山东建筑大学毕业设计- 0 -外文文献：1.IntroductionCryptography is the science of keeping secrets secret.Assume a sender referred to here and in what follows as Alice(as is commonly used)wants tosend a message m to a receiver referred to as Bob. She uses an insecure communication channel.For example,the channel could be a computer network or a telephone line.There is a problem if the message contains confidential information. The message could be intercepted and read by an eavesdropper.Or,even worse,the adversary,as usual referred to here as Eve,might be able to modify the message during transmission in such a way that the legitimate recipient Bob does not detect the manipulation.One objective of cryptography is to provide methods for preventing such attacks.Other objectives are discussed in Section 1.2.1.1 Encryption and SecrecyThe fundamental and classical task of cryptography is to provide confidentiality by encryption methods.The message to be transmittedit can be some text,numerical data,an executable program or any other kind of informationis called the plaintext.Alice encrypts the plaintext m and obtains the ciphertext c.The ciphertext c is transmitted to Bob.Bob turns the ciphertext back into the plaintext by decryption.To decrypt,Bob needs some secret information,a secret decryption key. Adversary Eve still may intercept the ciphertext. However,the encryption should guarantee secrecy and prevent her from deriving any information about the plaintext from the observed ciphertext.Encryption is very old.For example,Caesars shift cipher was introduced more than 2000 years ago.Every encryption method provides an encryption algorithm E and a decryption algorithm D.In classical encryption schemes,both algorithms depend on the same secret key .This key k is used for both encryption and decryption.These encryption methods are therefore kcalled symmetric.For example,in Caesars cipher the secret key is the offset of the shift.We haveSymmetric encryption and the important examples DES(data encryption standard)and AES 山东建筑大学毕业设计- 1 -(advanced encryption standard)are discussed in Chapter 2.In 1976,W.Diffie and M.E.Hellman published their famous paper,New Directions in Cryptography (DifHel76).There they introduced the revolutionary concept of public-key cryptography. They provided a solution to the long standing problem of key exchange and pointed the way to digital signatures.The public-key encryption methods(comprehensively studied in Chapter 3)are asymmetric.Each recipient of messages has his personal key ,consisting of two parts: is the encryption key and is made public, is the ),(skppk skdecryption key and is kept secret. If Alice wants to send a message m to Bob,she encrypts m by use of Bobs publicly known encryption key .Bob decrypts the ciphertext by use of his decryption key ,which is known only to him.We haveskMathematically speaking,public-key encryption is a so-called one-way function with a trapdoor. Everyone can easily encrypt a plaintext using the public key ,but the other direction pkis difficult.It is practically impossible to deduce the plaintext from the ciphertext,without knowing the secret key (which is called the trapdoor information).skPublic-key encryption methods require more complex computations and are less efficient than classical symmetric methods.Thus symmetric methods are used for the encryption of large amounts of data.Before applying symmetric encryption,Alice and Bob have to agree on a key.To keep this key secret,they need a secure communication channel.It is common practice to use public-key encryption for this purpose.1.2 The Objectives of CryptographyProviding confidentiality is not the only objective of cryptography.Cryptography is also used to provide solutions for other problems:1.Data integrity.The receiver of a message should be able to check whether the message was modified during transmission,either accidentally or deliberately.No one should be able to substitute a false message for the original message,or for parts of it.2.Authentication.The receiver of a message should be able to verify its origin.No one should be able to send a message to Bob and pretend to be Alice(data origin authentication).When initiating 山东建筑大学毕业设计- 2 -a communication,Alice and Bob should be able to identify each other(entity authentication).3.Non-repudiation.The sender should not be able to later deny that she sent a message.If messages are written on paper,the medium-paper-provides a certain security against manipulation. Handwritten personal signatures are intended to guarantee authentication and non-repudiation.If electronic media are used,the medium itself provides no security at all,since it is easy to replace some bytes in a message during its transmission over a computer network,and it is particularly easy if the network is publicly accessible,like the Internet.So,while encryption has a long history,the need for techniques providing data integrity and authentication resulted from th