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2 Session Plan Overview of Classical encryption techniques: Symmetric cipher model Substitution Transposition Steganography

11/21/ Cryptography Crypto secret key public key cryptographic hashes Used for authentication, integrity protection, encryption

11/21/ plaintext - original message ciphertext - coded message cipher - algorithm for transforming plaintext to ciphertext key - info used in cipher known only to sender/receiver encipher (encrypt) - converting plaintext to ciphertext decipher (decrypt) - recovering ciphertext from plaintext cryptography - study of encryption principles/methods cryptanalysis (codebreaking) - study of principles/ methods of deciphering ciphertext without knowing key cryptology - field of both cryptography and cryptanalysis

11/21/ Cryptography Crypto secret key public key cryptographic hashes Used for authentication, integrity protection, encryption

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7 Components of symmetric encryption Plain text Encryption algorithm Secret key Cipher text Decryption algorithm

11/21/2015 NETWORK SECURITY 8 Important factors of symmetric encryption Type of operations(substitution & transpositions) Number of keys used Processing mode ( block cipher & stream cipher) Attacks on encryption system Crypt analysis – To get plain text or key using algorithm Brute-force attack – Try every possible key to get plain text/key Cryptography + cryptanalysis = cryptology

11/21/2015 NETWORK SECURITY 9 Important factors of symmetric encryption Type of operations(substitution & transpositions) Number of keys used Processing mode ( block cipher & stream cipher) Attacks on encryption system Crypt analysis – To get plain text or key using algorithm Brute-force attack – Try every possible key to get plain text/key Cryptography + cryptanalysis = cryptology

11/21/2015 NETWORK SECURITY 10 Important factors of symmetric encryption Type of operations(substitution & transpositions) Number of keys used Processing mode ( block cipher & stream cipher) Attacks on encryption system Crypt analysis – To get plain text or key using algorithm Brute-force attack – Try every possible key to get plain text/key Cryptography + cryptanalysis = cryptology

11/21/2015 NETWORK SECURITY 11 ciphertext only only knows algorithm & ciphertext known plaintext know/suspect plaintext & ciphertext chosen plaintext select plaintext and obtain ciphertext chosen ciphertext select ciphertext and obtain plaintext chosen text select plaintext or ciphertext to en/decrypt

11/21/2015 NETWORK SECURITY 12 unconditional security no matter how much computer power or time is available, the cipher cannot be broken since the ciphertext provides insufficient information to uniquely determine the corresponding plaintext computational security given limited computing resources (eg time needed for calculations is greater than age of universe), the cipher cannot be broken

11/21/2015 NETWORK SECURITY 13 always possible to simply try every key most basic attack, proportional to key size assume either know / recognise plaintext Key Size (bits)Number of Alternative Keys Time required at 1 decryption/µs Time required at 10 6 decryptions/µs = 4.3  µs= 35.8 minutes2.15 milliseconds = 7.2  µs= 1142 years10.01 hours = 3.4  µs= 5.4  years 5.4  years = 3.7  µs= 5.9  years 5.9  years 26 characters (permutation) 26! = 4   µs= 6.4  years 6.4  10 6 years

11/21/2015 NETWORK SECURITY 14 Caesar cipher Mono alphabetic cipher Playfair cipher Ployalphabetic ciphers One-time pad

11/21/2015 NETWORK SECURITY 15 Each character of a message is replaced by a character three po- sition down in the alphabet. plaintext: are you ready ciphertext: DUH BRX UHDGB ciphertext can be expressed as c = E(3, p) = (p + 3) mod 26 where E() stands for encryption c = E(k, p) = (p + k) mod 26 The formula for decryption would be p = D(k, c) = (c − k) mod 26

11/21/2015 NETWORK SECURITY 16 In a monoalphabetic cipher, our substitution characters are a random permutation of the 26 letters of the alphabet: plaintext letters: a b c d e f..... substitution letters: t h i j a b..... Note that there are 26! permutations of the alphabet. That is a number larger than 4 × Pairs of adjacent characters are referred to as digrams, and triples of characters as trigrams.

