ICaptchaTokenProtector Interface

This document explains the ICaptchaTokenProtector interface and how to use it to encrypt and decrypt captcha tokens. It's intended for developers who need to implement custom token protection for their captcha functionality.

ICaptchaTokenProtector Interface Overview

The ICaptchaTokenProtector interface defines the contract for encrypting and decrypting captcha tokens. Implementing this interface allows you to use a custom encryption mechanism to protect the integrity and confidentiality of the tokens, preventing tampering and unauthorized access.

Interface Methods

The ICaptchaTokenProtector interface defines two methods:

Encrypt(string token)

• Purpose: Encrypts the provided token.
• Parameters:
  • token: The token to encrypt (string).
• Returns: The encrypted token (string).
• Exceptions:
  • System.Security.Cryptography.CryptographicException: Thrown when an error occurs during encryption (e.g., invalid token, key not found, algorithm failure). Handle this exception appropriately in your implementation.

Decrypt(string token)

• Purpose: Decrypts the provided token.
• Parameters:
  • token: The encrypted token to decrypt (string).
• Returns: The decrypted token (string).
• Exceptions:
  • System.Security.Cryptography.CryptographicException: Thrown when an error occurs during decryption (e.g., invalid token, tampering, key not found). Handle this exception appropriately in your implementation.

Implementing ICaptchaTokenProtector

To use a custom token protection mechanism, you must create a class that implements the ICaptchaTokenProtector interface. This class will contain your specific encryption and decryption logic.

public class MyCustomTokenProtector : ICaptchaTokenProtector
{
    private readonly IDataProtectionProvider _dataProtectionProvider; // Example: Using Data Protection

    public MyCustomTokenProtector(IDataProtectionProvider dataProtectionProvider)
    {
        _dataProtectionProvider = dataProtectionProvider;
    }

    public string Encrypt(string token)
    {
        // Example using Data Protection API:
        var protector = _dataProtectionProvider.CreateProtector("MyCaptchaPurpose");
        return protector.Protect(token);

        // Or your custom encryption logic:
        // ...
    }

    public string Decrypt(string token)
    {
        // Example using Data Protection API:
        var protector = _dataProtectionProvider.CreateProtector("MyCaptchaPurpose");
        return protector.Unprotect(token);

        // Or your custom decryption logic:
        // ...
    }
}

This example demonstrates using the ASP.NET Core Data Protection API for encryption. You can replace this with your own encryption algorithm (e.g., AES, RSA) if needed. Be very careful when implementing your own cryptography. Ensure it's done correctly to avoid security vulnerabilities.

Registering the Custom Implementation

After creating your custom ICaptchaTokenProtector implementation, you need to register it with the dependency injection container. Use the AddCaptchaTokenEncryptionService method in your Program.cs (for .NET 6 and later) or Startup.cs (for older ASP.NET versions):

builder.Services.AddCaptcha(options => { /* ... captcha options ... */ })
       .AddCaptchaTokenEncryptionService<MyCustomTokenProtector>(); // Register your custom protector

This registers your MyCustomTokenProtector with the service collection, making it available for use by the captcha library. It is keyed with "DataProtection" which is used to retrieve it later. If you do not register a custom ICaptchaTokenProtector, a default implementation using DefaultDataProtectionService will be used.

Important Considerations

• Security: Token protection is crucial for preventing captcha manipulation. Use a strong encryption algorithm and manage your encryption keys securely. If you're implementing your own cryptography, consult with a security expert to ensure its robustness. The ASP.NET Core Data Protection API is a good option for most cases, as it handles key management and provides robust encryption.
• Dependency Injection: Ensure you correctly register your custom ICaptchaTokenProtector implementation with the dependency injection container.
• Error Handling: Implement proper error handling for CryptographicException in your Encrypt and Decrypt methods. Log the exceptions or take other appropriate actions.
• Key Management: If you're using a symmetric encryption algorithm, secure key management is essential. Don't hardcode keys in your application. Use a secure key storage mechanism. The Data Protection API helps with this.

By correctly implementing and registering your ICaptchaTokenProtector, you can significantly enhance the security of your captcha implementation. Always prioritize security best practices when dealing with sensitive data like captcha tokens.