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Index of Coincidence Formula:
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The Index of Coincidence (IC) is a statistical measure used in cryptography to analyze the frequency distribution of letters in a text. It helps determine whether a text is likely to be a natural language or random characters, and can assist in breaking classical ciphers.
The calculator uses the Index of Coincidence formula:
Where:
Explanation: The formula calculates the probability that two randomly selected letters from the text will be identical. Higher values indicate text is more likely to be in a natural language.
Details: The Index of Coincidence is crucial in cryptanalysis for determining the type of cipher used, estimating key length in polyalphabetic ciphers, and distinguishing between meaningful text and random characters.
Tips: Enter letter frequencies as comma-separated values (e.g., "8,2,3,4,7") and the total number of letters. For English text, typical IC values range from 0.06 to 0.07, while random text gives approximately 0.0385.
Q1: What is a typical IC value for English text?
A: For English text, the Index of Coincidence is typically around 0.0667. Different languages have characteristic IC values.
Q2: How is IC used in cryptanalysis?
A: IC helps identify monoalphabetic vs. polyalphabetic ciphers and can estimate the period (key length) of polyalphabetic ciphers like Vigenère.
Q3: What does a low IC value indicate?
A: Low IC values (close to 0.0385) suggest the text is random or encrypted with a polyalphabetic cipher using a long key.
Q4: Can IC be used for languages other than English?
A: Yes, different languages have different characteristic IC values due to their unique letter frequency distributions.
Q5: What are the limitations of IC analysis?
A: IC becomes less reliable with shorter texts and may not work well with modern encryption methods or when the text contains many non-letter characters.