Determine Cipher Type - Monoalphabetic vs Polyalphabetic Analysis
The Index of Coincidence (IC) is a statistical measure that helps identify the type of cipher used. It measures how likely it is that two randomly selected letters from a text are the same.
calculateIC("your text here")
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The Index of Coincidence was developed by William F. Friedman in the 1920s as a method to break polyalphabetic ciphers. It remains one of the most fundamental tools in cryptanalysis.
Natural language has characteristic letter frequencies (E is common, Z is rare). Monoalphabetic ciphers preserve these frequencies, just with different letters. Polyalphabetic ciphers flatten the frequency distribution by using multiple alphabets, making the text appear more random.
IC is calculated as: IC = Ξ£(ni(ni-1)) / (N(N-1))
Where ni is the frequency of each letter and N is the total number of letters.