【正文】 objectives are discussed in Section . Encryption and Secrecy The fundamental and classical task of cryptography is to provide confidentiality by encryption message to be transmitted– it can be some text,numerical data,an executable program or any other kind of information– is called the encrypts the plaintext m and obtains the ciphertext ciphertext c is transmitted to turns the ciphertext back into the plaintext by 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 example,Caesar’s shift cipher was introduced more than 2020 years encryption method provides an encryption algorithm E and a decryption algorithm classical encryption schemes,both algorithms depend on the same secret key k .This key k is used for both encryption and encryption methods are therefore called example,in Caesar’ s cipher the secret key is the offset of the have Symmetric encryption and the important examples DES(data encryption standard)and AES (advanced encryption standard)are discussed in Chapter 2. 山東建筑大學(xué) 畢業(yè)設(shè)計 2 In 1976, and published their famous paper,New Directions in Cryptography ([DifHel76]).There they introduced the revolutionary concept of publickey cryptography. They provided a solution to the long standing problem of key exchange and pointed the way to digital publickey encryption methods(prehensively studied in Chapter 3)are recipient of messages has his personal key ),( skpkk? ,consisting of two parts:pk is the encryption key and is made public,sk is the decryption key and is kept secret. If Alice wants to send a message m to Bob,she encrypts m by use of Bob’s publicly known encryption keypk .Bob decrypts the ciphertext by use of his decryption key sk ,which is known only to have Mathematically speaking,publickey encryption is a socalled oneway function with a trapdoor. Everyone can easily encrypt a plaintext using the public keypk ,but the other direction is is practically impossible to deduce the plaintext from the ciphertext,without knowing the secret key sk (which is called the trapdoor information). Publickey encryption methods require more plex putations and are less efficient than classical symmetric symmetric methods are used for the encryption of large amounts of applying symmetric encryption,Alice and Bob have to agree on a keep this key secret,they need a secure munication is mon practice to use publickey encryption for this purpose. The Objectives of Cryptography Providing confidentiality is not the only objective of is also used to provide solutions for other problems: receiver of a message should be able to check whether the message was modified during transmission,either accidentally or one should be able to substitute a false message for the original message,or for parts of it. receiver of a message should be able to verify its one should be able to send a message to Bob and pretend to be Alice(data origin authentication).When initiating a munication,Alice and Bob should be able to identify each other(entity authentication). 山東建筑大學(xué) 畢業(yè)設(shè)計 3 sender should not be able to later deny that she sent a message. If messages are written on paper,the mediumpaperprovides a certain security against manipulation. Handwritten personal signatures are intended to guarantee authentication and 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 puter work,and it is particularly easy if the work is publicly accessible,like the Inter. So,while encryption has a long history,the need for techniques providing data integrity and authentication resulted from the rapidly increasing significance of electronic munication. There are symmetric as well as publickey methods to ensure the integrity of symmetric methods require a secret key k that is shared by sender and message m is augmented by a message authentication code(MAC).The code is generated by an algorithm and depends on the secret augmented message )),(,( mkMACm is protected against receiver may test the integrity of an ining message(m,m)by checking whether Message authentication codes may be implemented by keyed hash functions (see Chapter 3). Digital signatures require publickey methods(see Chapter 3 for examples and details).As with classical handwritten signatures,they are intended to provide authentication and that nonrepudiation is an indispensable feature if digital signatures are used to sign signatures depend on the secret key of the signerthey can be generated only by the other hand,anyone can check whether a signature is valid,by applying a publicly known verification algorithm Verify,which depends on the public key of the Alice wants to sign the message m,she applies the algorithm Sign with her secret key sk and gets the signature Sign ),( msk .Bob receives a signature s for message m,and may then check the signature by testing whether with Alice’ s public key pk . It is mon not to sign the message itself,but to apply a cryptograp hic hash function(see 山東建筑大學(xué) 畢業(yè)設(shè)計 4 Section )first and then sign the hash schemes like the famous RSA(named after its inventors:Rivest,Shamir and Adleman),the decryption algorithm is used to generate signatures and the encryption algorithm is used to verify approach to digital signatures is therefore often referred to as the“ hashthendecrypt” paradigm(see for details).More sophisticated signature schemes,like the probabilistic signature scheme(PSS),require more the hash value by pseudorandom sequences turns signing into a probabilistic pr