Network Security

1. Network Security Prof. N.N.GAIKWAD M.P.A.S.C COLLEGE PANVEL DIST-RAIGAD

2. ONE TIME PAD/ VERNAM CIPHER • Is perfect secrecy possible? • If each PT symbol encrypted with a randomly chosen key (Shannon) • This idea used by Vernam to create One time pad • Key has same length as the PT and key chosen randomly • Known as Perfect Cipher • Attacks impossible for sufficiently long PT msgs

3. ONE TIME PAD/ VERNAM CIPHER • Generally not usable in practice • Key is random & as long as the PT msg • Can be used to store some secret data in a public computer • Scenario: President of a country needs to send a secret msg to president of another country • Send a trusted envoy with the random key b4 sending the msg

4. ROTOR CIPHER • Basic idea: mono aphabetic substitution, but each time the (PT char to CT char) mapping is changed

5. ROTOR CIPHER • Initial position = Secret Key • The first PT char is encrypted with the initial setting, the second with the setting after first rotation, and so on.. • If the rotor is stationary then ?? • Encrypt ”bee” with the given key

6. TRANSPOSITION CIPHERS • Change location of the symbols in PT to get CT • An ”a” at the first position of the PT may appear at the tenth position in CT • Does permutation of characters

7. Keyless transposition ciphers • RAIL FENCE CIPHER • Consider the PT msg ”Meet me at the park” • CT msg = ”MEMATEAKETETHPR” • Cryptanalysis very easy => 2 rows only & no key

8. Keyless transposition ciphers • Another method • PT = row by row • CT = col by col • Cryptanalysis • Easy if attacker knows number of cols.

9. Keyed transposition ciphers • Idea : divide PT chars into groups of predetermined size called 'blocks' and use a permutation key • Eg:- Consider ”enemy attacks tonight” • Group chars into blocks of size 5 (an agreed-upon value by both Alice and Bob) • Use a bogus char (say z) to ”fill” the last block • Result: e n e m y a t t a c k s t o n i g h t z

10. Keyed transposition ciphers • Permutation key Encryption Decryption • Cipher text • EEMYN TAACT TKONS HITZG

11. Combining keyed & keyless • Better scrambling • Encryption & Decryption done in 3 steps • PT msg written into a table row by row • Permutation done by reordering cols (based on key) • The new table read col by col ( ==> CT msg )

12. Combining keyed & keyless

13. Combining keyed & keyless • Keys • Table is not necessary • Eg: enc key : 3 1 4 5 2 & dec key: 2 5 1 3 4 • Finding Enc/Dec key from the other • Write index below • Swap values with indices • Sort values in ascending order of index • Try for: 2 6 3 1 4 7 5

14. Combining keyed & keyless • Key as a matrix

15. Combining keyed & keyless • Cryptanalysis • Trans. Ciphers don't change the frequency of chars • But frequency of digrams & trigrams hidden • Brute force attack extremely difficult

16. Double transposition cipher • Double transposition cipher

17. STREAM & BLOCK CIPHERS • Literature divides symmetric ciphers into two broad categories – stream ciphers and block ciphers • Stream cipher – encryption and decryption done one symbol at a time • Block ciphers – enc and dec done on a bolck of symbos at a time

18. Stream Cipher • There is a plain text stream • P = P1P2P3. . . • There is a cipher text stream • C = C1C2C3. . . • There is a key stream • K = (k1, k2, k3, . . . )

19. Stream Cipher

20. Stream cipher • Examples • Additive cipher • K = (k, k, k, . . . ) • Monoalphabetic substitution cipher • K = mapping of the current PT char to CT char, . . . • Vigenere cipher • K = (k1, k2, . . . , km, k1, k2, . . .)

21. Block cipher

22. Block cipher • Examples • Play fair cipher (block size = 2) • Hill cipher

23. Stream & Block Combined • Idea: • Blocks of PT encrypted individually • Use a stream of keys to encrypt blocks (separate key for each block) • Cipher is a block cipher when looking at the individual blocks • Cipher is a stream cipher when looking at the whole msg considering each block as a single unit

24. Thank You