Symmetric Encryption .

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Essential Terminology. plaintext - the first message ciphertext - the coded message figure - calculation for changing plaintext to ciphertext key - data utilized as a part of figure known just to sender/recipient encipher (encode) - changing over plaintext to ciphertext disentangle (unscramble) - recouping ciphertext from plaintextcryptography - investigation of encryption standards/methodscryptanalysis (codebreaking) - the
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Symmetric Encryption or routine/private-key/single-key sender and beneficiary share a typical key all established encryption calculations are private-key was just sort before creation of open key in 1970\'s

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Basic Terminology plaintext - the first message ciphertext - the coded message figure - calculation for changing plaintext to ciphertext key - information utilized as a part of figure known just to sender/collector encipher (scramble) - changing over plaintext to ciphertext translate (unscramble) - recouping ciphertext from plaintext cryptography - investigation of encryption standards/techniques cryptanalysis (codebreaking) - the investigation of standards/strategies for disentangling ciphertext without knowing key cryptology - the field of both cryptography and cryptanalysis

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Symmetric Cipher Model

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Requirements two necessities for secure utilization of symmetric encryption: a solid encryption calculation a mystery key known just to sender/recipient Y = E K ( X ) X = D K ( Y ) expect encryption calculation is known infers a safe channel to disperse key

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Cryptography can portray by: kind of encryption operations utilized substitution/transposition/item number of keys utilized single-key or private/two-key or open path in which plaintext is prepared piece/stream

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Types of Cryptanalytic Attacks ciphertext just know calculation/ciphertext, measurable, can distinguish plaintext known plaintext know/suspect plaintext & ciphertext to assault figure picked plaintext select plaintext and acquire ciphertext to assault figure picked ciphertext select ciphertext and get plaintext to assault figure picked content select either plaintext or ciphertext to en/decode to assault figure

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Brute Force Search constantly conceivable to just attempt each key most fundamental assault, corresponding to key size accept either know/perceive plaintext

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Classical Substitution Ciphers where letters of plaintext are supplanted by different letters or by numbers or images or if plaintext is seen as a succession of bits, then substitution includes supplanting plaintext bit designs with ciphertext bit designs

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Caesar Cipher soonest known substitution figure by Julius Caesar initially verified use in military undertakings replaces every letter by third letter on case: meet me after the frock party PHHW PH DIWHU WKH WRJD SDUWB

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Caesar Cipher can characterize change 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 numerically give every letter a number 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 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 then have Caesar figure as: C = E( p ) = ( p + k ) mod (26) p = D(C) = (C – k ) mod (26)

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Cryptanalysis of Caesar Cipher just have 26 conceivable figures A maps to A,B,..Z could basically attempt each thusly an animal compel seek given ciphertext, simply attempt all movements of letters do need to perceive when have plaintext eg. break ciphertext "GCUA VQ DTGCM"

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Brute-Force Cryptanalysis of Caesar Cipher

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Monoalphabetic Cipher as opposed to simply moving the letter set could rearrange (clutter) the letters discretionarily each plaintext letter maps to an alternate irregular ciphertext letter subsequently key is 26 letters in length Plain: abcdefghijklmnopqrstuvwxyz Cipher: DKVQFIBJWPESCXHTMYAUOLRGZN Plaintext: ifwewishtoreplaceletters Ciphertext: WIRFRWAJUHYFTSDVFSFUUFYA

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Monoalphabetic Cipher Security now have a sum of 26! = 4 x 1026 keys with such a large number of keys, may believe is secure yet would be !!!WRONG!!! issue is dialect qualities

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Language Redundancy and Cryptanalysis human dialects are repetitive eg "th lrd s m shphrd shll nt wnt" letters are not similarly regularly utilized as a part of English e is by a wide margin the most well-known letter then T,R,N,I,O,A,S different letters are genuinely uncommon cf. Z,J,K,Q,X have tables of single, twofold & triple letter frequencies

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English Letter Frequencies

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Use in Cryptanalysis key idea - monoalphabetic substitution figures don\'t change relative letter frequencies found by Arabian researchers in 9 th century compute letter frequencies for ciphertext think about tallies/plots against known qualities if Caesar figure search for regular pinnacles/troughs tops at: An E-I triple, NO match, RST triple troughs at: JK, X-Z for monoalphabetic must distinguish every letter tables of basic twofold/triple letters help

