C#

var password = "thisIsAReallllllllyLongPasswordForTesting";
var passPhrase = "E546C8DF278CD5931069B522E695D4F2";
var initVector = "HR$2pIjHR$2pIj12";
var encrypted = Cryptography.EncryptString(password, passPhrase, initVector);

Console.WriteLine("Encrypted password");
Console.WriteLine(encrypted.ToString());

string decrypted = Cryptography.DecryptString(encrypted, passPhrase, initVector);

Console.WriteLine("decrypted password");
Console.WriteLine(decrypted);

public static class Cryptography
{
    public static string EncryptString(string plainText, string passPhrase, string initVector)
    {
        string encrypted = "";

        // Create an Aes object with the specified key and IV.
        using (Aes aes = Aes.Create())
        {
            aes.Key = Encoding.ASCII.GetBytes(passPhrase);
            aes.IV = Encoding.ASCII.GetBytes(initVector);

            // Create a new MemoryStream object to contain the encrypted bytes.
            using (var memoryStream = new MemoryStream())
            {
                // Create a CryptoStream object to perform the encryption.
                using (var cryptoStream = new CryptoStream(memoryStream, aes.CreateEncryptor(), CryptoStreamMode.Write))
                {
                    // Encrypt the plaintext.
                    using (var streamWriter = new StreamWriter(cryptoStream))
                    {
                        streamWriter.Write(plainText);
                    }

                    encrypted = Encoding.UTF8.GetString(memoryStream.ToArray());
                }
            }
        }

        return encrypted;
    }

    public static string DecryptString(string encryptedText, string passPhrase, string initVector)
    {
        string decrypted;

        // Convert the input string to bytes
        var inputBytes = Encoding.ASCII.GetBytes(encryptedText);

        //PadToMultipleOf(ref inputBytes, 16);

        // Create an Aes object with the specified key and IV.
        using (Aes aes = Aes.Create())
        {
            aes.Key = Encoding.ASCII.GetBytes(passPhrase);
            aes.IV = Encoding.ASCII.GetBytes(initVector);

            // Create a new MemoryStream object to contain the decrypted bytes.
            using (var memoryStream = new MemoryStream(inputBytes))
            {
                // Create a CryptoStream object to perform the decryption.
                using (var cryptoStream = new CryptoStream(memoryStream, aes.CreateDecryptor(), CryptoStreamMode.Read))
                {
                    // Decrypt the ciphertext.
                    using (var streamReader = new StreamReader(cryptoStream))
                    {
                        decrypted = streamReader.ReadToEnd();
                    }
                }
            }
        }

        return decrypted;
    }

    private static void PadToMultipleOf(ref byte[] src, int pad)
    {
        int len = (src.Length + pad - 1) / pad * pad;
        Array.Resize(ref src, len);
    }
}

The input data is not a complete block.'

PadToMultipleOf(ref inputBytes, 16);

Padding is invalid and cannot be removed.'

textBox_taskMsg.Text = "Task is being performed.. This may take some time.";

string[] files = { textBox_Source1.Text, textBox_Source2.Text }; // Vids to be merged
string output = textBox_Source1.Text + "_merged.mp4"; // Output vid name
var conversion = await FFmpeg.Conversions.FromSnippet.Concatenate(output, files); // Merge
await conversion.Start();

// SHOW MERGING PROGRESS

textBox_taskMsg.Text = "Merge Complete.";

--------
//Create a string array to store file paths
string[] filepaths = new string[10];
filepaths.SetValue(textBox_Source1.Text, 0);
filepaths.SetValue(textBox_Source2.Text, 1);
//Call the video merging function
MergeVideos(filepaths, filepaths[0] + "_merged");

------
using (ITimeline timeline = new DefaultTimeline())
{
    IGroup group = timeline.AddVideoGroup(32, 720, 576); // Set bitrate, width, height

    var firstClip = group.AddTrack().AddVideo(videoPaths[0]);
    var secondClip = group.AddTrack().AddVideo(videoPaths[1], firstClip.Duration /* offset */);

    using (AviFileRenderer renderer = new AviFileRenderer(timeline, outputFilePath))
    {
        renderer.Render();
    }
}