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Ignoring the 676 factor of the notch settings ignores the fact that the notches changes the rotor setting at some point in the message. So the 17,576 rotor settings can change after the first character of the message and must change by the 26th character. These changes are additions to the key length, when viewed over the entire message. That means that for a message length of 1 character, the key length is indeed 1.58 X 10^20, but for a real message longer than 1 character, the key length would be 1.07 X 10^23. The fact that the movement of the rotors is regular (like an odometer) makes the cryptoanalysis easier but does not reduce the factor of 676.