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The DRYAD Numeral Cipher / Authentication System ( KTC 1400 D ) is a simple, paper cryptographic system currently in use by the U. S. military for authentication and for encryption of short, numerical messages.
Every unit with a radio is given a set of DRYAD code sheets.
A single sheet is valid for a limited time ( e. g. 6 hours ), called a cryptoperiod.

1.814 seconds.