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Versions: 00 01 02

Network Working Group                                        K.W. Burgin
Internet Draft                                  National Security Agency
Intended Status: Standards Track                               M.A. Peck
Expires: January 12, 2012                          The MITRE Corporation
                                                           July 11, 2011

              AES-CBC Mode with HMAC-SHA2 For Kerberos 5
               draft-burgin-kerberos-aes-cbc-hmac-sha2-01


Abstract

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

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on January 12, 2012.

Copyright and License Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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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, but do not use
   ciphertext stealing as in [RFC3962].  Instead, the messages are
   padded to a multiple of the AES block size as described in Section
   6.3 of [RFC5652].

   The new types use the PBKDF2 algorithm for key generation from
   strings, with a modification to the use in [RFC3962] that the hash
   algorithm in the pseudorandom function used by PBKDF2 will be SHA-256
   instead of SHA-1.

   The new types use key derivation to produce keys for encryption,
   integrity protection, and checksum operations as in [RFC3962].
   However, a key derivation function from [SP800-108] which uses the
   SHA-256 or SHA-384 hash algorithm is used in place of the DK key
   derivation function used in [RFC3961].

   The new types use the HMAC algorithm with a hash from the SHA-2
   family for integrity protection and checksum operations.

2.  Conventions used in this Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
    document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Protocol Key Representation

   The AES key space is dense, so we can use random or pseudorandom
   octet strings directly as keys.  The byte representation for the key
   is described in [FIPS197], where the first bit of the bit string is
   the high bit of the first byte of the byte string (octet string).

4.  Key Generation from Pass Phrases

   We use a variation on the key generation algorithm specified in
   Section 4 of [RFC3962] with the following changes:

   *  The pseudorandom function used by PBKDF2 will be the SHA-256 HMAC
      of the passphrase and salt, instead of the SHA-1 HMAC of the
      passphrase and salt. The salt SHOULD contain at least 128 random
      bits as recommended in [SP800-132].  It MAY also contain other



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      information such as the principal's realm and name components.

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

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

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

     saltp = enctype-name | 0x00 | salt
     tkey = random2key(PBKDF2(passphrase, saltp,
                              iter_count, keylength))
     key = KDF-HMAC-SHA2(tkey, "kerberos") where "kerberos" is the
           byte string {0x6b 0x65 0x72 0x62 0x65 0x72 0x6f 0x73}.

   where the pseudorandom function used by PBKDF2 is the SHA-256 HMAC of
   the passphrase and salt, the value for keylength is the AES key
   length, and the algorithm KDF-HMAC-SHA2 is defined in Section 5.


5.  Key Derivation Function

   We use a key derivation function from Section 5.1 of [SP800-108]
   which uses the HMAC algorithm as the PRF.  The counter i is expressed
   as four octets in big-endian order.  The length of the output key in
   bits (denoted as k) is also represented as four octets in big-endian
   order.  The "Label" input to the KDF is the usage constant supplied
   to the key derivation function, and the "Context" input is null.

   When the encryption type is aes128-cbc-hmac-sha256-128:

     n = 1
     K1 = HMAC-SHA-256(key, 00 00 00 01 | constant | 0x00 | 00 00 00 80)
     DR(key, constant) = First 128 bits of K1
     KDF-HMAC-SHA2(key, constant) = random-to-key(DR(key, constant))

   When the encryption type is aes256-cbc-hmac-sha384-192:

     n = 1
     K1 = HMAC-SHA-384(key, 00 00 00 01 | constant | 0x00 | 00 00 01 00)
     DR(key, constant) = First 256 bits of K1
     KDF-HMAC-SHA2(key, constant) = random-to-key(DR(key, constant))




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6.  Kerberos Algorithm Protocol Parameters

   The following parameters apply to the encryption types aes128-cbc-
   hmac-sha256-128 and aes256-cbc-hmac-sha384-192.

   The key-derivation function described in the previous section is used
   to produce the three intermediate keys.  CBC mode [SP800-38A]
   requires the input be padded to a multiple of the encryption
   algorithm block size, which is 128 bits for AES.  The input will be
   padded as described in Section 6.3 of [RFC5652] in which the value of
   each added octet equals the number of octets that are added.

   Each encryption will use a freshly generated 16-octet initialization
   vector generated at random by the message originator.

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

   Decryption is performed by removing the HMAC, decrypting the
   remainder, verifying the HMAC, and verifying and removing the
   padding.

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

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

               Encryption Mechanism for AES-CBC-HMAC-SHA2
------------------------------------------------------------------------

protocol key format       128- or 256-bit string

specific key structure    Three protocol-format keys: { Kc, Ke, Ki }.

required checksum         As defined below.
mechanism

key-generation seed       key size (128 or 256 bits)
length

cipher state              Initial vector of length c (128 bits)




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initial cipher state      All bits zero

encryption function       N = random nonce of length c (128 bits)

                          pad  = Shortest string of non-zero length to
                                 bring plaintext to a length that is a
                                 multiple of m.  The value of each added
                                 octet equals the number of octets that
                                 are added.

