Block-oriented encryption algorithms such as DES encry ption are based on the fact that they encrypt entire text blocks of fixed length. In this process, the plaintext block containing the key text is converted into the ciphertext
. In this conversion, the plaintext is converted into the ciphertext using one or more mathematical operations. The various block ciphers therefore specify several modes of operation for the algorithms used to generate the ciphertext blocks. The electronic code book (ECB) is one of them, and the key block chain
(CBC) is another.
In the electronic code book, a block cipher, each plaintext block to be ciphered is encrypted with the corresponding cipher block. For a plaintext block with 2expN possible plaintext strings there are 2expN possible key strings. Since only whole blocks are ever encrypted, the last part of a plaintext string must be padded. Identical plaintext blocks result in identical ciphertext blocks, generating identical ciphertexts for certain applications. Therefore, this DES mode of operation does not appear to be sufficiently secure, since a text section that appears multiple times in the text always results in the same ciphertext section.