ANSI X3.106 Cipher Block Chaining (CBC) method

ANSI standard X3.106 defines four modes of operation for ciphering, and one of these modes, Cipher Block Chaining (CBC), defines the basic method for ciphering multiple 8-byte data strings.

Figure 1 and Figure 2 show CBC using the Encipher and Decipher verbs. A plaintext data string that must be a multiple of eight bytes is processed as a series of 8-byte blocks. The ciphered result from processing an 8-byte block is XORed with the next block of 8 input bytes. The last 8-byte ciphered result is defined as an output chaining value (OCV). The security server stores the OCV in bytes 0 - 7 of the chaining_vector variable.

An ICV is XORed with the first block of eight bytes. When you call the Encipher or Decipher verb, specify the INITIAL or CONTINUE keywords. If you specify the INITIAL keyword, the default, the initialization vector from the verb parameter is XORed with the first eight bytes of data. If you specify the CONTINUE keyword, the OCV identified by the chaining_vector parameter is XORed with the first eight bytes of data.