interoperate with the M-94 if suitable strips were provided. The original design used an aluminum base. William F. Friedman describes the problem of getting them manufactured and how it was overcome: "It soon became apparent to both Army and Navy cryptologists that a great increase in cryptosecurity would be obtained if the alphabets of the M-94 device could be made variant instead of invariant. There began efforts in both services to develop a practical instrument based upon this principle. I won't take time to show all these developments but only the final form of the one adopted by the Army, Strip Cipher Device Type, M-138-A. This form used an aluminum base into which channels with overhanging edges were cut to hold cardboard strips of alphabets which could be slid easily within the channels. It may be of interest to you to learn that after I had given up in my attempts to find a firm which would or could make such aluminum grooved devices in quantity, Mrs. Friedman, by womanly wiles and cajolery on behalf of her own group in the U.S. Coast Guard, succeeded in inducing or enticing one firm to make them for her. And it's how the first models of strip cipher devices made of aluminum by the extrusion process came about, and how the U.S. Army, by administrative cooperation on an inter-Service level and technical cooperation on a marital level, found it practical to develop and produce in quantity its Strip Cipher Device, Type M-138-A. This was used from 1935 to 1941 or 1942 by the Army, the Navy, the Marine Corps, the Coast Guard, et al, including the Treasury and State Departments. It was used as a back-up system even after the Armed Services as well as the Department of State began employing much better and more sophisticated cipher machines of high speed and security." Another problem was that aluminum was in short supply early in the war and attempts were made to make the strip cipher bases out of plastic or mahogany, with limited success. Fortunately, aluminum became available again for this use in the fall of 1943. Cryptanalysis Like most classical ciphers, strip ciphers can be easily cracked if there is enough intercepted ciphertext. However, this takes time and specialized skills, so the M-94 was still good enough during the early years of World War II for its intended use as a "tactical