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How does DES works in Information Security?
DES is a block cipher. The encryption procedure is create of two permutations (Pboxes) that can be defined initial and final permutations, and 16 Feistel rounds. Each round need a different 48-bit round key produced from the cipher key in accordance with a predefined algorithm.
DES is based on the two essential attributes of cryptography including substitution (also known as confusion) and transposition (also known as diffusion). DES includes 16 steps, each of which is known as round.
Each round implements the steps of substitution and transposition. DES is a symmetric cryptosystem. A block is be enciphered is inflicted to an initial permutation IP and thus to a complex key-dependant computation and lastly to a permutation which is the inverse of the initial permutation IP−1.
Let us discuss the following steps in DES.
Initial Permutation (IP) − An initial permutation is required only once at the starting of the encryption process. In DES, after the complete plaintext is divided into blocks of 64 bits each, IP is required on each of them. This initial permutation is a step in the transposition procedure.
Rounds − DES uses 16 rounds. Each round of DES is a Feistel cipher. The round takes LI−1and RI−1from previous round and produce LI and RI, which go to the next round. It can consider that each round has two cipher component (mixer and swapper).
Each of these component is invertible. The swapper is certainly invertible. It can swaps the left half of the text with the right half. The mixer is invertible through the XOR operation. All non-invertible component are collected within the function f(RI−1, KI).
Final Permutation (FP) − At the end of the 16th rounds, the final permutation is implemented. This is a simple transposition is depend on the table. For example, the 40th input takes the position of the 1st output bit, etc. The output of the final permutation is the 64-bit encrypted block.
| 40 | 8 | 48 | 16 | 56 | 24 | 64 | 32 | 39 | 7 | 47 | 15 | 55 | 23 | 63 | 31 |
| 38 | 6 | 46 | 14 | 54 | 22 | 62 | 30 | 37 | 5 | 45 | 13 | 53 | 21 | 61 | 29 |
| 36 | 4 | 44 | 12 | 52 | 20 | 60 | 28 | 35 | 3 | 43 | 11 | 51 | 19 | 59 | 27 |
| 34 | 2 | 42 | 10 | 50 | 18 | 58 | 26 | 33 | 1 | 41 | 9 | 49 | 17 | 57 | 25 |
DES Decryption − It is an extremely complex encryption scheme and thus the decryption using DES can employ an ultimately different approach. The similar algorithm is used for encryption in DES also works for decryption.
The values of the multiple tables and the operations as well as their sequence are so carefully selected that the algorithm is reversible. The only difference between the encryption and the decryption procedure is the reversal of key portions.
If the original key K was divided into K1, K2, K3….K16 for the 16 encryption rounds, therefore for decryption, the key must be used as K16, K15, and K14…k1.
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