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I was learning about the finding the key length reading the following web site... http://practicalcryptography.com/cryptanalysis/stochastic-searching/cryptanalysis-vigenere-cipher/

and I really don't understand some part of the example shown in the web. They assume the key length is 2 and extract the two sequence 1,3,5,7.... and 2,4,6,8... and from these two sequences they calculate the I.C.

I wasn't sure how they calculate the I.C. for each sequence. Do they find the frequency count of each letter from a to z, and calculate the I.C using the following formula?

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CodesInChaos
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# For calculating the I.C.index of coincidence for each sequence

I was learning about the finding the key length reading the following web site... http://practicalcryptography.com/cryptanalysis/stochastic-searching/cryptanalysis-vigenere-cipher/

and I really don't understand some part of the example shown in the web. They assume the key length is 2 and extract the two sequence 1,3,5,7.... and 2,4,6,8... and from these two sequences they calculate the I.C. I

I wasn't sure how they calculate the I.C. for each sequence. Do they find the frequency count of each letter from a to z, and calculate the I.C using the following formula???http://www.cs.mtu.edu/~shene/NSF-4/Tutorial/VIG/EQN-IOC-4.jpg

Formula for the index of coincidence http://www.cs.mtu.edu/%7Eshene/NSF-4/Tutorial/VIG/EQN-IOC-4.jpg

# For calculating the I.C. for each sequence

I was learning about the finding the key length reading the following web site... http://practicalcryptography.com/cryptanalysis/stochastic-searching/cryptanalysis-vigenere-cipher/

and I really don't understand some part of the example shown in the web. They assume the key length is 2 and extract the two sequence 1,3,5,7.... and 2,4,6,8... and from these two sequences they calculate the I.C. I wasn't sure how they calculate the I.C. for each sequence. Do they find the frequency count of each letter from a to z, and calculate the I.C using the following formula???http://www.cs.mtu.edu/~shene/NSF-4/Tutorial/VIG/EQN-IOC-4.jpg