Multithreading the AES encryption in .NET

I have written a program for encrypting files with AES. The encryption process is done on file streams.

The encryption method:

public static async Task EncryptUsingAESAsync(FileStream fileReadStream, FileStream fileWriteStream, byte[] key, byte[] iv)
{
    using(Aes aesAlg = Aes.Create())
    {
        aesAlg.Key = key;
        aesAlg.IV = iv;

        ICryptoTransform encryptor = aesAlg.CreateEncryptor();

        using(CryptoStream cryptoStream = new CryptoStream(fileWriteStream, encryptor, CryptoStreamMode.Write))
        {
            await fileReadStream.CopyToAsync(cryptoStream);
        }
    }
}

And this method will be called on,

public static async Task EncryptFileStreamAsync(string absolutePath, byte[] key, byte[] iv)
{
    using(FileStream fReadStream = new FileStream(absolutePath, FileMode.Open, FileAccess.Read))
    {
        using(FileStream fWriteStream = new FileStream($"{absolutePath}.EWW", FileMode.CreateNew, FileAccess.Write))
        {
            await fWriteStream.WriteAsync(iv, 0, iv.Length);
            await CryptoProvider.EncryptUsingAESAsync(fReadStream, fWriteStream, key, iv);
        }
    }

    File.Delete(absolutePath);
}

I have read that AES implementation in .NET actually uses Windows cryptographic APIs. So can I await these methods (especially EncryptUsingAESAsync method) like asynchronous IO or should I explicitly call it in TaskFactoy.StartNew() to get it done on a seperate thread? or is there any other way to get the benefit of multithreading?

It depends what your goal is, and your environment concerns. If you are limited by disk IO, you won't see any benefit from running these activities in parallel.

Personally, I'd get a few typical sets of files, in a similar environment, and benchmark this with a few implementations, but I'd also add List<Task>.WhenAny (or .WhenAll) to your list of potential implementations to check.

I'd probably go the Async route if you need a solution quickly, as this will then be managed by the runtime (and you've already built the tasks), it'll run multiple tasks in parallel if it has the environmental overhead to manage it. I'm not sure if the CryptoProvider.EncryptUsingAESAsync is internally multithreaded though, this could add further considerations.