Generate Captcha Using C++

A guide on how to create a Captcha using C++ with graphics libraries to generate random text and images, providing protection against automated attacks for web applications or software.

In this article, we will explore how to create a simple Captcha using C++. The process involves generating a random string, adding noise, and displaying it on an image.

C++ Code

#include <iostream>
#include <cstdlib>
#include <ctime>
#include <opencv2/opencv.hpp>

std::string generateCaptcha(int length) {
    std::string captcha;
    const char charset[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
    int maxIndex = sizeof(charset) - 1;

    for (int i = 0; i < length; ++i) {
        captcha += charset[rand() % maxIndex];
    }

    return captcha;
}

void createCaptchaImage(const std::string& captcha) {
    int width = 200;
    int height = 80;
    cv::Mat image(height, width, CV_8UC3, cv::Scalar(255, 255, 255));

    // Set font and draw text
    int fontFace = cv::FONT_HERSHEY_SIMPLEX;
    double fontScale = 1;
    int thickness = 2;
    cv::Point textOrg(10, 50);

    cv::putText(image, captcha, textOrg, fontFace, fontScale, cv::Scalar(0, 0, 0), thickness);

    // Add noise
    for (int i = 0; i < 1000; ++i) {
        int x = rand() % width;
        int y = rand() % height;
        image.at<cv::Vec3b>(y, x) = cv::Vec3b(rand() % 256, rand() % 256, rand() % 256);
    }

    cv::imwrite("captcha.png", image);
    std::cout << "Captcha image saved as captcha.png" << std::endl;
}

int main() {
    srand(time(0));
    std::string captcha = generateCaptcha(6);
    std::cout << "Generated Captcha: " << captcha << std::endl;
    createCaptchaImage(captcha);
    return 0;
}

Detailed explanation:

1. generateCaptcha(): Generates a random Captcha string from alphanumeric characters.
2. createCaptchaImage(): Creates a Captcha image using OpenCV, adding the Captcha text and noise.
3. main(): Calls generateCaptcha to create the Captcha string and createCaptchaImage to generate the corresponding image.

System Requirements:

• C++ Compiler (GCC, MSVC, ...)
• OpenCV library version 4.0 or above

How to install the libraries needed to run the C++ code above:

1. Install OpenCV: OpenCV Installation Guide
2. Compile with OpenCV:

   g++ -o captcha captcha.cpp `pkg-config --cflags --libs opencv4`
   

Tips:

• Choose harder-to-guess characters and colors for better security.
• Add more noise and text distortion to increase difficulty in recognition.

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