Kamstrup kem file decryptor for .NET

Program.cs

namespace kemdecrypt
{
    using System;
    using System.IO.Compression;
    using System.Linq;
    using System.Text;
    using System.Threading;
    using System.Threading.Tasks;
    using System.Xml.Linq;
    using McMaster.Extensions.CommandLineUtils;

    public class Program
    {
        public static Task<int> Main(string[] args)
        {
            return CommandLineApplication.ExecuteAsync<Program>(args);
        }

        [Option(Description = "File is zipped.", Inherited = false, LongName = "zip", ShortName = "z", ShowInHelpText = true)]
        public bool IsZipped { get; }

        [Option(Description = "Password for decrypting the kem file.", Inherited = false, LongName = "password", ShortName = "pass", ShowInHelpText = true)]
        public string Password { get; private set; }

        [Option(Description = "The path of the file.", Inherited = false, LongName = "path", ShortName = "p", ShowInHelpText = true)]
        public string Path { get; }

        private async Task<int> OnExecuteAsync(CommandLineApplication app)
        {
            if (string.IsNullOrEmpty(Path))
            {
                app.ShowHelp();
                return 0;
            }

            byte[] encrypted;

            // TODO check if it actually is a zip file, and so on.
            if (IsZipped)
            {
                XElement element = await XElement
                    .LoadAsync(
                        ZipFile.OpenRead(Path).Entries.First( x=> x.Name.EndsWith(".kem"))
                        .Open(),
                    LoadOptions.None,
                    new CancellationToken()) ;

                encrypted = Convert.FromBase64String(element.Value);
            }
            else
            {
                encrypted = Convert.FromBase64String(XElement.Load(Path).Value);
            }

            if (string.IsNullOrEmpty(Password))
            {
                Console.WriteLine("No password provided. Please input password now.");
                Password = Prompt.GetPassword("Password: ");
            }

            var decryptor = new AesDecryptor(Encoding.UTF8.GetBytes(Password));
            var decrypted = decryptor.Decrypt(encrypted);

            // Kamstrup seems to pad the file with \b - We trim it away, since it is invalid xml.
            var decoded = Encoding.UTF8.GetString(decrypted);
            var parsed = decoded.Substring(0, decoded.LastIndexOf(">") + 1);

            Console.WriteLine(XElement.Parse(parsed));

            return 0;
        }
    }
}

namespace kemdecrypt
{
    using System;
    using System.Security.Cryptography;
    public class AesDecryptor
    {
        private readonly byte[] _decryptionKey;
        public AesDecryptor(byte[] decryptionKey)
        {
            if (decryptionKey == null)
            {
                throw new ArgumentNullException(nameof(decryptionKey));
            }

            // Password can be less than 16 bytes, so we pad some extra 0's to the end of the key to ensure it has the correct length.
            _decryptionKey = new byte[16];
            Buffer.BlockCopy(decryptionKey, 0, _decryptionKey, 0, decryptionKey.Length);
        }

        public byte[] Decrypt(byte[] payload)
        {
            using (var algorithm = new AesCryptoServiceProvider
            {
                Padding = PaddingMode.None,
                Mode = CipherMode.CBC,
                IV = _decryptionKey, // initialization vector is the same as the key.
                Key = _decryptionKey
            })
            {
                return algorithm.CreateDecryptor().TransformFinalBlock(payload, 0, payload.Length);
            }
        }
    }
}