Computer Security and Cryptography

4.5 FINDING THE KEY OF VERNAM–VIGENÈRE CIPHERTEXT WITH KNOWN PERIOD BY CORRELATION

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                       cipherEx4.1
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xeedt nerye rthti lpxtl xpbae itrxe eucoy wqrup wmdbd odfrx
oiqhz jxeei dcpht hawlz ikeht cleaa znnsr qaoih mxeca bayxb
rerzq trtqg devbn alcsy qiztw cypep uzvqr nppyi xxswh dygea
eecsh rcucr fekke ilxij ezidj mkazr tepoe bdcxw blqre vmzif
nmmpi smcot evsxx awllt qalrh xidat rioee tczeq iacdc wqeyh
sezbb qtyqe aebdd wmylq qjgsj pgipv wfnuc oywqr krzqt rtqgd
gsktd dwqez hucpx sllep yhgee yxnep mlmce wgfez itwxp uetns
qmuft cwxla zpwcw bejep vmjez ilphx tmszg xlrev prioa ftnvs
psetn xmlnj glcwm ioifv ippen nlsio sxdxw piyjw exbmq ceepm
rarpw wbsyp yriaa zsfrq xnzto wtxcq titpl rmits rtoga oleod
xnmit lsexm pitif wzyxq hqpdw mptmc niscc abayx bredy xlbfd
xgspl uehth izoye fxios tpgif bezec skoay bphxl pxpjk ejeeh
fglxs npnok xmydy eract tdw
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Example 4.1

cipherEx4.1 of length 623 letters is the encipherment of ASCII plaintext by a Vigenère substitution with period 7. The plaintext x and ciphertext y are each divided into 7 plain- and ciphertext files consisting of the letters separated by 7 places:

TABLE 4.2 Letter Counts in Each Subfile of cipherEx4.1

The ith subfiles x_i and y_i are each of length n_i; each letter ...