This publication describes the design and operation of the NBS testbed that is used for the validation of hardware implementations of the Federal Information Processing Data Encryption Standard (DES). A particular implementation is verified if it correctly performs a set of 291 test cases that have been defined to exercise every basic element of the algorithm. As a further check on the correctness of the implementation, an extensive Monte-Carlo test is performed. This publication includes the full specification of the DES algorithm, a complete listing of the DES test set, and a detailed description of the interface to the testbed.