Question

The Index of Coincidence is

(a) 0.065

(b) 0.048

(c) 0.067

(d) 0.044

I got this question in class test.

This interesting question is from Overview topic in portion Cryptography Overview, TCP/IP and Communication Networks of Cryptograph & Network Security

Answer 1

Right choice is (d) 0.044

To elaborate: Number of letters = 145.From this, IC=0.0438697 .This is very strong evidence that  the message came from a polyalphabetic ciphering scheme.