Changing RSA key

[Yony]

im trying to change the rsa key for my server..
i've saw somewhere link to rsakeygenerator so i tried to generate a key,
it asks me which key length to generate (i think its 309) and which encryption exponent to use. one more thing, this is the private key..
how do i generate from that the public key?

edit:
the code is encrypted with e,d,m.. but in the source the letters are p,q,d... is it ok?

one more thing, how do i compile Simone's code:

void makeKeys()
{ 

        #define BITSTRENGTH  1024 
 /* size of modulus (n) in bits */ 
   #define PRIMESIZE    (BITSTRENGTH / 2)  
   /* size of the primes p and q  */
mpz_t d,e,n;
 mpz_t M,c;
 mpz_init(d); 
 mpz_init(e); 
 mpz_init(n);  
 mpz_init(M);  
 mpz_init(c);   
   mpz_t p,q;  
   mpz_init(p); 
     mpz_init(q);  
char* p_str = new char[PRIMESIZE+1];    
char* q_str = new char[PRIMESIZE+1]; 
p_str[0] = '1';    q_str[0] = '1';   
for(int i=1;i<PRIMESIZE;i++)        
p_str[i] = (int)(2.0*rand()/(RAND_MAX+1.0)) + 48;
    for(int i=1;i<PRIMESIZE;i++) 
           q_str[i] = (int)(2.0*rand()/(RAND_MAX+1.0)) + 48;
               p_str[PRIMESIZE] = '\0';
                   q_str[PRIMESIZE] = '\0';
                       mpz_set_str(p,p_str,2);
                           mpz_set_str(q,q_str,2);
                               mpz_nextprime(p,p);
                                   mpz_nextprime(q,q);
                                       mpz_get_str(p_str,10,p);
                                           mpz_get_str(q_str,10,q); 
                                              printf("Random Prime 'p' = %s\n",p_str);
                                                  printf("Random Prime 'q' = %s\n",q_str); 
    /*    *  Step 2 : Calculate n (=pq) ie the 1024 bit modulus    *  and x (=(p-1)(q-1)).    */ 
       char n_str[1000];
           mpz_t x;
               mpz_init(x);
/* Calculate n... */ 
   mpz_mul(n,p,q);
mpz_get_str(n_str,10,n);
    printf("\nn = %s\n",n_str); 
       /* Calculate x... */ 
          mpz_t p_minus_1,q_minus_1;
              mpz_init(p_minus_1); 
                 mpz_init(q_minus_1); 
                    mpz_sub_ui(p_minus_1,p,(unsigned long int)1);
 mpz_sub_ui(q_minus_1,q,(unsigned long int)1);
     mpz_mul(x,p_minus_1,q_minus_1); 
        /*    *  Step 3 : Get small odd integer e such that gcd(e,x) = 1.    */ 
           mpz_t gcd;
               mpz_init(gcd);
                   /*    *  Assuming that 'e' will not exceed the range 
                      *  of a long integer, which is quite a reasonable   
                       *  assumption.    */
   unsigned long int e_int = 65537; 
      while(true)
          {    
                  mpz_gcd_ui(gcd,x,e_int); 
                   if(mpz_cmp_ui(gcd,(unsigned long int)1)==0) 
                              break; 
                                     /* try the next odd integer... */  
                                           e_int += 2; 
          }   
                                               mpz_set_ui(e,e_int);
                                                   /*    *  Step 4 : Calculate unique d such that ed = 1(mod x)    */  
                                                     char d_str[1000];
                                                         if(mpz_invert(d,e,x)==0) 
                                                            {    
                                                                     printf("\nOOPS : Could not find multiplicative inverse!\n"); 
                                     } 
                                        mpz_get_str(d_str,10,d);
                                      printf("\n\n"); 
                                         /*    *  Print the public and private key pairs...    */  
                                           printf("\nPublic Keys (e,n): \n\n"); 
                                              printf("\nValue of 'e' : %ld",e_int); 
                                                 printf("\nValue of 'n' : %s ",n_str); 
                                                    printf("\n\n");  
                                                      printf("\nPrivate Key : \n\n"); 
                                                         printf("\nValue of 'd' : %s",d_str);


}