第2单元 经典加密技术.pptVIP

第2章 经典加密技术 Symmetric Encryption or conventional / private-key / single-key sender and recipient share a common key all classical encryption algorithms are private-key was only type prior to invention of public-key in 1970’s and by far most widely used Some Basic Terminology plaintext - original message ciphertext - 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) - study of principles/ methods of deciphering ciphertext without knowing key cryptology - 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 mathematically have: Y = EK(X) X = DK(Y) assume encryption algorithm is known implies a secure channel to distribute key Cryptography characterize cryptographic system by: type of encryption operations used substitution / transposition / product number of keys used single-key or private / two-key or public way in which plaintext is processed block / stream Cryptanalysis objective to recover key not just message general approaches: cryptanalytic attack brute-force attack Cryptanalytic Attacks ciphertext only only know algorithm ciphertext, is statistical, know or can identify plaintext known plaintext know/suspect plaintext ciphertext chosen plaintext select plaintext and obtain ciphertext chosen ciphertext select ciphertext and obtain plaintext chosen text select plaintext or ciphertext to en/decrypt More Definitions unconditional security no matter how much computer power or time is available, the cipher cannot be broken since the ciphertext provides insufficient information to uniquely determine the corresponding plain