GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc1829

Network Working Group P. Karn Request for Comments: 1829 Qualcomm Category: Standards Track P. Metzger

                                                              Piermont
                                                            W. Simpson
                                                            Daydreamer
                                                           August 1995
                     The ESP DES-CBC Transform

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Abstract

 This document describes the DES-CBC security transform for the IP
 Encapsulating Security Payload (ESP).

Table of Contents

   1.     Introduction ..........................................    1
      1.1       Keys ............................................    1
      1.2       Initialization Vector ...........................    1
      1.3       Data Size .......................................    2
      1.4       Performance .....................................    2
   2.     Payload Format ........................................    3
   3.     Algorithm .............................................    5
      3.1       Encryption ......................................    5
      3.2       Decryption ......................................    5
   SECURITY CONSIDERATIONS ......................................    6
   ACKNOWLEDGEMENTS .............................................    7
   REFERENCES ...................................................    8
   AUTHOR'S ADDRESS .............................................   10

Karn, Metzger & Simpson Standards Track [Page i] RFC 1829 ESP DES-CBC August 1995

1. Introduction

 The Encapsulating Security Payload (ESP) [RFC-1827] provides
 confidentiality for IP datagrams by encrypting the payload data to be
 protected.  This specification describes the ESP use of the Cipher
 Block Chaining (CBC) mode of the US Data Encryption Standard (DES)
 algorithm [FIPS-46, FIPS-46-1, FIPS-74, FIPS-81].
 All implementations that claim conformance or compliance with the
 Encapsulating Security Payload specification MUST implement this
 DES-CBC transform.
 This document assumes that the reader is familiar with the related
 document "Security Architecture for the Internet Protocol"
 [RFC-1825], which defines the overall security plan for IP, and
 provides important background for this specification.

1.1. Keys

 The secret DES key shared between the communicating parties is eight
 octets in length.  This key consists of a 56-bit quantity used by the
 DES algorithm.  The 56-bit key is stored as a 64-bit (eight octet)
 quantity, with the least significant bit of each octet used as a
 parity bit.

1.2. Initialization Vector

 This mode of DES requires an Initialization Vector (IV) that is eight
 octets in length.
 Each datagram contains its own IV.  Including the IV in each datagram
 ensures that decryption of each received datagram can be performed,
 even when other datagrams are dropped, or datagrams are re-ordered in
 transit.
 The method for selection of IV values is implementation dependent.
 Notes:
    A common acceptable technique is simply a counter, beginning with
    a randomly chosen value.  While this provides an easy method for
    preventing repetition, and is sufficiently robust for practical
    use, cryptanalysis may use the rare serendipitous occurrence when
    a corresponding bit position in the first DES block increments in
    exactly the same fashion.

Karn, Metzger & Simpson Standards Track [Page 1] RFC 1829 ESP DES-CBC August 1995

    Other implementations exhibit unpredictability, usually through a
    pseudo-random number generator.  Care should be taken that the
    periodicity of the number generator is long enough to prevent
    repetition during the lifetime of the session key.

1.3. Data Size

 The DES algorithm operates on blocks of eight octets.  This often
 requires padding after the end of the unencrypted payload data.
 Both input and output result in the same number of octets, which
 facilitates in-place encryption and decryption.
 On receipt, if the length of the data to be decrypted is not an
 integral multiple of eight octets, then an error is indicated, as
 described in [RFC-1825].

1.4. Performance

 At the time of writing, at least one hardware implementation can
 encrypt or decrypt at about 1 Gbps [Schneier94, p. 231].

Karn, Metzger & Simpson Standards Track [Page 2] RFC 1829 ESP DES-CBC August 1995

2. Payload Format

 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                Security Parameters Index (SPI)                |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 ~                   Initialization Vector (IV)                  ~
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 ~                          Payload Data                         ~
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           ... Padding           |  Pad Length   | Payload Type  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Security Parameters Index (SPI)
    A 32-bit value identifying the Security Parameters for this
    datagram.  The value MUST NOT be zero.
 Initialization Vector (IV)
    The size of this field is variable, although it is constant for
    all DES-CBC datagrams of the same SPI and IP Destination.  Octets
    are sent in network order (most significant octet first)
    [RFC-1700].
    The size MUST be a multiple of 32-bits.  Sizes of 32 and 64 bits
    are required to be supported.  The use of other sizes is beyond
    the scope of this specification.  The size is expected to be
    indicated by the key management mechanism.
    When the size is 32-bits, a 64-bit IV is formed from the 32-bit
    value followed by (concatenated with) the bit-wise complement of
    the 32-bit value.  This field size is most common, as it aligns
    the Payload Data for both 32-bit and 64-bit processing.
    All conformant implementations MUST also correctly process a
    64-bit field size.  This provides strict compatibility with
    existing hardware implementations.
       It is the intent that the value not repeat during the lifetime
       of the encryption session key.  Even when a full 64-bit IV is
       used, the session key SHOULD be changed at least as frequently
       as 2**32 datagrams.

