Index of Coincidence

Read this: https://www.dcode.fr/index-coincidence

"Iuhtxhqfb dqdobvlv lv wkh vwxgb ri wkh glvwulexwlrq ri wkh ohwwhuv lq d whaw. Dqdobvlv ri iuhtxhqflhv khos ghfubswlqj vxevwlwxwlrq-edvhg flskhuv xvlqj wkh idfw wkdw vrph ohwwhuv dssdulwlrqv duh ydublqj lq d jlyhq odqjxdjh : lq hqjolvk, ohwwhuv H, W ru D duh frpprq zkloh C ru T duh uduh."

  1. Explain how the index of coincidence can be used in cryptography.
  2. What is a transposition cipher?
  3. What is a monoalphabetic cipher?
  4. What is a polyalphabetic cipher?