added code for RSA in C

RSA_Cipher/Simple_RSA_Cipher.c

@@ -0,0 +1,159 @@
+//C program for encryption and decryption
+#include<stdio.h>
+#include<stdlib.h>
+#include<math.h>
+#include<string.h>
+
+int x, y, n, t, i, flag;
+long int e[50], d[50], temp[50], j;
+char en[50], m[50];
+char msg[100];
+int prime(long int); 
+void encryption_key();
+long int cd(long int);
+void encrypt();
+void decrypt();
+
+int main()
+{
+   printf("\nENTER FIRST PRIME NUMBER\n");
+   scanf("%d",&x);
+
+   //checking if prime or not
+   flag = prime(x);
+   if(flag == 0)
+   {
+      printf("INVALID INPUT\n");
+      exit(0);
+   }
+
+   printf("\nENTER SECOND PRIME NUMBER\n");
+   scanf("%d",&y);
+
+   flag = prime(y);
+   if(flag == 0 || x == y)
+   {
+      printf("INVALID INPUT\n");
+      exit(0);
+   }
+
+   printf("\nENTER MESSAGE TO ENCRYPT\n");
+   scanf("%s",msg);
+
+   for(i = 0; msg[i] != 0; i++)
+      m[i] = msg[i];
+      n = x * y;
+      t = (x - 1) * (y - 1);
+
+   encryption_key();
+   printf("\nCOMBINATION VALUES OF e AND d ARE\n");
+
+   for(i = 0; i < j - 1; i++)
+      printf("\n%ld\t%ld",e[i],d[i]);
+
+   encrypt();
+   decrypt();
+   return 0;
+} //end of the main program
+
+int prime(long int pr)
+{
+   int i;
+   j = sqrt(pr);
+   for(i = 2; i <= j; i++)
+   {
+      if(pr % i == 0)
+         return 0;
+   }
+   return 1;
+ }
+
+//function to generate encryption key
+void encryption_key()
+{
+   int k;
+   k = 0;
+   for(i = 2; i < t; i++)
+   {
+      if(t % i == 0)
+         continue;
+      flag = prime(i);
+      if(flag == 1 && i != x && i != y)
+      {
+         e[k] = i;
+         flag = cd(e[k]);
+         if(flag > 0)
+         {
+            d[k] = flag;
+            k++;
+         }
+         if(k == 99)
+         break;
+      }
+   }
+}
+
+long int cd(long int a)
+{
+   long int k = 1;
+   while(1)
+   {
+      k = k + t;
+      if(k % a == 0)
+         return(k/a);
+   }
+}
+
+//function to encrypt the message
+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;
+   printf("\n\nTHE ENCRYPTED MESSAGE IS\n");
+   for(i=0; en[i] != -1; i++)
+      printf("%c",en[i]);
+}
+
+//function to decrypt the message
+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;
+   printf("\n\nTHE DECRYPTED MESSAGE IS\n");
+   for(i = 0; m[i] != -1; i++)
+      printf("%c",m[i]);
+
+   printf("\n");
+}