1. Cryptology Terminology and Early History

2. Cryptology Terms Cryptology –The science of concealing the meaning of messages and the discovery of the meaning of hidden messages Plaintext –A message in ordinary language Codetext or ciphertext –The plain text message transformed using a code or cipher so that it cannot be read without knowing the code or cipher

3. Cryptology Terms Cryptography –The study of how to conceal the meaning of messages (encryption) using codes and ciphers. –Sometimes more generally used as a synonym for Cryptology Cryptanalysis –The study of how to discover the meaning of hidden messages (decryption)

4. Cryptology Terms Code –A system for altering a text by substituting different words or symbols for words or phrases in the plaintext Cipher –A system for altering a text by substituting different symbols for the plaintext symbols (letters), for example by substituting different letters for the plaintext letters according to some system. Nomenclator –Combination of code and cipher

5. Cryptology Terms Steganography –The science of hiding the existence of a message (rather than changing the form of the message so that it cannot be read)

6. Highlights in the History of Cryptology Timeline of Cryptology

7. How would you keep a message secret? Steganography – hide it Transposition – scramble it Substitution –Cipher, substituting for each letter –Code, substituting for words or phrases

8. Cryptanalysis How do your break a code so you can read it? Example: Sherlock Holmes’ problem of the Dancing Men. Example: our work in class

9. Monoalphabetic Ciphers Summary

10. Caesar Cipher Shift of alphabet (additive) Same order of letters, just shifted Analysis can include patterns of frequency

11. Affine Cipher Two keys –Add s to plaintext value –Multiply result by t –Reduce mod 26 Susceptible to frequency analysis –Single letter –Digrams, Trigrams Brute force

12. Keyword scramble, with shift Keyword and letter Scrambled alphabet starts with keyword (duplicate letters eliminated) followed by rest of alphabet, with keyword letters left out. Can be shifted so keyword begins at letter key Susceptible to frequency analysis, with guesses at keyword when some letters are found

13. Summary Monalphabetic not secure because of frequency analysis Easy to remember keys (Caesar, single number; Affine, two numbers; keyword, letter)