The HC-9 was a mechanical cipher device manufactured by Swedish company AB Transvertex. The HC-9 was designed after World War II and likely remained in use up to the 1970s. The machine was used for low-level communications.
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- Photograph of an HC-9
- Another HC-9 photograph with a description
- A description of the machine and a simulator in QBASIC
- Discussion of the machine's operation
- Cipher A. Deavours and Louis Kruh, The Swedish HC-9 Ciphering Machine, Cryptologia, Vol. 13(3), July 1989, pp. 251–265
- Cipher A. Deavours and Louis Kruh, The Swedish HC-9 Ciphering Machine Challenge, Cryptologia, Vol. 14(2), April 1990, pp. 139–144
- H. P. Greenough, Cryptanalysis of the Swedish HC-9: A Known-Plaintext Approach, Cryptologia, 1997, 21(4), pp353–367.