As you know, cryptology consists of cryptography and cryptanalysis.
The subject of studying cryptography is the encryption of information in order to protect it from unauthorized access. From the original document (plain text, digital image, sound signal, etc.), which is called plain text, when encrypted, its encrypted version is formed, which is called ciphertext, closed text, cryptogram. If encryption of text is a direct task, then decryption, i.e. converting the ciphertext into open text is the opposite problem.
The second part of cryptology - cryptanalysis - studies the methods of breaking ciphers, their application. This term was first introduced into cryptology by William Friedman in 1920.
It is believed that the history of cryptography goes back about 4 thousand years. If we take the technological characteristics of the encryption methods used as a basis, then at the moment there are five periods of development of cryptography:
I period - from about the 3rd millennium BC e.
The dominance of mono-alphabetic ciphers (the basic principle is the replacement of the alphabet of the original text by another alphabet through the replacement of letters with other letters or symbols).
Almost four thousand years ago, in the city of Menet Khufu on the banks of the Nile, a certain Egyptian scribe drew hieroglyphs that told the story of his master's life. In doing so, he became the pioneer of the documented history of cryptography. To classify his inscription, the Egyptian scribe did not use any full-fledged cipher. An inscription that has survived to this day, carved around 1900 BC. e. on the tomb of a noble man named Khnumhotep, only in some places it consists of unusual hieroglyphic symbols instead of more familiar hieroglyphs. The unnamed scribe tried not to obstruct the reading of the text, but only to give it great importance. He did not use cryptography, but used one of the essential elements of encryption, the deliberate transformation of written characters. This is the oldest known text that has undergone such changes.
Reconstruction of a special stick for inscriptions on different surfaces.
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II period - from the 9th century in the Middle East and from the 15th century. in Europe before the beginning of the 20th century
An introduction to the use of polyalphabetic ciphers.
III period - from the beginning to the middle of the XX century
Implementation of electromechanical devices in the work of encryptors. At the same time, the use of polyalphabetic ciphers continued.
IV period - from the middle to the 70s of the XX century
The transition to mathematical cryptography, when in the work of Claude Shannon there are strict mathematical definitions of the amount of information, entropy, encryption function.
Until 1975, cryptography remained "classical" or, more correctly, secret key cryptography.
V period - from the late 1970s to the present
The modern period of development of cryptography is distinguished by the emergence and development of a new direction - public key cryptography. Its appearance is marked not only by new technical capabilities, but also by the relatively widespread use of cryptography for use by individuals.
Modern cryptography forms a separate scientific direction at the intersection of mathematics and computer science - works in this area are published in scientific journals, and regular conferences are organized.
Stream ciphers are preferred by military organizations around the world, and banking structures are preferred by block ciphers, since stream ciphers are of little use for software implementation due to bitwise encryption and decryption. The software implementation of block ciphers is simpler, since instead of manipulating bits, they operate on large blocks of data.