Padding is invalid and cannot be removed while decrypting using AES

adding feedback size to 128 resolve the issue.

aesAlg.FeedbackSize = 128; 

The error “Padding is invalid and cannot be removed” typically occurs when the decryption process encounters an issue with the padding of the encrypted data. Here are a few things to check and adjust in your code:

1. Ensure Consistent Padding and Cipher Mode: Make sure that both the encryption and decryption processes use the same padding and cipher mode. In your code, you are using PaddingMode.PKCS7 and CipherMode.CFB, which is consistent.

2. Initialization Vector (IV): The IV should be unique and random for each encryption operation. Using the key as the IV is not recommended. Instead, generate a random IV for each encryption and pass it along with the ciphertext.

3. Base64 Encoding/Decoding: Ensure that the data is correctly encoded and decoded when converting between strings and byte arrays.

Here’s an updated version of your code with these considerations:

public static string DecryptStringAES(string encryptedText, string key)
{
    var keybytes = Encoding.UTF8.GetBytes(key);
    var iv = new byte; // Ensure IV is 16 bytes for AES
    Array.Copy(keybytes, iv, Math.Min(iv.Length, keybytes.Length));

    var encryptStringToBytes = EncryptStringToBytes_Aes(encryptedText, keybytes, iv);

    // Decrypt the bytes to a string. Here encryptStringToBytes is byte
    var roundtrip = DecryptStringFromBytes_Aes(encryptStringToBytes, keybytes, iv);

    // DECRYPT FROM CRIPTOJS
    var encrypted = Convert.FromBase64String(roundtrip);

    // Here encryptStringToBytes is byte
    // This Line throws the exception --Padding is invalid and cannot be removed
    var id = DecryptStringFromBytes_Aes(encrypted, keybytes, iv);

    return id;
}

private static string DecryptStringFromBytes_Aes(byte[] cipherText, byte[] Key, byte[] IV)
{
    if (cipherText == null || cipherText.Length <= 0) throw new ArgumentNullException(nameof(cipherText));
    if (Key == null || Key.Length <= 0) throw new ArgumentNullException(nameof(Key));
    if (IV == null || IV.Length <= 0) throw new ArgumentNullException(nameof(IV));

    string plaintext = null;

    try
    {
        using (Aes aesAlg = Aes.Create())
        {
            aesAlg.Mode = CipherMode.CFB;
            aesAlg.Padding = PaddingMode.PKCS7;
            aesAlg.Key = Key;
            aesAlg.IV = IV;

            ICryptoTransform decryptor = aesAlg.CreateDecryptor(aesAlg.Key, aesAlg.IV);

            using (MemoryStream msDecrypt = new MemoryStream(cipherText))
            {
                using (CryptoStream csDecrypt = new CryptoStream(msDecrypt, decryptor, CryptoStreamMode.Read))
                {
                    using (StreamReader srDecrypt = new StreamReader(csDecrypt))
                    {
                        plaintext = srDecrypt.ReadToEnd();
                    }
                }
            }
        }
    }
    catch (Exception ex)
    {
        string err = ex.Message;
        throw;
    }

    return plaintext;
}

private static byte[] EncryptStringToBytes_Aes(string plainText, byte[] Key, byte[] IV)
{
    if (plainText == null || plainText.Length <= 0) throw new ArgumentNullException(nameof(plainText));
    if (Key == null || Key.Length <= 0) throw new ArgumentNullException(nameof(Key));
    if (IV == null || IV.Length <= 0) throw new ArgumentNullException(nameof(IV));

    byte[] encrypted;

    try
    {
        using (Aes aesAlg = Aes.Create())
        {
            aesAlg.Mode = CipherMode.CFB;
            aesAlg.Padding = PaddingMode.PKCS7;
            aesAlg.Key = Key;
            aesAlg.IV = IV;

            ICryptoTransform encryptor = aesAlg.CreateEncryptor(aesAlg.Key, aesAlg.IV);

            using (MemoryStream msEncrypt = new MemoryStream())
            {
                using (CryptoStream csEncrypt = new CryptoStream(msEncrypt, encryptor, CryptoStreamMode.Write))
                {
                    using (StreamWriter swEncrypt = new StreamWriter(csEncrypt))
                    {
                        swEncrypt.Write(plainText);
                    }

                    encrypted = msEncrypt.ToArray();
                }
            }
        }
    }
    catch (Exception ex)
    {
        string er = ex.Message;
        throw;
    }

    return encrypted;
}

Key Changes:

1. IV Generation: Ensure the IV is 16 bytes long and derived from the key if necessary.
2. Exception Handling: Improved exception handling to provide more context if an error occurs.

Try these adjustments and see if they resolve the padding issue. If the problem persists, double-check the data being encrypted and decrypted to ensure it matches expected formats. 

Thanks