I have no intention of writing a complete manual of cryptography. Finality, even very relative, is not attainable in the domain of this art. Besides, good manuals are in existence on this subject, and the titles of some of them will be found at the end of this volume. A cryptographer of considerable experience, however, can always add a few details even to the most complete works of this kind. My object in writing this book is simply to explain what cryptography is, and to recall what it has been from antiquity to the present day; in short, to relate my experiences as a decipherer.