The same plaintext with a single comma being altered would yield a fully different hash, immediately betraying any forgery. The hashing algorithm also makes it extremely difficult for Eve to forge a plaintext while yielding the same hash (referred to as hash attack). For a basic introduc- tion to PKC-based signature, authentication, and hash algorithms, see my earlier book. 14 As also outlined in the reference, there exist many futuristic applications of cryptogra- phy, which are likely to mark the next generations, starting with present times: certified email, anonymous digital cash, simultaneous secret exchange, and contract signature and global electronic polling or voting, to quote a few. Such not-so-distant applications of cryptography are only a hint of the vastness and complexity of the subject, especially when it comes to the border of individual rights and privacy protection issues. It would be naive to trust that rights and privacy would be better protected if under the full control of computing systems. The two examples of electronic voting and digital cash point to future controversies and new forms of potential abuse. A so-called perfect electronic voting or digital cash system could be hacked, causing the nullity of elections or cash transactions, and putting democracy and private rights at an unprecedented level of risk. And this observation remains true and accurate despite any means of "provably secure" transmission of data! Quantum cryptography, to be described in the next section, should, therefore, be addressed within the full scope of communications security and its applications, a field that relies essentially on classical cryptoalgorithms. While quantum cryptography represents a true revolution in the field, it only solves some of the many issues faced by communications security. This is the correct perception I seek to convey here by having made this chapter a comprehensive description of cryptography from classical to quantum, rather than exclusively focusing on the second, which would ha