Chapter 2 Cryptography,2012,Foundation of Information Security,1,2,3,4,Overview,Overview of Cryptography,Classical Ciphers,Block Ciphers,5,6,1 Overview of Cryptography,1.1 History of Cryptography,1.2 Cryptosystem,1.3 Cryptanalysis,1.4 Cryptography,Before 1949 Classical Encryption古典加密 Before computer was invented, cryptography was art more than science. There were some cipher algorithms, encryption machines & simple cryptanalysis密码分析 ways. The main encryption objects are alphabet character. The security of data is based on the secrecy of algorithms.,1.1 History of Cryptography密码学历史,The Phaistos (1700 BC),Encryption Machines in Early 20th Century,19491976 Shannon published “The Communication Theory of Secret Systems” in 1949, which indicated cryptography became a formal subject. The development of computer enabled ciphers on complex computing. The security of data is based on the secrecy of secret key instead of cipher algorithm.,History of Cryptography (cont d),After 1976 Diffie & Hellman published “New Directions in Cryptography” in 1976, in which put forward asymmetric cryptography不对称密码体制. Rivest, Shamir & Adleman bring forward RSA public key algorithm. Public key cryptography enables secret communication without key transmission between sender & receiver which is well fit for digital signature 数字签名.,History of Cryptography (cont d),Diffie & Hellman published “New Directions in Cryptography” in 1976Government of the United States enacted 颁布“Data Encryption Standard”-DES in 1977Government of the United States enacted new encryption standard-EES in 1993Doctor from the Bell Lab broke EES in 1995DES was broken in 1997Government of the United States recruited 征集 new standard for computer encryption-AES all over the world in 1997AES was enacted in 2001 after strict selection,Memorabilia大事记 of Modern Cryptography,Hypothesize that attacker knows the cipher algorithm usedSecurity of a cryptosystem should rely on the secrecy of the key instead of the cipher algorithmAs a result, the design of cryptosystem should follow the public principle,Kerckhoffs Principle (1883),Definition Cryptography密码编制学 study of encryption principles/methods Cryptanalysis密码分析学 study of principles/ methods of deciphering ciphertext密文 without knowing key Cryptology密码学 the field of both cryptography and cryptanalysisPrinciple Camouflaging伪装 message, preventing unauthorized user from knowing what it means,1.2 Cryptosystem密码体制,Plaintext(Message)明文 - the original message Ciphertext密文 - the coded message Key密钥 - info used in encryption & decryption, known only to sender/receiver, which should be kept secret K=Encipher加密算法 (encrypt) - converting plaintext to ciphertext CE(M，Ke)Decipher解密算法 (decrypt) - recovering plaintext from ciphertext MD(C，Kd),Component of Cryptosystem,Two requirements for secure use of symmetric encryption对称加密:- a strong encryption algorithm- a secret key known only to sender / receiverAssume encryption algorithm is knownExist a secure channel to distribute key,Requirements,CiphertextC xlcm,CiphertextC xlcm,cryptanalyst,Source,Destination,encryption key,decryptionkey,Plaintext M love,secure channel,Ke,Kd,Key,Encryption,PlaintextMlove,channel,Decryption,Contents of keep secret - Restricted algorithm: secrecy of algorithmClassical Cipher - Key-based algorithm: secrecy of keyModern CipherNumber of keys used- Hash functions: no key- Secret key cryptography: one key (Symmetric Cipher对称密码/ Conventional Cipher传统密码/ Single-Key Cipher单钥密码)- Public key cryptography: two keys - public, private (Asymmetric Cipher非对称密码/ Public-Key Cipher公钥密码/ Two-Key Cipher双钥密码)Way in which plaintext is processed- Block分组密码: process input & output as block- Stream流密码/序列密码: process input & output as bit or character,Classification of Cryptography,Symmetric Cipher Ke=KdAsymmetric CipherKeKd Ke KdSo, make Ke public, keep Kd secret,Symmetric Cipher & Asymmetric Cipher,Conventional Cipher- Block CipherDES IDEA EES AES - Stream CipherRC4Public Key Cipher - RSA ElGamal ECC,Examples of Modern Cipher Type,Advantages of Conventional Cipher- speedy for encryption & decryptionDisadvantages of Conventional Cipher - hard to distribute & manage key, realize digital signatureAdvantages of Public Key Cipher - easy to distribute & manage key, realize digital signatureDisadvantages of Public Key Cipher - hard to generate key, slow for encryption & decryption,Advantages & Disadvantages,Definition The process of attempting to discover M or K or both is known as cryptanalysis.Brute Force Attack 暴力破解攻击(穷举攻击)Statistics Analyse Attack 统计分析攻击Mathematics Analyse Attack 数学分析攻击,1.3 Cryptanalysis,Decrypt ciphertext by trying every possible key according to the length of key space, until getting possible right plaintextCan break any cryptosystem by theoryOn average, half of all possible keys must be tried to achieve success,