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      全国计算机技术与软件专业技术资格（水平）考试，这门新开的信息安全工程师分属该考试“信息系统”专业，位处中级资格。教材《信息安全工程师教程》及考试大纲于7月1日出版，希赛小编整理了信息安全工程师教程学习笔记之Cryptography，供大家参考学习。

      Cryptography Introduction

      Cryptography(or cryptology;derived from Greekκρυπτόςkryptós"hidden,"and the verbγρφωάgráfo"write").In modern times,it has become a branch of information theory,as the mathematical study of information and especially its transmission from place to place.The noted cryptographer Ron Rivest has observed that"cryptography is about communication in the presence of adversaries."It is a central contributor to several fields:information security and related issues,particularly,authentication,and access control.One of cryptography's primary purposes is hiding the meaning of messages,not usually the existence of such messages.In modern times,cryptography also contributes to computer science.Cryptography is central to the techniques used in computer and network security for such things as access control and information confidentiality.Cryptography is also used in many applications encountered in everyday life;the security of ATM cards,computer passwords,and electronic commerce all depend on cryptography.

      As shown in Figure 2.1,the German Lorenz cipher machine used in World War II for encryption of very high-level general staff messages

      Terminology

      The term is often used to refer to the field as a whole,as is cryptology("the study of secrets").The study of how to circumvent the confidentiality sought by using encryption is called cryptanalysis or,more loosely,"codebreaking."The field is a rich source of jargon,some of it humorous.

      Until modern times,cryptography referred almost exclusively to encryption,the process of converting ordinary information(plaintext)into something unintelligible;this is a ciphertext.Decryption is the reverse,moving from unintelligible ciphertext to plaintext.A cipher(or cypher)is a pair of algorithms which perform this encryption and the reversing decryption.The detailed operation of a cipher is controlled both by the algorithm and,in each instance,by a key.This is a secret parameter(known only to the communicants)for the cipher algorithm.Keys are important as ciphers without variable keys are trivially breakable and so rather less than useful.Historically,ciphers were often used directly for encryption or decryption without additional procedures.

      In colloquial use,the term"code"is often used to mean any method of encryption or concealment of meaning.However,within cryptography,code has a more specific meaning;it means the replacement of a unit of plaintext(i.e.,a meaningful word or phrase)with a code word(for example,apple pie replaces attack at dawn).Codes are no longer used in serious cryptography—except incidentally for such things as unit designations(eg,'Bronco Flight')—since properly chosen ciphers are both more practical and more secure than even the best codes,and better adapted to computers as well.

      Some use the English terms cryptography and cryptology interchangeably,while others use cryptography to refer to the use and practice of cryptographic techniques,and cryptology to refer to the subject as a field of study.In this respect,English usage is more tolerant of overlapping meanings than are several European languages.

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    希赛软考网，拥有十四年软考培训经验，希赛网一直坚持自主研发，将丰富的软考培训经验有效融入教程研发过程，自成体系的软考在线题库（软考历年真题）、软考培训教材和软考视频教程，多样的培训方式包括在线辅导、面授、和，使考生的学习更具系统性，辅导更具针对性。采用全程督学机制，，软考平均通过率在全国。