Lecture 3 1

TRANSPOSITION CIPHERS 2

Transposition Ciphers now consider classical transposition or permutation ciphers these hide the message by rearranging the letter order without altering the actual letters used can recognise these since have the same frequency distribution as the original text 3

Rail Fence cipher write message letters out diagonally over a number of rows then read off cipher row by row eg. write message out as: m e m a t r h t g p r y e t e f e t e o a a t giving ciphertext MEMATRHTGPRYETEFETEOAAT 4

Decryption of Rail Fence Divide the statement by 2 If odd number let the first part is more than 1 Put he first part above And the second part below Read the statement as diagonal 5

Reverse cipher Write the message backwards Ex: Plain: I came I saw I conq uered Cipher: d ereu q noci w asie maci 6

Decryption of Reverse cipher Write the cipher message backwards 7

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. 8

Row Transposition Ciphers Make the statement in 5 columns: “the simplest possible transpositions” THESI MPLES TPOSS IBLET RANSP OSITI ONSXX

Row Transposition Ciphers THESI MPLES TPOSS IBLET RANSP OSITI ONSXX STIEH EMSLP STSOP EITLB SRPNA TOIIS XOXSN

Row Transposition Ciphers Make the statement in 5 columns: 11 Key: Cipher: STIEH EMSLP STSOP EITLB SRPNA TOIIS XOXSN STIEH EMSLP STSOP EITLB SRPNA TOIIS XOXSN

The same plain with another key THESI MPLES TPOSS IBLET RANSP OSITI ONSXX TIESH MSLEP TSOSP ITLEB RPNSA OIITS OXSXN The key : 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:

Row transposition : Example 1 " laser beams can be modulated to carry more intelligence than radio” EBRESAL BNACSMA ALUDOME ACOTDET EROMYRR ILLETNI HTECNEG OIDARNA

Row transposition : Example 1 KEY : EBRESAL BNACSMA ALUDOME ACOTDET EROMYRR ILLETNI HTECNEG OIDARNA ERALESB BAMACSN AUMEDOL AOETTDC EORRMYR ILNIETL HEEGCNT ODNAARI

Row transposition : Example 1 Solution is : “bselare nscamab lodemua cdtteoa rymrroe lteinli tncg eeh iraando” ERALESB BAMACSN AUMEDOL AOETTDC EORRMYR ILNIETL HEEGCNT ODNAARI

Another Example: 2 Let key : COMPUTER PLAIN: “a convenient way to express the permutation “ 17

Another Example Let key : COMPUTER Key will be: 18 COMPUTER

Another Example 19 Key: Plain: a convenient way to the permutation Cipher: ANOVINCE EW TAOTNY TPEEUMHR TITOXXAN ACONVENI ENTWAYTO THEPERMU TATIONXX ANOVINCE EWTAOTNY 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: 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 20

Decryption of a Row Transposition cipher consists of: writing the message out in rows reading off the message by reordering columns 21

CIPHER TEXT : LHEL VOEE BRYOXDYX THE solution : hello every body LEHL EEOV OYRB XYDX

Decryption of a Row Transposition cipher example: Cipher text = OANTTOSRGINCHRPE Key: IVAN 23

The solution is : not a strong cipher 24

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 25

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 26 3 =17576 alphabets 26

Block Ciphers Dr. Nermin Hamza 27

Modern Cryptographic Techniques Modern cipher system : ◦ Symmetric ◦ Asymmetric Symmetric cryptography: ◦ Stream cipher ◦ Block cipher 28

Modern Cryptographic Techniques 29

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. 30

Stream cipher 31 Stream ciphers:- process messages a bit or byte at a time when en/decrypting

Stream cipher 32 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 33

Stream cipher 34 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: y i = e si (x i ) ≡ x i +s i mod 2. Decryption: x i = d si (y i ) ≡ y i +s i 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 35

Block cipher 36 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 37

Block modes Electronic Codebook (ECB) mode Cipher Block Chaining (CBC) mode Cipher Feedback (CFB) Output Feedback (OFB) mode 38

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 39

Block modes 40

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. 41

Block modes 42

Block modes 43

Block modes Cipher Block Chaining (CBC) mode 44

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. 45

Block modes 46

Block modes 47

Block modes Cipher Feedback (CFB) 48

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. 49

Block modes 50

Block modes 51

Block modes Output Feedback (OFB) mode 52

Block vs Stream Ciphers many current ciphers are block ciphers broader range of applications 53

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. 54

Write a program for encrypt or / Decryption using Row transposition 55