The simplest block cipher mode of operation is the electronic codebook mode (ECB). One block of plain-text always produces the same block of cipher-text. If crypt-analysts learn that the block ``8d226acd'' encrypts to the cipher-text block ``1c7ed351'', they can immediately decrypt that cipher-text block whenever it appears in a message. This vulnerability is greatest at the beginning and end of messages, where well-defined headers and footers contain information about the sender, receiver, date, etc. This problem is solved by the modes described below.