Cryptography is the science of transforming messages to make them secure and immune to attack. The literal meaning of cryptography is “secret writing”. If the original message is sent through a network, then any hacker can get access and change its meaning. To ensure the security of the message, the original message is transformed to ciphertext using an encryption algorithm by the sender. And the receiver uses a decryption algorithm to transform the ciphertext back into plaintext.

Encryption and decryption algorithms are called ciphers. And those algorithms operate on a set of numbers called Key. To encrypt a message, we need an encryption algorithm, encryption key and the plain text. These create the ciphertext. Similarly to decrypt a message, we need a decryption algorithm, decryption key and the ciphertext. These reveal the plaintext. In cryptography three characters (Alice, Bob, and Eve) are very popular. Alice is the person who sends message and Bob is the recipient. Eve disturbs this communication by intercepting message to uncover the data. There are 2 types of ciphers (cryptography algorithms) such as.

1-    Symmetric-key or Secret key Cryptography
2-    Asymmetric-key or Public key Cryptography #### 1) Symmetric-Key Cryptography

In Symmetric-Key Cryptography, the same key is used by both sender and receiver. So the key must be known to both parties. The traditional ciphers are substitution cipher and transposition cipher. A substitution cipher substitutes one symbol with another. For example, we can replace character B with G and F with X. In a transposition cipher, location of a character is changed in the ciphertext. Let’s discuss some important symmetric ciphers.

DES (Data Encryption Standard) - DES is a symmetric-key block cipher designed by IBM. A block cipher divides the plaintext into blocks and uses the same key to encrypt and decrypt the blocks. DES encrypts a 64-bit plaintext block using a 64-bit key. It is widely used in military, aerospace and intelligence institutions because of its fast operation and security.

Triple DES – It is advanced from DES since three keys are used in succession to encrypt a message. It has also increased the key size to 112 bits.

– AES is a very complex round cipher with three different key sizes: 128, 192, or 156 bits.

IDEA (International Data Encryption Algorithm) – It was developed by Xuejia Lai and james Massey. The block size is 64 bits and key size is 128 bits.

Blowfish – It was developed by Bruce Schneier . The block size is 64 and key size between 32 and 448.

CAST-128 – It was developed by Carlisle Adams and Stafford Tavares. The block size is 64 bits and key size is 128 bits.

RC5 – RC5 was designed by Ron Rivest. It has different block size and key sizes.

#### 2) Asymmetric-Key Cryptography

This uses two keys: a private key and a public key. Public key is used to encrypt to message whereas private key is used to decrypt. The public encryption key is made available to whoever wants to use it, but the private key is kept secret by the key owner. The process is explained below:

-    If A wants to send a message to B, the message is encrypted by A using B’s public key.
-    If B receives the message, the message is decrypted by using B’s private key. No other recipient can decrypt the message.

RSA – It is most commonly used public key algorithm. It is named by its inventors name Rivest, Shamir, and Adelman (RSA). It uses two numbers as the public and private keys. RSA is useful for short messages and also used in digital signatures. But it is very slow if the message size becomes long.