Friedman Test

Friedman Test:

• Statistical test to determine if text is encrypted
• Calculates Friedman K to estimate cipher key length
• Compares Index of Coincidence to expected values
• Distinguishes between monoalphabetic and polyalphabetic ciphers

Formula: K = (IC_english - IC_random) / (IC_text - IC_random)

Where IC_english ≈ 0.067, IC_random ≈ 0.038

Interpretation:

• K approximately 1: Random text or monoalphabetic cipher
• K greater than 1: Polyalphabetic cipher (K approximately key length)
• K less than 1: Unusual or highly encrypted text

History: Developed by William F. Friedman in 1922

Named one of the most important statistical tests in cryptanalysis.