AES Encryption | InvalidCipherTextException: MAC check in GCM failed, while decrypting the cipher text at c# side

AES Encryption | InvalidCipherTextException: MAC check in GCM failed, while decrypting the cipher text at C# side

If you are encountering the "InvalidCipherTextException: MAC check in GCM failed" error while decrypting AES encrypted cipher text in C#, don't worry! This article will help you understand the issue and guide you through the troubleshooting steps to resolve it.

Understanding AES Encryption and GCM Mode

AES (Advanced Encryption Standard) is a widely used symmetric encryption algorithm that ensures secure transmission and storage of sensitive data. It operates on blocks of data, typically 128 bits in size, and uses a secret key to encrypt and decrypt the data.

GCM (Galois/Counter Mode) is a mode of operation for AES that provides both confidentiality and integrity. It includes an authentication tag, also known as a Message Authentication Code (MAC), which is used to verify the integrity of the cipher text during decryption.

The InvalidCipherTextException: MAC check in GCM failed Error

The "InvalidCipherTextException: MAC check in GCM failed" error occurs when the MAC check fails during the decryption process. This indicates that the cipher text has been tampered with or corrupted, and the integrity of the data cannot be verified.

Possible Causes of the Error

There are a few potential causes for this error:

• The cipher text may have been modified or corrupted during transmission or storage.
• The encryption and decryption processes may be using different keys.
• The encryption and decryption processes may be using different initialization vectors (IVs).
• The encryption and decryption processes may be using different authentication tags.

Troubleshooting Steps

To resolve the "InvalidCipherTextException: MAC check in GCM failed" error, follow these steps:

1. Verify the Cipher Text: Ensure that the cipher text has not been modified or corrupted. If possible, compare the received cipher text with the original cipher text to identify any discrepancies.
2. Check the Key: Make sure that the encryption and decryption processes are using the same key. If you are using a key management system, ensure that the correct key is being used for decryption.
3. Check the Initialization Vector (IV): Verify that the encryption and decryption processes are using the same IV. The IV should be randomly generated and unique for each encryption operation.
4. Check the Authentication Tag: Ensure that the encryption and decryption processes are using the same authentication tag. The authentication tag is typically generated during encryption and appended to the cipher text. It is used to verify the integrity of the data during decryption.
5. Update Libraries and Dependencies: If you are using any third-party libraries or dependencies for AES encryption and decryption, make sure they are up to date. Sometimes, outdated libraries can cause compatibility issues and result in the "InvalidCipherTextException" error.

Example Code

Here is an example of C# code for AES encryption and decryption using GCM mode:

using System;
using System.Security.Cryptography;

public class AesGcmExample
{
    public static byte[] Encrypt(byte[] plainText, byte[] key, byte[] iv)
    {
        using (AesGcm aesGcm = new AesGcm(key))
        {
            byte[] cipherText = new byte[plainText.Length];
            aesGcm.Encrypt(iv, plainText, cipherText, null);
            return cipherText;
        }
    }

    public static byte[] Decrypt(byte[] cipherText, byte[] key, byte[] iv)
    {
        using (AesGcm aesGcm = new AesGcm(key))
        {
            byte[] plainText = new byte[cipherText.Length];
            aesGcm.Decrypt(iv, cipherText, null, plainText);
            return plainText;
        }
    }
}
  

Conclusion

The "InvalidCipherTextException: MAC check in GCM failed" error can be resolved by ensuring the integrity of the cipher text and verifying that the encryption and decryption processes are using the same key, IV, and authentication tag. By following the troubleshooting steps provided in this article, you should be able to overcome this issue and successfully decrypt the AES encrypted cipher text in C#.

References