making analysis of the machine more complex. The reduced number of inputs ensured that the generated key would not be biased. The rotor stepping was also made more complex. The slowest two rotors, which originally were unlikely to step during the course of an encipherment, were redesigned so that they stepped depending on the output of the previous key output. One rotor, designated the "fast bump" rotor, would step if the fourth and fifth bits of the previous output were both true; and similarly the "slow bump" rotor would do the same for the first, second and third bits. Certain of the rotor stepping arrangements were discovered to be weaker than others, and so these were ruled out for key lists. This redesigned version of the M-228 was put into service by April 1943. However, the machine was judged to be only secure enough to handle traffic up to SECRET by landline, and CONFIDENTIAL by radio. The machine was also shared with the United Kingdom for joint communications. A further-modified version of the M-228 that could be used for the highest level traffic, was designated M-228-M, or SIGHUAD. From that point on, the Army monitored the communications of its high-level systems to ensure that good operational procedure was being followed, even for highly secure devices such as the SIGABA and SIGTOT devices. As a result, poor operator practices, such as transmitting messages in depth, were largely eliminated. References Stephen J. Kelley, "The SIGCUM Story: Cryptographic Failure, Cryptographic Success", in Cryptologia 21(4), October 1997, pp289–316. External links Converter M-228 or SIGCUM by John Savard Rotor machines Encryption devices