According to his family,[citation needed] he found conditions there uncongenial and requested transfer to the Military Historical Institute (Wojskowy Instytut Historyczny) in Warsaw.
As a historian, Kozaczuk indignantly refuted Cold-War-inspired allegations in the anticommunist Paris-based Polish-language periodical Kultura that his books were actually works of collective authorship that were merely published under his name.
Kozaczuk was the first to reveal (in his book, Bitwa o tajemnice, Battle for Secrets, 1967) that the German Enigma-machine cipher had been broken before World War II by Polish cryptologists.
Winterbotham's The Ultra Secret, Kozaczuk participated in international conferences devoted to World War II military intelligence and Enigma decryption.
He is perhaps best known outside Poland for the 1984 English-language book, Enigma: How the German Machine Cipher Was Broken, and How It Was Read by the Allies in World War Two, edited and translated by Christopher Kasparek.
The 2004 Enigma is largely an abridgment of the 1984 book, with Marian Rejewski's appendices replaced with contributions by other authors, of uneven quality and, for the most part, comparatively little importance.