There are several types of captchas, such as image-based captchas, text-based captchas, reCAPTCHA (Google's captcha service), audio captchas, and more. For mobile devices, it's recommended to use a mobile-friendly captcha type that is easy to interact with on smaller screens. Text-based or image-based captchas are commonly used on mobile platforms.
Choose a programming language that is suitable for mobile development. Common choices include Java for Android and Swift/Objective-C for iOS. Ensure you have the necessary development tools and SDKs installed on your computer.
Design a clean and user-friendly interface for the captcha. It should be simple and easy to solve for legitimate users. Avoid complex captchas that might frustrate users.
Write code to generate random captcha challenges, such as random strings of characters or simple arithmetic problems (e.g., 5 + 3 = ?). The challenges should be straightforward for a human to solve but challenging for automated bots.
Keep a record of the correct solutions to the captcha challenges on the server-side. The solution should be associated with a unique identifier or token.
When a user submits the captcha response, validate the input on the server-side. Compare the submitted solution with the correct solution stored in the server database. If they match, the user is verified as human.
Provide appropriate error messages and handling to guide users if they fail to solve the captcha correctly. Allow users to request a new captcha challenge in case they find it too difficult to read.
To prevent replay attacks, set an expiration time for each captcha challenge. If the user takes too long to submit the captcha response, consider the challenge expired, and require them to request a new one.
Test the captcha thoroughly on different mobile devices and screen sizes to ensure it works as expected. Test for usability, security, and accessibility to make sure it is not overly burdensome for legitimate users.
Remember that the client-side captcha code alone is not enough to prevent all bot attacks. Implement additional server-side security measures to protect your application from more sophisticated attacks.
Remember that captcha codes are not foolproof and can be bypassed by determined attackers. However, by following the steps above, you can significantly reduce the risk of automated bot abuse and protect your mobile application's integrity and user experience.