ClassicalEncryptionTechniques.pptVIP

Chapter 2 – Classical EncryptionTechniques Symmetric encryption Secret key encryption Shared key encryption Symmetric Encryption or conventional / secret-key / single-key sender and recipient share a common key was the only type of cryptography, prior to invention of public-key in 1970’s Basic Terminology plaintext - the original message ciphertext - the coded message cipher - algorithm for transforming plaintext to ciphertext key - info used in cipher known only to sender/receiver encipher (encrypt) - converting plaintext to ciphertext decipher (decrypt) - recovering ciphertext from plaintext cryptography - study of encryption principles/methods cryptanalysis (codebreaking) - the study of principles/ methods of deciphering ciphertext without knowing key cryptology - the field of both cryptography and cryptanalysis Symmetric Cipher Model Requirements Two requirements for secure use of symmetric encryption: a strong encryption algorithm a secret key known only to sender / receiver Y = EK(X) X = DK(Y) assume encryption algorithm is known implies a secure channel to distribute key Cryptography can be characterized by: type of encryption operations used substitution / transposition / product number of keys used single-key or secret-key vs two-key or public-key way in which plaintext is processed block / stream Types of Cryptanalytic Attacks ciphertext only only know algorithm / ciphertext, statistical, can identify plaintext known plaintext know/suspect plaintext ciphertext to attack cipher chosen plaintext select plaintext and obtain ciphertext to attack cipher chosen ciphertext select ciphertext and obtain plaintext to attack cipher chosen text select either plaintext or ciphertext to en/decrypt to attack cipher Brute Force Search always possible to simply try every key most basic attack, proportional to key size assume either know / recognise plaintext More Definitions unconditional security no matter how much computer power is available, the cip