## Introduction

Cryptography or encryption designates a procedure that translates plain text (plain text or plain text) into an unintelligible sequence of characters using a key. The goal is that the content of the resulting secret text or cryptogram (ciphertext) is only accessible to those with the key to decrypt it. Although “plain text” or “cypher text” comes from military strategy, cryptographic methods can also applied to other types of electronic information such as voice messages, image files or programming codes, and email notifications.

## How is Data Encrypt?

An encryption method consists of two elements, a cryptographic algorithm and one or more secret keys. While the algorithm describes the encryption method (e.g., “shifts each letter through the alphabet sequence”), the key provides the parameter (“C = three positions”). It allows encryption to  define as a procedure by plaintext. A key delivered to the cryptographic algorithm, and an encrypt text is obtain.

### Digital Key

In modern encryption, sequences of bits are used as digital keys. An essential criterion for encryption security is the bit length of the keys, which indicates the logarithmic measure of the number of possible combinations that can be use. One also speaks in this case of critical possibility space or essential space, which, the more comprehensive. The more resistant it is against the so-called brute force attacks

## The Expansion Of The Key

Users interested in Cryptography Or Encryption Designates or decrypting data do not generally work with keys but with more manageable sequences and passwords. A strong password comprises a series of 8 to 12 characters. It made up of letters, numbers and symbols has a decisive advantage over bit sequences in that people  remember them without much effort.

A prevalent method for calculating keys from passwords is represent by the Password-Base Key Derivation Function 2 ( PBKDF2 ). A process by which a semi-random sequence of bits, the so-called salt value, is add to the password. And, using cryptographic hash functions, the series of bits with the desired length is derived.

### The Key Exchange Problem

A central issue in cryptography concerns how information is encrypt in one place and decrypt in another. Julius Caesar already had to face this problem in such a way that, if the leader wanted to send, for example. An encrypted message from Rome to the Germanic front, he had to make sure that the recipient could decipher it.

### Symmetric Cryptography

Information encryption was based on symmetric cryptography, whose origins date back to ancient methods such as the Caesar above code. The basic principle behind it is that encryption and decryption done with the help of a single secret key. It implies that the two interlocutors must have a copy of the key. Which must be protect from third parties so as not to risk the secret information

## Conclusion

A prevalent combination in hybrid cryptography consists of symmetric encryption of the user data using AES. And subsequent asymmetric decryption of the session key with RSA. The session key can also be manage with the Diffie-Hellman method. Which, although its Ephemeral Diffie-Hellman (EDH) variant can provide Forward Secrecy.