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Example Cryptanalysis given ciphertext: UZQSOVUOHXMOPVGPOZPEVSGZWSZOPFPESXUDBMETSXAIZ VUEPHZHMDZSHZOWSFPAPPDTSVPQUZWYMXUZUHSX EPYEPOPDZSZUFPOMBZWPFUPZHMDJUDTMOHMQ check relative letter frequencies (see content) figure P & Z are e and t figure ZW is th and henceforth ZWP is the procedure with experimentation at last get: it was unveiled yesterday that few casual however coordinate contacts have been made with political agents of the viet cong in moscow

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Playfair Cipher not even the expansive number of keys in a monoalphabetic figure gives security one way to deal with enhancing security was to encode numerous letters the Playfair Cipher is a case developed by Charles Wheatstone in 1854, yet named after his companion Baron Playfair

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Playfair Key Matrix a 5X5 grid of letters in light of a catchphrase fill in letters of watchword (sans copies) fill rest of framework with different letters eg. utilizing the catchphrase MONARCHY MONAR CHYBD EFGIK LPQST UVWXZ

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Encrypting and Decrypting plaintext scrambled two letters at once: if a couple is a rehashed letter, embed a filler like \'X\', eg. "balloon" scrambles as "ba lx lo on" if both letters fall in a similar column, supplant each with letter to right (wrapping back to begin from end), eg. "ar" encodes as "RM" if both letters fall in a similar segment, supplant each with the letter underneath it (again wrapping to best from base), eg. "mu" scrambles to "CM" generally every letter is supplanted by the one in its line in the section of the other letter of the combine, eg. "hs" encodes to "BP", and "ea" to "IM" or "JM" (as coveted)

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Security of the Playfair Cipher security greatly enhanced over monoalphabetic since have 26 x 26 = 676 digrams would require a 676 passage recurrence table to investigate (verses 26 for a monoalphabetic) and correspondingly more ciphertext was generally utilized for a long time (eg. US & British military in WW1) it can be broken, given a couple of hundred letters since still has quite a bit of plaintext structure

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Polyalphabetic Ciphers another way to deal with enhancing security is to utilize numerous figure letter sets called polyalphabetic substitution figures makes cryptanalysis harder with more letters in order to figure and compliment recurrence dissemination utilize a key to choose which letter set is utilized for every letter of the message utilize every letters in order thusly rehash from begin after end of key is come to

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Vigenère Cipher most straightforward polyalphabetic substitution figure is the Vigenère Cipher successfully various caesar figures key is different letters long K = k1 k2 ... kd i th letter determines i th letters in order to utilize every letters in order thus rehash from begin after d letters in message unscrambling just works backward

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Example compose the plaintext out compose the catchphrase rehashed above it utilize every key letter as a caesar figure key scramble the relating plaintext letter eg utilizing watchword misleading key: deceptivedeceptivedeceptive plaintext: wearediscoveredsaveyourself ciphertext:ZICVTWQNGRZGVTWAVZHCQYGLMGJ

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Autokey Cipher in a perfect world need a key the length of the message Vigenère proposed the autokey figure with catchphrase is prefixed to message as key knowing watchword can recuperate the initial few letters utilize these thusly on whatever remains of the message yet at the same time have recurrence attributes to assault eg. given key tricky key: deceptivewearediscoveredsav plaintext: wearediscoveredsaveyourself ciphertext:ZICVTWQNGKZEIIGASXSTSLVVWLA

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One-Time Pad if a genuinely irregular key the length of the message is utilized, the figure will be secure called a One-Time cushion is unbreakable since ciphertext bears no measurable relationship to the plaintext since for any plaintext & any ciphertext there exists a key mapping one to other can just utilize the key once however have issue of safe dissemination of key

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Transposition Ciphers now consider traditional transposition or stage figures these conceal the message by improving the letter arrange without modifying the real letters utilized can perceive these since have an indistinguishable recurrence conveyance from the first content

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Transposition Ciphers now consider established transposition or change figures these shroud the message by reworking the letter arrange without adjusting the real letters utilized can perceive these since have an indistinguishable recurrence appropriation from the first content

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Rail Fence figure compose message letters out askew over various columns then read off figure push by line eg. compose message out as: m e m a t r h t g p r y e t e f e t e o an a t giving ciphertext MEMATRHTGPRYETEFETEOAAT

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Row Transposition Ciphers a more intricate plan compose letters of message out in lines over a predetermined number of segments then reorder the segments as indicated by some key before perusing off the columns Key: 3 4 2 1 5 6 7 Plaintext: a 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

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Product Ciphers figures utilizing substitutions or transpositions are not secure as a result of dialect qualities subsequently consider utilizing a few figures in progression to

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