What do you mean by Data Encryption Standard

The data encryption standard (DES) is a common standard for data encryption and a form of secret key cryptography (SKC), which uses only one key for encryption and decryption. Public key cryptography (PKC) uses two keys, i.e., one for encryption and one for decryption.

In 1972, the National Bureau of Standards (NBS) approached the Institute for Computer Sciences and Technology (ICST) to devise an encryption algorithm to secure stored and transmitted data. The algorithm would be publicly available, but its key would be top secret. The National Security Agency (NSA) assisted with the cryptographic algorithm evaluation processes, and in 1973, submission invitations were posted in the Federal Register. However, the submissions were unacceptable. In 1974, a second invitation was posted, which resulted in a submission from IBM. In 1975, technical specifications were published for comments in the Federal Register, and analysis and review commenced. In 1977, NBS issued the algorithm, i.e., DES, as Federal Information Processing Standards (FIPS) 46. Shortly thereafter, the U.S. Department of Defense (DoD) implemented DES. Specifications are outlined in FIPS publication 46-3, FIPS 81, ANSI X3.92 and ANSI X3.106. For security reasons, the U.S. government has never authorized exports of this encryption software. There are at least 72 quadrillion DES key possibilities. In 1993, NIST recertified DES, and the Advanced Encryption Standard (AES) became its unofficial replacement.

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#include <bits/stdc++.h>

using namespace std;

string hex2bin(string s)

{

    unordered_map<char, string> mp;

    mp['0'] = "0000";

    mp['1'] = "0001";

    mp['2'] = "0010";

    mp['3'] = "0011";

    mp['4'] = "0100";

    mp['5'] = "0101";

    mp['6'] = "0110";

    mp['7'] = "0111";

    mp['8'] = "1000";

    mp['9'] = "1001";

    mp['A'] = "1010";

    mp['B'] = "1011";

    mp['C'] = "1100";

    mp['D'] = "1101";

    mp['E'] = "1110";

    mp['F'] = "1111";

    string bin = "";

    for (int i = 0; i < s.size(); i++) {

        bin += mp[s[i]];

    }

    return bin;

}

string bin2hex(string s)

{

    unordered_map<string, string> mp;

    mp["0000"] = "0";

    mp["0001"] = "1";

    mp["0010"] = "2";

    mp["0011"] = "3";

    mp["0100"] = "4";

    mp["0101"] = "5";

    mp["0110"] = "6";

    mp["0111"] = "7";

    mp["1000"] = "8";

    mp["1001"] = "9";

    mp["1010"] = "A";

    mp["1011"] = "B";

    mp["1100"] = "C";

    mp["1101"] = "D";

    mp["1110"] = "E";

    mp["1111"] = "F";

    string hex = "";

    for (int i = 0; i < s.length(); i += 4) {

        string ch = "";

        ch += s[i];

        ch += s[i + 1];

        ch += s[i + 2];

        ch += s[i + 3];

        hex += mp[ch];

    }

    return hex;

}

string permute(string k, int* arr, int n)

{

    string per = "";

    for (int i = 0; i < n; i++) {

        per += k[arr[i] - 1];

    }

    return per;

}

string shift_left(string k, int shifts)

{

    string s = "";

    for (int i = 0; i < shifts; i++) {

        for (int j = 1; j < 28; j++) {

            s += k[j];

        }

        s += k[0];

        k = s;

        s = "";

    }

    return k;

}

string xor_(string a, string b)

{

    string ans = "";

    for (int i = 0; i < a.size(); i++) {

        if (a[i] == b[i]) {

            ans += "0";

        }

        else {

            ans += "1";

        }

    }

    return ans;

}

string encrypt(string pt, vector<string> rkb,

               vector<string> rk)

{

    pt = hex2bin(pt);

    int initial_perm[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 };

    pt = permute(pt, initial_perm, 64);

    cout << "After initial permutation: " << bin2hex(pt)

         << endl;

    string left = pt.substr(0, 32);

    string right = pt.substr(32, 32);

    cout << "After splitting: L0=" << bin2hex(left)

         << " R0=" << bin2hex(right) << endl;

    int exp_d[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 };

