e-PG Pathshala Information Technology Information security Module 4: polyalphabetic ciphers –Vigenere Cipher, Autokey system, One- Time pad, Transposition techniques, steganography Homophonic substitution ciphers: Homophonic substitution cipher maps a character to more than one code. For example, “A” could correspond to 3, 15, 23; while for “B” this could be 5, 17, 21. This type of substitution makes cryptanalysis or brute force harder but it doesn’t hide all statistical properties. With the help of powerful computer this can be broken in a few seconds. Polyalphabetic Cipher: Monoalphabetic substitution rules are used. Key decides which monoalphabetic substitution is to be performed. Vigenere Cipher: P=p0p1p2……pn-1 K=k0k1k2……km-1 C=C0C1C2……Cn-1 C=E(K,P) = E[(k0k1k2……km-1, p0p1p2……pn-1)] =(p0+k0) mod 26, (p1+k1) mod 26, …(pm-1+km-1) mod 26, (pm+k0) mod 26 … K is of length m. For m letters of plaintext , key is used. For the next m letters of plaintext, the key letters are repeated until all plaintext letters are used. Pi=(Ci – ki mod m) mod 26 Numeric value for each alphabet: a b c d e f g h i j k l m 0 1 2 3 4 5 6 7 8 9 10 11 12 e-PG Pathshala Information Technology Information security n o p q r s t u v w x y z 13 14 15 16 17 18 19 20 21 22 23 24 25 Vigenere Cipher: Key : mask Plaintext : meetmeatfield Ciphertext : key 12 0 18 10 12 0 18 10 12 0 18 10 12 plaintext 12 4 4 19 12 4 0 19 5 8 4 11 3 ciphertext 24 4 22 3 24 4 12 19 23 18 16 11 21 Strength of Vigenere Cipher: The letter frequency is masked. But key is repeated so for the same plaintext letter and key letter – same ciphertext letter is achieved. For meet and me – me generates same ciphertext ye because key is repeated. Autokey system: Periodic nature of the keyword can be overcomed by nonrepeating keyword that is as long as an autokey system. Key word is concatenated with plaintext to provide a running key. Key : mask meet meatf e-PG Pathshala Information Technology Information security Plaintext : meetmeatfield Ciphertext : key 12 0 18 10 12 4 4 19 12 4 0 10 5 plaintext 12 4 4 19 12 4 0 19 5 8 4 11 3 ciphertext 24 4 22 3 24 8 4 13 17 12 4 21 8 VERNAM cipher: Cipher works on binary data(bits) ci=pi ki Where Pi = ith binary digit of plaintext Ki = ith binary digit of key ci = ith binary digit of ciphertext = exclusive – or (XOR) operation pi=ci ki One – Time pad: The key is used to encrypt and decrypt a single message, then key is discarded. Each new message requires a new key of the same length as the new message. Such a scheme, is known as a one-time pad. One time pad is unbreakable. No statistical relationship to the plaintext. Includes space so 27 characters scheme. One – Time pad Example: e-PG Pathshala Information Technology Information security Plaintext : mr mustard key : pxlmvmsydo Ciphertext : ANKYODKYUR Plaintext : miss scarl Key : pftgpmiydg Ciphertext : ANKYODKYUR Cryptanalyst can find keys and can not decide which plaintext is true and which key is correct. One Time pad – Difficulties in using: Practically difficult to generate large quantities of random keys. How to distribute key and protect them. Because for every message , equal length of key is to be sent to sender and receiver. One time pad gives perfect secrecy. But limited use . Used only for highly secure low bandwidth channels. Transposition Techniques. – permutation of plaintext letters : Rail fence cipher: Message “ all gather at lawn”. Rail fence depth 2 a l a h r t a n l g t e a l W The encrypted message : ALAHRTANLGTEALW. Cryptanalyst can break this. e-PG Pathshala Information Technology Information security Transposition Techniques. –– Complex scheme: permutation of plaintext letters Write the message row by row , read column by column. Permutation is done by order of the column. Plaintext : discusstheformulainmeeting Key :3214 3 2 1 4 Plaintext d i s c u s s t h e f o r m u l a i n m e e t i n g x y Ciphertext : ssfuntxisemiegduhraenctolmiy For key 3214, start with column labeled 1- column 3 in table. Write all the letters then column labeled 2 – column 2 in the table. Then column labeled 3 and 4. This type of transposition cipher is not secure because letter frequencies in ciphertext are same as plaintext. Cryptanalyst can put ciphertext in a matrix and play with column position. Stegnography: Plaintext message is hidden. e-PG Pathshala Information Technology Information security Simple method – An arrangement of words or letters in the message can give real message. Text : Driveon Road Open passage. Hidden message : First letter of each word - DROP Stegnography techniques: Character Marking – Selected letters are overwritten in pencil and not visible unless hold at an angle to bright light Invisible ink – leave no visible trace until heat or some chemical is applied to the paper. Pin punctures – pin punctures on selected letters. Not visible unless the paper is held up in light. Advantage : hide the secret communication Disadvantage : Lot of overhead to hide few bits of information .