Introduction to Cryptography with Coding Theory, 3rd Edition

Chapter 14 What Can Go Wrong

The mathematics behind cryptosystems is only part of the picture. Implementation is also very important. The best systems, when used incorrectly, can lead to serious problems. And certain design considerations that might look good at the time can later turn out to be bad.

We start with a whimsical example that (we hope) has never been used in practice. Alice wants to send a message to Bob over public channels. They decide to use the three-pass protocol (see Section 3.6).

Here is a physical description. Alice puts her message in a box, puts on her lock, and send the box to Bob. Bob puts on his lock and sends the box back to Alice, who takes her lock off the box and sends the box to Bob. He takes off his lock and ...