 |
| A Siemens Teleprinter (left ) was used in conjunction with the M-190 device at the right. The placard indicates the designation of KRYPTOFORSATS which is a generic expression meaning "crypto-accessory"or "add on". M-190 and most of the other keytape driven on-line systems never had a system name associated with them. (Photo by Henrik Teller) |
 |
| Combiner: The keystream tape was read by the upper reader. The plain text tape to be enciphered or the ciphered tape to be deciphered was placed on the bottom reader. (Photo by Henrik Teller) |
The M-190 produces a Vernam cipher by exclusive OR'ing the input from the cleartext tape reader with the input from the keytape reader. When the random keytapes are used only once and only to one destination, the Vernam cipher cannot be broken and is just as secure as a One Time Pad.
There are a couple of small differences when compared to the T-37 ICA. The M-190 had the option to be used off-line to produce an encoded tape or to decode a tape. Encoding was done by typing out the message on any teletype and producing a clear text tape. This was then mounted on the M-190 and the keytape set to the next unused sequence number. The yellow "No Page Copy" button and the "Tape Encoded " buttons were then pressed simultaneously. This would make the T-100 produce tape-only copy so one would not waste paper on printing the random garbage that was an encoded tape. The Start button would now be pressed and the teletype attached to the M-190 would produce an encoded tape.
To decode an encoded tape, the operator set the keytape to the correct position, pressed the "Tape Decoded" and Start buttons, and the machine would print the decoded message on the attached teletype. The message indicators for messages encrypted this way, were simply the sequence number on the keytape telling the operator on the other end where on the keytape to start the process. No sequence number was ever to be used twice.
TEMPEST rules decreed that no crypto gear was to be used directly on a TELEX circuit, since clear text could "leak" from the encoding/decoding process even with the TELEX set to local mode. Consequently all Danish M-190's for TELEX use were set up with a local T-100 or Olivetti teletype, and the encoded tapes were hand carried to the TELEX terminal for transmission.
Like the T-37 ICA, the M-190 had illuminated buttons and two big lamps. The green lamp signified enciphered mode and the red lamp for clear text mode. The two screws between the green and red lamps and the buttons adjust the speed of transmission between 50 and 75 baud. These were almost never used, since in those days machines were rarely moved between circuits of differing speeds. On dedicated point-to-point circuits, of course, the M-190 was used in its on-line mode, the procedure being just like the one outlined on the T-37 document. The M-190 was approved for all classifications and was unclassified when the keytape was not mounted.
Most Danish M-190's were replaced during the late 1980's by the Kongsberg Defence Communications Omnicoder. The Omnicoder, in its turn, was replaced by the KG-84 in the mid-to-late 90's, but it is still operational in, among other places, the Norwegian Foreign Service, where it is now due to be replaced by the KDC Nordcoder.
