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Versions: 00 01 RFC 2405

Network Working Group                                IPsec Working Group
INTERNET DRAFT                                                 C. Madson
Expires in Six Months                                Cisco Systems, Inc.
                                                            N. Doraswamy
                                                      Bay Networks, Inc.
                                                           November 1997

                    The ESP DES-CBC Cipher Algorithm
                            With Explicit IV

Status of this Memo

   This document is a submission to the IETF Internet Protocol Security
   (IPSEC) Working Group. Comments are solicited and should be addressed
   to the working group mailing list (ipsec@tis.com) or to the authors.

   This document is an Internet-Draft.  Internet Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working Groups. Note that other groups may also distribute
   working documents as Internet Drafts.

   Internet-Drafts draft documents are valid for a maximum of six months
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   To learn the current status of any Internet-Draft, please check the
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   ftp.isi.edu (US West Coast).

   Distribution of this memo is unlimited.


   This document describes the use of the DES Cipher algorithm in Cipher
   Block Chaining Mode, with an explicit IV, as a confidentiality
   mechanism within the context of the IPSec Encapsulating Security
   Payload (ESP).

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1. Introduction

   This document describes the use of the DES Cipher algorithm in Cipher
   Block Chaining Mode as a confidentiality mechanism within the context
   of the Encapsulating Security Payload.

   DES is a symmetric block cipher algorithm. The algorithm is described
   in [FIPS-46][FIPS-46-1][FIPS-74][FIPS-81]. [Schneier96] provides a
   general description of Cipher Block Chaining Mode, a mode which is
   applicable to several encryption algorithms.

   As specified in this draft, DES-CBC is not an authentication
   mechanism. [Although DES-MAC, described in [Schneier96] amongst other
   places, does provide authentication, DES-MAC is not discussed here.]

   For further information on how the various pieces of ESP fit together
   to provide security services, refer to [ESP] and [Thayer97].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC-2119].

2. Algorithm and Mode

   DES-CBC is a symmetric secret-key block algorithm. It has a block
   size of 64 bits.

   [FIPS-46][FIPS-46-1][FIPS-74] and [FIPS-81] describe the DES
   algorithm, while [Schneier96] provides a good description of CBC

2.1 Performance

   Phil Karn has tuned DES-CBC software to achieve 10.45 Mbps with a 90
   MHz Pentium, scaling to 15.9 Mbps with a 133 MHz Pentium.  Other DES
   speed estimates may be found in [Schneier96].

3. ESP Payload

   DES-CBC requires an explicit Initialization Vector (IV) of 8 octets
   (64 bits).  This IV immediately precedes the protected (encrypted)
   payload. The IV SHOULD be chosen at random.

   Including the IV in each datagram ensures that decryption of each
   received datagram can be performed, even when some datagrams are
   dropped, or datagrams are re-ordered in transit.

   Implementation note:

      Common practice is to use random data for the first IV and the
      last 8 octets of encrypted data from an encryption process as the
      IV for the next encryption process; this logically extends the CBC
      across the packets. It also has the advantage of limiting the

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      leakage of information from the random number genrator. No matter
      which mechnism is used, the receiver MUST NOT assume any meaning
      for this value, other than that it is an IV.

   The payload field, as defined in [ESP], is broken down according to
   the following diagram:

      |                                                               |
      +                   Initialization Vector (IV)                  +
      |                                                               |
      |                                                               |
      ~              Encrypted Payload (variable length)              ~
      |                                                               |
       1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8

3.1 Block Size and Padding

   The DES-CBC algorithm described in this document MUST use a block
   size of 8 octets (64 bits).

   When padding is required, it MUST be done according to the
   conventions specified in [ESP].

4. Key Material

   DES-CBC is a symmetric secret key algorithm. The key size is 64-bits.
   [It is commonly known as a 56-bit key as the key has 56 significant
   bits; these 56 bits are stored in an 8-byte (64- bit) value, where
   each byte has seven significant bits from the 56-bit value and the
   least significant bit is used as a parity bit.]

   [ESP] describes the general mechanism to derive keying material for
   the ESP transform. The derivation of the key from some amount of
   keying material does not differ between the manually- and
   automatically-keyed security associations.

   The mechanism MUST derive a 64-bit key value for use by this cipher.
   This derived value MUST be adjusted for parity as necessary. Weak key
   checks will be performed; if a weak key is dicovered, the key will be
   rejected and IPSEC will request a new SA.

