# Introduction

A Secret key Algorithm is used for encrypting and decrypting electronic information, in cryptography. A secret key algorithm (symmetric algorithm) is a cryptographic algorithm that uses the shared (same) key to encrypt and decrypt data. Data are translated to a type using symmetric encryption algorithms which can not be understood by anyone who has not the secret key to decrypt it. The high level of security is provided by the symmetrical algorithm. Symmetric keys that can be exchanged between two routers supporting a VPN.

Symmetric encryption algorithms characteristics:

• The key length is 80 to 256 bits.

• A sender and receiver must share a secret key.

• They are usually quite fast (wire speed), because these algorithms are based on simple mathematical operations.

• DES, 3DES, AES, IDEA, RC2/4/5/6, and Blowfish are examples of symmetric encryption algorithms.
• Symmetric encryption algorithms use the same pre-shared secret key to encrypt and decrypt data. The pre-shared key is known by the sender and receiver before any encrypted communications begins.
• Because both parties are guarding a shared secret, the encryption algorithms used can have shorter key lengths. Shorter key lengths for faster execution.
• For this reason symmetric algorithms are generally much less computationally intensive than asymmetric algorithms.

Symmetric Encryption Techniques

There are two types of encryption method used:

• Block Ciphers
• Stream Ciphers

# Block ciphers

Block ciphers transform a fixed-length block of plain text into a common block of ciphertext of 64 or 128 bits.
Block size refers to how much data is encrypted at any one time.
• The key length refers to the size of the encryption key that is used.
• This ciphertext is decrypted by applying the reverse transformation to the ciphertext block, using the same secret key.

Common block ciphers include:
DES with a 64-bit block size
AES with a 128-bit block size
RSA with a variable block size

# Stream ciphers

Stream ciphers encrypt plaintext one byte or one bit at a time.
• Think of it like a block cipher with a block size of one bit.
• The Vigenère cipher is an example of a stream cipher.
• Can be much faster than block ciphers, and generally do not increase the message size.

Common stream ciphers include:
A5 used to encrypt GSM cell phone communications.
RC4 cipher.
DES can also be used in stream cipher mode.