It's been over 40 years since Abraham Sinkov wrote this wonderful little book. As he mentions in his introduction (Preface to the first edition, above) this is a book concerning elementary cryptographic systems. Much has happened in the cryptographic world since this book first appeared. Notably, public-key systems have changed the landscape entirely. In bringing this book up to date, I've included the RSA method (Chapter 6), reasoning that understanding its underpinnings requires relatively elementary number theory and so would be a useful addition. The difficulty in breaking RSA leads to the question of what is a perfectly secure system, and so I've also added a chapter on one-time pads.

Otherwise, I've tried to change very little in the original text. Some terminology I've brought up-to-date: “direct standard” alphabets and ciphers are now “additive”, “decimated” is now “multiplicative”. Sinkov's original exercises I've left unaltered. Their subjects are rather dated, reflecting the cold war era (there are references to nuclear testing and communists), but leaving them I think does no harm to the material being studied and might be now thought of as “quaint”. In any case, decrypting them presents the same challenge as if more modern messages were used.

It's hard to find discussion of some of these topics anymore, methods you can think of as paper-and-pencil methods. Sinkov still presents the best discussion available on how to break columnar ciphers of unequal length, I feel.