4.1 Weak Keys

   DES has 64 known weak keys, including so-called semi-weak keys and
   possibly-weak keys (from [Schneier96] -- corrected version provided
   by William Allan Simpson -- shown here in hex with parity bits):

      0101 0101  0101 0101
      1f1f 1f1f  0e0e 0e0e
      e0e0 e0e0  f1f1 f1f1
      fefe fefe  fefe fefe

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      semi-weak key pairs:

      01fe 01fe  01fe 01fe    fe01 fe01  fe01 fe01
      1fe0 1fe0  0ef1 0ef1    e0f1 e0f1  f10e f10e
      01e0 01e0  01f1 01f1    e001 e001  f101 f101
      1ffe 1ffe  0efe 0efe    fe1f fe1f  fe0e fe0e
      011f 011f  010e 010e    1f01 1f01  0e01 0e01
      e0fe e0fe  f1fe f1fe    fee0 fee0  fef1 fef1

      possibly-weak keys:

      1f1f 0101  0e0e 0101    e001 01e0  f101 01f1
      011f 1f01  010e 0e01    fe1f 01e0  fe0e 01f1
      1f01 011f  0e01 010e    fe01 1fe0  fe01 0ef1
      0101 1f1f  0101 0e0e    e01f 1fe0  f10e 0ef1
      e0e0 0101  f1f1 0101    fe01 01fe  fe01 01fe
      fefe 0101  fefe 0101    e01f 01fe  f10e 01fe
      fee0 1f01  fef1 0e01    e001 1ffe  f101 0efe
      e0fe 1f01  f1fe 0e01    fe1f 1ffe  fe0e 0efe
      fee0 011f  fef1 010e    1ffe 01e0  0efe 01f1
      e0fe 011f  f1fe 010e    01fe 1fe0  01fe 0ef1
      e0e0 1f1f  f1f1 0e0e    1fe0 01fe  0ef1 01fe
      fefe 1f1f  fefe 0e0e    01e0 1ffe  01f1 0efe

      fe1f e001  fe0e f101    0101 e0e0  0101 f1f1
      e01f fe01  f10e fe01    1f1f e0e0  0e0e f1f1
      fe01 e01f  fe01 f1e0    1f01 fee0  0e01 fef1
      e001 fe1f  f101 fe0e    011f fee0  010e fef1
      01e0 e001  01f1 f101    1f01 e0fe  0e01 f1fe
      1ffe e001  0efe f101    011f e0fe  010e f1fe
      1fe0 fe01  0ef1 fe01    0101 fefe  0101 fefe
      01fe fe01  01fe fe01    1f1f fefe  0e0e fefe
      1fe0 e01f  0ef1 f10e    fefe e0e0  fefe f1f1
      01fe e01f  01fe f10e    e0fe fee0  f1fe fef1
      01e0 fe1f  01f1 fe0e    fee0 e0fe  fef1 f1fe
      1ffe fe1f  0efe fe0e    e0e0 fefe  f1f1 fefe

   Implementations SHOULD take care not to select weak keys [CN94],
   although the likelihood of picking one at random is negligible.

4.2 Key Lifetime

   There are no current recommendations for key lifetime.

5. Interaction with Authentication Algorithms

   As of this writing, there are no known issues which preclude the use
   of the DES-CBC algorithm with any specific authentication algorithm.

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6. Security Considerations

   [Much of this section was originally written by William Allen Simpson
   and Perry Metzger.]

   Users need to understand that the quality of the security provided by
   this specification depends completely on the strength of the DES
   algorithm, the correctness of that algorithm's implementation, the
   security of the Security Association management mechanism and its
   implementation, the strength of the key [CN94], and upon the correct-
   ness of the implementations in all of the participating nodes.

   [Bell95] and [Bell96] describe a cut and paste splicing attack which
   applies to all Cipher Block Chaining algorithms. This attack can be
   addressed with the use of an authentication mechanism.

   The use of the cipher mechanism without any corresponding
   authentication mechanism is strongly discouraged. This cipher can be
   used in an ESP transform that also includes authentication; it can
   also be used in an ESP transform that doesn't include authentication
   provided there is an companion AH header. Refer to [ESP], [AH],
   [arch], and [Thayer97] for more details.

   When the default ESP padding is used, the padding bytes have a
   predictable value.  They provide a small measure of tamper detection
   on their own block and the previous block in CBC mode.  This makes it
   somewhat harder to perform splicing attacks, and avoids a possible
   covert channel.  This small amount of known plaintext does not create
   any problems for modern ciphers.

   At the time of writing of this document, [BS93] demonstrated a dif-
   ferential cryptanalysis based chosen-plaintext attack requiring 2^47
   plaintext-ciphertext pairs, where the size of a pair is the size of a
   DES block (64 bits). [Matsui94] demonstrated a linear cryptanalysis
   based known-plaintext attack requiring only 2^43 plain- text-
   ciphertext pairs.  Although these attacks are not considered
   practical, they must be taken into account.

