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

A dryad (; , sing.: ) is a tree nymph, or tree spirit, in Greek mythology. In Greek drys signifies "oak." Thus, dryads are specifically the nymphs of oak trees, though the term has come to be used for all tree nymphs in general. "Such deities are very much overshadowed by the divine figures defined through poetry and cult," Walter Burkert remarked of Greek nature deities. They were normally considered to be very shy creatures, except around the goddess Artemis, who was known to be a friend to most nymphs.