Dönitz revived Hermann Bauer's idea of grouping several submarines together into a "Rudeltaktik" ("pack tactic", commonly called "wolfpack") to overwhelm a merchant convoy's escorts. Implementation of wolfpacks had been difficult in World War I owing to the limitations of available radios. In the interwar years, Germany had developed ultrahigh frequency transmitters, while the Enigma cipher machine was believed to have made communications secure. A 1922 paper written by "Kapitäinleutnant" Wessner of the "Wehrabteilung" (Defence Ministry) pointed to the success of surface attacks at night and the need to coordinate operations with multiple boats to defeat the escorts. Dönitz knew of the paper and improved the ideas suggested by Wessner. This tactic had the added advantage that a submarine on the surface was undetectable by Asdic. Dönitz claimed after the war he would not allow his service to be intimidated by British disclosures about Asdic and the course of the war had proven him right. In reality, Dönitz harboured fears stretching back to 1937 that the new technology would render the U-boat impotent. Dönitz published his ideas on night attacks in January 1939 in a booklet called "Die U-Bootwaffe" which apparently went unnoticed by the British. The Royal Navy's overconfidence in Asdic encouraged the Admiralty to suppose it could deal with submarines whatever strategy they adopted—in this they were proven wrong; submarines were difficult to locate and destroy under operational conditions.