外文文献：
1.Introduction
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Cryptography is the science of keeping crets cret.Assume a nder referred to here and in what follows as Alice(as is commonly ud)wants tond a message m to a receiver referred to as Bob. She us an incure 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 wor,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 discusd in Section 1.2.
1.1 Encryption and Secrecy
The fundamental and classical task of cryptography is to provide confidentiality by encryption methods.The message to be transmitted–it can be some text,numerical data,an executable program or any other kind of information–is 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 cret information,a cret decryption key. Adversary Eve still may intercept the ciphertext.
However,the encryption should guarantee crecy and prevent her from deriving any information about the plaintext from the obrved ciphertext.
Encryption is very old.For example,Caesar’s 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 cret key杨村糕干小学唐诗.This key k is ud for both encryption and decryption.The encryption methods are therefore called symmetric.For example,in Caesar’s cipher the cret key is the offt of the shift.We have
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Symmetric encryption and the important examples DES(data encryption standard)and AES (advanced encryption standard)are discusd 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 decryption key and is kept cret. If Alice wants to nd a message m to Bob,she encrypts m by u of Bob’s publicly known encryption key.Bob decrypts the ciphertext by u of his decryption key,which is known only to him.We have
Mathematically 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 is difficult.It is practically impossible to deduce the plaintext from the ciphertext,without knowing the cret key 高强度间歇性训练>地瓜粥 (which is called the trapdoor information).
Public-key encryption methods require more complex computations and are less efficient than classical symmetric methods.Thus symmetric methods are ud 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 cret,they need a cure communication channel.It is common practice to u public-key encryption for this purpo.
1.2 The Objectives of Cryptography
书柜用英语怎么说Providing confidentiality is not the only objective of cryptography.Cryptography is also ud 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 fal 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 nd a message to Bob and pretend to be Alice(data origin authenticati
on).When initiating a communication,Alice and Bob should be able to identify each other(entity authentication).
3.Non-repudiation.The nder should not be able to later deny that she nt a message.
If messages are written on paper,the medium-paper-provides a certain curity against manipulation. Handwritten personal signatures are intended to guarantee authentication and non-repudiation.If electronic media are ud,the medium itlf provides no curity 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.
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