CryptosystemAcryptosystem is composedofa set(Ee:eEX)consisting of eneiphering transformations and the corresponding set(E-l:eEX)=(Dd:deX)of deciphering transformations.In other words,for eachee,there eaists aunique deX suich that Da=E-, so that Da(Ee(m)=m for all meMThe keys (e,d) are called a key pair where possibly e = d. A cryptosystem isalso called a cipher
Cryptosystem
问题3：你能否说说一个“好”的cryptosystem应该是什么样的？
Kerckhoffs'Rulesfor Field Ciphers1. The cryptosystem should be practically unbreakable (breakable in theory,perhaps,butnotinpractice).2.A compromised cryptosystem should not inconveniencethe correspondents.(Thisistheonefromwhichhisprinciple,displayedonpage96,seemstobederived since it says that the enemy may know the cryptosystem, but oneshould still be able to send messages sincethe enemy cannot cryptanalyzewiththisknowledgeand withoutthekey.)3.Thekey should be easy to both remember and change at will4. Cryptosystems must be amenable to being sent by telegraph.5. The mechanisms of the cryptosystem must be easily portable and entirelyoperablebya singleentity.6. The cryptosystem must beeasy to use without reference to any manuals ortheneed fordeep mental effort