Network Security - Lecture 12

Network SecurityLecture 12Presented by: Dr. Munam Ali Shah Part 2 (b)Cryptography as a Network Security ToolSummary of the previous lectureWe had overviewed the 3-dimensions of a cryptographic system, i.e., type of operation to transform the plain text into cipher text, number of keys used and the way in which plain text is processedWe also discussed unconditional and computational security. One example of unconditional security is OTPThe difference between Cryptanalysis and Brute Force attacks, were also discussed.And lastly, we practically perform cryptography through the Shift CipherOutlines of today’s lectureWe will:Explore a couple of more examples of Shift CipherContinue the discussion on Mono-alphabetic Cipher with an example will also be discussedPerform a cryptanalysis on a cipher text to reveal the plain text.ObjectivesYou would be able to present an overview of more cryptographic schemes and you will understand the limitation of each scheme.You would be able to break the code (ideally..)ConceptsA private key cipher is composed of two algorithmsencryption algorithm Edecryption algorithm DThe same key K is used for encryption & decryptionK has to be distributed beforehandCaesar CipherIf each letter is assigned a number (a=0, z=25), Encryption/Decryption defined as:C = E(p) = (P + 3) mod (26)P = D(c) = (C – 3) mod (26)Example:meet me after the toga partyphhw ph diwhu wkh wrjd sduwbCaesar Cipher: Encryption ExampleK = 7P = Rome is the greatest empireC = yvtl pz aol nylhalza ltwpylCaesar Cipher: Decryption ExampleK = 7C = yvtl dhz uva ibpsa pu h khfP = Rome was not built in a dayCaesar Cipher: Decryption with Unknown Key C=tfnriujuzvdrepkzdvjsvwfivkyvziuvrkyjkyvmrczrekevmvikrjkvfwuvrkyslkfetvtfnriuj uzv drep kzdvj svwfiv kyvzi uvrkyj; kyv mrczrek evmvi krjkv fw uvrky slk fetvP = Cowards die many times before their deaths; the valiant never taste of death but once. (K = 17)Julius Caesar by William ShakespeareCryptanalysis of Caesar Cipher Only have 26 possible ciphers A maps to A,B,..Z Could simply try each in turn A brute force search Given ciphertext, just try all shifts of lettersDo need to recognize when have plaintextMonoalphabetic CipherInstead of substituting each letter in a sequential order (shift), substitute the letters arbitrarily Each plaintext letter maps to a unique ciphertext letter Hence key is 26 letters long Monoalphabetic Cipher SecurityHow many total keys are there?26! = 4 x 1026 keys With so many keys, is it secure?No It is secure against brute force attack but problem lies in language characteristicsCalled frequency analysis attackLanguage Redundancy and CryptanalysisHuman languages are redundant Thats why we can compress text filesLetters are not equally commonly used Which is the most common letter?E Which is the least common letter?Z English Letter FrequenciesLanguage Redundancy and CryptanalysisHave tables of single, double & triple letter frequencies for various languagesWhich is the most common digram?TH Which is the most common trigram?THE Use in CryptanalysisKey concept – mono-alphabetic substitution ciphers do not change relative letter frequencies Each occurrence of a particular plaintext letter maps to the same ciphertext letter So attack is easy:Calculate letter frequencies for ciphertextCompare counts/plots against known values Example CryptanalysisGiven ciphertext uzqsovuohxmopvgpozpevsgzwszopfpesxudbmetsxaizvuephzhzshzowsfpappdtsvpquzwymxuzuhsxepyepopdzszufpombzwpfupzhmdjudtmohmqFrequency AnalysisP 13.33H 5.83F 3.33B 1.67C 0.00Z 11.67 D 5.00 W 3.33 G 1.67 K 0.00 S 8.33 E 5.00 Q 2.50 Y 1.67 L 0.00 U 8.33 V 4.17 T 2.50 I 0.83 N 0.00 O 7.50 X 4.17 A 1.67 J 0.83 R 0.00 M 6.67 Example CryptanalysisGuess P & Z are E and T, respectivelyutqsovuohxmoevgeoteevsgtwstoefeesxudbmetsxaitvueehthtshtowsfeaeedtsvequtwymxutuhsxeeyeeoedtstufeombtwefuethmdjudtmohmqExample CryptanalysisAmong digrams starting with Z, ZW has the highest occurrence (3 times)Guess ZW is THUtqsovuohxmoevgeoteevsgthstoefeesxudbmetsxaitvueehthtshtowsfeaeedtsvequthymxutuhsxeeyeeoedtstufeombthefuethmdjudtmohmqHence ZWP is THEExample CryptanalysisUtqsovuohxmoevgeoteevsgthstoefeesxudbmetsxaitvueehthtshtowsfeaeedtsvequthymxutuhsxeeyeeoedtstufeombthefuethmdjudtmohmqGuess S is AUtqaovuohxmoevgeoteevagthatoefeeaxudbmetaxaitvueehthtahtowafeaeedtavequthymxutuhaxeeyeeoedtatufeombthefuethmdjudtmohmqExample CryptanalysisU, V and M may correspond to O, I and NContinuing with trial and error, we finally get the following plaintext It was disclosed yesterday that several informal but direct contacts have been made with political representatives of the Viet Cong in MoscowSummary of today’s lectureWe discussed more examples of Shift/Ceaser CipherWe also discussed examples of mono-alphabetic cipher and poly-alphabetic cipherNext lecture topicsOur discussion on more cryptographic schemes will continue. We will explore One Time Pad and OTPAn example of Vigenere Cipher will also be discussed with its limitations.Transposition Cipher with an example of Rail Fence Cipher will form part of our next lecture.The End