18[1].古典替换密码cont

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1、第四讲古典替换密码1。多字母表替换密码(Polyalphabeticciphers)增强密码安全性的方法是使用多字母替换密码hencethenamepolyalphabeticciphers使得密码分析更困难，因为有较多的密码表去猜测打乱了字母出现的频率用一个密钥决定对每个字母使用什么字母变换表密钥的第I个字母指定第I个字母表ithletterofkeyspecifiesithalphabettouse依次使用每个字母表useeachalphabetinturnrepeatfromstartafterendofkeyisreached2.VigenèreCipher最简单

2、的多字母替换密码（VigenèreCipher）——多重caesarciphers密钥是多字母长度K=k1k2...kd数学表示：EncryptionisdoneusingEki(a):a->a+ki(mod26)DecryptionisdoneusingDki(a):a->a-ki(mod26)3.VigenèreExamplewritetheplaintextoutunderitwritethekeywordrepeatedthenusingeachkeyletterinturnasacaesarcipherkeyencryptthecorrespondingplai

3、ntextletterPlaintextTHISPROCESSCANALSOBEEXPRESSEDKeywordCIPHERCIPHERCIPHERCIPHERCIPHEPlaintextVPXZTIQKTZWTCVPSWFDMTETIGAHLH3.VigenèreExampleC->CDEFGHIJKLMNOPQRSTUVWXYZABI->IJKLMNOPQRSTUVWXYZABCDEFGHP->PQRSTUVWXYZABCDEFGHIJKLMNOH->HIJKLMNOPQRSTUVWXYZABCDEFGE->EFGHIJKLMNOPQRSTUVWXYZABCDR->R

4、STUVWXYZABCDEFGHIJKLMNOPQABCDEFGHIJKLMNOPQRSTUVWXYZtomaptheaboveplaintextletters.'T'useskey'C'mapsto'V''H'useskey'I'mapsto'P''I'useskey'P'mapsto'X'etc4.BeaufordCipher类似Vigenère密码，但顺序相反数学描述:EncryptionisdoneusingEki(p):p->ki-p(mod26)DecryptionisdoneusingDki(c):c->ki-c(mod26)eg.ifusingasingl

5、ekeyletterof'd'havetranslationalphabetPlain:ABCDEFGHIJKLMNOPQRSTUVWXYZCipher:DCBAZYXWVUTSRQPONMLKJIHGFE5.Variant-BeaufordCipher类似Vigenère密码，但是移位向左实际上是Vigenère逆密码(decryptsit)数学表示：EncryptionisdoneusingEki(a):a->a-ki(mod26)DecryptionisdoneusingDki(a):a->a+ki(mod26)eg.Variant-Beaufordkey'c'(s

6、hiftleft2places)sameasVigenèrekey'y'(shiftright24places)6.AutokeyCipher更多的字母替换表可以提高密码的安全性Vigenère提出了自动密钥密码（autokeycipher）密钥字作为消息的前缀组成密钥knowingkeywordcanrecoverthefirstfewlettersthenusetheseinturnontherestofthemessageeg.givenkey"DECEPTIVE"andmessage"WEAREDISCOVEREDSAVEYOURSELF"key:DECEPTIV

7、EWEAREDISCOVEREDSAVplaintext:WEAREDISCOVEREDSAVEYOURSELFciphertext:ZICVTWQNGKZEIIGASXSTSLVVWLAProblemswithautokeyciphersAutorkeycipher看起来好象比较安全实际上仍然有语言特征characteristicsareusedbythekeyasthemessageie.akeyof'E'willbeusedmoreoftenthana'T'etchencean'E'encryptedwithakeyof