In 1954–55, following the Korean War, Kozaczuk carried out armistice-related duties in Korea. In 1955–58 he served in the Polish Ministry of Internal Affairs (Ministerstwo Spraw Wewnętrznych). In 1957–58 he saw duty with the International Control Commission in Vietnam.
In 1958–69 he served in Polish military counter-intelligence (Wojskowa Służba Wewnętrzna). According to his family, 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. After France's Gustave Bertrand gave further details of prewar and wartime Franco-Polish collaboration on Enigma, and after the cipher's momentous wartime breaking achieved worldwide notoriety with F.W. Winterbotham's The Ultra Secret, Kozaczuk participated in international conferences devoted to World War II military intelligence and Enigma decryption. After the publication of his 1984 English-language book, Enigma, he visited the United States on a publicity tour.
In his latter years, Kozaczuk devoted much attention to setting up and operating his own publishing firm in Warsaw. In the course of these activities, he was set upon and robbed and beaten; thereafter, according to his family, he was never again quite the same.
Aside from his history books and articles, Kozaczuk also published some poems.
Kozaczuk was decorated with the Knight's Cross of the Order of Polonia Restituta.
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, Frederick, MD, University Publications of America. The book incorporates much additional research and documentation beyond what was available in the earlier Polish-language edition, W kręgu Enigmy, and has been described as "the Bible" on the Polish aspects of the history of Enigma-cipher decryption.