Repeating Text

Computer Science Level 3

In an ancient tower far far away, Chris and Agnishom found an old ciphered text. Curious, they start to decrypt the text, but to no avail...

Chris : "I tried frequency analysis and it doesn't work!"
Agnishom : "I noticed that the word $\text{ORRWXAGKIOK}$ keeps repeating. They occur starting in position 2, 28 and 67."

If the text is encrypted using the Vigenère Cipher , which options is most likely to be the keyword?

no old transmissions focal mississippi

1 solution

Noel Lo
May 11, 2017

Observe where are the starting positions of the word - 2, 28 and 67. Take their differences as follows:

$28-2=26$

$67-28=39$

or if you want, $67-2=65$

Considering that $26=2*13$ , $39=3*13$ and that $65=5*13$ , $13$ is indeed a common factor of $26, 39, 65$ so the keyword is 13-letters long. Therefore,

$=>\boxed{transmissions}$

Repeating Text

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