Karn, Metzger & Simpson Standards Track [Page 3] RFC 1829 ESP DES-CBC August 1995

 Payload Data
    The size of this field is variable.
    Prior to encryption and after decryption, this field begins with
    the IP Protocol/Payload header specified in the Payload Type
    field.  Note that in the case of IP-in-IP encapsulation (Payload
    Type 4), this will be another IP header.
 Padding
    The size of this field is variable.
    Prior to encryption, it is filled with unspecified implementation
    dependent (preferably random) values, to align the Pad Length and
    Payload Type fields at an eight octet boundary.
    After decryption, it MUST be ignored.
 Pad Length
    This field indicates the size of the Padding field.  It does not
    include the Pad Length and Payload Type fields.  The value
    typically ranges from 0 to 7, but may be up to 255 to permit
    hiding of the actual data length.
    This field is opaque.  That is, the value is set prior to
    encryption, and is examined only after decryption.
 Payload Type
    This field indicates the contents of the Payload Data field, using
    the IP Protocol/Payload value.  Up-to-date values of the IP
    Protocol/Payload are specified in the most recent "Assigned
    Numbers" [RFC-1700].
    This field is opaque.  That is, the value is set prior to
    encryption, and is examined only after decryption.
       For example, when encrypting an entire IP datagram (Tunnel-
       Mode), this field will contain the value 4, which indicates
       IP-in-IP encapsulation.

Karn, Metzger & Simpson Standards Track [Page 4] RFC 1829 ESP DES-CBC August 1995

3. Algorithm

 In DES-CBC, the base DES encryption function is applied to the XOR of
 each plaintext block with the previous ciphertext block to yield the
 ciphertext for the current block.  This provides for
 re-synchronization when datagrams are lost.
 For more explanation and implementation information for DES, see
 [Schneier94].

3.1. Encryption

 Append zero or more octets of (preferably random) padding to the
 plaintext, to make its modulo 8 length equal to 6.  For example, if
 the plaintext length is 41, 5 octets of padding are added.
 Append a Pad Length octet containing the number of padding octets
 just added.
 Append a Payload Type octet containing the IP Protocol/Payload value
 which identifies the protocol header that begins the payload.
 Provide an Initialization Vector (IV) of the size indicated by the
 SPI.
 Encrypt the payload with DES in CBC mode, producing a ciphertext of
 the same length.
 Octets are mapped to DES blocks in network order (most significant
 octet first) [RFC-1700].  Octet 0 (modulo 8) of the payload
 corresponds to bits 1-8 of the 64-bit DES input block, while octet 7
 (modulo 8) corresponds to bits 57-64 of the DES input block.
 Construct an appropriate IP datagram for the target Destination, with
 the indicated SPI, IV, and payload.
 The Total/Payload Length in the encapsulating IP Header reflects the
 length of the encrypted data, plus the SPI, IV, padding, Pad Length,
 and Payload Type octets.

3.2. Decryption

 First, the SPI field is removed and examined.  This is used as an
 index into the local Security Parameter table to find the negotiated

Karn, Metzger & Simpson Standards Track [Page 5] RFC 1829 ESP DES-CBC August 1995

 parameters and decryption key.
 The negotiated form of the IV determines the size of the IV field.
 These octets are removed, and an appropriate 64-bit IV value is
 constructed.
 The encrypted part of the payload is decrypted using DES in the CBC
 mode.
 The Payload Type is removed and examined.  If it is unrecognized, the
 payload is discarded with an appropriate ICMP message.
 The Pad Length is removed and examined.  The specified number of pad
 octets are removed from the end of the decrypted payload, and the IP
 Total/Payload Length is adjusted accordingly.
 The IP Header(s) and the remaining portion of the decrypted payload
 are passed to the protocol receive routine specified by the Payload
 Type field.

