The following is an English sentence encrypted by means of a

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.

Solution

The Key length is 9.

The Vigen`ere keyword is ANIELTXYW

and plain text is

ILEARNRDHOWTOCAYCULATETHRAMOUNTOFCAPERNEEDRDFORAROOZWHENIWASNTSCHOOLYBUMULTIPLLTHESQUARRFOOTAGEOSTHEWALLSOYTHECUBIPCONTENTSBFTHEFLOOEANDCEILIAGCOMBINEQANDDOUBLRITYOUTHEAALLOWHALSTHETOTALSOROPENINTSSUCHASWVNDOWSANDQOORSTHENLOUALLOWTUEOTHERHAYFFORMATCUINGTHEPAGTERNTHENLOUDOUBLEGHEWHOLETUINGAGAINGOGIVEAMAEGINOFERRBRANDTHENLOUORDERTUEPAPER