Cryptography, the study of ciphers, for a long time was a secret sci-
ence because it was used primarily to ensure security of state and
military secrets. Presently, cryptographic methods and tools are used
not only by the state, but also in the private or corporate life, and
classified information is not necessarily the main object of protection.
The amount of information presented in the digital form and spread
all over the world is now very large, and this information requires
protection against nonfriendly intrusion, collection, substitution, fal-
sification, and so on. Cryptography provides the most efficient tools
for defending against these threats.
Nowadays cryptographic algorithms still remain secret for ordi-
nary users, although many people have already applied some crypto-
graphic means like encrypting electronic mail or smart bank cards,
and so on. Naturally, the main question the user asks is whether a
given cryptographic tool provides sufficient defense. However, even a
precise formulation of this question is not easy. Whom are we pro-
tecting from? What are the capabilities of our opponents? What
goals do they pursue? How to measure the level of security? The
list of these questions can be extended. Answers to them require a
knowledge of the very basics of cryptography.
The goal of this book is to give a popular explanation of these
basics (we touch only upon the "nonstate" part of the science; the
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