Cracking A Stream Cipher Breaking the Code Worksheet-Mercy College .

Breaking the Code Objectives All encryption algorithms can be broken. Even the algorithm considered the strongest by the U.S. government, AES (Advanced Encryption Standard), can be broken, although that might take a while. According to NIST, if you build a machine that can break 255 DES (Data Encryption Standard) keys per second, it will take that machine an estimated 172 trillion years to crack a 128-bit AES key. Early cryptographic algorithms were simple. The simplest schemes are stream ciphers in which one symbol of plaintext is converted to one symbol of ciphertext during encryption. Of stream ciphers, the monoalphabetic substitution ciphers, where only one symbol stands for only one letter, are the easiest to crack. An example of a monoalphabetic substitution cipher would be a scheme where 1 5 A, 2 5 B, 3 5 C, and so on. The Caesar cipher took this idea a little further: A 5 D, B 5 E, C 5 F, and so on. Lv wkdw vr kdug? One way to approach substitution ciphers is to bear in mind the frequency with which letters are used in the English language. “E” is by far the most commonly used letter in com- mon words. “T” is second, and tied for third are “A,” “O,” “I,” “S,” and “N.” Of course, if the ciphertext were made up of words, there would need to be some symbol representing a space and, particularly in longer ciphertexts, the space would be the most common symbol. In this lab, you crack a stream cipher. The following is a sentence encrypted with a mono alphabetic substitution cipher. Your task is to decrypt it. Take some time to examine the cipher text. Make notes of your findings. 54:68:69:73:20:69:73:20:6e:6f:74:20:61:20:73:65:63:75:72:65:20:6d:65:73:73: 61:67:65:20:62:65:63:61:75:73:65:20:62:6f:74:68:20:68:65:78:61:64:65:63:69: 6d:61:6c:20:61:6e:64:20:41:53:43:49:49:20:61:72:65:20:77:65:6c:6c:20:6b: 6e:6f:77:6e:20:63:68:61:72:61:63:74:65:72:20:73:65:74:73:20:61:6e:64:20: 74:68:65:72:65:20:61:72:65:20:61:75:74:6f:6d:61:74:69:63:20:63:6f:6e:76:65:72:74:65: 72:73:20:6f:6e:6c:69:6e:65:2e