Foundations of Computer SecurityLecture 38: Cryptography IIDr. Bill YoungDepartment of Computer SciencesUniversity of Texas at AustinLecture 38: 1 Cryptography IISome TerminologyEncryption and decryption are functions which transform one textinto another. In functional notation:C = E (P) and P = D(C)where C denotes ciphertext, E is the encryption rule, D is thedecryption rule, P is the plaintext. In this case, we also have:P = D(E(P))It is obviously important to be able to recover the original messagefrom the ciphertext.Lecture 38: 2 Cryptography IIKeyed AlgorithmsOften the encryption and decryption algorithms use a key K . Thekey selects a specific algorithm from the family of algorithmsdefined by E .We write this dependence as:C = E (P, KE) and P = D(C, KD)If KE= KD, then the algorithm is called symmetric. If not, then itis called asymmetric. In general,P = D(E(P, KE), KD)An algorithm that does not use a key is called a keyless cipher.Lecture 38: 3 Cryptography IISome NotationOften the notation E (P, K ) and D(C, K) becomes cumbersome.An alternative notation is often used, particularly in cryptographicprotocols.We’ll often use {P}Kto denote E(P, K), and sometimes to denoteD(P, K). For example,P = D(E(P, KE), KD) = {{P}KE}KD.This is usually appropriate since, in many important commercialcryptosystems, the same algorithm is used for both encryption anddecryption (i.e., the algorithm is its own inverse).Lecture 38: 4 Cryptography IICryptanalysis TasksA cryptanalyst may attempt to do any or all of the following:to break a single message;to recognize patterns in encrypted messages;to infer some meaning without breaking the algorithm;to deduce the key;to find weaknesses in the implementation or environment orthe use of encryption;to find weaknesses in the algorithm, without necessarilyhaving intercepted any messages.Lecture 38: 5 Cryptography IICryptanalysis ToolsThe analyst works with:encrypted messages,known encryption algorithms,intercepted plaintext,data items known or suspected to be in a ciphertext message,mathematical and statistical tools and techniques,properties of languages,computers,ingenuity and luck.Lecture 38: 6 Cryptography IILessonsEncryption is designed to obscure the meaning of text.Redundancy is the enemy of secure encryption because itprovides leverage to the attacker.Next lecture: Properties of CiphersLecture 38: 7 Cryptography
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