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rfc:rfc1851

Network Working Group P. Karn Request for Comments: 1851 Qualcomm Category: Experimental P. Metzger

                                                              Piermont
                                                            W. Simpson
                                                            Daydreamer
                                                        September 1995
                    The ESP Triple DES Transform

Status of this Memo

 This document defines an Experimental Protocol for the Internet
 community.  This does not specify an Internet standard of any kind.
 Discussion and suggestions for improvement are requested.
 Distribution of this memo is unlimited.

Abstract

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

Table of Contents

   1.     Introduction ..........................................    2
      1.1       Keys ............................................    2
      1.2       Initialization Vector ...........................    2
      1.3       Data Size .......................................    3
      1.4       Performance .....................................    3
   2.     Payload Format ........................................    4
   3.     Algorithm .............................................    6
      3.1       Encryption ......................................    6
      3.2       Decryption ......................................    7
   SECURITY CONSIDERATIONS ......................................    7
   ACKNOWLEDGEMENTS .............................................    8
   REFERENCES ...................................................    9
   AUTHOR'S ADDRESS .............................................   11

Karn, et al Experimental [Page 1] RFC 1851 ESP 3DES September 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 a variant of
 of the Cipher Block Chaining (CBC) mode of the US Data Encryption
 Standard (DES) algorithm [FIPS-46, FIPS-46-1, FIPS-74, FIPS-81].
 This variant, known as Triple DES (3DES), processes each block of the
 plaintext three times, each time with a different key [Tuchman79].
 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 3DES key shared between the communicating parties is
 effectively 168-bits long.  This key consists of three independent
 56-bit quantities used by the DES algorithm.  Each of the three 56-
 bit subkeys 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 3DES 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, et al Experimental [Page 2] RFC 1851 ESP 3DES September 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 3DES 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

 Three DES-CBC implementations may be pipelined in series to provide
 parallel computation.  At the time of writing, at least one hardware
 implementation can encrypt or decrypt at about 1 Gbps [Schneier94, p.
 231].

Karn, et al Experimental [Page 3] RFC 1851 ESP 3DES September 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 3DES 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, et al Experimental [Page 4] RFC 1851 ESP 3DES September 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, et al Experimental [Page 5] RFC 1851 ESP 3DES September 1995

3. Algorithm

 The 3DES algorithm is a simple variant on the DES-CBC algorithm.  The
 DES function is replaced by three rounds of that function, an
 encryption followed by a decryption followed by an encryption, each
 with independant keys, k1, k2 and k3.
 Note that when all three keys (k1, k2 and k3) are the same, 3DES is
 equivalent to DES-CBC.  This property allows the 3DES hardware
 implementations to operate in DES mode without modification.
 For more explanation and implementation information for Triple 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 Triple DES (EDE 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.

Karn, et al Experimental [Page 6] RFC 1851 ESP 3DES September 1995

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
 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 Triple DES (DED
 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 Triple
 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 for any of the three DES rounds, although the odds
 of picking one at random are low [Schneier94, p. 233].
 It was originally thought that DES might be a group, but it has been
 demonstrated that it is not [CW92].  Since DES is not a group,
 composition of multiple rounds of DES is not equivalent to simply
 using DES with a different key.
 Triple DES with independent keys is not, as naively might be
 expected, as difficult to break by brute force as a cryptosystem with
 three times the keylength.  A space/time tradeoff has been shown
 which can brute-force break triple block encryptions in the time

Karn, et al Experimental [Page 7] RFC 1851 ESP 3DES September 1995

 naively expected for double encryption [MH81].
 However, 2DES can be broken with a meet-in-the-middle attack, without
 significantly more complexity than breaking DES requires [ibid], so
 3DES with independant keys is actually needed to provide this level
 of security.  An attack on 3DES using two independent keys that is
 somewhat (sixteen times) faster than any known for independent keys
 has been shown [OW91].
 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].
 Although it is widely believed that 3DES is substantially stronger
 than DES, as it is less amenable to brute force attack, it should be
 noted that real cryptanalysis of 3DES might not use brute force
 methods at all.  Instead, it might be performed using variants on
 differential [BS93] or linear [Matsui94] cryptanalysis.  It should
 also be noted that no encryption algorithm is permanently safe from
 brute force attack, because of the increasing speed of modern
 computers.
 As with all cryptosystems, those responsible for applications with
 substantial risk when security is breeched should pay close attention
 to developments in cryptography, and especially cryptanalysis, and
 switch to other transforms should 3DES prove weak.

Acknowledgements

 Some of the text of this specification was derived from work by
 Randall Atkinson for the SIP, SIPP, and IPv6 Working Groups.
 Comments should be submitted to the ipsec@ans.net mailing list.

Karn, et al Experimental [Page 8] RFC 1851 ESP 3DES September 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.
 [CW92]   Campbell, K.W., and Wiener, M.J., "Proof that DES Is Not a
          Group", Advances in Cryptology -- Crypto '92 Proceedings,
          Berlin: Springer-Verlag, 1993, pp 518-526.
 [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.
 [MH81]   Merle, R.C., and Hellman, M., "On the Security of Multiple
          Encryption", Communications of the ACM, v. 24 n. 7, 1981,
          pp. 465-467.

Karn, et al Experimental [Page 9] RFC 1851 ESP 3DES September 1995

 [OW91]   van Oorschot, P.C., and Weiner, M.J.  "A Known-Plaintext
          Attack on Two-Key Triple Encryption", Advances in Cryptology
          -- Eurocrypt '90 Proceedings, Berlin: Springer-Verlag, 1991,
          pp. 318-325.
 [RFC-1800]
          Postel, J., "Internet Official Protocol Standards", STD 1,
          RFC 1800, USC/Information Sciences Institute, July 1995.
 [RFC-1700]
          Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC
          1700, USC/Information Sciences Institute, October 1994.
 [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
 [Tuchman79]
          Tuchman, W, "Hellman Presents No Shortcut Solutions to DES",
          IEEE Spectrum, v. 16 n. 7, July 1979, pp. 40-41.

Karn, et al Experimental [Page 10] RFC 1851 ESP 3DES September 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, et al Experimental [Page 11]

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