The Data Encryption Standard is a block cipher operating on 64-bit data blocks. DES was designed by IBM and adopted by the U.S. government as the standard encryption method for nonmilitary and nonclassified use. The algorithm encrypts a 64-bit plaintext block using a 64- bit key, as shown in Figure

DES has two transposition blocks (P-boxes) and 16 complex round ciphers (they are repeated). Although the 16 iteration round ciphers are conceptually the same, each uses a different key derived from the original key. The initial and final permutations are keyless straight permutations that are the inverse of each other. The permutation takes a 64-bit input and permutes them according to predefined values. Each round of DES is a complex round cipher, as shown in the Figure below. Note that the structure of the encryption round ciphers is different from that of the decryption one.

Asymmetric Key Cryptography:

Some examples of public-key cryptosystems are :

Elgamal (named for its inventor, Taher Elgamal),

RSA (named for its inventors, Ron Rivest, Adi Shamir, and Leonard Adleman),

Diffie-Hellman (named for its inventors),

DSA ,the Digital Signature Algorithm (invented by David Kravitz).