1. Row Transposition Ciphers
a more complex transposition
write letters of message out in rows over a specified number of columns (key length)
Then reorder the columns according to some key before reading off the rows
Key :
Plain text : the simplest possible transpositions.

2. Row Transposition Ciphers
Make the statement in 5 columns:
“the simplest possible transpositions” 1 2 3 4 5 T H E S I M P L O B R A N X

3. Row Transposition Ciphers4 1 5 3 2 S T I E H M L P O B R N A X 1 2 3 4 5 T H E S I M P L O B R A N X

4. Row Transposition Ciphers
Make the statement in 5 columns: 4 1 5 3 2 S T I E H M L P O B R N A X
Key: 41532
Cipher:
STIEH EMSLP STSOP EITLB SRPNA TOIIS XOXSN
A more complex transposition cipher is to write the message in a rectangle, row by row, and read the message off shuffling the order of the columns in each row.
A pure transposition cipher is easily recognized because it has the same letter frequencies as the original plaintext. For the type of columnar transposition just shown, cryptanalysis is fairly straightforward and involves laying out the ciphertext in a matrix and playing around with column positions. Digram and trigram frequency tables can be useful.

5. The same plain with another key1 2 3 4 5 T H E S I M P L O B R A N X 1 5 3 4 2 T I E S H M L P O B R N A X
The key : 15342
The cipher is :TIESH MSLEP TSOSP ITLEB RPNSA OIITS OXSXN

6. Row transposition : Example 1
Plain text : " laser beams can be modulated to carry more intelligence than radio”
Key is:

7. Row transposition : Example 1
" laser beams can be modulated to carry more intelligence than radio” 7 6 5 4 3 2 1 E B R S A L N C M U D O T Y I H G

8. Row transposition : Example 17 6 5 4 3 2 1 E B R S A L N C M U D O T Y I H G
KEY : 7 5 2 1 4 3 6 E R A L S B M C N U D O T Y I H G

9. Row transposition : Example 17 5 2 1 4 3 6 E R A L S B M C N U D O T Y I H G
Solution is :
“bselare nscamab lodemua cdtteoa rymrroe lteinli tncg eeh iraando”

10. Another Example: 2 Let key : COMPUTER PLAIN:
“a convenient way to express the permutation “

11. Another Example Let key : COMPUTER Key will be: C O M P U T E R 1 4 35 8 7 2 6

12. Another Example 1 2 3 4 5 6 7 8 A C O N V E I T W Y H P R M U X 1 4 3
Key:
Plain: a convenient way to the permutation
Cipher: ANOVINCE EW TAOTNY TPEEUMHR TITOXXAN

13. Row Transposition Ciphers
a more complex transposition
write letters of message out in rows over a specified number of columns
then reorder the columns according to some key before reading off the rows
Key:
Plaintext: a t t a c k p
o s t p o n e
d u n t i l t
w o a m x y z
Ciphertext: TTNAAPTMTSUOAODWCOIXKNLYPETZ

14. Decryption of a Row Transposition cipher
consists of:
writing the message out in rows
reading off the message by reordering columns

15. CIPHER TEXT : LHEL VOEE BRYOXDYX THE solution : hello every body4 2 1 3 L E H O V Y R B X D

16. Decryption of a Row Transposition cipher example:
Cipher text = OANTTOSRGINCHRPE Key: K=IVAN

17. The solution is : not a strong cipher

18. Product Ciphers
ciphers using substitutions or transpositions are not secure because of language characteristics
hence consider using several ciphers in succession to make harder:
two substitutions make a more complex substitution
two transpositions make more complex transposition
but a substitution followed by a transposition makes a new much harder cipher
this is bridge from classical to modern ciphers

19. Product Ciphers
before modern ciphers, rotor machines were most common product cipher
were widely used in WW2
German Enigma
implemented a very complex, varying substitution cipher
used a series of cylinders, each giving one substitution, which rotated and changed after each letter was encrypted
with 3 cylinders have 263 =17576 alphabets

20. Lecture 3 Block Ciphers Dr. Nermin Hamza

21. Modern Cryptographic Techniques
Modern cipher system :
Symmetric
Asymmetric
Symmetric cryptography:
Stream cipher
Block cipher

22. Modern Cryptographic Techniques

23. Stream cipher
Stream ciphers: where plaintext bits are combined with a pseudorandom cipher bit stream (key stream), typically by an
exclusive-or (xor) operation. In a stream cipher, the plaintext digits are encrypted one at a time, and the transformation of successive digits varies during the encryption.

24. Stream cipher
Stream ciphers:- process messages a bit or byte at a time when en/decrypting

25. Stream cipher
2 types:
Synchronous stream
Asynchronous stream
Synchronous stream ciphers where the key stream depends only on the key,
Asynchronous stream ones where the key stream also depends on the ciphertext.

26. Stream cipher

27. Stream cipher Definition Stream Cipher Encryption and Decryption
The plaintext, the ciphertext and the key stream consist of individual bits, i.e., xi,yi, si ∈ {0,1}.
Encryption: yi = esi (xi) ≡ xi+si mod 2.
Decryption: xi = dsi (yi) ≡ yi+si mod 2.

28. Modern Block Ciphers look at modern block ciphers
one of the most widely used types of cryptographic algorithms
provide secrecy /authentication services
focus on DES (Data Encryption Standard)
to illustrate block cipher design principles

29. Block cipher
Block cipher scheme :encrypts one block of data at a time using the same key on each block.
In general, the same plaintext block will always be encrypted to the same cipher text if using the same key in a block cipher whereas the same plaintext will be encrypted to different cipher text in a stream cipher.

30. Block cipher
block ciphers: process messages in blocks, each of which is then en/decrypted
like a substitution on very big characters
64-bits or more

31. Block modes Electronic Codebook (ECB) mode
Cipher Block Chaining (CBC) mode
Cipher Feedback (CFB)
Output Feedback (OFB) mode

32. Block modes Electronic Codebook (ECB) mode:
The simplicity of the encryption modes is the electronic codebook (ECB) mode, in which the message is split into blocks and each is encrypted separately

33. Block modes

34. Block modes Cipher Block Chaining (CBC) mode
In the cipher-block chaining (CBC) mode, each block of the plaintext is XORed with the previous cipher text block before being encrypted. This way, each cipher text block is dependent on all plaintext blocks up to that point.

35. Block modes

36. Block modes

37. Block modes
Cipher Block Chaining (CBC) mode

38. Block modes Cipher Feedback (CFB)
Cipher feedback mode converts the block cipher into a stream cipher: they generate key-stream blocks, which then are XORed with the plaintext blocks to get the cipher-text. Just as with other stream ciphers, flipping a bit in the cipher-text produces a flipped bit in the plaintext at the same location. With cipher feedback, a key-stream block is computed by encrypting the previous cipher-text block.

39. Block modes

40. Block modes

41. Block modes
Cipher Feedback (CFB)

42. Block modes Output Feedback (OFB) mode
OFB is similar to CFB but with small differences, where the Output feedback generates the next key-stream block by encrypting the last one.

43. Block modes

44. Block modes

45. Block modes
Output Feedback (OFB) mode

46. Block vs Stream Ciphers
many current ciphers are block ciphers
broader range of applications

47. Block vs Stream Ciphers
Block ciphers work a on block / word at a time, which is some number of bits. All of these bits have to be available before the block can be processed.
Stream ciphers work on a bit or byte of the message at a time, hence process it as a “stream”.
Block ciphers are currently better analysed, and seem to have a broader range of applications, hence focus on them.