RFC 1851 (rfc1851) - Page 2 of 11


The ESP Triple DES Transform



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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


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