Fundamentals of Computer Security

1. Fundamentals of Computer Security Simple Substitution Ciphers and Elementary Cryptanalysis CSCI 379 Fundamentals of Computer Security

2. Cryptology encryption key decryption key Cryptography encryption algorithm decryption algorithm ciphertext Cryptanalysis plaintext plaintext plaintext CSCI 379 Fundamentals of Computer Security

3. Substitution Ciphers CSCI 379 Fundamentals of Computer Security

4. The Additive Cipher(Caesar Cipher = Additive Cipher with key=3) Algorithm: for each letter P in the plaintext, compute a corresponding letter C of the ciphertext using the equation below. The key value k determines a shift in the cipher text alphabet. Example: PLAINTEXT ciphertext (key k=3) CSCI 379 Fundamentals of Computer Security

5. The Multiplicative Cipher Algorithm: for each letter P in the plaintext, compute a corresponding letter C of the ciphertext using the equation below. Caution: For key = k, one should choose k such that GCD(k,26) = 1. PLAINTEXT ciphertext (key: k=11) CSCI 379 Fundamentals of Computer Security

6. The Affine Cipher Algorithm: for each letter P in the plaintext, compute a corresponding letter C of the ciphertext using the equation below. Caution: For key = (m,a), one should choose m such that GCD(m,26) = 1. PLAINTEXT ciphertext (key: m=5, a=8) CSCI 379 Fundamentals of Computer Security

7. The “Random” Cipher Algorithm: for each letter P in the plaintext, look up a corresponding letter C of the ciphertext in the ciphertext alphabet, which is a random permutation of the plaintext alphabet. To generate the random permutation follow the code excerpt below: for i=1 to 25 do victim = random(1, 26-i) temp = alphabet[26-i] alphabet[26-i] = alphabet[victim] alphabet[victim] = temp end Key: Pseudo-random number generators (RNGs) need to be primed with a seed value. To decipher a message successfully, the recipient must prime its RNG with the same key used for encipherment. CSCI 379 Fundamentals of Computer Security

8. The Multiliteral Cipher Algorithm: for each letter P in the plaintext, find the cell in the table that containst it and substitute it for TWO letters in the ciphertext: the first letter corresponds to the row of the cell found and the the second to its column. Example: If key = “power”, E(“Juan is cool”) = oeerppwwoeewpwwewewp. CSCI 379 Fundamentals of Computer Security

9. Secret Writing Codes Steganography Ciphers Public Key Symmetric Key Unkeyed Classical Stream Block Transposition Substitution ? ? Additive Multiplicative Affine Random Multiliteral CSCI 379 Fundamentals of Computer Security

10. Secret Writing Codes Steganography Ciphers Public Key Symmetric Key Unkeyed Classical Stream Block Transposition Substitution Monoalphabetic Polyalphabetic Additive Multiplicative Affine Random Multiliteral CSCI 379 Fundamentals of Computer Security

11. Attacks on Cryptosystems CSCI 379 Fundamentals of Computer Security

12. Case (1): Ciphertext-Only Attack Given the ciphertext of several messages enciphered with the same algorithm, recover the plaintext of as many messages as possible, or yet recover the key(s) used. CSCI 379 Fundamentals of Computer Security

13. Case (2): Chosen-Plaintext Attack Given the ciphertext of several messages enciphered with the same algorithm, and their corresponding plaintexts, the cryptanalist can choose plaintexts that get encrypted. CSCI 379 Fundamentals of Computer Security

14. Case (3): Known-Plaintext Attack The cryptanalist has the ciphertext of several messages and knowledge of the plaintext used to generate them. CSCI 379 Fundamentals of Computer Security

15. Case (4): Rubber hose cryptanalysis - Sometimes, it may be much easier to obtain encryption keys by means of threats, torture, blackmail, espionage, etc. Since the human element may be considered the weakest link in the chain that security is built on, it may be the easiest to break. CSCI 379 Fundamentals of Computer Security

16. Ciphertext-Only Attack by Frequency Analysis English Language (Sources: “Moby Dick”, by Herman Melville and “The Picture of Dorian Gray”, by Oscar Wilde) CSCI 379 Fundamentals of Computer Security

17. Ciphertext-Only Attack by Frequency Analysis English Language Caesar Cipher CSCI 379 Fundamentals of Computer Security

18. Ciphertext-Only Attack by Frequency Analysis English Language Multiplicative Cipher k=11 CSCI 379 Fundamentals of Computer Security

19. Ciphertext-Only Attack by Frequency Analysis Multiplicative Cipher k=11 Affine Cipher k=(m=11,a=5) CSCI 379 Fundamentals of Computer Security

20. Additional Statistics(Sources: “Moby Dick”, by Herman Melville and “The Picture of Dorian Gray”, by Oscar Wilde) ING 14442 THE 9619 HAT 7309 THA 6487 HER 6063 ERE 5911 TER 4821 THI 4308 VER 4214 ENT 4174 ITH 3918 WIT 3663 GHT 3254 WHA 3173 HIN 3118 ION 2993 OME 2984 EVE 2947 EAR 2907 AIN 2891 INT 2834 AVE 2823 OUL 2811 HOU 2800 ESS 2756 IGH 2756 NCE 2755 TED 2728 HEN 2690 ULD 2578 THER 4412 OULD 2578 IGHT 2383 HERE 2332 THIN 2192 TION 2114 HING 2084 OUGH 1910 WHAL 1712 EVER 1629 HALE 1596 TING 1499 RING 1340 THOU 1298 ERED 1257 WOUL 1239 LING 1226 NING 1225 OUND 1185 OTHE 1175 KING 1067 ANCE 1060 SELF 1031 ERIN 1022 MENT 1016 DING 998 EATH 992 ATHE 986 ATIO 984 NDER 977 Repeated LettersTextbook: SS, EE, TT, LL, MM, OOTotal letters in text: 1764911A 10B 517C 612D 741E 7506F 1876G 452H 5I 52K 3L 10374M 908N 1085O 5514P 1607R 2040S 4766T 2801U 1X 21Z 66 TH 52563 HE 44727 ER 29017 IN 28537 AN 27957 RE 23086 HA 21373 ND 21089 ED 19388 OU 19306 NG 17848 HI 17183 AT 16986 EN 16622 ON 15453 ST 14235 AR 14030 TE 13669 LE 13419 ES 13348 OR 13327 SE 12663 IS 12375 EA 12333 AL 11941 VE 11308 AS 10470 LL 10374 NE 10348 NT 10292 Most repeated letters digrams trigrams 4-grams CSCI 379 Fundamentals of Computer Security

21. Fun Reading on Frequency Analysys (ERES) The Adventure of the Dancing Men, Sir Arthur Conan Doyle The Gold Bug, Edgan Allan Poe CSCI 379 Fundamentals of Computer Security