Cryptanalysis

Close-up of the rotors in a Fialka cipher machine
"Cryptanalysis (from the Greek kryptós, 'hidden', and analýein, 'to loosen' or 'to untie') is the study of analyzing information systems in order to study the hidden aspects of the systems. Cryptanalysis is used to breach cryptographic security systems and gain access to the contents of encrypted messages, even if the cryptographic key is unknown. In addition to mathematical analysis of cryptographic algorithms, cryptanalysis includes the study of side-channel attacks that do not target weaknesses in the cryptographic algorithms themselves, but instead exploit weaknesses in their implementation. ... Cryptanalysis has coevolved together with cryptography, and the contest can be traced through the history of cryptography—new ciphers being designed to replace old broken designs, and new cryptanalytic techniques invented to crack the improved schemes. In practice, they are viewed as two sides of the same coin: secure cryptography requires design against possible cryptanalysis. ..."
Wikipedia, W - History of cryptography
W - Martin and Mitchell defection, W - List of cryptographers
Glenn’s Computer Museum
YouTube: What is Cryptography - Introduction to Cryptography - Lesson 1, Introduction to Cryptographic Keys and Certificates, Cryptography Lesson #1 - Block Ciphers

(Left) William Martin and Bernon Mitchell (Center) tell Moscow press why they defected

2015 April: The Imitation Game (2014), 2016 August: Cryptography - Neal Stephenson (1999)