The Index of Coincidence and Its Applications in Cryptanalysis

by William F. Friedman

A revision of a paper with the same title published in 1922 by the Riverbank Laboratories, this material was classified Confidential for over 50 years. This book provides two examples of cryptanalytic solution by means of tabulating coincidences and calculating their significance. One example deals with a substitution cipher wherein a series of messages employing up to 125 random mixed secondary alphabets can be solved without assuming a plaintext value for a single cipher letter. Another example deals with a "multiple alphabet, combined substitution-transposition" cipher, solved from but a single message of fair length.

99 pages, Paperback

First published April 1, 1996