# Cryptographic methods

**Cryptographic methods** are the most effective means of protecting information in a computer system. The most important indicator of the reliability of the cryptographic method of closing information is its strength - the minimum amount of cipher text that can be opened by statistical analysis. Thus, the strength of the cipher determines the amount of information that can be encrypted using one key.

The complexity of the encryption method is determined by the number of elementary operations required to encrypt one character of the original text.

Basic requirements for the cryptographic method: the complexity and strength of the cryptographic data closure should be selected depending on the volume and degree of data secrecy; the reliability of the closure must be such that the secrecy is not violated even if the attacker becomes aware of the encryption method; the cryptographic method, the set of keys used, and the distribution mechanisms should not be too complex; the execution of the forward and backward transformation procedures must be formal; errors arising during the conversion process should not be spread throughout the text; the redundancy introduced by the protection procedures should be minimal. Some of these methods are discussed below.

*Replacement encryption (substitution)*. This is the simplest cryptographic encryption method. The characters of the encrypted text are replaced with other characters taken from one (mono-alphabetical substitution) or several (polyalphabetic substitution) alphabets. However, such a cipher has low strength. The ciphertext has the same statistical characteristics as the original one, therefore, using the frequency dictionary of the occurrence of characters in the language in which the message is written, and selecting the frequencies of occurrence of characters in the encrypted message, it is possible to restore the replacement table.

The use of polyalphabetic substitutions increases the strength of the cipher. Several alphabets are used to replace characters, and the alphabets are changed sequentially and cyclically.

Permutation encryption This cryptographic method consists in the fact that the characters of the encrypted text are permuted according to certain rules inside the encrypted block of characters.

*Encryption methods using keys*. These cryptographic techniques assume knowledge of the key in encryption and decryption. In this case, an important task is the secure transmission of the key, which is usually also encrypted. Considering the short length of the phrase containing the key, the strength of the key cipher is significantly higher than that of the main text.

*Public key systems*. The most promising systems for cryptographic data protection are currently public key systems. These systems use one key to encrypt data and another to decrypt. The first key is not secret and can be published for use by all system users who encrypt data. To decrypt the data, the recipient uses a second key, which is secret. The decryption key cannot be determined from the encryption key.

*Using hash functions*. Hashing functions are widely used to encrypt user passwords and to create electronic signatures. They map a message of any length to a fixed-size string. A feature of its application is the fact that there is no function that could restore the original message from a compressed display - it is a one-way hash function.

Having received at his disposal a file that stores user passwords converted by a hash function, an attacker is not able to get the passwords themselves from them, but must sort out password combinations of characters, apply a hash function to them and check for a match between the received string and the string from the hashed password file... This work is made difficult by the fact that he does not know the length of the password either.

*Electronic digital signature*. When exchanging electronic documents, it is very important to establish the authorship, authenticity and integrity of the information in the received document. The solution of these tasks is assigned to the digital signature accompanying the electronic document. It is functionally similar to a regular handwritten signature.

An electronic digital signature is a relatively small amount of additional information transmitted with a document. Typically, a digital signature is encrypted using public key cryptographic techniques and links the content of the document, the signature itself, and the key pair. Changing at least one of these elements makes it impossible to verify the authenticity of the digital signature.