Ferris, John. “Behind the Enigma: The Authorized History of GCHQ, Britain’s Secret Cyber-Intelligence Agency”. The Journal of Intelligence, Conflict, and Warfare 3, no. 3 (March 24, 2021): 131–134. Accessed April 20, 2024. https://journals.lib.sfu.ca/index.php/jicw/article/view/2795.