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**CLASSICAL ENCRYPTION TECHNIQUES**

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**Encryption Techniques**

Symmetric Encryption - Substitution Cipher - Transposition Cipher Asymmetric Encryption - RSA Cipher

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Basic Terminology Encryption: Process of encoding a message so that its meaning is not obvious. Decryption: The reverse process of Encryption. Transforming an encrypted message into its normal, plaintext form. Cryptosystem: A system for Encryption and Decryption. • plaintext - the original message • cipher text - the coded message • cipher - algorithm for transforming plaintext to cipher text • key - info used in cipher known only to sender/receiver

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**Symmetric Encryption a message( i.e. sender and recipient share a**

use the same (secret) key to encrypt and decrypt a message( i.e. sender and recipient share a common key ) All classical encryption algorithms are private-key Symmetric Encryption also called as conventional / private-key / single-key Encryption Techniques

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**Cryptanalysis •Cryptanalysis is the process of trying to find the**

plaintext or key •Two main approaches – Brute Force ( try all possible keys) – Exploit weaknesses in the algorithm or key ( e.g. key generated from password entered by user, where user can enter bad password )

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**Cryptanalysis: Types of Attack**

• Cipher text only – Just have cipher text to be decoded • Known plaintext – Also have one or more plaintext-cipher text pairs • Chosen plaintext – Cryptanalyst can choose plaintext and get corresponding Cipher text • Chosen cipher text – Cryptanalyst can choose cipher text and get corresponding plaintext • Chosen text – Combination of chosen plaintext and chosen cipher text

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**Cryptanalysis: Brute Force Attack (Exhaustive Search)**

Try all possible keys until code is broken • On average, need to try half of all possible keys • Infeasible if key length is sufficiently long Key size No of Keys Time required at Time required at 106 (bits) encryptions per ? s encryption per ? s x minutes 2 milliseconds x years hours x x 1024 years x 1018 years x x 1036 years x 1030 years Age of universe: 1010 years Note: DES has a 56 bit key; AES key has 128+ bits

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**Classical Encryption Techniques**

Substitution techniques The letters of the message are replaced by other letters or by numbers or symbols. Transposition techniques Performing some sort of permutation on the messages letters

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**More Definitions • unconditional security**

– no matter how much computer power is available, the cipher cannot be broken since the cipher text provides insufficient information to uniquely determine the corresponding plaintext. • computational security – given limited computing resources (e.g. time needed for calculations is greater than age of universe), the cipher cannot be broken

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**Substitution Ciphers where letters of plaintext are replaced by other**

letters or by numbers or symbols if plaintext is viewed as a sequence of bits, then substitution involves replacing plaintext bit patterns with cipher text bit patterns

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Caesar Cipher The caesar Cipher earliest known substitution cipher, developed by Julius Caesar • First attested use in military affairs • Replaces each letter by 3rd letter. Example Text : HELLO WORLD Key : Shift each ASCII character right by 3 Encrypted text: KHOOR#ZRUOG To Decrypt : Shift each character left by 3

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**Caesar Cipher • can define transformation as:**

a b c d e f g h i j k l m n o p q r s t u v w x y z D E F G H I J K L M N O P Q R S T U V W X Y Z A B C • mathematically give each letter a number a b c d e f g h i j k l m n o p q r s t u v w x y Z • then have Caesar cipher as: C = E(p) = (p + k) mod (26) p = D(C) = (C – k) mod (26)

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**Attacking Caesar Cipher**

Brute force – Key is just one letter (or number between 1 and 25) – Try all 25 keys – Easy!

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**One-Time Pad • Unbreakable substitution cipher**

• Uses random key that is as long as the message • Can use key only once

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**Mono alphabetic Cipher**

Use any permutation of the 26 alphabetic characters a b c d e f g h i j k l m n o p q r s t u v w x y z q e r y u i o p a s d f g w h j k l z x c v b n m t Example : Plain text :under attack we need help Cipher text :cwyul qxxqrd bu wuuy pufj

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**Attacking Mono alphabetic substitution**

Brute force – Very difficult; Key is 26 letters long – No. of possible keys = 26! = 4 x 1026 • Algorithm weaknesses: – Frequency of letters in English language is well known • Can deduce plaintext->cipher text mapping by analyzing frequency of occurrence • e.g. on analyzing plenty of cipher text, most frequent letter probably corresponds to ‘E’ – Can spot diagrams and trigrams • Diagram: common 2-letter sequence; e.g. ‘th’, ‘an’, ‘ed’ • Trigram: common 3-letter sequence: e.g. ‘ing’, ‘the’, ‘est’

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**Frequency of Letters in English**

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**Poly alphabetic Cipher**

Using different mono alphabetic substitution message: wearediscoveredsaveyourself key: deceptivedeceptivedeceptive Ciphertext: ZICVTWQNGRZGVTWAVZHCQYGLMGJ

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**Transposition Ciphers**

• Classical Transposition or permutation ciphers • These hide the message by re arranging the letter order without altering the actual letters used. • can recognize these since have the same frequency distribution as the original text

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**Rail Fence cipher Write message letters out diagonally over a**

number of rows • Then read off cipher row by row • e.g.. Message : “ATTACK AT MIDNIGHT” write message out as: A T C A M D I H T A K T I N G T • Cipher text is ATCAMDIHTAKTINGT

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**Row Transposition Ciphers**

• A more complex scheme • 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 word : NETWORK Message : ATTACKISPOSTPONED N E T W O R K Key : Plaintext : A T T A C K I S P O S T P O N E D A B C D Cipher text: TPEIODASNCTBKPCTODASA

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Product Ciphers • Ciphers using substitutions or transpositions are not secure because of language characteristics • Hence consider using several ciphers in succession to make harder, but: – 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

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