資管三C網路建構實習課 加解密概念 3/18
埃及人的故事 Every Egyptian received two names, which were known respectively as the true name and the good name, or the great name and the little name; and while the good or little name was made public, the true or great name appears to have been carefully concealed. —The Golden Bough, Sir James George Frazer
基本名詞 • 鑰匙（Key） • 公鑰（Public Key） • 私鑰（Private Key）
私鑰密碼學
私鑰密碼學 • traditional private/secret/single key cryptography uses one key • shared by both sender and receiver • if this key is disclosed communications are compromised • also is symmetric, parties are equal • hence does not protect sender from receiver forging a message & claiming is sent by sender
公鑰密碼學
公鑰密碼學 • probably most significant advance in the 3000 year history of cryptography • uses two keys – a public & a private key • asymmetric since parties are not equal • uses clever application of number theoretic concepts to function • complements rather than replaces private key cryptography
公鑰密碼學 • public-key/two-key/asymmetric cryptography involves the use of two keys: • a public-key, which may be known by anybody, and can be used to encrypt messages, and verify signatures • a private-key, known only to the recipient, used to decrypt messages, and sign (create) signatures • is asymmetric because • those who encrypt messages or verify signatures cannot decrypt messages or create signatures
加密系統 • 對稱式 (Symmetric) • ex: DES(Data Encryption Standard) • 非對稱式 (Asymmetric) • ex: RSA(Rivest-Shamir-Adelman)
DES • Data Encryption Standard (DES) is a widely-used method of data encryption using a private (secret) key that was judged so difficult to break by the U. S. government that it was restricted for exportation to other countries. • Symmetric!
RSA • RSA is an Internet encryption and authentication system that uses an algorithm developed in 1977 by Ron Rivest, Adi Shamir, and Leonard Adleman. The RSA algorithm is the most commonly used encryption and authentication algorithm and is included as part of the Web browsers from Microsoft and Netscape. • Asymmetric!
RSA演算法 e • C=P mod n d • P=C mod n C: Ciphertext 密文 P: Plaintext 明文 e: Encryption 加密 d: Decryption 解密
RSA範例 Encryption Plaintext 88 7 88 mod 187 = 11 Decryption Ciphertext 11 KU = 7, 187 23 11 mod 187 = 88 KR = 23, 187 KU：公鑰 KR：私鑰 Plaintext 88
作業一 • 請將下列名詞，詳細加以介紹，並附上心得 • digital certificate • SSL(Secure Sockets Layer) • SET(Secure Electronic Transaction) • CA(Certificate Authority)
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