Subject: Mobile Computing

Difficulty: Medium

Marks: 4 Marks

An encryption system in which the sender and receiver of a message share a single, common key that is used to encrypt and decrypt the message. Symmetric-key systems are simpler and faster, but their main drawback is that the two parties must somehow exchange the key in a secure way. Public-key encryption avoids this problem because the public key can be distributed in a non-secure way, and the private key is never transmitted. Symmetric-key cryptography is sometimes called secret-key cryptography. The most popular symmetric-key system is the Data Encryption Standard (DES).

The Key components of symmetric key Cryptography

1. Plaintext: The original data or text is called plaintext.

2. Ciphertext: The original message changed to another unreadable format using some algorithm is called Ciphertext.

3. Key: Key is a number on which algorithm is based, like the Caesar cipher uses key no 3.

4. Encryption algorithm: This algorithm is required at sender‘s side for changing the original message (Plaintext) to unreadable format (Ciphertext) to protect the data from other non- valid receivers.

5. Decryption algorithm: Required at receiver‘s side for retrieving the original message that is to change the ciphertext to plaintext.

6. Hashed message Authentication code: In this case the copy of the key is added along with data and combination is hashed using the key less hash function such as SHA 1.Result of this Is HMAC which is then again prep ended with that same key and result is again hashed using that algorithm. At receiver side the receiver creates its own HMAC and compares it with delivered to validate and check for authentication.

7. Digital signature: Like in case of banks when user signs a cheque, they check your signature for authentication to see that the user is valid. To understand the concept of Digital signature, let us take an example there are two users A and B.A send message to B and B checks that the message came from A not anyone else. B can ask A to sign the message so that it can be proved that A is the actual sender and B verifies the authenticity. This is known as digital signature.

For implementing cryptography we need encryption algorithm for changing plaintext to ciphertext and decryption algorithm at receiver‘s side for changing Ciphertext to plaintext.