Kullback, a leading mathematician in the U.S. Signal Intelligence Service, lays out the statistical foundations for analysing cipher systems: probability distributions of plaintext and ciphertext, frequency analysis, the mathematics of substitution and transposition systems, and the statistical behaviour of enciphered languages. He formalises methods for detecting patterns, estimating key lengths, measuring randomness, and exploiting deviations from ideal encryption. Much of the work grew out of wartime cryptanalytic practice and later became central to both classical and computer‑age cryptography. Today, the book is prized as a primary source on how statistical reasoning became a weapon in intelligence work.