hip existing between the character in the plaintext and the character Secure Communication in the ciphertext. Monoalphabetic substitution is simple but the code can be attacked easily. b. Polyalphabetic substitution: A character in the plaintext can be converted to many characters in the 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 sam