The CBC mode uses an initial chaining vector (ICV) in its processing. The CBC mode only processes blocks of data in exact multiples of the blocksize. The ICV is exclusive ORed with the first block of plaintext prior to the encryption step; the block of ciphertext just produced is exclusive-ORed with the next block of plaintext, and so on. You must use the same ICV to decipher the data. This disguises any pattern that may exist in the plaintext. CBC mode is the default for encrypting and decrypting data using the Encipher and Decipher callable services. Cipher Processing Rules describes the CBC-specific processing rules in detail.