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【信息安全工程师202211】【71-75】Perhaps the most obvious difference between private-key and public-key encryption is that the former assumes complete secrecyof all cryptographic keys， whereas the latter requires secrecy foronly the private key.Although this may seem like a minor distinction ，the ramifications are huge： in the private-key setting thecommunicating parties must somehow be able to share the（71）keywithout allowing any third party to learn it， whereas in the public-key setting the（72）key can be sent from one party to the other overa public channel without compromising security.For partiesshouting across a room or， more realistically ， communicating overa public network like a phone line or the nternet， public-keyencryption is the only option.
Another important distinction is that private-key encryption schemes use the（73）key for both encryption and decryption，whereas public-key encryption schemes use（74）keys for each operation.That is public-key encryption is inherently asymmetri C．This asymmetry in thepublic-key setting means that the roles of sender and receiver are notinterchangeable as they are in the private-key setting； a single key-pair allows communication in one direction only.（Bidirectionalcommunication can be achieved in a number of ways； the point is thata single invocation of a public-key encryption scheme forces adistinction between one user who acts as a receiver and other userswho act as senders.）ln addition， a single instance of a（75）encryptionscheme enables multiple senders to communicate privately with asingle receiver，in contrast to the private-key case where a secret keyshared between two parties enables private communication onlybetween those two parties.

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【信息安全工程师201905】【71-75】The modern study of symmetric-key ciphers relates mainly to the study of block ciphers and stream ciphers and to their applications. A block cipher is， in a sense， a modern embodiment of Alberti's polyalphabetic cipher： block ciphers take as input a block of （71）and a key， and output a block of ciphertext of the same size. Since messages are almost always longer than a single block， some method of knitting together successive blocks is required. Several have been developed， some with better security in one aspect or another than others. They are the mode of operations and must be carefully considered when using a block cipher in a cryptosystem.
The Data Encryption Standard （DES） and the Advanced Encryption Standard （AES） are（72）designs which have been designated cryptography standards by the US government （though DES's designation was finally withdrawn after the AES was adopted）. Despite its deprecation as an official standard， DES （especially its still-approved and much more secure triple-DES variant） remains quite popular; it is used across a wide range of applications， from ATM encryption to e-mail privacy and secure remote access. Many other block ciphers have been designed and released， with considerable variation in quality. Many have been thoroughly broken. See Category： Block ciphers.
Stream ciphers， in contrast to the ‘block’type， create an arbitrarily long stream of key material， which is combined （73）the plaintext bit-by-bit or character-by-character， somewhat like the one-time pad. In a stream cipher， the output（74）is created based on an internal state which changes as the cipher operates. That state change is controlled by the key， and， in some stream ciphers， by the plaintext stream as well. RC4 is an example of a well-known， and widely used， stream cipher; see Category： Stream ciphers.
yptographic hash functions （often called message digest functions） do not necessarily use keys， but are a related and important class of cryptographic algorithms. They take input data （often an entire message）， and output a short fixed length hash， and do so as a one-way function. For good ones，（75）（two plaintexts which produce the same hash） are extremely difficult to find.
Message authentication codes （MACs） are much like cryptographic hash functions， except that a secret key is used to authenticate the hash value on receipt. These block an attack against plain hash functions.

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【信息安全工程师202011】【71-75】Symmetric-key cryptosystems use the（71） key for encryption and decryption of a message,though a message or group of messages may have a different key than others. A significant disadvantage of symmetric ciphers is the key management necessary to use them securely.Each distinct pair of communicating parties must, ideally, share a different key, and perhaps each ciphertext exchanged as well. The number of keys required increases as the square of the number of network members,which very quickly requires complex key management schemes to keep them all straight and secret. The difficulty of securely establishing a secret（72）between two communicating parties, when a secure channel doesn't already exist between them, also presents a chicken-and-egg problem which is a considerable practical obstacle for cryptography users in the real worl
Whitfield Difñie and Martin Hellman, authors of the first paper on public-key cryptography.
In a groundbreaking 1976 paper, Whitfield Diffie and Martin Hellman proposed the notion of public-key (also, more generally, called asymmetric key) cryptography in which two different but mathematically related keys are used-a public key and a private key. A public key system is so constructed that calculation of one key (the private key) is computationally infeasible（73）the other (the public key), even though they are necessarily relate Instead, both keys are generated secretly, as an interrelated pair. The historian David Kahn described public-key cryptography as “the most revolutionary new concept in the field since poly-alphabetic substitution emerged in the Renaissance”.
In public-key cryptosystems,the（74）key may be freely distributed,while its paired private key must remain secret. The public key is typically used for encryption, while the private or secret key is used for decryption. Diffie and Hellman showed that public-key cryptography was possible by presenting the Diffie-Hellman key exchange protocol.
In 1978, Ronald Rivest, Adi Shamir, and Len Adleman invented（75）,another public-key system.
In 1997, it finally became publicly known that asymmetric key cryptography had been invented by James H. Ellis at GCHQ,a British intelligence organization, and that, in the early 1970s,both the Diffie-Hellman and RSA algorithms had been previously developed(by Malcolm J. Williamson and Clifford Cocks, respectively).

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