ue for hiding a secret message within a larger document or picture in such a way that others cannot contents of the hidden message is called discern the presence or The earliest known, and simplest use, of a substitution cipher was called the letter of the alphabet with the letter standing three places further down the alphabet. 2. cipher and involved replacing each The two types of attack on an encryption algorithm are cryptanalysis, based on properties of the encryption algorithm, and which involves trying all possible keys. 3. 4. All encryption algorithms are based on two general principles: substitution and 5. A cipher processes the input one block of elements at a time, producing an output block for each input block, whereas a stream cipher processes the input elements continuouly, producing output one element at a time. 6. Two alternatives to DES are AES and are threas of concern regarding the level of security provided by DES are the e nature of the algorithm and the attack exploits the fact that an encryption ferenon algorithm often takes slightly different amounts of tine encryption on different inputs. 9. Public-key Separate keryptography is asymmetric, involving the use of two keys, in contrast to which uses only one key d cryptographic algorithm applied to the messager of data that is a function of the message. vate key Signing is achievirnder signs' a message with its private key. Signing achieved by a by a small block to a description of a 1. Which of the following is an attack that involves trying every possible key until an intelligible translation of the ciphertext is obtained? a) Brute-force b) Ciphertext only c) Caesar attack d) Chosen plaintext 2. Which of the following is the most widely used secret-key cryptosystem a) Steganography b) DES c) Caesar Cipher d) RSA 3. Which of the following attacks is the easiest to defend against because the opponent has the least amount of information to work with? a) Ciphertext-only b) Chosen ciphertext c) Chosen plaintext d) Known plaintext 4. A scheme known as ais unbreakable because it produces random output that bears no statistical relationship to the plaintext. a) Unbreakable cryptosystem b) one-time pad c) RSA d) DES 5. Which of the following cipher is one that encrypts a digital data stream one bit or one byte at a time? a) Block b) Stream c) Linear d) Substitution