                          N1 = N + cipherState (+ denotes XOR)
                          C = E(Ke, plaintext | pad, N1)
                          H = HMAC(Ki, N | plaintext | pad)
                          ciphertext =  N | C | H[1..h]
                          cipherState = N

decryption function       (N,C,H) = ciphertext
                          (P, pad) = D(Ke, C, N + cipherState)
                          if (H != HMAC(Ki, N | P | pad)[1..h]
                                   or pad is bad)
                             report error
                          cipherState = N

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

key generation functions:

string-to-key function    tkey = random2key(PBKDF2(passphrase, saltp,
                                                   iter_count,
                                                   keylength))
                          base-key = KDF-HMAC-SHA2(tkey, "kerberos")

                          where the pseudorandom function used by PBKDF2
                          is the SHA-256 HMAC of the passphrase and salt

default string-to-key     00 00 80 00
parameters

random-to-key function    identity function

key-derivation function   KDF-HMAC-SHA2 as defined in Section 5.  The
                          key usage number is expressed as four octets
                          in big-endian order.

                          Kc = KDF-HMAC-SHA2(base-key, usage | 0x99);
                          Ke = KDF-HMAC-SHA2(base-key, usage | 0xAA);
                          Ki = KDF-HMAC-SHA2(base-key, usage | 0x55);



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                Checksum Mechanism for AES-CBC-HMAC-SHA2
------------------------------------------------------------------------
associated cryptosystem   AES-128-CBC or AES-256-CBC as appropriate

get_mic                   HMAC(Kc, message)[1..h]

verify_mic                get_mic and compare

   Using this profile with each key size gives us two each of encryption
   and checksum algorithm definitions.

  +--------------------------------------------------------------------+
  |                         encryption types                           |
  +--------------------------------------------------------------------+
  |         type name                  etype value          key size   |
  +--------------------------------------------------------------------+
  |   aes128-cbc-hmac-sha256-128           TBD1               128      |
  |   aes256-cbc-hmac-sha384-192           TBD2               256      |
  +--------------------------------------------------------------------+

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

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

7.  IANA Considerations

   IANA is requested to assign:

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

     Etype   encryption type              Reference
     -----   ---------------              ---------
     TBD1    aes128-cbc-hmac-sha256-128   [I.D.burgin-kerberos-aes-
                                           cbc-hmac-sha2]
     TBD2    aes256-cbc-hmac-sha384-192   [I.D.burgin-kerberos-aes-
                                           cbc-hmac-sha2]

   2.  Checksum type numbers for hmac-sha256-128-aes128 and hmac-sha384-



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       192-aes256 in the Kerberos Checksum Type Numbers registry.

     Sumtype   Checksum type            Size   Reference
     -------   -------------            ----   ---------
     TBD3      hmac-sha256-128-aes128   16     [I.D.burgin-kerberos-
                                                aes-cbc-hmac-sha2]
     TBD4      hmac-sha384-192-aes256   24     [I.D.burgin-kerberos-
                                                aes-cbc-hmac-sha2]

8.  Security Considerations

   Padding oracle attacks were introduced by Vaudenay in [POA].  The
   attack relies on an oracle that decrypts messages that were encrypted
   using CBC mode with PKCS#5 padding and returns an answer to the
   sender about whether the padding is correct.  This information allows
   an attacker to recover the plaintext from an encrypted message
   through repeated inquiries to the oracle even though the encryption
   key is never learned by the attacker.  The attack can be mitigated by
   returning a single error type when decryption fails and not
   distinguishing between failed MAC check and failed padding check.

   This specification requires implementations to generate random
   values.  The use of inadequate pseudo-random number generators
   (PRNGs) can result in little or no security.  The generation of
   quality random numbers is difficult.  NIST Special Publication 800-90
   [SP800-90] and [RFC4086] offer random number generation guidance.

   This document specifies a mechanism for generating keys from pass
   phrases or passwords.  The salt and iteration count resist brute
   force and dictionary attacks, however, it is still important to
   choose or generate strong passphrases.

9  References

9.1  Normative References

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

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

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

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



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   [RFC5652]    Housley, R., "Cryptographic Message Syntax (CMS)", STD
                70, RFC 5652, September 2009.

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


9.2  Informative References

   [SP800-38A]  National Institute of Standards and Technology,
                "Recommendation for Block Cipher Modes of Operation -
                Methods and Techniques", NIST Special Publication 800-
                38A, February 2001.

   [SP800-90]   National Institute of Standards and Technology,
                Recommendation for Random Number Generation Using
                Deterministic Random Bit Generators (Revised), NIST
                Special Publication 800-90, March 2007.

   [SP800-108]  National Institute of Standards and Technology,
                "Recommendation for Key Derivation Using Pseudorandom
                Functions", NIST Special Publication 800-108, October
                2009.

   [SP800-132]  National Institute of Standards and Technology,
                "Recommendation for Password-Based Key Derivation, Part
                1: Storage Applications", NIST Special Publication 800-
                132, June 2010.

   [POA]        Vaudenay, Serge, "Security Flaws Induced by CBC Padding
                Applications to SSL, IPSEC, WTLS...", EUROCRYPT 2002.

Appendix A.  AES-CBC Test Vectors

   TBD

Authors' Addresses

   Kelley W. Burgin
   National Security Agency

   EMail: kwburgi@tycho.ncsc.mil

   Michael A. Peck
   The MITRE Corporation

   EMail: mpeck@mitre.org



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