Program to Generate CAPTCHA and Verify User Using Java

CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) is a security mechanism widely used to differentiate between human users and automated bots. It presents a challenge to users in the form of distorted text or images that must be deciphered correctly. In this section, we will explore how to generate unique CAPTCHAs and verify user humanity in Java.

Generating Unique CAPTCHAs

To create a unique CAPTCHA, we follow a process that involves generating random characters. In Java, we can achieve this as follows:

Character Set: Start by defining a character set from which you want to generate the CAPTCHA. Typically, it includes a mix of uppercase letters, lowercase letters, and numbers. In this example, we will use a character array named characters to represent the character set.
Random Generation: Generate random characters by selecting a random index from the character set array. The Math. random() function can be utilized to achieve the same.
Build CAPTCHA: Iterate through the selected indices and append the corresponding characters to build the CAPTCHA string. We can specify the desired length of the CAPTCHA by controlling the number of iterations.
Return CAPTCHA: Return the generated CAPTCHA string.

CaptchaGenerator.java

public class CaptchaGenerator {
    public static String generateCaptcha(int length) {
        char[] characters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789".toCharArray();
        StringBuilder captcha = new StringBuilder();
        for (int i = 0; i < length; i++) {
            int index = (int) (Math.random() * characters.length);
            captcha.append(characters[index]);
        }
        return captcha.toString();
    }
    public static void main(String[] args) {
        int captchaLength = 9; // Enter the required length of CAPTCHA
        String capt = generateCaptcha(captchaLength);
        System.out.println("CAPTCHA: " + capt);
    }
}

Output:

CAPTCHA: svGvwyc4m

Confirming Humanity of Users

Following the creation and presentation of the CAPTCHA to the user, we must confirm that they are human. Usually, it includes:

User input: Encouraging the user to input the CAPTCHA that appears in the picture that is displayed.
Comparing the user's input with the CAPTCHA that was created.
Success or Failure: The user is deemed human if their input matches the CAPTCHA that was produced. If not, they can be urged to give it another go.

It is an illustration of how to compare the user's input with the CAPTCHA that was generated:

public static boolean isCaptchaMatch(String userResponse, String generatedCaptcha) {
    return userResponse.equals(generatedCaptcha);
}

CaptchaMatch.java

import java.util.*;
import java.io.*;
public class CaptchaMatch {
    // Returns true if two strings are the same
    static boolean checkCaptcha(String captcha, String userCaptcha) {
        return captcha.equals(userCaptcha);
    }
    // Generates a CAPTCHA of a given length
    static String generateCaptcha(int length) {
        // Characters to be included
        String characters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
        StringBuilder captcha = new StringBuilder();
        for (int i = 0; i < length; i++) {
            int index = (int) (Math.random() * characters.length());
            captcha.append(characters.charAt(index));
        }
        return captcha.toString();
    }
    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("Generated CAPTCHA: " + captcha);
        // Ask the user to enter a CAPTCHA
        System.out.print("Enter the CAPTCHA: ");
        String userCaptcha = reader.readLine();
        // Notify the user about the matching status
        if (checkCaptcha(captcha, userCaptcha)) {
            System.out.println("CAPTCHA Matched");
        } else {
            System.out.println("CAPTCHA Not Matched");
        }
    }
}

Output:

Generated CAPTCHA: QDf06rJYI
Enter the CAPTCHA: QDf06rJYI
CAPTCHA Matched

Explanation

It creates a random CAPTCHA, asks the user to enter it, and then verifies that the input provided by the user matches the produced CAPTCHA. It prints "CAPTCHA Matched" if the CAPTCHAs match; if not, it publishes "CAPTCHA Not Matched."

Complexity

Time Complexity: O(n)

Space Complexity: O(1)

Putting CAPTCHA to Use in an Online Application

We may show the produced CAPTCHA inside a form, get the user's answer, and use the verification function to validate it in order to incorporate CAPTCHA into a web application. For more sophisticated and secure CAPTCHA solutions, we may also leverage pre-existing Java libraries, like Google's reCAPTCHA.

Conclusion

In conclusion, improving the security of online applications requires creating distinct CAPTCHAs and confirming the humanity of users in Java. we may defend your online application from possible dangers by creating CAPTCHAs that can distinguish between automated bots and people using the technique described above.

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