SYMMETRIC ENCRYPTION

Symmetric encryption uses the same key to encrypt and decrypt. Because this is a shared key, the keys have to be kept private. Symmetric Encryption is known as Private Key encryption. Remember, "YOU HAVE TO KEEP YOUR PRIVATES: PRIVATE".

With symmetric encryption, you do not want to send the key with the message, if the message were to be intercepted, they would have access to the key and be able to read (decrypt) the message.

The keys are exchanged out-of-band. For a secure exchange of symmetric keys, you would use some form of Diffie Hellman (DH, ECDHE, DHE).

Symmetric encryption uses less overhead than asymmetric encryption and decryption.

Stream ciphers and block ciphers are forms of symmetric encryption, The following are all symmetric encryptions:

Stream Cipher: 

RC4 (encrypts one bit at a time, used for audio & video streaming. WEP, WPA (TKIP), and SSL/TLS are built on RC4). Considered weak encryption and has been deprecated.

Block Ciphers:
DES: Data Encryption Standard - a symmetric block cipher that encrypts in 64-bit blocks. It uses a 56-bit key. Considered weak encryption and has been deprecated.

3DES: Triple-DES - uses 3 different keys to encrypt 3 different times. Encrypts in 64-bit blocks, easy upgrade from DES without changing hardware. Considered weak encryption and has been deprecated.

AES: Advanced Encryption Standard - encrypts data in 128-bit blocks. It has 3 different key sizes, 128-bit, 192-bit, and 256-bit. The most widely used encryption.

Blowfish: Encrypts in 64-bit blocks and uses key sizes between 32-bit and 64-bit.

Twofish: Encrypts data in 128-bit blocks and uses key sizes of 128-bit, 192-bit, or 256-bit.

IF THE ENCRYPTION IS A STREAM CIPHER OR BLOCK CIPHER, THEN IT IS SYMMETRIC ENCRYPTION: