Program to generate CAPTCHA and verify user

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.
Examples: 

CAPTCHA: x9Pm72se
Input: x9Pm62es
Output: CAPTCHA Not Matched

CAPTCHA: cF3yl9T4
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.

Algorithm:

1. First declare and define the checkCaptcha() function that takes two string parameters and returns a boolean value.
2. Within the checkCaptcha() function, compare the two string parameters using the compare() function and return true if they are the same; otherwise, return false.
3. Declare and define the generateCaptcha() function that takes an integer parameter and returns a string value.
4. Within the generateCaptcha() function, initialize a time variable using the time() function and seed the random number generator using the srand() function.
5. Declare a character array containing all the characters to be included in the CAPTCHA and assign it to a char pointer variable.
6. Generate a random CAPTCHA string of the specified length by repeatedly appending random characters from the character array to a string variable using the push_back() function.
7. Return the generated CAPTCHA string.
8. Within the main() function, declare a string variable named captcha and call the generateCaptcha() function with a length of 9 to generate a random CAPTCHA string.
9. Print the generated CAPTCHA string on the console. 
    

// C++ program to automatically generate CAPTCHA and
// verify user
#include <bits/stdc++.h>
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;
    srand((unsigned)time(&t));

    // Characters to be included
    char* chrs = "abcdefghijklmnopqrstuvwxyzABCDEFGHI"
                 "JKLMNOPQRSTUVWXYZ0123456789";

    // Generate n characters from above set and
    // add these characters to captcha.
    string captcha = "";
    while (n--)
        captcha.push_back(chrs[rand() % 62]);

    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");
    else
        printf("\nCAPTCHA Not Matched");

    return 0;
}

// Java pprogram to automatically generate CAPTCHA and
// verify user
import java.util.*;
import java.io.*;

class GFG
{
    
    // Returns true if given two strings are same
    static boolean checkCaptcha(String captcha, String user_captcha)
    {
        return captcha.equals(user_captcha);
    }
    
    // Generates a CAPTCHA of given length
    static String generateCaptcha(int n)
    {
        //to generate random integers in the range [0-61]
        Random rand = new Random(62); 
        
        // Characters to be included
        String chrs = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
     
        // Generate n characters from above set and
        // add these characters to captcha.
        String captcha = "";
        while (n-->0){
            int index = (int)(Math.random()*62);
            captcha+=chrs.charAt(index);
        }
          
        return captcha;
    }
    
    // Driver code
    public static void main(String[] args)throws IOException
    {
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
        
        // Generate a random CAPTCHA
        String captcha = generateCaptcha(9);
        System.out.println(captcha);
     
        // Ask user to enter a CAPTCHA
        System.out.println("Enter above CAPTCHA: ");
        String usr_captcha = reader.readLine();
     
        // Notify user about matching status
        if (checkCaptcha(captcha, usr_captcha))
            System.out.println("CAPTCHA Matched");
        else
            System.out.println("CAPTCHA Not Matched");
    }
}

// This code is contributed by shruti456rawal

# Python program to automatically generate CAPTCHA and
# verify user
import random

# Returns true if given two strings are same
def checkCaptcha(captcha, user_captcha):
    if captcha == user_captcha:
        return True
    return False

# Generates a CAPTCHA of given length
def generateCaptcha(n):
    
    # Characters to be included
    chrs = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
    
    # Generate n characters from above set and
    # add these characters to captcha.
    captcha = ""
    while (n):
        captcha += chrs[random.randint(1, 1000) % 62]
        n -= 1
    return captcha

# Driver code

# Generate a random CAPTCHA
captcha = generateCaptcha(9)
print(captcha)

# Ask user to enter a CAPTCHA
print("Enter above CAPTCHA:")
usr_captcha = input()

# Notify user about matching status
if (checkCaptcha(captcha, usr_captcha)):
    print("CAPTCHA Matched")
else:
    print("CAPTCHA Not Matched")

# This code is contributed by shubhamsingh10

using System;
using System.Text;

class GFG
{
  // Returns true if given two strings are same
  static bool CheckCaptcha(string captcha, string user_captcha)
  {
    return captcha.Equals(user_captcha);
  }

  // Generates a CAPTCHA of given length
  static string GenerateCaptcha(int n)
  {
    // to generate random integers in the range [0-61]
    Random rand = new Random();

    // Characters to be included
    string chrs = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";

    // Generate n characters from above set and
    // add these characters to captcha.
    StringBuilder captcha = new StringBuilder();
    while (n-- > 0)
    {
      int index = rand.Next(62);
      captcha.Append(chrs[index]);
    }

    return captcha.ToString();
  }

  // Driver code
  static void Main(string[] args)
  {
    // Generate a random CAPTCHA
    string captcha = GenerateCaptcha(9);
    Console.WriteLine(captcha);

    // Ask user to enter a CAPTCHA
    Console.WriteLine("Enter above CAPTCHA: ");
    string usr_captcha = Console.ReadLine();

    // Notify user about matching status
    if (CheckCaptcha(captcha, usr_captcha))
      Console.WriteLine("CAPTCHA Matched");
    else
      Console.WriteLine("CAPTCHA Not Matched");
  }
}

// JavaScript program to automatically generate CAPTCHA and
// verify user

// Returns true if given two strings are same
function checkCaptcha(captcha, user_captcha) {
return captcha.localeCompare(user_captcha) == 0;
}

// Generates a CAPTCHA of given length
function generateCaptcha(n) {
// Characters to be included
const chrs = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
let captcha = "";
for (let i = 0; i < n; i++) {
captcha += chrs[(Math.floor(Math.random() * chrs.length))];
}
return captcha;
}

// Driver code
function main() {
// Generate a random CAPTCHA
const captcha = generateCaptcha(9);
console.log(captcha);

// Ask user to enter a CAPTCHA
const usr_captcha = prompt("Enter above CAPTCHA:");

// Notify user about matching status
if (checkCaptcha(captcha, usr_captcha))
console.log("CAPTCHA Matched");
else
console.log("CAPTCHA Not Matched");
}

main();

Output: 

CAPTCHA: cF3yl9T4
Enter CAPTCHA: cF3yl9T4
CAPTCHA Matched

Time Complexity: O(n)
Space Complexity: O(1)