R R R Encryption A Brief Overview CSE870 Advanced Software Engineering Cheng Sp 2003 1 R R Encryption R Encryption Definition mechanisms to disguise the message so that if the intermission is intercepted diverted the content of the message will not be understood Impact foundational building block to security based computing CSE870 Advanced Software Engineering Cheng Sp 2003 2 R R Terminology R Scenario S wants to send the message T to R where an outsider O wants the message and tries to access it S Sender R Receiver T Transmission Medium O Interceptor or Intruder 4 ways O might try to access message Block it prevent T from reaching R availability Intercept it read or listen to message secrecy Modify it obtaining message and changing it Fabricate generate an authentic looking message to be delivered to R appearing to come from S CSE870 Advanced Software Engineering Cheng Sp 2003 3 R R R Terminology Encryption process of encoding a message so that its meaning is not obvious Decryption transforming encrypted message back to its normal form Encode decode translating phrases to other words or phrases Encipher decipher translating letters or symbols individually Plaintext original form of message P p1 p2 pn Ciphertext encrypted form of message C c1 c2 cn Encryption decryption relationships C E P P D C P D E P CSE870 Advanced Software Engineering Cheng Sp 2003 4 R R R Encryption Algorithms Some encryption algs use a key K C E K P E is a SET of encryption algs Key K selects specific one Symmetric Encryption P D K E K P encryption decryption keys are the same Asymmetric Encryption P D KD E KE P CSE870 Advanced Software Engineering Cheng Sp 2003 5 R R Pictorial Representation R Symmetric Encryption Key Plaintext Encryption Ciphertext Decryption Original Plaintext Asymmetric Encryption Encryption Key Decryption Key KE Plaintext KD Encryption Ciphertext CSE870 Advanced Software Engineering Cheng Sp 2003 Decryption Original Plaintext 6 R R R More Terms Cryptography hidden writing Practice of using encryption to conceal text Cryptanalyst Person who studies encryption and encrypted messages Intent find hidden meaning Cryptographer and Cryptanalyst Both attempt to translate coded material to original form Cryptographer works on behalf of legitimate sender or receiver Cryptanalyst Works on behalf of unauthorized interceptor Cryptology research study into encryption decryption Includes cryptography and cryptanalysis CSE870 Advanced Software Engineering Cheng Sp 2003 7 R R Cryptanalysis R Objective Break an encryption Deduce the meaning of a ciphertext mesg Determine decrypting algorithm that matches an encrypting algorithm Possible techniques break single message Recognize patterns in encrypted mesgs break subsequent mesgs with straightforward decryption alg Find general weaknesses in encryption alg Without necessarily intercepting any mesgs Tools Encrypted mesgs known encryption algs intercepted plaintext data elements known suspected of being in ciphertext mathematical statistical techniques props of languages computers and luck CSE870 Advanced Software Engineering Cheng Sp 2003 8 R R Breakable Encryption R Encryption algorithm is BREAKABLE Given enough time and data an analyst could determine the algorithm Practicality is issue For given cipher scheme may have 1030 possible decipherments Select one from 1030 Current technology perform 1010 ops sec Require 1020 secs 1012 years Reality Check Cryptanalyst won t just try the hard ways Ex more clever approach might only take 10 15 ops 1010 ops sec 1015 ops will take about one day Breakability estimates are based on CURRENT technology CSE870 Advanced Software Engineering Cheng Sp 2003 9 R R Character Representations R Study ways to encrypt any computer material ASCII EBCDIC chars Binary data or Object code Control stream A B C D E F G H I J K L 0 9 10 11 12 1 2 3 4 5 6 N O P Q R S T 7 8 U V W X Y M Z 13 14 15 16 17 18 19 20 21 22 23 24 25 CSE870 Advanced Software Engineering Cheng Sp 2003 10 R R R Substitution based Encryption Monoalphabetic Ciphers Caesar Cipher ci E pi pi 3 wuhdwb lpsrvvleoh wklv phvvdjh lv qrw wrr kdug wr euhdn Easy to perform in field no written instructions Permutation reordering of the elements ci a pi Use a key A B C D E F G H I J K L M N O P Q R S T U V WX Y Z A B C D E F G H I J K L K E YWeakness study frequency distribution S P E C T A U L R B D F G H I M N O P Q R S T U V W X J K N N O Q V W X Y Z CSE870 Advanced Software Engineering Cheng Sp 2003 11 R R R Polyalphabetic Substitution Ciphers Desire flat distribution Combine distributions that are high with low ones Encipher T as a and sometimes as b Also encipher X as a and sometimes as b Use two separate encryption alphabets Tables for odd and even positions mod 26 mod 26 TREAT YIMPO SSIBL E Fumnf dyvtf czysh h CSE870 Advanced Software Engineering Cheng Sp 2003 12 R R Substitution Discussion R Major weakness frequency distribution index of coincidence measure of variation between frequencies in a distribution Some letters are just used more frequently than others Numerous enciphering techniques still can make it difficult to hide these patterns Kasiski Method find number of alphabets used Identify repeated patterns of 3 or more chars For each pattern write down position at which each instance of pattern begins Compute difference between start points of success instances Determine all factors of each difference If polyalphabetic subst used key length will be one of the factors that appears often in previous step CSE870 Advanced Software Engineering Cheng Sp 2003 13 R R R Transpositions Permutations Definition encryption where letters are rearranged Goal diffusion spread info from message or key out widely across the ciphertext Try to break established patterns CSE870 Advanced Software Engineering Cheng Sp 2003 14 R R Transposition Techniques R Columnar Transpositions Rearrangement of chars of plaintext into cols C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 Etc T H I S I S A M E S S A G E T O S H O W H O W A C O L U M N A R T R A N S P O S I T I O N W O R K S tssoh oaniw haaso lrsto imghw CSE870 Advanced Software 15 seeoa mrook istwc nasns Engineering utpir Cheng Sp 2003 R R R Transpositions Digram patterns of adjacent letters Study 2 and 3 letter combinations of adj letters Double Transposition Alg Involves 2 columnar transpositions With different number of columns applied sequentially Fractionated Morse keyed monoalphabetic cipher Result is subsequently blocked clustered
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