   More disturbingly, [Weiner94] has shown the design of a DES cracking
   machine costing $1 Million that can crack one key every 3.5 hours.
   This is an extremely practical attack.

   One or two blocks of known plaintext suffice to recover a DES key.
   Because IP datagrams typically begin with a block of known and/or
   guessable header text, frequent key changes will not protect against
   this attack.

   It is suggested that DES is not a good encryption algorithm for the
   protection of even moderate value information in the face of such
   equipment.  Triple DES is probably a better choice for such purposes.

   However, despite these potential risks, the level of privacy provided
   by use of ESP DES-CBC in the Internet environment is far greater than
   sending the datagram as cleartext.

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7. References

      [Bell95]  Bellovin, S., "An Issue With DES-CBC When Used Without
      Strong Integrity", Presentation at the 32nd Internet Engineering
      Task Force, Danvers Massachusetts, April 1995.

      [Bell96]  Bellovin, S., "Problem Areas for the IP Security Protocols",
      Proceedings of the Sixth Usenix Security Symposium, July 1996.

      [BS93]  Biham, E., and Shamir, A., "Differential Cryptanalysis of
      the Data Encryption Standard", Berlin: Springer-Verlag, 1993.

      [CN94]  Carroll, J.M., and Nudiati, S., "On Weak Keys and Weak Data:
      Foiling the Two Nemeses", Cryptologia, Vol. 18 No. 23 pp.
      253-280, July 1994.

      [FIPS-46]  US National Bureau of Standards, "Data Encryption Standard",
      Federal Information Processing Standard (FIPS) Publication 46,
      January 1977.

      [FIPS-46-1]  US National Bureau of Standards, "Data Encryption Standard",
      Federal Information Processing Standard (FIPS) Publication 46-1,
      January 1988.

      [FIPS-74]  US National Bureau of Standards, "Guidelines for
      Implementing and Using the Data Encryption Standard", Federal
      Information Processing Standard (FIPS) Publication 74, April 1981.

      [FIPS-81]  US National Bureau of Standards, "DES Modes of Operation"
      Federal Information Processing Standard (FIPS) Publication 81,
      December 1980.

      [Matsui94]  Matsui, M., "Linear Cryptanalysis method for DES Cipher,"
      Advances in Cryptology -- Eurocrypt '93 Proceedings, Berlin:
      Springer-Verlag, 1994.

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

      [Schneier96] Schneier, B., "Applied Cryptography Second Edition",
      John Wiley & Sons, New York, NY, 1996.  ISBN 0-471-12845-7.

      [Weiner94]  Wiener, M.J., "Efficient DES Key Search", School of
      Computer Science, Carleton University, Ottawa, Canada, TR-244, May
      1994.  Presented at the Rump Session of Crypto '93.

      [ESP]  Kent, S., Atkinson, R., "IP Encapsulating Security Payload
      (ESP)", draft-ietf-ipsec-esp-v2-02.txt, work in progress, November

      [AH]  Kent, S., Atkinson, R., "IP Authentication Header (AH)",
      draft-ietf-ipsec-auth-header-03.txt, work in progress, November

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      [arch] Kent, S., Atkinson, R., "Security Architecture for the
      Internet Protocol", draft-ietf-ipsec-arch-sec-02.txt, work in
      progress, November 1997.

      [Thayer97] Thayer, R., Doraswamy, N., Glenn, R., "IP Security
      Document Roadmap", draft-ietf-ipsec-doc-roadmap-02.txt, work in
      progress, November, 1997.

8. Acknowledgments

   Much of the information provided here originated with various ESP-DES
   documents authored by Perry Metzger and William Allen Simpson,
   including the data entry of the known weak key values, and especially
   the Security Considerations section.

   This document is also derived in part from previous works by Jim
   Hughes, those people that worked with Jim on the combined DES-
   CBC+HMAC-MD5 ESP transforms, the ANX bakeoff participants, and the
   members of the IPsec working group.

   Thanks also to Rob Glenn for assisting with the nroff formatting.

   The IPSec working group can be contacted via the IPSec working
   group's mailing list (ipsec@tis.com) or through its chairs:

        Robert Moskowitz
        Chrysler Corporation

        Theodore Y. Ts'o
        Massachusetts Institute of Technology

9. Editors' Addresses

        Cheryl Madson
        Cisco Systems, Inc.

        Naganand Doraswamy
        Bay Networks, Inc.

Madson, Doraswamy                                               [Page 7]

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