11/21/2015 NETWORK SECURITY 17 human languages are redundant eg "th lrd s m shphrd shll nt wnt" letters are not equally commonly used in English E is by far the most common letter followed by T,R,N,I,O,A,S other letters like Z,J,K,Q,X are fairly rare have tables of single, double & triple letter frequencies for various languages

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11/21/2015 NETWORK SECURITY 19 In Playfair cipher, you first choose an encryption key. You then enter the letters of the key in the cells of a 5 × 5 matrix in a left to right fashion starting with the first cell at the top- left corner.

11/21/2015 NETWORK SECURITY 20 Rules you must insert a chosen “filler” letter (let’s say it is ‘x’) between any repeating letters in the plaintext. Two plaintext letters that fall in the same row of the 5 × 5 matrix are replaced by letters to the right of each in the row. Two plaintext letters that fall in the same column are replaced by the letters just below them in the column. Otherwise, for each plaintext letter in a pair, replace it with the letter that is in the same row but in the column of the other letter.

11/21/2015 NETWORK SECURITY 21 if a truly random key as long as the message is used, the cipher will be secure called a One-Time pad is unbreakable since ciphertext bears no statistical relationship to the plaintext since for any plaintext & any ciphertext there exists a key mapping one to other can only use the key once though problems in generation & safe distribution of key

11/21/2015 NETWORK SECURITY 22 Mapping is achieved permutation on plain text letters. Transposition means rearranging the order of appearance of the elements of the plaintext. Rail fence Row transposition

11/21/2015 NETWORK SECURITY 23 A very simple form of [transposition cipher] is the rail fence, named for its fencelike appearance, which is the result of aligning rows of letters, then shifting them. The key is the number of rails and the order in which they are taken off.

11/21/2015 NETWORK SECURITY 24 Three rows

11/21/2015 NETWORK SECURITY 25 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

11/21/2015 NETWORK SECURITY 26 before modern ciphers, rotor machines were most common complex ciphers in use widely used in WW2 German Enigma, Allied Hagelin, Japanese Purple 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

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11/21/2015 NETWORK SECURITY 29 Steganography is the science of hiding information One of the oldest stego schemes was to shave the head of a messenger and tattoo a message on the messenger's head. After the hair grows back, the messenger can be sent to the intended recipient, where the messenger's head can be shaved and the message recovered. This method is decidingly clever, patient, and very low-tech, and goes right to the heart of steganography's literal meaning of "covered writing."

11/21/2015 NETWORK SECURITY 30 Character marking – visible at an angle to bright light Invisible ink – visible on heat or chemical is applied pin punctures – visible in front of light Type writer correction ribbon – visible in strong light Deliberate misspelling to mark words in the message Use of small changes in spacing to indicate significant letters or words in a hidden message Use of a slightly different font in a typeset message to indicate the hidden letters Not only restricted to written forms of communication

11/21/2015 NETWORK SECURITY 31 Consider this cablegram that might have been sent by a journalist/spy from the U.S. to Europe during World War I: PRESIDENT'S EMBARGO RULING SHOULD HAVE IMMEDIATE NOTICE. GRAVE SITUATION AFFECTING INTERNATIONAL LAW. STATEMENT FORESHADOWS RUIN OF MANY NEUTRALS. YELLOW JOURNALS UNIFYING NATIONAL EXCITEMENT IMMENSELY. The first letters of each word form the character string: PERSHINGSAILSFROMNYJUNEI. A little imagination and some spaces yields the real message: PERSHING SAILS FROM NY JUNE I.

11/21/2015 NETWORK SECURITY 32 Template (e.g., a piece of paper with holes cut in it) or a set of preselected locations on the page to hide a message THE MOST COMMON WORK ANIMAL IS THE HORSE. THEY CAN BE USED TO FERRY EQUIPMENT TO AND FROM WORKERS OR TO PULL A PLOW. BE CAREFUL, THOUGH, BECAUSE SOME HAVE SANK UP TO THEIR KNEES IN MUD OR SAND, SUCH AS AN INCIDENT AT THE BURLINGTON FACTORY LAST YEAR. BUT HORSES REMAIN A SIGNIFICANT FIND. ON A FARM, AN ALTERNATE WORK ANIMAL MIGHT BE A BURRO BUT THEY ARE NOT AS COMFORTABLE AS A TRANSPORT ANIMAL Applying a template or rule as to which words to read to this message might yield the following: HORSE FERRY SANK IN BURLINGTON FIND ALTERNATE TRANSPORT

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