Jim Choate skribis:

On pp. 43 - 44 [of Hagerman's "The American Civil War..."] ... a 'cipher disk' whereby the Union was able to change their telegraph codes on an hourly schedule. It was apparently very effictive in dealing with taps and such.

Anyone know of another source that discusses this disk? I looked in Applied

There's a very brief mention of a disk in "Masked Dispatches: Cryptograms and Cryptology in American History, 1775-1900", available from the NSA Central Security Service. (Series 1, Pre-World War 1, Volume 1, United States Cryptologic History), Center for Cryptologic History, by Ralph E. Weber, 1993. On page 110 he says: "The North, on the other hand, adopted a handy "on-line" means of changing the basic flag code by prearrangement or at will, even within the act of transmission. This was done with a disk, in which the alphabet on the inner disk revolved against an outer ring of flag combination, enabling an instant change of code." My impression from this and from the chapter summary is that the disk was used for flag codes but not telegraphy. For telegraphy the Union apparently used a word-based route transposition that Friedman found unimpressive. It's clearly different from the brass cipher disk used by the Confederates, which used letters rather than flag positions as the ciphertext alphabet. Weber doesn't include pictures of the disk. Jim Gillogly Sterday, 2 Yule S.R. 1998, 05:45 12.19.4.14.0, 10 Ahau 18 Mac, First Lord of Night