The Index of Coincidence for English language is approximately 0.0667.
- Explanation:
The Index of Coincidence (IoC) is a measure used in cryptography to assess the likelihood that two randomly selected letters in a text are the same. In the English language, the IoC is approximately 0.0667, meaning that there is a 6.67% chance that two randomly selected letters in a text will be the same.
The IoC is calculated by taking the sum of the squares of the frequencies of each letter in the text, divided by the square of the total number of letters in the text. This value ranges from 0 (no coincidence) to 1 (complete coincidence).
In English, the IoC is influenced by the frequency of letters in the language. For example, the letter 'e' is the most common letter in English, so the IoC for English will be affected by this high frequency.
The IoC can be used in cryptanalysis to determine the key length of a Vigenère cipher, a type of polyalphabetic substitution cipher. By analyzing the IoC of a ciphertext, cryptanalysts can make educated guesses about the key length and decrypt the message.
Overall, the IoC is a valuable tool in cryptanalysis for assessing the likelihood of coincidence in a text and can help decipher encrypted messages.