implantation of RSA algorithm in c++ [closed] - c++

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I am working on a project to implement RSA algorithm in c++ and i have no idea about c++ before but I'm still learning,
My question in RSA is how to encode characters to numbers from 0-25:
a encode to 0,
b to 1,
c to 2,
.
.
z to 25,
here is my code:
/*
* C++ Program to Implement the RSA Algorithm
*/
#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]);
}

Answer 1

Not sure I get your question right but my bet is you're asking how to convert between ASCII and number representation of characters.
Encoding/Decoding has nothing to do with RSA. You just use dynamic range shift. As the alphabet is in increasing order in ASCII so the only thing left is to offset the a or A to zero.
for lowercase char to number conversion try:
char c='m'; // c is you character m for example
int i=c-`a`; // i is output number
if you got both lowercase and uppercase letters then you need to change it to :
char c='q'; // c is you character q for example
int i; // i is output number
if ((c>='a')&&(c<='z')) i=c-'a';
else i=c-'A';`
where c is your character and i is output number
For number to char conversion try:
c=i+`a`;
or for uppercase:
c=i+`A`;