3

I am trying to interpret the formula for index of coincidence from here…

Index of coincidence

I am not understanding $f_i$. How can I calculate $f_i$ in this context? Is this the appearance of single word like (a) in the whole sentence or what ? I am trying to solve it manually without using the mentioned website tool, but I am getting the wrong answer.

Mike Edward Moras
  • 18,161
  • 12
  • 87
  • 240
ARG
  • 143
  • 5

1 Answers1

2

Here $f_i$ is simply the number of times the character $i$ appears in the ciphertext of length $N$ and where $Z$ is the alphabet size.

If you had ciphertext ADCXU ZMDYZ DXZUM and which was derived from English plaintext then $N=15$ and $f_A=1,f_B=0,f_C=1, f_D=3,\ldots, f_Z=2.$

kodlu
  • 25,146
  • 2
  • 30
  • 63