Security Considerations

 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 key management mechanism and its implementation, the
 strength of the key [CN94], and upon the correctness of the
 implementations in all of the participating nodes.
 Among other considerations, applications may wish to take care not to
 select weak keys, although the odds of picking one at random are low
 [Schneier94, p 233].
 The cut and paste attack described by [Bell95] exploits the nature of
 all Cipher Block Chaining algorithms.  When a block is damaged in
 transmission, on decryption both it and the following block will be
 garbled by the decryption process, but all subsequent blocks will be
 decrypted correctly.  If an attacker has legitimate access to the
 same key, this feature can be used to insert or replay previously
 encrypted data of other users of the same engine, revealing the
 plaintext.  The usual (ICMP, TCP, UDP) transport checksum can detect
 this attack, but on its own is not considered cryptographically
 strong.  In this situation, user or connection oriented integrity
 checking is needed [RFC-1826].
 At the time of writing of this document, [BS93] demonstrated a

Karn, Metzger & Simpson Standards Track [Page 6] RFC 1829 ESP DES-CBC August 1995

 differential cryptanalysis based chosen-plaintext attack requiring
 2^47 plaintext-ciphertext pairs, and [Matsui94] demonstrated a linear
 cryptanalysis based known-plaintext attack requiring only 2^43
 plaintext-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.

Acknowledgements

 This document was reviewed by the IP Security Working Group of the
 Internet Engineering Task Force (IETF).  Comments should be submitted
 to the ipsec@ans.net mailing list.
 Some of the text of this specification was derived from work by
 Randall Atkinson for the SIP, SIPP, and IPv6 Working Groups.
 The use of DES for confidentiality is closely modeled on the work
 done for SNMPv2 [RFC-1446].
 Steve Bellovin, Steve Deering, Karl Fox, Charles Lynn, Craig Metz,
 Dave Mihelcic and Jeffrey Schiller provided useful critiques of
 earlier versions of this draft.

Karn, Metzger & Simpson Standards Track [Page 7] RFC 1829 ESP DES-CBC August 1995

References

 [Bell95]  Bellovin, S., "An Issue With DES-CBC When Used Without
          Strong Integrity", Proceedings of the 32nd IETF, Danvers,
          MA, April 1995.
 [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 dor DES Cipher,"
          Advances in Cryptology -- Eurocrypt '93 Proceedings, Berlin:
          Springer-Verlag, 1994.
 [RFC-1446]
          Galvin, J., and McCloghrie, K., "Security Protocols for
          Version 2 of the Simple Network Management Protocol
          (SNMPv2)", RFC-1446, DDN Network Information Center, April
          1993.
 [RFC-1700]
          Reynolds, J., and Postel, J., "Assigned Numbers", STD 2,

Karn, Metzger & Simpson Standards Track [Page 8] RFC 1829 ESP DES-CBC August 1995

          RFC-1700, USC/Information Sciences Institute, October 1994.
 [RFC-1800]
          Postel, J., "Internet Official Protocol Standards", STD 1,
          RFC-1800, USC/Information Sciences Institute, July 1995.
 [RFC-1825]
          Atkinson, R., "Security Architecture for the Internet
          Protocol", RFC-1825, Naval Research Laboratory, July 1995.
 [RFC-1826]
          Atkinson, R., "IP Authentication Header", RFC-1826, Naval
          Research Laboratory, July 1995.
 [RFC-1827]
          Atkinson, R., "IP Encapsulating Security Protocol (ESP)",
          RFC-1827, Naval Research Laboratory, July 1995.
 [Schneier94]
          Schneier, B., "Applied Cryptography", John Wiley & Sons, New
          York, NY, 1994.  ISBN 0-471-59756-2
 [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.

Karn, Metzger & Simpson Standards Track [Page 9] RFC 1829 ESP DES-CBC August 1995

Author's Address

 Questions about this memo can also be directed to:
    Phil Karn
    Qualcomm, Inc.
    6455 Lusk Blvd.
    San Diego, California  92121-2779
    karn@unix.ka9q.ampr.org
    Perry Metzger
    Piermont Information Systems Inc.
    160 Cabrini Blvd., Suite #2
    New York, NY  10033
    perry@piermont.com
    William Allen Simpson
    Daydreamer
    Computer Systems Consulting Services
    1384 Fontaine
    Madison Heights, Michigan  48071
    Bill.Simpson@um.cc.umich.edu
        bsimpson@MorningStar.com

Karn, Metzger & Simpson Standards Track [Page 10]

/data/webs/external/dokuwiki/data/pages/rfc/rfc1829.txt · Last modified: 1995/08/08 20:15 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki