RFC 1829 (rfc1829) - Page 3 of 10

The ESP DES-CBC Transform

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