Index of Coincidence Calculator
Determine Cipher Type - Monoalphabetic vs Polyalphabetic Analysis
π What is Index of Coincidence?
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.
IC Value Reference:
- English Text: IC β 0.067 (6.7%)
- Monoalphabetic Cipher: IC β 0.067 (preserves letter frequency)
- Polyalphabetic Cipher: IC β 0.038-0.045 (flattens frequency)
- Random Text: IC β 0.038 (completely flat)
π― How to Interpret Results:
- IC > 0.060: Likely monoalphabetic substitution (Caesar, Atbash, simple substitution)
- IC β 0.038-0.050: Likely polyalphabetic cipher (VigenΓ¨re, Beaufort)
- IC < 0.038: Possibly columnar transposition or very complex cipher
π Analysis Results
Visual IC Scale
0.038 Polyalphabetic
0.045 English
0.067
π Letter Frequency Distribution
π’ Statistical Details
π― Recommended Next Steps
π How to Use This Tool
- Input Text: Paste your ciphertext into the text area
- Calculate: Click "Calculate IC" to analyze the text
- Interpret: Review the IC value and visual scale to determine cipher type
- Choose Method: Use recommendations to select the appropriate decryption tool
calculateIC("your text here")
Quickly calculate IC from the browser console
π Understanding Index of Coincidence
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.
Why IC Works:
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.
The Mathematics:
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.
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