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Program to Implement the RSA Algorithm

Program to Implement the RSA Algorithm

This C++ program encodes any message using RSA Algorithm. Input is case sensitive and works only for all characters. RSA is one of the first practicable public-key cryptosystems and is widely used for secure data transmission. In such a cryptosystem, the encryption key is public and differs from the decryption key which is kept secret. In RSA, this asymmetry is based on the practical difficulty of factoring the product of two large prime numbers, the factoring problem. RSA stands for Ron Rivest, Adi Shamir and Leonard Adleman.

    #include<iostream>

    #include<math.h>

    #include<string.h>

    #include<stdlib.h>

    using namespace std;

    long int p, q, n, t, flag, e[100], d[100], temp[100], j, m[100], en[100], i;

    char msg[100];

    int prime(long int);

    void ce();

    long int cd(long int);

    void encrypt();

    void decrypt();

    int prime(long int pr)

    {

        int i;

        j = sqrt(pr);

        for (i = 2; i <= j; i++)

        {

            if (pr % i == 0)

                return 0;

        }

        return 1;

    }

    int main()

    {

        cout << "\nenter first prime number\n";

        cin >> p;

        flag = prime(p);

        if (flag == 0)

        {

            cout << "\nwrong input\n";

            exit(1);

        }

        cout << "\nenter another prime number\n";

        cin >> q;

        flag = prime(q);

        if (flag == 0 || p == q)

        {

            cout << "\nwrong input\n";

            exit(1);

        }

        cout << "\nenter message\n";

        fflush(stdin);

        cin >> msg;

        for (i = 0; msg[i] != NULL; i++)

            m[i] = msg[i];

        n = p * q;

        t = (p - 1) * (q - 1);

        ce();

        cout << "\npossible values of e and d are\n";

        for (i = 0; i < j - 1; i++)

            cout << e[i] << "\t" << d[i] << "\n";

        encrypt();

        decrypt();

        return 0;

    }

    void ce()

    {

        int k;

        k = 0;

        for (i = 2; i < t; i++)

        {

            if (t % i == 0)

                continue;

            flag = prime(i);

            if (flag == 1 && i != p && i != q)

            {

                e[k] = i;

                flag = cd(e[k]);

                if (flag > 0)

                {

                    d[k] = flag;

                    k++;

                }

                if (k == 99)

                    break;

            }

        }

    }

    long int cd(long int x)

    {

        long int k = 1;

        while (1)

        {

            k = k + t;

            if (k % x == 0)

                return (k / x);

        }

    }

    void encrypt()

    {

        long int pt, ct, key = e[0], k, len;

        i = 0;

        len = strlen(msg);

        while (i != len)

        {

            pt = m[i];

            pt = pt - 96;

            k = 1;

            for (j = 0; j < key; j++)

            {

                k = k * pt;

                k = k % n;

            }

            temp[i] = k;

            ct = k + 96;

            en[i] = ct;

            i++;

        }

        en[i] = -1;

        cout << "\nthe encrypted message is\n";

        for (i = 0; en[i] != -1; i++)

            printf("%c", en[i]);

    }

    void decrypt()

    {

        long int pt, ct, key = d[0], k;

        i = 0;

        while (en[i] != -1)

        {

            ct = temp[i];

            k = 1;

            for (j = 0; j < key; j++)

            {

                k = k * ct;

                k = k % n;

            }

            pt = k + 96;

            m[i] = pt;

            i++;

        }

        m[i] = -1;

        cout << "\nthe decrypted message is\n";

        for (i = 0; m[i] != -1; i++)

            printf("%c", m[i]);

    }