CSP-845. Note that the strips in the picture are replicas and do not have the correct designators on either the left or right sides.
M-138A, an Army version of the strip cipher.
HISTORY OF CSP-845 a.k.a. M-138A:Strip cipher systems, both prior to and during WW II, played an important role in classified communications. Before and at the very beginning of the war a great deal of reliance was placed on strip systems because of the shortage of cipher machines. They remained in use as backup systems in the event a cipher machine system could not be used and where the holders were not authorized to use high grade cipher machines throughout the war.
Strip ciphers evolved from rotary wheel ciphers such as the CSP-488, both work on the same poly-alphabetic substitution principle. Colonel Parker Hitt invented the M-138 strip system in 1916. This precursor to M-138A was an aluminum board with twenty-five channels in which sliding strips of mixed alphabets were used. It was basically a flat version of M-94 (CSP-488) in which the alphabets could be changed more easily. During 1935 the first M-138 was deployed. In 1939 the device was changed to include 30 channels with paper strips and became M-138A in the Army, and then CSP-845 in the Navy. By summer of 1940 most of the United States defense units were using this cipher system. During the war at times when aluminum was scarce unsuccessful substitutions of wood or plastic were made, but both eventually warped and the basic aluminum design was successful throughout the war.
DESCRIPTION OF CSP-845:Cipher Device CSP-845 is a hinged aluminum board (14" x 12") into which are milled thirty grooved channels (5/16" wide). Those channels are designed to hold changeable paper strips containing random mixed alphabets. The horizontal divisions between the channels are 1/8" wide. At the bottom of the channeled board is a groove 1/2" wide. In this groove is a small metal slide to which a guide rule is riveted. Two aluminum strips, 1" wide, are attached to the board in a vertical position approximately 2 inches from the edges of the board. The aluminum strips are attached to the board by rivets at about every fifth division between the channels. The left aluminum strip acts as a stop for the guide rule. However, the guide rule is prevented from ever touching the right aluminum strip by a small metal square which is riveted in the groove at the bottom of the board. If the guide rule is pushed to the extreme right and is thus stopped by this small metal square, one column of letters aligned on the alphabet strips is visible to the right of the guide rule.
The space between the aluminum strips is such that, when the guide rule is pushed to the extreme left and the letters on the alphabet strips are aligned in the vertical column, 27 of letters are visible between the guide rule and the right aluminum strip. At the top of the board is a series of numbers from 1 to 25, preceded and followed by the letter "X". These numbers are so positioned that when alphabet strips are inserted in the channels and a column of letters is aligned under either "X", the resultant 25 columns of letters are correctly numbered. In front of the number series at the top of the board is the designation "C.S.P.845", (When the board was first issued, it was classified CONFIDENTIAL and was so marked. However, this inscription was deleted when the classification was removed from the board in Nov 1944.)
The channeled board is made in two sections connected by hinges. These hinges are located in the center are part of the aluminum strips. The guide rule is also hinged, thus allowing the operator to fold the board. A slender cylindrical rod crosses the space between the two aluminum strips at a point just above the channels. This small rod is attached by means of a screws. The purpose of this small rod is to keep the top half of the guide rule close to the board when the top half of the board is being opened or closed.
OPERATION OF CSP-845:The method of operation of the strip cipher changed several times during the war in attempts to improve the security of the system. The Army and Navy had different instructions of the use of the system that were not resolved until 1946 when a Navy style strip elimination indicator system was adopted for both Army and Navy use.
The system involves the use of the CSP-845, a key list, and 100 random-alphabet
strips. The keying sequence of 30 strips is changed daily, and five of
these 30 strips are eliminated at random for each message.
Army instructions for operation may be found in:
REFERENCES:The information enclosed here is excerpted from:
Historical Survey Of Strip Cipher Systems. This is available from NARA; NSA Historical Collections 190/37/7/1, NR 3525 CBRK24 12957A 19450000.
SRH-366 History of Army Strip Cipher. This is available from NARA; RG 0457: NSA/CSS Finding Aid A1, 9020 US Navy Records Relating to Cryptology 1918-1950 Stack 190 Begin Loc 36/12/04 Location 1-19
Special Instructions For Using The Strip Cipher Device (1945) This is available from NARA; NSA Historical Collections 190/37/7/1, NR 2288 CBLL35 12804A 19450205 (Box 798, F: 2288, pp 12)
CSP-845 photograph by Russell Booth, San Francisco, CA.
M-138A photograph is from the National Security Agency.
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