The following set of cryptanalytic tools is based on the GW BASIC code in appendix F of this declassified Basic Cryptanalysis field manual.

Ciphertext:

**Monographic IC:**

**Digraphic IC:**

**Length of text:**

For frequency analysis, please use the improved

**frequency analysis tool**.

The above ciphertext with repeated sequences underlined.

Periodic Index of Coincidence for a period length of (change for different period)

If the average periodic IC is high, then the chosen period length is likely to be correct, or a multiple of the correct period length. This analysis is helpful for determining the keyword length if a periodic key encryption system like Vigenere Cipher is in use.

Monographic Index of Coincidence - a measure of the likelihood that this ciphertext is a letter-for-letter substitution.

Digraphic Index of Coincidence - a measure of the likelihood that this ciphertext uses two letters to represent one plaintext character.