1. What Is The Index Of Coincidence?

The Index of Coincidence (IC) is a statistical measure used in cryptanalysis to identify the likelihood of coincidences in a text. It compares the observed frequency of coinciding characters to the expected frequency in a random text, helping to determine if a text is natural language or encrypted.

2. How Does The Calculator Work?

The calculator uses the Index of Coincidence formula:

Where:

• \( IC \) — Index of Coincidence (dimensionless)
• \( f_i \) — Frequency of letter i
• \( N \) — Total number of letters in the text

Explanation: The formula calculates the probability that two randomly selected characters from the text are identical, normalized by the expected probability in a random text.

3. Importance Of IC Calculation

Details: IC is crucial in cryptanalysis for determining the period of polyalphabetic ciphers, identifying the language of encrypted text, and distinguishing between random and meaningful text. It helps cryptanalysts break classical ciphers like Vigenère.

4. Using The Calculator

Tips: Enter any text (encrypted or plain) in the input field. The calculator will automatically remove non-alphabetic characters and calculate the IC. For accurate results, use texts with sufficient length (minimum 50 characters recommended).

5. Frequently Asked Questions (FAQ)

Q1: What do different IC values indicate?
A: English text typically has IC ≈ 0.065, random text ≈ 0.038, and other languages have characteristic values that help identify the source language.

Q2: How is IC used in cryptanalysis?
A: By comparing IC values at different shifts, cryptanalysts can determine the key length in polyalphabetic ciphers and identify patterns in encrypted text.

Q3: What is the minimum text length for reliable IC calculation?
A: While calculable for any text ≥2 characters, reliable results typically require at least 50 characters to minimize statistical fluctuations.

Q4: Does IC work for all types of ciphers?
A: IC is most effective against classical polyalphabetic ciphers. Modern encryption methods like AES are not vulnerable to IC analysis.

Q5: How does IC vary between languages?
A: Different languages have different characteristic IC values due to their unique letter frequency distributions (e.g., English: 0.065, French: 0.074, German: 0.072).