The US authorities used up to mid 1943 the Bell Labs A-3 speech scrambler, a device that utilized band-splitting and inversion. The A-3 was not secure and in the period 1941-44 the Germans were able to decode the conversations in real-time. American cryptologists knew that the A-3 was insecure and they developed the SIGSALY, a device that was a quantum leap in terms of security. However SIGSALY weighed over 50 tons and thus could only be used at prepared sites.For field communications a speech privacy system was urgently needed but at the time it was not possible to combine a high level of crypto-security with a small and compact design. According to the postwar history of the US Army’s SSA-Signal Security Agency, in page 45, a portable speech scrambler called AN/GSQ-1 - SIGJIP was developed by Bell Telephone Laboratories in collaboration with the Signal Corps and in 1943 the responsibility for its further development was transferred to the SSA. The device was tested and its security was found wanting, however the need for such a device was so great that in 1944 several units were sent to the European, Mediterranean and South Pacific theaters.
According to TICOM reports the Germans were able to retrieve such a device from a downed Mustang fighter and although they solved the cryptologic system, in practice it was thought that finding the ‘key’ used for each mission would be difficult due to the time element.
From TICOM I-31 ‘Detailed interrogations of Dr. Hüttenhain, formerly head of research section of OKW/Chi, at Flensburg on 18-21 June 1945’
From TICOM I-58 ‘Interrogation of Dr. Otto Buggisch of OKW/Chi’
For those who want more info on the AN/GSQ-1- SIGJIP in the US National Archives and Records Administration - RG 457 - Entry 9032 - box 792- there is the file NR 2228 CBLL24 6144A 19450927 ‘PERFORMANCE SPEECH EQUIPMENT AN/GSQ-1 AN/GSQ-1A SIGJIP-SIGMAR’.