Basics of cryptography
Introduction

Cryptography is a mathematical study dealing with methods of secretion of messages into forms which prevent unauthorized entities from understanding of the meaning. . Many security applications are, in fact, based on the use of cryptographical algorithms to encrypt and decrypt data.

Encryption is a process (transformation) of changing data so that if an unauthorized person accesses the encrypted data, this data will be unrecognizable and useless. Decryption is converting data back to its original form.

Cryptographic system (comprising encryption, decryption and key) enables to store sensitive information or transmit it across insecure environments (like the Internet) so that it cannot be read by anyone except the intended recipient. Cryptography has now become an industry standard for providing information security, trust, controlling access to resources, and electronic transactions.

This technique is used in everyday actions, such as making or receiving a call from a mobile phone, paying with a credit or debit card, withdrawing money from an ATM or logging on to a computer with a password.

A cryptographic algorithm, or cipher, is a mathematical function used in the encryption and decryption process. A cryptographic algorithm works in combination with one or several keys – a word, number, or phrase – to encrypt plaintext. The same plaintext encrypts to different ciphertext when different keys are used. The security of encrypted data is entirely dependent on two things: the strength of the cryptographic algorithm and the secrecy of the key.

A strong cryptographic algorithm needs to fulfill the following criteria: