Appearance and dimensions
- The device is made of brass.
- The outer disc is 2.25″ (57mm) in diameter and 1mm thick. “CSA” is stamped in red in the center, over the smaller letters “S. S.” (which appear to also have been red originally, and are commonly thought to mean “secret service”). The regular alphabet is stamped in black around the edge.
- The inner disc is 1.6″ (41mm) in diameter, 1mm thick and also has a regular alphabet around the edge.
- The third disc, on the back, is 1.8″ (46mm) in diameter and is stamped with the text “F. LABARRE / RICHMOND.VA:”. Its thickness is unknown but may also be 1mm.
- The axle is about 1/4″ (6mm) in diameter.
- A knurled flat nut about 3/4″ (20mm) in diameter holds the parts together.
- The device is 0.375″ (9.5mm) thick at its thickest.
- The 3 existing devices that have been described show Roman numerals (“III”, “X”, and “XIIII”) scratched on the back. No information was found about these markings; they might be a production number made by Francis Labarre, or indicate the Army regiment where they were used.
Cryptology
The device essentially performs a Vigenere polyalphabetic substitution, with discs instead of a table or sliding rulers.
The strength of this system depends heavily on the key used. For it to be considered relatively secure, the ratio [key length] / [message length] must be high enough and the key must not be made up of existing words.