CS595-Cryptography and Network Security Cryptography and Network Security Xiang-Yang Li CS595-Cryptography and Network Security CS595-Cryptography and Network Security Introduction The art of war teaches us not on the likelihood of the enemy’s not coming, but on our own readiness to receive him; not on the chance of his not attacking, but rather on the fact that we have made our position unassailable. The art of War, Sun Tzu CS595-Cryptography and Network Security Information Transferring CS595-Cryptography and Network Security Attack: Interruption CS595-Cryptography and Network Security Attack: Interception CS595-Cryptography and Network Security Attack: Modification CS595-Cryptography and Network Security Attack: Fabrication CS595-Cryptography and Network Security Attacks, Services and Mechanisms ! Security Attacks " Action compromises the information security ! Security Services " Enhances the security of data processing and transferring ! Security mechanism " Detect, prevent and recover from a security attack CS595-Cryptography and Network Security Important Features of Security ! Confidentiality, authentication, integrity, non-repudiation, non-deny, availability, identification, …… [...]... service CS595 -Cryptography and Network Security Network Security Model Trusted Third Party principal principal Security transformation Security transformation opponent CS595 -Cryptography and Network Security Cryptography ! Cryptography is the study of " ! Secret (crypto-) writing (-graphy) Concerned with developing algorithms: " " " Conceal the context of some message from all except the sender and recipient... concatenated together CS595 -Cryptography and Network Security Encryption and Decryption Plaintext ciphertext Encipher C = E(K)(P) Decipher P = D(K)(C) Key source CS595 -Cryptography and Network Security Key Management ! Using secret channel ! Encrypt the key ! Third trusted party ! The sender and the receiver generate key " The key must be same CS595 -Cryptography and Network Security Attacks ! Recover... code-book CS595 -Cryptography and Network Security Encryption and Decryption Decipher P = D(K2)(C) ciphertext Plaintext Encipher C = E(K1)(P) K1, K2: from keyspace CS595 -Cryptography and Network Security Security ! Two fundamentally different security " Unconditional # security No matter how much computer power is available, the cipher cannot be broken " Computational # security Given limited computing... secrecy), and/ or Verify the correctness of a message to the recipient (authentication) Form the basis of many technological solutions to computer and communications security problems CS595 -Cryptography and Network Security Basic Concepts ! Cryptography " ! Plaintext " ! The art or science encompassing the principles and methods of transforming an intelligible message into one that is unintelligible, and. .. change relative letter frequencies do need a moderate amount of ciphertext (100+ letters) CS595 -Cryptography and Network Security Letter Frequency Analysis ! Single Letter " A,B,C,D,E,… ! Double Letter " TH,HE,IN,ER,RE,ON,AN,EN,… ! Triple Letter " THE ,AND, TIO,ATI,FOR,THA,TER,RES,… CS595 -Cryptography and Network Security Modular Arithmetic Cipher ! Use a more complex equation to calculate the ciphertext... Caesar cipher: a=1 CS595 -Cryptography and Network Security Cryptanalysis ! Key space:23*26 " Brute force search ! Use letter frequency counts to guess a couple of possible letter mappings " frequency pattern not produced just by a shift " use these mappings to solve 2 simultaneous equations to derive above parameters CS595 -Cryptography and Network Security Playfair Cipher Used in WWI and WWII s e f o v i/j... Chosen text " Algorithm, ciphertext, chosen plaintext and ciphertext CS595 -Cryptography and Network Security Steganography ! Conceal the existence of message " Character marking " Invisible ink " Pin punctures " Typewriter correction ribbon ! Cryptography renders message unintelligible! CS595 -Cryptography and Network Security Contemporary Equiv ! Least significant bits of picture frames " 2048x3072 pixels... useless if system is known CS595 -Cryptography and Network Security Caesar Cipher ! Replace each letter of message by a letter a fixed distance away (use the 3rd letter on) ! Reputedly used by Julius Caesar ! Example: L FDPH L VDZ L FRQTXHUHG I CAME I SAW I CONGUERED " The mapping is ABCDEFGHIJKLMNOPQRSTUVWXYZ DEFGHIJKLMNOPQRSTUVWXYZABC CS595 -Cryptography and Network Security Mathematical Model ! Description... D(k) : i → i - k mod 26 " Encryption CS595 -Cryptography and Network Security Cryptanalysis: Caesar Cipher ! Key space: 26 " ! Exhaustive key search Example " " " GDUCUGQFRMPCNJYACJCRRCPQ HEVDVHRGSNQDOKZBDKDSSDQR Plaintext: JGXFXJTIUPSFQMBDFMFUUFSTKHYGYKUJVGRNCEGNG VVGTU Ciphertext: LIZHZLVKWRUHSODFHOHWWHUVMJAIAMWXSVITPEGI PIXXIVW CS595 -Cryptography and Network Security Character Frequencies ! In most... CS595 -Cryptography and Network Security Basic Concepts ! Cryptanalysis " ! Cryptology " ! The study of principles and methods of transforming an unintelligible message back into an intelligible message without knowledge of the key Also called codebreaking Both cryptography and cryptanalysis Code " An algorithm for transforming an intelligible message into an unintelligible one using a code-book CS595-Cryptography . CS595 -Cryptography and Network Security Cryptography and Network Security Xiang-Yang Li CS595 -Cryptography and Network Security CS595 -Cryptography and Network. Tzu CS595 -Cryptography and Network Security Information Transferring CS595 -Cryptography and Network Security Attack: Interruption CS595 -Cryptography and Network