1. Prof. N.N.GAIKWAD M.P.A.S.C COLLEGE PANVEL DIST-RAIGAD
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 3 1 4 5 2
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 : & dec key:
Finding Enc/Dec key from the other
Write index below
Swap values with indices
Sort values in ascending order of index
Try for:

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

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