1. Welcome to Cryptology 1 st Semester – Room B310

2. Syllabus

3. Cryptology Is the science (and to some extent the art) of building and analyzing different encryption-decryption methods. (Spillman, 2005, p.3) “Cryptanalysis is the science of discovering weaknesses in existing methods so that the plaintext can be recovered without knowledge of the key.” “Cryptography is the science of building new more powerful and efficient encryption- decryption methods.”

6. Codes and Ciphers Code – Substitution and no algorithm Cipher – Uses an algorithm and key Steganography – method used to hide information which conceal the existence of the ciphertext.

7. Example of Steganography http://www.garykessler.net/library/fsc_stego.html

8. Cipher Evaluation First General Principle of Cryptography “The eavesdropper has knowledge of the underlying algorithm used to encrypt data.” (Spillman, 2005, p.4)

9. Kerckhoffs’ 6 fundamentals to any cryptographic algorithm The system should be unbreakable in practice if not theoretically unbreakable. Compromise of the system should not inconvenience the correspondents. The key should be easy to remember without notes and should be easy to change. The cryptograms should be transmissible by telegraphy. The apparatus or documents should be portable and operable by a single person. The system should be easy, neither requiring knowledge of a long list of rules nor involving mental strain. (Spillman, 2005, p.4)

10. Economics Ciphers do not have to be “unbreakable” to be secure. If (value of information) < (cost of breaking the cipher) then it is secure. If (time to break) > (lifetime of information) then it is secure.

11. Cryptanalysis Ethics 3 methods of attack –Ciphertext-only –Known-plaintext –Chosen-plaintext

12. Brief history of codes and ciphers The Codebreaker, by David Kahn Ancient Egyptian tombs Hebrews Greeks Spartans Arabs Europe (post Dark Ages) –Black Chambers –English Decyphering Branch MI-8 Bletchley Park NSA

13. Skytale tkdrhybmityisatlalhaneetetsaxhpraiaym srmpwtelaasesnsousafsfgseoe. This an example of skytale. This was used by the Spartans for milatary message.

14. Classical and Contemporary Ciphers

15. Caesar Cipher Shift letters by 3 positions ABCDEFGHIJKLMNOPQRSTUVWXYZ DEFGHIJKLMNOPQRSTUVWXYZABC Plaintext Ciphertext

16. Cryptology Languages Mathematics Physics Computer Science

17. Questions What is the difference between a code and a cipher? Why are codes rarely used today? Bob and Alice decide that because Eve can break a simple shift cipher they will use it twice—that is, they will create ciphertext by shifting every letter by 3. Then they will encipher the first ciphertext using a shift cipher with a key of 5. Is this a good idea? Why or why not?