RFC 2405 (rfc2405) - Page 2 of 10

The ESP DES-CBC Cipher Algorithm With Explicit IV

RFC 2405            The ESP DES-CBC Cipher Algorithm       November 1998


   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC-2119].

2. Algorithm and Mode

   DES-CBC is a symmetric secret-key block algorithm. It has a block
   size of 64 bits.

   [FIPS-46-2][FIPS-74] and [FIPS-81] describe the DES algorithm, while
   [Schneier96] provides a good description of CBC mode.

2.1 Performance

   Phil Karn has tuned DES-CBC software to achieve 10.45 Mbps with a 90
   MHz Pentium, scaling to 15.9 Mbps with a 133 MHz Pentium.  Other DES
   speed estimates may be found in [Schneier96].

3. ESP Payload

   DES-CBC requires an explicit Initialization Vector (IV) of 8 octets
   (64 bits).  This IV immediately precedes the protected (encrypted)
   payload. The IV MUST be a random value.

   Including the IV in each datagram ensures that decryption of each
   received datagram can be performed, even when some datagrams are
   dropped, or datagrams are re-ordered in transit.

   Implementation note:

      Common practice is to use random data for the first IV and the
      last 8 octets of encrypted data from an encryption process as the
      IV for the next encryption process; this logically extends the CBC
      across the packets. It also has the advantage of limiting the
      leakage of information from the random number genrator. No matter
      which mechnism is used, the receiver MUST NOT assume any meaning
      for this value, other than that it is an IV.

      To avoid ECB encryption of very similar plaintext blocks in
      different packets, implementations MUST NOT use a counter or other
      low-Hamming distance source for IVs.

   The payload field, as defined in [ESP], is broken down according to
   the following diagram:







Madson & Doraswamy          Standards Track