# Cryptography - Symmetric Cipher

### Table of Contents

## About

A Symmetric Cipher is cipher that uses shared key (shared secret between the two parties.

Because the key is shared, the cipher is said to be symmetric in contrario to asymmetric where the secret is known from only one side.

The most known symmetric cipher is the Advanced Encryption Standard (AES)

Encryption algorithms which use the same and single key for both encryption and decryption are known as:

- symmetric key algorithms.
- or secret-key encryption

## Usage

### Connection encryption

The symmetric cipher is quicker than an asymmetric one that's why they are generally used in a second phase of a SSL connection when the shared secret has been exchanged between the two parties via a connection encrypted by asymmetric cipher.

### Data encryption

Symmetric Cipher are also used in data transition. For instance, you may send an email verification with a link that has an encrypted payload. If the user clicks on the link, you receive the data and you can verify that it was the data you send.

## Symmetric postal analogy

- Alice would puts the secret message in a box
- Alice would lock the box using a padlock to which she has a key.
- Alice would send the box to Bob through regular mail.
- Bob would use an identical copy of Alice's key to open the box
- Bob would reads the message.
- Bob would use the same padlock to send his secret reply.

## Symmetric Concept

The single secret key must be shared and kept private by both:

- the sender (for encryption)
- and the receiver (for decryption).

To use a symmetric encryption scheme, the sender and receiver must securely share the key in advance. This is normally done after a connection is secured with an asymmetric cipher (because a symmetric cipher is less expensive in computational power). See section below asymmetric cipher.

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