The following is an English sentence encrypted by means of a Vigen`ere cipher. I
ID: 3791010 • Letter: T
Question
The following is an English sentence encrypted by means of a Vigen`ere cipher.
IYMEC GOBDO JBSNT VAQLN BIEAO YIOHV XZYZY LEEVI PWOBB
OEIVZ HWUDE AQALL KROCU WSWRY SIUYB MAEIR DEFYY LKODK
OGIKP HPRDE JIPWL LWPHR KYMBM AKNGM RELYD PHRNP ZHBYJ
DPMMW BXEYO ZJMYX NYJDQ WYMEO GPYBC XSXXY HLBEL LEPRD
EGWXL EPMNO CMRTG QQOUP PEDPS LZOJA EYWNM KRFBL PGIMQ
AYTSH MRCKT UMVST VDBOE UEEVR GJGGP IATDR ARABL PGIMQ
DBCFW XDFAW UWPPM RGJGN OETGD MCIIM EXTBE ENBNI CKYPW
NQBLP GIMQO ELICM RCLAC MV
(i) Compute its index of coincidence.
(ii) Use this to estimate the length of the keyword.
(iii) Use Kasiski’s method to estimate the length of the keyword.
Find the Vigen`ere keyword (which need not be an English word) and find the
plaintext.
Explanation / Answer
INDEX OF COINCIDENCE: Itis a number that cn be used in Cryptanalysis to help determine how a ciphertext has been encrypted.
The Index Of Coincidence for the given text is 1.11798152097
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