 ciphertext. There is a one-to-many relationship existing between the character in the plaintext and the character in the ciphertext. The code can be broken through trial and error method. B. Transpositional cipher: In this method the transpositional cipher retains the characters in their plaintext form but changes their position to make the ciphertext. The text is organized to form a two-dimensional table. The key defines what columns should be swapped and results in an interchanged column pattern. Here also the plaintext can be found out through a trial-and- error method. C. Block cipher: The block ciphers take a number of bytes and encrypt them as a single unit. Two common implementations are P-box and S-box. P-boxes and S-boxes can be combined to obtain a more complex cipher block called the product block. P-box: Transposition at the bit level can be performed by the P-box where P stands for permutation. P-box implementation in hardware is faster than in software. The same number of ones and zeros are present in both plaintext and ciphertext. The encryption algorithm, decryption algo- rithm, and the key are embedded in the hardware. S-box: Substitution at the bit level can be performed by the S-box where one decimal digit is substituted by another. Here S stands for substitution. Let us take DES and Triple DES as examples of symmetric key cryptography. I. Data encryption standard (DES) is a complex block cipher where using a 56-bit key the algorithm encrypts a 64-bit plaintext (see Figure 31.3). A64-bit ciphertext is created by putting the text through complex swap and transposi- tion procedures. Using complex encryption and decryption algorithms DES can substitute eight characters at a time. Encryption and the key are the same for each segment, implying that, if the data are four equal segments then the result will also have four equal segments. II. Triple DES is an algorithm compatible with DES that uses three DES blocks and 56-bit keys and operates on eight character segments. The encryption process in tri