What is the DES algorithm?
Data Encryption Standard (DES) is a block cipher algorithm that takes plain text in blocks of 64 bits and converts them to ciphertext using keys of 48 bits. It is a symmetric key algorithm, which means that the same key is used for encrypting and decrypting data.
Steps for generating keys
There are 16 rounds of encryption in the algorithm, and a different key is used for each round. How keys are generated is listed below.
Bits are labeled from 1 to 64 starting from the most significant bit and going to the least significant bit.
// The array elements denote the bit numbersint pc1[56] = {57,49,41,33,25,17,9,1,58,50,42,34,26,18,10,2,59,51,43,35,27,19,11,3,60,52,44,36,63,55,47,39,31,23,15,7,62,54,46,38,30,22,14,6,61,53,45,37,29,21,13,5,28,20,12,4};
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Divide the result into two equal parts: C and D.
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C and D are left-shifted circularly. For encryption rounds 1, 2, 9, and 16 they are left shifted circularly by 1 bit; for all of the other rounds, they are left-circularly shifted by 2.
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The result is compressed to 48 bits in accordance with the following rule:
int pc2[48] = {14,17,11,24,1,5,3,28,15,6,21,10,23,19,12,4,26,8,16,7,27,20,13,2,41,52,31,37,47,55,30,40,51,45,33,48,44,49,39,56,34,53,46,42,50,36,29,32};
- The result of step 3 is the input for the next round of key generation.
Steps for encryption
- Transpose the bits in the 64-block according to the following:
// 58 means that the 58th bit should be considered// the first bit, 50th bit the second bit and so on.int initial_permutation_table[64] = {58,50,42,34,26,18,10,2,60,52,44,36,28,20,12,4,62,54,46,38,30,22,14,6,64,56,48,40,32,24,16,8,57,49,41,33,25,17,9,1,59,51,43,35,27,19,11,3,61,53,45,37,29,21,13,5,63,55,47,39,31,23,15,7};
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Divide the result into equal parts: left plain text (1-32 bits) and right plain text (33-64 bits)
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The resulting parts undergo 16 rounds of encryption in each round.
The right plain text is expanded using the following expansion table:
// The array elements denote the bit numbersint expansion_table[48] = {32,1,2,3,4,5,4,5,6,7,8,9,8,9,10,11,12,13,12,13,14,15,16,17,16,17,18,19,20,21,20,21,22,23,24,25,24,25,26,27,28,29,28,29,30,31,32,1};
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The expanded right plain text now consists of 48 bits and is XORed with the 48-bit key.
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The result of the previous step is divided into 8 boxes. Each box contains 6 bits. After going through the eight substitution boxes, each box is reduced from 6 bits to 4 bits. The first and last bit of each box provides the row index, and the remaining bits provide the column index. These indices are used to look-up values in a substitution box. A substitution box has 4 rows, 16 columns, and contains numbers from 0 to 15.
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The result is transposed in accordance with the following rule:
// The array elements denote the bit numbersint permutation_table[32] = {16,7,20,21,29,12,28,17,1,15,23,26,5,18,31,10,2,8,24,14,32,27,3,9,19,13,30,6,22,11,4,25};
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XOR the left half with the result from the above step. Store this in the right plain text.
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Store the initial right plain text in the left plain text.
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These halves are inputs for the next round. Remember that there are different keys for each round.
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After the 16 rounds of encryption, swap the left plain text and the right plain text.
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Finally, apply the inverse permutation (inverse of the initial permutation), and the ciphertext will be generated.
Steps for decryption
The order of the 16 48-bit keys is reversed such that key 16 becomes key 1, and so on. Then, the steps for encryption are applied to the ciphertext.
