Cryptography (a term composed of two Greek words: “kryptós”, which means hidden, and “graphein”, which means to write) is a technique used to obfuscate or encode information, in order to ensure that it can only be read by people in possession of the key to decipher them. In contrast, cryptanalysis aims to decipher hidden texts without knowing a priori how they were hidden. In the field of computer security, cryptography and cryptanalysis represent two sides of the same coin and are studied by cryptology.

#### Characteristics of cryptography

The characteristics of cryptography are the following:

**Confidentiality**: the information is accessible only to the person for whom it is intended and no other person except him can access it.**Integrity**: Information cannot be changed during storage or in transition between the sender and the intended recipient without any addition to the information being detected.**Non-repudiation**: The creator/sender of the information cannot deny his intention to send information at a later stage.**Authentication**: The identities of the sender and recipient are confirmed. The destination/origin of the information is also confirmed.

#### How encryption works

Encryption is based on an algorithmAlgorithm A procedure applied to solve a given problem. and a cryptographic key. In its most basic form, two people, often called Alice and Bob, agree on a particular secret key. Later, Alice sends a secret message to Bob (or Bob sends a message to Alice). The key is used to transform the original message (which is usually called plaintext) into an encrypted form, called ciphertext, that is incomprehensible to anyone without the key. When Bob receives the ciphertext, he can use the key to transform the ciphertext back into the original plaintext; this is the decryption process. This type of encryption is called symmetric key encryption.

#### Types of cryptography

There are three types of cryptography:

- Symmetric key cryptography
- Asymmetric key cryptography
- HashHash The cryptographic function that identifies blocks in the blockchain. function

**Symmetric Key Cryptography** is an encryption system where the sender and receiver of message use a single common key to encrypt and decrypt messages. Symmetric Key Systems are faster and simpler but the problem is that sender and receiver have to somehow exchange key in a secure manner. The most popular symmetric key cryptography system are Data Encryption System(DES) and Advanced Encryption System(AES).

**Asymmetric cryptography**, or Public-key cryptography, is an encryption system that use pairs of related keys. Each key pair consists of a public key and a corresponding private key. A receiver’s public key is used for encryption and a receiver’s private key is used for decryption. Even if the public key is known by everyone, only the intended receiver can decode it because he alone know his private key. The most popular asymmetric key cryptography algorithm is RSA algorithm.

A **hash function** is a mathematical function that converts any digital data into an output string with a fixed number of characters. This is a one-way operation: converting data (called a message) to output (called a hash). The reverse operation is not possible. No key is used in this algorithm. Many operating systems use hash functions to encrypt passwords. Some examples of hash functions are: MD2, MD4, MD5 and SHA-1.

#### Applications of cryptography

Cryptography is used in password management in computers, in home wireless communications, in cellular networksNetwork The set of computers connected to each other, called nodes, on which the blockchain of a specific cryptocurrency is based., on the Internet to obscure data communication in transit between clients and servers, in electronic signatures, in financial-banking transactionsTransaction Exchange of value, property, or data between two parties., in cryptocurrencies, in pay per view to prevent non-subscribers from viewing paid audiovisual content, in short, in practically all cases where data confidentiality is required.

#### History of cryptography

Cryptography has been used for thousands of years to help provide confidential communications between mutually trusted parties. There are traces of cryptography in the ancient ciphers used by the Jews in the atbash code; the Spartans had their own particular system of communicating secret messages, the scytala; Gaius Julius Caesar is credited with using the so-called Caesar cipher. At the beginning of the 20th century, the invention of electromechanical devices, such as the rotor Enigma machine, raised encryption to more sophisticated and efficient levels; the subsequent introduction of electronics and computers has allowed the use of increasingly complex encryption schemes, many of which cannot be obtained with pen and paper. The evolution of cryptographic systems, combined with the evolution of theoretical physics, have made it possible to create cryptographic techniques based on the use of quantum mechanics in the key exchange phase. The advantage of this technique is that it makes man in the middle attacks unusable: that is, if during the exchange of the key someone managed to intercept it, this would become immediately evident to both the person sending and receiving the message.