C++ Program to generate CAPTCHA and verify user


What is CAPTCHA?

A CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) is a test to determine whether the user is human or not.

So, the task is to generate unique CAPTCHA every time and to tell whether the user is human or not by asking user to enter the same CAPTCHA as generated automatically and checking the user input with the generated CAPTCHA.

So before writing a c++ program to generate CAPTCHA we must understand with the help of example.


CAPTCHA: x9Pm72se
Input: x9Pm62es
Output: CAPTCHA Not Matched

Input: cF3yl9T4
Output: CAPTCHA Matched

The set of characters to generate CAPTCHA are stored in a character array chrs[] which contains (a-z, A-Z, 0-9), therefore size of chrs[] is 62.

To generate a unique CAPTCHA every time, a random number is generated using rand() function (rand()%62) which generates a random number between 0 to 61 and the generated random number is taken as index to the character array chrs[] thus generates a new character of captcha[] and this loop runs n (length of CAPTCHA) times to generate CAPTCHA of given length.

C++ program to generate CAPTCHA and verify user:


// C++ program to automatically generate CAPTCHA and
// verify user
using namespace std;

// Returns true if given two strings are same
bool checkCaptcha(string &captcha, string &user_captcha)
    return captcha.compare(user_captcha) == 0;

// Generates a CAPTCHA of given length
string generateCaptcha(int n)
    time_t t;

    // Characters to be included
    char *chrs = "abcdefghijklmnopqrstuvwxyzABCDEFGHI"

    // Generate n characters from above set and
    // add these characters to captcha.
    string captcha = "";
    while (n--)

    return captcha;

// Driver code
int main()
    // Generate a random CAPTCHA
    string captcha = generateCaptcha(9);
    cout << captcha;

    // Ask user to enter a CAPTCHA
    string usr_captcha;
    cout << "nEnter above CAPTCHA: ";
    cin >> usr_captcha;

    // Notify user about matching status
    if (checkCaptcha(captcha, usr_captcha))
        printf("nCAPTCHA Matched");
        printf("nCAPTCHA Not Matched");

    return 0;


Enter CAPTCHA: cF3yl9T4


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