Data Encryption Standard is a 56-bit Block Ccipher that makes use of the same key to decrypt or decrypt information. The underlying mechanism involves a number of permutations and shifting of binary data into encrypted format as well as mixing and substituting. The procedure is repeated 16 times for each round in order to confuse the relationship between plaintext and the encrypted ciphertext that results.
DES is difficult to break. Each round involves key-mixing, a XOR with a subkey of 48 bits, expansion permutation and S-box-permutation. The security of DES is heavily dependent on the latter. The 32-bit half-block can be divided into eight 6 bit parts and then processed by a set of eight nonlinear substitutions called S-boxes. These are then tabulated in a function called f. Each S-box replaces one of the input bits with four output bits which makes the cipher more difficult to break.
After the S-box procedure the expanded right plain text (RPT) is merged with the left plain text (LPT) to create a final ciphertext block of 64-bits. This is the initial of a series XOR operations that utilize a 48-bit ciphertext key that will serve as the basis for a XOR operation utilizing the original key.
The XOR operation that uses the 48-bit key obscures as much as is possible the original connection between plaintext and the ciphertext. This is the reason why DES is so secure. The encryption algorithm also has options, like a counter which increments with every new ciphertext, but they aren’t essential in securing the data.