In the history of cryptography, the Biuro Szyfrów was the Cipher Bureau of Polish Military Intelligence. On 8 May 1919, a Cipher Section (Sekcja Szyfrów) of the Polish Army was formed by Lt. Józef Staślicki, and a few months later was renamed Biuro Szyfrów. It contributed substantially to the defeat of the Soviets by Piłsudski during the Polish-Soviet War of 1920, as Soviet military cryptography of the time was less than unbreakable. Later on, German communications became its primary focus; "BS4" was the section dealing with German ciphers, headed by Maksymilian Ciezki .

Ciezki hired three young Polish mathematicians early in the 1930s: Jerzy Różycki, Henryk Zygalski and Marian Rejewski. Rejewski made one of the biggest advances in cryptographic history by applying methods of pure mathematics in breaking the German Army's new cryptosystem, Enigma, in 1932. The staff of the Biuro also designed, and had built, the cyclometer device to assist in the Enigma work, and later on the bomba kryptologiczna to do the same.

As it became clear that Poland could not survive the Nazi attack, most of the staff of the Biuro were evacuated. Many made it to France and some of them worked on German communications with French Intelligence in Paris and later at PC Bruno. After PC Bruno was shut down many of the Polish staff, including Rejewski, were again evacuated this time to Britain.

One of the Biuro's staff, Antoni Palluth, was part owner of a small machine/radio shop (Wytwórnia Radiotechniczna AVA) which did much of the fabrication of the cyclometers, the bombas, and other devices. Some of the workers at that machine shop were not evacuated for one reason or another as Poland was overrun in September 1939 and were captured by the Germans. Somehow they managed to avoid saying anything which might lead the Germans to suspect that the Biuro had broken the Enigma.