TheCodeBreakers
unique key, application of the ideal system would require shipping out
on tape at the very least the equivalent of the total communications
volume of a war. In fact, however, considerable extra key material would
have to be supplied. A group of subordinate units may possess some tape
in common for intercommunication, but once one unit uses a roll of
keytape, the others must cancel their identical rolls. In practice, this step
is the most difficult. It is virtually impossible in the hubbub of battle to
monitor the messages of a dozen other units to determine what keytapes
they have used.
In general, the physical problems bar employing a onetime system in
a fluid situation, such as military operations in the field. These
difficulties do not hold for more stable situations, such as exist at high
military headquarters, at diplomatic posts, or in a two-way spy
correspondence— and in such situations one-time systems are
practicable and are used
annaabi.ee 
