VII Международная студенческая научная конференция Студенческий научный форум - 2015

For thousands years, cryptography has been the art of providing secure communication over insecure channels, and cryptanalysis has been the dual art breaking into such communications. Historically, cryptology has been almost exclusively in the hands of the military and diplomats. With the advent of the computer revolution, and more importantly of a society in which vast amounts of personal, financial, commercial and technological information are stored in computer data banks and transferred over computer networks, the necessity for civilian cryptography has become overwhelming.

The purpose of the present work is to give introduction in cryptography.

First, consider the basic concepts.

Cryptography is the study of secret (crypto-) writing (-graphy)

Cryptography is the art or science encompassing the principles and methods of transforming an intelligible message into one that is unintelligible, and then retransforming that message back to its original form.

Cryptanalysis is the study of principles and methods of transforming an unintelligible message back into an intelligible message without knowledge of the key. Also called codebreaking.

Cryptology is both cryptography and cryptanalysis

Cryptographic systems are characterized along three independent dimensions:

1. The type of operations used for transforming plaintext to cipher text. All encryption algorithms are based on two general principles: substitution, in which each element in the plaintext (bit, letter, group of bits or letters) is mapped into another element, and transposition, in which elements in the plaintext are rearranged. The fundamental requirement is that no information be lost (that is, that all operations are reversible). Most systems, referred to as product systems, involve multiple stages of substitutions and transpositions.

2. The number of keys used. If both sender and receiver use the same key, the system is referred to as symmetric, single-key, secret-key, or conventional encryption. If the sender and receiver use different keys, the system is referred to as asymmetric, two-key, or public-key encryption.

3. The way in which the plaintext is processed. A block cipher processes the input one block of elements at a time, producing an output block for each input block. A stream cipher processes the input elements continuously, producing output one element at a time, as it goes along.

Modern cryptography is heavily based on mathematical theory and computer science practice; cryptographic algorithms are designed around computational hardness assumptions, making such algorithms hard to break in practice by any adversary. It is theoretically possible to break such a system, but it is infeasible to do so by any known practical means. These schemes are therefore termed computationally secure; theoretical advances, e.g., improvements in integer factorization algorithms, and faster computing technology require these solutions to be continually adapted. There exist information-theoretically secure schemes that provably cannot be broken even with unlimited computing power—an example is the one-time pad—but these schemes are more difficult to implement than the best theoretically breakable but computationally secure mechanisms.

REFERENCES:

1. Cryptography and network security: principles and practice. William Stallings.

2. Cryptography and Network Security. Xiang-Yang Li

3. Lecture Notes in Computer Science. G. Goos, J. Hartmanis

4. Wikipedia.org