【文章內容簡介】 ial piece of data called the key. 如果接收到加密數據的人要看原來的數據 ， 就必須把數據轉換為原來的形式 ， 這個過程稱為 解密 。 解密是加密過程的逆過程 。 為了進行解密 ， 接收者必須有稱為密鑰的特殊數據 。 The two main peting cryptography schemes are known as the secretkey (symmetric) system and the publickey (asymmetric) system. The secretkey system uses a single, wholly secret sequence both to encrypt and to decrypt messages. The publickey system uses a pair of mathematically related sequences, one each for encryption and decryption [1]. 現在有兩種主要的 、 相互競爭的密碼術：秘鑰 （ 對稱 ） 和公鑰 （ 不對稱 ） 系統 。 秘鑰系統采用單一的絕密序列 ， 對報文進行加密和解密 。 公鑰系統 采用一對數學上相關的序列 ， 一個用于加密 ， 另一個用于解密 。 Chapter 10 Computer and Network Security 計算機專業(yè)英語 1017 Modern Cryptography Data Encryption Secretkey encryption One of the most popular secretkey encryption schemes is IBM’s Data Encryption System (DES), which became the . federal standard in 1997. the standard form uses a 56bit key to encrypt 64bit data blocks. The following is a notation for relating plaintext, ciphertext, and keys. We will use C=E k (P) to mean that the encryption of the plaintext P using key k gives the ciphertext C. similarly, P=D k (C) represents of decryption of C to get the plaintext again. It then follows that D k ( E k (P))=P 密鑰加密 IBM的數據加密系統 (DES)是最流行的密鑰加密方案之一 。 1977年 ， 該方案成為美國聯邦標準 。 該標準形式采用 56位的密鑰對 64位的數據塊進行加密 。 下面是有關明文 、 密文和密鑰關系的表示法 。 我們用 C=E k (P)表示用密鑰 K對明文 P加密 ， 得到密文 C。 類似的 ， P=D k (C)代表對 C解密得到明文 。 因而遵循： D k (E k (P))=P Chapter 10 Computer and Network Security 計算機專業(yè)英語 1018 Modern Cryptography Data Encryption DES has been studied by many of the world’s leading cryptographers, but no weaknesses have been uncovered. To crack a DESencrypted message a hacker or mercial spy would need to try 255 possible keys. This type of search would need days of puter time on the world’s fastest superputers. Even then, the message may not be cracked if the plaintext is not easily understood [2]. 為了打開一個 DES加密的報文 ， 黑客或商業(yè)間諜需要試驗 255種可能的密鑰 ， 這種搜索在世界上最快的巨型機上也需好幾天的計算機時間 。 如果未加密的 “ 明文 ” 是不易理解的 ， 即使算出報文也可能解不開 。 Chapter 10 Computer and Network Security 計算機專業(yè)英語 1019 Modern Cryptography Data Encryption Developers using DES can improve security by changing the keys frequently, using temporary session keys, or using tripleencryption DES. With triple DES, each 64bit block is encrypted under three different DES keys. Recent research has confirmed that tripleDES is indeed more secure than singleDES. The User Data Masking Encryption Facility is an exportgrade algorithm substituted for DES in several IBM products, such as the Distributed Computing Environment (DCE) [3]. 使用 DES的開發(fā)人員可以通過頻繁更改密鑰 ， 使用臨時的會話密鑰或使用三重加密 DES來提高安全性 。 使用三重 DES時 ， 每個 64位數據塊用三種不同的 DES密鑰加密 。 最新研究已確認三重 DES確實比單重 DES更安全 。 Chapter 10 Computer and Network Security 計算機專業(yè)英語 1020 Modern Cryptography Data Encryption Publickey encryption The key distribution problem has always been the weak link in the secretkey systems. Since the encryption key and decryption key are the same( or easily derived from one another) and the key has to be distributed to all users of the system, it seemd as if there was an inherent builtin problem: keys had to be protected from theft, but they also had to be distributed, so they could not just be locked up in a bank vault. 公鑰加密 密鑰的分布問題在秘鑰系統中一直是一個薄弱環(huán)節(jié) 。 因為加密密鑰和解密密鑰是相同的 （ 或彼此容易推出來 ） 并且這個密鑰必須分配給該秘鑰系統的所有用戶 ， 這好像是存在一個固有的內部問題 ， 必須保護密鑰不被偷竊 ， 但又必須分布出去 ， 所以它們不可能只是鎖在銀行的地下室里 。 Chapter 10 Computer and Network Security 計算機專業(yè)英語 1021 Modern Cryptography Data Encryption Encryption can be used to protect data in transit as well as data in storage. Some vendors provide hardware encryption devices that can be used to encrypt and decrypt data. There are also software encryption packages which are available either mercially or as free software. 加密可以用來保護傳輸中的數據和存儲器中的數據 。 一些廠家提供硬件加密設備 ， 用來加密和解密數據 。 也可買到軟件加密程序包或作為自由軟件免費獲得 。 Encryption can be defined as the process of tasking information that exists in some reada