Explain the purpose, nature, function and operations of Symmetric Encryption Principles. What are the essential ingredients of a symmetric cipher? Differentiate between encryption and decryption.

Symmetric encryption is a class of algorithms for cryptography that use the same cryptographic keys for both encryption of plaintext and decryption of ciphertext. The keys may be identical or there may be a simple transformation to go between the two keys. The keys, in practice, represent a shared secret between two or more parties that can be used to maintain a private information link.

Encryption methods can be extremely efficient, requiring minimal processing, and Both sender and receiver must possess encryption key. If either copy of key is compromised, an intermediate can decrypt and read messages

A symmetric encryption has two components a block cipher and stream cipher. (i) A block cipher operates on groups of bits – typically groups of 64. If the final block of the plaintext message is shorter than 64 bits, it is padded with some regular pattern of 1s and 0s to make a complete block. Block ciphers encrypt each block independently, so the plaintext does not have to be processed in a sequential manner. (ii) A stream cipher generally operates on one bit of plaintext at a time, although some stream ciphers operate on bytes. A component called a keystream generator generates a sequence of bits, usually known as a keystream. In the simplest form of stream cipher, a modulo-2 adder (exclusive-OR or XOR gate) combines each bit in the plaintext with each bit in the keystream to produce the ciphertext. At the receiving end, another modulo-2 adder combines the ciphertext with the keystream to recover the plaintext.

A symmetric encryption has 5 ingredients, which are the following:

1.                 Plaintext: this is the original message or data that is fed into the algorithm as input.

2.                 Encryption algorithm: this algorithm performs various substitutions and transformations on the plaintext.

3.                 Secret Key: is also input to algorithm. The exact substitutions and transformations performed by the algorithm depend on the key.

4.                 Ciphertext: this is the scrambled message produced as output. It depends on the plaintext and secret key.

5.                 Decryption algorithm: this is essentially the encryption algorithm run in reverse. It takes the ciphertext and the same secret key and produces the original plaintext.