    int s[8][4][16] = {

        { 14, 4,  13, 1, 2,  15, 11, 8,  3,  10, 6,  12, 5,

          9,  0,  7,  0, 15, 7,  4,  14, 2,  13, 1,  10, 6,

          12, 11, 9,  5, 3,  8,  4,  1,  14, 8,  13, 6,  2,

          11, 15, 12, 9, 7,  3,  10, 5,  0,  15, 12, 8,  2,

          4,  9,  1,  7, 5,  11, 3,  14, 10, 0,  6,  13 },

        { 15, 1,  8,  14, 6,  11, 3, 4,  9,  7,  2,  13, 12,

          0,  5,  10, 3,  13, 4,  7, 15, 2,  8,  14, 12, 0,

          1,  10, 6,  9,  11, 5,  0, 14, 7,  11, 10, 4,  13,

          1,  5,  8,  12, 6,  9,  3, 2,  15, 13, 8,  10, 1,

          3,  15, 4,  2,  11, 6,  7, 12, 0,  5,  14, 9 },

        { 10, 0,  9,  14, 6,  3,  15, 5,  1,  13, 12,

          7,  11, 4,  2,  8,  13, 7,  0,  9,  3,  4,

          6,  10, 2,  8,  5,  14, 12, 11, 15, 1,  13,

          6,  4,  9,  8,  15, 3,  0,  11, 1,  2,  12,

          5,  10, 14, 7,  1,  10, 13, 0,  6,  9,  8,

          7,  4,  15, 14, 3,  11, 5,  2,  12 },

        { 7,  13, 14, 3,  0,  6,  9,  10, 1,  2, 8,  5,  11,

          12, 4,  15, 13, 8,  11, 5,  6,  15, 0, 3,  4,  7,

          2,  12, 1,  10, 14, 9,  10, 6,  9,  0, 12, 11, 7,

          13, 15, 1,  3,  14, 5,  2,  8,  4,  3, 15, 0,  6,

          10, 1,  13, 8,  9,  4,  5,  11, 12, 7, 2,  14 },

        { 2,  12, 4, 1,  7,  10, 11, 6, 8,  5,  3,  15, 13,

          0,  14, 9, 14, 11, 2,  12, 4, 7,  13, 1,  5,  0,

          15, 10, 3, 9,  8,  6,  4,  2, 1,  11, 10, 13, 7,

          8,  15, 9, 12, 5,  6,  3,  0, 14, 11, 8,  12, 7,

          1,  14, 2, 13, 6,  15, 0,  9, 10, 4,  5,  3 },

        { 12, 1,  10, 15, 9,  2,  6,  8,  0,  13, 3, 4, 14,

          7,  5,  11, 10, 15, 4,  2,  7,  12, 9,  5, 6, 1,

          13, 14, 0,  11, 3,  8,  9,  14, 15, 5,  2, 8, 12,

          3,  7,  0,  4,  10, 1,  13, 11, 6,  4,  3, 2, 12,

          9,  5,  15, 10, 11, 14, 1,  7,  6,  0,  8, 13 },

        { 4,  11, 2,  14, 15, 0,  8, 13, 3,  12, 9,  7,  5,

          10, 6,  1,  13, 0,  11, 7, 4,  9,  1,  10, 14, 3,

          5,  12, 2,  15, 8,  6,  1, 4,  11, 13, 12, 3,  7,

          14, 10, 15, 6,  8,  0,  5, 9,  2,  6,  11, 13, 8,

          1,  4,  10, 7,  9,  5,  0, 15, 14, 2,  3,  12 },

        { 13, 2,  8, 4,  6,  15, 11, 1,  10, 9, 3, 14, 5,

          0,  12, 7, 1,  15, 13, 8,  10, 3,  7, 4, 12, 5,

          6,  11, 0, 14, 9,  2,  7,  11, 4,  1, 9, 12, 14,

          2,  0,  6, 10, 13, 15, 3,  5,  8,  2, 1, 14, 7,

          4,  10, 8, 13, 15, 12, 9,  0,  3,  5, 6, 11 }

    };

    int per[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 };

    cout << endl;

    for (int i = 0; i < 16; i++) {

        string right_expanded = permute(right, exp_d, 48);

        string x = xor_(rkb[i], right_expanded);

        string op = "";

        for (int i = 0; i < 8; i++) {

            int row = 2 * int(x[i * 6] - '0')

                      + int(x[i * 6 + 5] - '0');

            int col = 8 * int(x[i * 6 + 1] - '0')

                      + 4 * int(x[i * 6 + 2] - '0')

                      + 2 * int(x[i * 6 + 3] - '0')

                      + int(x[i * 6 + 4] - '0');

            int val = s[i][row][col];

            op += char(val / 8 + '0');

            val = val % 8;

            op += char(val / 4 + '0');

            val = val % 4;

            op += char(val / 2 + '0');

            val = val % 2;

            op += char(val + '0');

        }

        op = permute(op, per, 32);

        x = xor_(op, left);

        left = x;

        if (i != 15) {

            swap(left, right);

        }

        cout << "Round " << i + 1 << " " << bin2hex(left)

             << " " << bin2hex(right) << " " << rk[i]

             << endl;

    }

    string combine = left + right;

    int final_perm[64]

        = { 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 };

    string cipher

        = bin2hex(permute(combine, final_perm, 64));

    return cipher;

}

int main()

{

    string pt, key;

    pt = "123456ABCD132536";

    key = "AABB09182736CCDD";

    key = hex2bin(key);

    int keyp[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 };

    key = permute(key, keyp, 56);

    int shift_table[16] = { 1, 1, 2, 2, 2, 2, 2, 2,

                            1, 2, 2, 2, 2, 2, 2, 1 };

    int key_comp[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 };

    string left = key.substr(0, 28);

    string right = key.substr(28, 28);

    vector<string> rkb;

    vector<string> rk;

    for (int i = 0; i < 16; i++) {

        left = shift_left(left, shift_table[i]);

        right = shift_left(right, shift_table[i]);

        string combine = left + right;

        string RoundKey = permute(combine, key_comp, 48);

        rkb.push_back(RoundKey);

        rk.push_back(bin2hex(RoundKey));

    }

    cout << "\nEncryption:\n\n";

    string cipher = encrypt(pt, rkb, rk);

    cout << "\nCipher Text: " << cipher << endl;

    cout << "\nDecryption\n\n";

    reverse(rkb.begin(), rkb.end());

    reverse(rk.begin(), rk.end());

    string text = encrypt(cipher, rkb, rk);

    cout << "\nPlain Text: " << text << endl;

}