rfc1829.txt

RFC 的详细文档！

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