As a young mathematician, I learned how to make and break simple codes from Sinkov's beguiling introduction to cryptanalysis. In the intervening forty years there has been a revolution in cryptography, and it is fitting that for a modern edition of Sinkov's classic, Todd Feil has indicated the nature of this revolution by adding a lucid introduction to RSA public-key encryption and the number theory which underlies it. If you know a young code breaker who might become a young mathematician, buy her or him this book! Or buy it for yourself and try decoding those messages."" - Phil Straffin, Professor Emeritus of Mathematics and Computer Science at Beloit College""Sinkov's Elementary Cryptanalysis is an eminently readable classic that introduces the reader to both the techniques and the spirit of cryptanalysis - the art and science of reading secret messages. Todd Feil has done a fine job of modernizing the language of the original. His chapters on RSA encryption and one-time pads give added value to the original, especially the very nice treatment of RSA that assumes almost no prior knowledge of number theory."" - Ezra Brown, Virginia Tech University

• Part I. Monoalphabetic Ciphers: 1. The Caesar cipher; 2. Modular arithmetic; 3. Additive alphabets; 4. Solution of additive alphabets; 5. Frequency considerations; 6. Multiplications; 7. Solution of multiplicative alphabets; 8. Affine ciphers; Part II. General Substitution: 9. Mixed alphabets; 10. Solution of mixed alphabet ciphers; 11. Solution of five-letter groupings; 12. Monoalphabets with symbols; Part III. Polyalphabetic Substitution: 13. Polyalphabetic ciphers; 14. Recognition of polyalphabetic ciphers; 15. Determination of number of alphabets; 16. Solutions of additive subalphabets; 17. Mixed plain sequences; 18. Matching alphabets; 19. Reduction to a monoalphabet; 20. Mixed cipher sequences; 21. General comments; Part IV. Polygraphic Systems: 22. Linear transformations; 23. Multiplication of matrices - inverses; 24. Involutory transformations; 25. Recognition of digraphic ciphers; 26. Solution of a linear transformation; 27. How to make the Hill system more secure; Part V. Transposition: 28. Columnar transposition; 29. Completely filled rectangles; 30. Incompletely filled rectangles; 31. Probable word method; 32. General case; 33. Identical length messages; Part VI. RSA Encryption: 34. Public-key encryption; 35. The RSA method; 36. Creating the RSA keys; 37. Why RSA works - Fermat's Little Theorem; 38. Computational considerations; 39. Maple and Mathematica for RSA; 40. Breaking RSA and signatures; Part VII. Perfect Security - One-Time Pads: 41. One-time pads; 42. Pseudo-random number generators; A. Tables; B. ASCII codes; C. Binary numbers; D. Solutions to exercises; Further readings; Index.