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CS426Fall 2010/Lecture 21 Computer Security CS 426 Lecture 2 Cryptography: Terminology & Classic Ciphers.

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Presentation on theme: "CS426Fall 2010/Lecture 21 Computer Security CS 426 Lecture 2 Cryptography: Terminology & Classic Ciphers."— Presentation transcript:

1 CS426Fall 2010/Lecture 21 Computer Security CS 426 Lecture 2 Cryptography: Terminology & Classic Ciphers

2 Announcements Join class mailing list: CS426_Fall2010@cs.purdue.edu CS426_Fall2010@cs.purdue.edu CS426Fall 2010/Lecture 22

3 CS426Fall 2010/Lecture 23 Security Goals Confidentiality (secrecy, privacy) –only those who are authorized to know can know Integrity –only modified by authorized parties and in authorized ways Availability –those authorized to access can get access

4 CS426Fall 2010/Lecture 24 Tools for Information Security Cryptography Access control Hardware/software architecture for separation Processes and tools for developing more secure software Monitoring and analysis Recovery and response

5 CS426Fall 2010/Lecture 25 Goals of Cryptography The most fundamental problem cryptography addresses: ensure security of communication over insecure medium What does secure communication mean? –confidentiality (privacy, secrecy) only the intended recipient can see the communication –integrity (authenticity) the communication is generated by the alleged sender What does insecure medium mean? –the adversary can eavesdrop –the adversary has full control over the communications

6 CS426Fall 2010/Lecture 26 Approaches to Secure Communication Steganography –“covered writing” –hides the existence of a message –depends on secrecy of method Cryptography –“hidden writing” –hide the meaning of a message –depends on secrecy of a short key, not method

7 Cryptography, cryptanalysis, and cryptology Cryptography, –Traditionally, designing algorithms/protocols –Nowadays, often synonym with cryptology Cryptanalysis –Breaking cryptography Cryptology: both cryptography & cryptanalysis –Becoming less common, CS426Fall 2010/Lecture 27

8 CS426Fall 2010/Lecture 28 History of Cryptography 2500+ years An ongoing battle between codemakers and codebreakers Driven by communication & computation technology –paper and ink –cryptographic engine & telegram, radio –modern cryptography: computers & digital communication

9 CS426Fall 2010/Lecture 29 Basic Terminology Plaintext original message Ciphertext transformed message Key secret used in transformation Encryption Decryption Cipher algorithm for encryption/decryption

10 CS426Fall 2010/Lecture 210 Shift Cipher The Key Space: –[1.. 25] Encryption given a key K: –each letter in the plaintext P is replaced with the K’th letter following corresponding number (shift right) Decryption given K: –shift left History: K = 3, Caesar ’ s cipher

11 CS426Fall 2010/Lecture 211 Shift Cipher: Cryptanalysis Can an attacker find K? –YES: by a bruteorce attack through exhaustive key search, –key space is small (<= 26 possible keys). Once K is found, very easy to decrypt

12 CS426Fall 2010/Lecture 212 General Mono-alphabetic Substitution Cipher The key space: all permutations of  = {A, B, C, …, Z} Encryption given a key  : –each letter X in the plaintext P is replaced with  (X) Decryption given a key  : –each letter Y in the cipherext P is replaced with  -1 (Y) Example: 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  = B A D C Z H W Y G O Q X S V T R N M L K J I P F E U BECAUSE  AZDBJSZ

13 CS426Fall 2010/Lecture 213 Strength of the General Substitution Cipher Exhaustive search is difficult –key space size is 26!  4×10 26 Dominates the art of secret writing throughout the first millennium A.D. Thought to be unbreakable by many back then

14 CS426Fall 2010/Lecture 214 Cryptanalysis of Substitution Ciphers: Frequency Analysis Basic ideas: –Each language has certain features: frequency of letters, or of groups of two or more letters. –Substitution ciphers preserve the language features. –Substitution ciphers are vulnerable to frequency analysis attacks.

15 CS426Fall 2010/Lecture 215 Frequency of Letters in English

16 CS426Fall 2010/Lecture 216 Security Principles Security by obscurity doesn’t work Should assume that the adversary knows the algorithm; the only secret the adversary is assumed to not know is the key

17 CS426Fall 2010/Lecture 217 Readings for This Lecture Required readings: –Cryptography on WikipediaCryptography on Wikipedia Optional Readings: –Security in Computing Chapter 2: Basic Encryption and Decryption Interesting reading –The Code Book by Simon SinghThe Code Book

18 CS426Fall 2010/Lecture 218 Coming Attractions … Cryptography: One-time Pad, Informational Theoretical Security, Stream Ciphers

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