4, and 6 will be effective. Any bar on the drum with a lug in any of those positions will be slid to the left, and that bar will participate in the variable-toothed gear driving the output of the machine. According to the given settings, bars 1, 2, 3, and 5 through 21 will be slid to the left, for a total of 20 bars, or 20 "teeth" on the variable-toothed gear. The encoding for this letter will use a shift of 20. The M-209 uses a reciprocal substitution cipher or Beaufort scheme; the alphabet used in the plaintext message is mapped to the same alphabet in reverse (atbash): If shifting is not considered, "A" becomes "Z", "B" becomes "Y", "C" becomes "X" and so on. Shifting proceeds in a reverse direction; for instance, a plaintext "P" maps to ciphertext "K"; shifting by three positions, to the left, gives ciphertext "N". The shift is circular, so when a shift steps off the left side, it continues again on the right. This approach is self-inversing, meaning that deciphering uses the same table in the same way: a ciphertext "N" is entered as if it were plaintext; this maps to "M" in the ciphertext alphabet, or "P" after shifting three positions, thus giving the original plaintext back. Continuing the example above, the initial letter to be encoded was "A", which maps to "Z" in ciphertext. The shift given by the variable-toothed gear was 20; shifting to the left 20 positions gives the final ciphertext letter "T", which is the same as the first digit in the check string. At the end of the encoding cycle, all six key wheels are advanced by one position. The key wheels will then read "BBBBBB", and the active pins will be "QPONML". A new set of guide arms will interact with the drum, resulting in a different shift for the next encoding operation, and so on. Security The security of the M-209 was good for its time, but it was by no means perfect. As with the Lorenz Electric teletypewriter cipher machine (codenamed Tunny by the Allies), if a codebreaker got hold of two overlapping sequences, he would have a fingerhold into the M-209 settings, and its operation had some distinctive quirks that could be exploited. As of early 1943, German code breaking in World War II was able to read 10-30% of M-209 messages. It