The RSA cipher is a popular cipher that is widely used on the Internet. This cipher and other modern ciphers are different from the ciphers that were once used only by some organizations during wartime, and as a fundamental technology to ensure information security, they give us peace of mind as we live in an information network society. Some of us may be using it unconsciously, and it has already become an indispensable part of our society.

In addition to the RSA cipher, there are many other modern ciphers such as DES and AES. While many of them are complex in design, the essence of the most distinctive RSA cipher is based on a very simple and interesting theory. Wouldn’t it be a shame if you don’t know what this “trick” is all about?

This reading material is an easy-to-understand explanation of modern cryptography, focusing on the RSA cipher. Please read this book if you are interested in this subject with your current knowledge and the explanation so far. The target readers are junior and senior high school students, college students who are purely liberal arts, and science majors who could not understand RSA cryptography from technical books. This is not a book for those who can understand RSA cryptography with technical books, or for those who already understand it.

I wanted many people to feel the fun of the “magic of numbers” contained in the RSA cryptosystem, so I asked my friend in the humanities who is not good at mathematics why he could not understand the RSA cryptosystem in technical books. He replied, ” It’s fine as long as it’s expressed in words, but as soon as a difficult mathematical expression appears, it becomes incomprehensible.” In other words, “divide the remainder by two, ……. It says. As long as it says “divide the remainder by 2,…”, it’s fine, but if it says “mod 2”, my brain starts to panic. Also, even if only the formula is written, it doesn’t make you feel it because you don’t know why you have to do the calculation.

Therefore, in this book, I have tried to explain in words as much as possible, not only in formulas, even if it is a little complicated. It may be a bit tedious, but we have tried to explain in words as much as possible, not only in formulas, so that the reader can understand why such mathematical procedures are necessary. In some cases, our approach is different from that of technical books and does not require much mathematical knowledge. I believe that this book can be understood by ordinary high school students, junior high school students with guts, and hyper elementary school students.

— Masashi Ito (maitou)
August 10, 1997 First edition
August 07, 1998 Revised edition
March 1, 2003 Revised edition