption function       (N, C, H) = ciphertext
                          if (H != HMAC(Ki, N | C)[1..h])
                              stop, report error
                          IV = N + cipherState (+ denotes XOR)
                          P = D(Ke, C, IV)
                              using CBC-CS3-Decrypt defined
                              in [SP800-38A+]
                          cipherState = N

pseudo-random function    Kp  = KDF-HMAC-SHA2(protocol-key, "prf")
                          PRF = HMAC(Kp, octet-string)

key generation functions:

string-to-key function    tkey = random-to-key(PBKDF2(passphrase, saltp,
                                                   iter_count,
                                                   keylength))
                          base-key = KDF-HMAC-SHA2(tkey, "kerberos")

                          where the pseudorandom function used by PBKDF2
                          is 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-CTS-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-cts-hmac-sha256-128           TBD1               128      |
  |   aes256-cts-hmac-sha384-192           TBD2               256      |
  +--------------------------------------------------------------------+

  +--------------------------------------------------------------------+
  |                          checksum types                            |
  +--------------------------------------------------------------------+
  |         type name                  sumtype value        length     |
  +--------------------------------------------------------------------+
  |    hmac-sha256-128-aes128              TBD3               128      |
  |    hmac-sha384-192-aes256              TBD4               192      |
  +--------------------------------------------------------------------+

   These checksum types will be used with the corresponding encryption
   types defined above.

7.  IANA Considerations

   IANA is requested to assign:

   1.  Encryption type numbers for aes128-cts-hmac-sha256-128 and
       aes256-cts-hmac-sha384-192 in the Kerberos Encryption Type
       Numbers registry.

     Etype   encryption type              Reference
     -----   ---------------              ---------
     TBD1    aes128-cts-hmac-sha256-128   [I.D.burgin-kerberos-aes-
                                           cbc-hmac-sha2]
     TBD2    aes256-cts-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

   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

   [SP800-38A+] National Institute of Standards and Technology,
                "Recommendation for Block Cipher Modes of Operation:
                Three Variants of Ciphertext Stealing for CBC Mode",
                Addendum to NIST Special Publication 800-38A, October
                2010.

   [RFC2119]    Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3961]    Raeburn, K., "Encryption and Checksum Specifications for
                Kerberos 5", RFC 3961, February 2005.

   [RFC3962]    Raeburn, K., "Advanced Encryption Standard (AES)
                Encryption for Kerberos 5", RFC 3962, February 2005.

   [RFC4086]    Eastlake 3rd, D., Schiller, J., and S. Crocker,
                "Randomness Requirements for Security", BCP 106,
                RFC 4086, June 2005.

   [FIPS197]    National Institute of Standards and Technology,
                "Advanced Encryption Standard (AES)", FIPS PUB 197,
                November 2001.



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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.


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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