::Stream ciphers

- a symmetric key cipher where plaintext bits are combined with a pseudorandom cipher bit stream (keystream).

- typically by an exclusive-or (xor) operation.

-In a stream cipher the plaintext digits are encrypted one at a time, and the transformation of successive digits varies during the encryption. An alternative name is a state cipher, as the encryption of each digit is dependent on the current state

::Block ciphers

- a block cipher is a symmetric key cipher operating on fixed-length groups of bits, termed blocks, with an unvarying transformation.

- A block cipher encryption algorithm might take (for example) a 128-bit block of plaintext as input, and output a corresponding 128-bit block of ciphertext.

::Data Encryption Standards (DES)

-The plaintext is divided into 64 bit blocks with a key of 56 bits(with 8 bit parity)

-through 16 round of Expansion, substitution, key mixing and permutation process

-Triple DES has been introduced