Comment chiffrer et déchiffrer symétriquement certaines données en C# à l'aide d'AES ?

J'essaie de chiffrer et de déchiffrer certaines données à l'aide d'AES. Mais je n'obtiens qu'une sortie brouillée. Qu'est-ce que je fais mal?

static void Test()
{
    byte[] myFileBytes; // Will contain encrypted data. First the IV, then the ciphertext.
    var myPassword = "helloworld";
    var dataToEncrypt = "this is a test";

    // STEP 1: Encrypt some data:

    byte[] key;

    using (var sha256 = SHA256.Create())
        key = sha256.ComputeHash(Encoding.UTF8.GetBytes(myPassword));

    using (var myFileStream = new MemoryStream())
    using (var aes = System.Security.Cryptography.Aes.Create())
    {
        aes.Key = key;
        myFileStream.Write(aes.IV); // Use the default created by AES, which is presumably non-pseudo random

        using (var cryptoStream = new CryptoStream(myFileStream, aes.CreateEncryptor(), CryptoStreamMode.Write))
        {
            cryptoStream.Write(Encoding.UTF8.GetBytes(dataToEncrypt));
            cryptoStream.Flush();

            myFileBytes = myFileStream.ToArray(); // We are done!

        } // Disposing CryptoStream disposes the underlying MemoryStream
    }

    // STEP 2: Decrypt it to verify that it works

    using (var aes = System.Security.Cryptography.Aes.Create())
    {
        using (var myFileStream = new MemoryStream(myFileBytes))
        {
            var iv = new byte[aes.IV.Length];

            myFileStream.Read(iv, 0, iv.Length);

            using (var cryptoStream = new CryptoStream(myFileStream, aes.CreateEncryptor(key, iv), CryptoStreamMode.Read))
            using (var copyStream = new MemoryStream())
            {
                cryptoStream.CopyTo(copyStream);

                var decrypted = Encoding.UTF8.GetString(copyStream.ToArray());

                Debug.Assert(dataToEncrypt == decrypted); // Fails!
            }
        }
    }
}