Encryption Decryption Question:

When we use a block cipher to encrypt a message of arbitrary length, we use techniques that are known as modes of operation for the block cipher. Modes must be at least as secure and as efficient as the underlying cipher. Modes may have properties in addition to those inherent in the basic cipher. The standard DES modes have been published in FIPS PUB 81 and as ANSI X3.106. A more general version of the standard generalized the four modes of DES to be applicable to a block cipher of any block size. The standard modes are Electronic Code Book (ECB), Cipher Block Chaining (CBC), Cipher Feedback (CFB), and Output Feedback (OFB).