These six lectures by Mr. William F. Friedman, dean of American cryptologists, were prepared in order to have the history of cryptology recorded by the most eminent pioneer in the application of scientific principles to the field – one who, without question, laid the foundation for our modern concepts. It is hoped that both new and old employees may be inspired with a feeling of belonging to a profession that abounds in drama and fascination and that has had a profound impact on history. The lectures, published for the first time in 1963,^[1] are now [1965] being reprinted to meet a continuing demand for an authoritative history of our craft.

Frank B. Rowlett Commandant, National Cryptologic School