At the end of the 19th century, thanks to the evolution of telecommunications, military and naval circles rediscover cryptology, also known as “science of secret writing”, and become more and more interested by it. Its quick development in wartime can be depicted by the creation or expansion of several units, called “Cipher services”, in France and in the United Kingdom. These services have two missions: protecting the national and allied communications, and attacking the secret codes of the enemy. Their growth during the First World War illustrates the emergence of a brand new branch of intelligence and its reflection in counter-espionage: signals intelligence, or SIGINT. A comparison between the French and British Cipher services within their armed forces shows that there were similarities in the establishment of these services, particularly in recruiting personnel whom were subject to secrecy, although the temporalities of certain missions differed. Within the Entente Cordiale, a secret, joint and allied cooperation was established between the various French and British signals intelligence services. This alliance included the creation of shared codes, as well as the sharing of information resulting from the interception and decrypting of enemy communications. It thus highlights the importance of cryptology for the Franco-British alliance in the fight against the Central Empires, which can also be observed in their other alliances, such as the one with the Americans. It also sheds light on the extent to which this intelligence specialty took on in the conduct of the war, which explains the shape taken by the French and British Ciphers after the war.