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 : 41532 Plain text : the simplest possible transpositions.

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

Row Transposition Ciphers 4 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

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.

The same plain with another key 1 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

Row transposition : Example 1 Plain text : " laser beams can be modulated to carry more intelligence than radio” Key is: 6 3 4 1 2 5 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

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

Row transposition : Example 1 7 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”

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

Another Example Let key : COMPUTER Key will be: C O M P U T E R 1 4 3 5 8 7 2 6

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: 14358726 Plain: a convenient way to the permutation Cipher: ANOVINCE EW TAOTNY TPEEUMHR TITOXXAN

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: 3 4 2 1 5 6 7 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

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

CIPHER TEXT : LHEL VOEE BRYOXDYX THE solution : hello every body 4 2 1 3 L E H O V Y R B X D

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

The solution is : not a strong cipher

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

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

Lecture 3 Block Ciphers Dr. Nermin Hamza

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

Modern Cryptographic Techniques

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.

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

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.

Stream cipher

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.

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

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.

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

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

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

Block modes

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.

Block modes

Block modes

Block modes Cipher Block Chaining (CBC) mode

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.

Block modes

Block modes

Block modes Cipher Feedback (CFB)

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.

Block modes

Block modes

Block modes Output Feedback (OFB) mode

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

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.