Bouncy Castle AES Encryption - providing input in blocks

I'm using Bouncy Castle library to encrypt some data in my Windows Store App. My EncryptHelper class:

public static class EncryptHelper
{
    private const string KEY = "chiaveAES";
    private const int SIZE = 16;
    private enum CipherMode
    { 
        Encrypt,
        Decrypt
    }

    private static PaddedBufferedBlockCipher InitCipher(CipherMode mode)
    {
        PaddedBufferedBlockCipher cipher = new PaddedBufferedBlockCipher(new CbcBlockCipher(new AesLightEngine()), new ZeroBytePadding());

        var key = new byte[32];
        var keyArray = KEY.ToCharArray();
        Buffer.BlockCopy(keyArray, 0, key, 0, Math.Min(keyArray.Length, key.Length));
        cipher.Init(mode == CipherMode.Encrypt, new KeyParameter(key));
        return cipher;
    }

    public static async Task Encrypt(Stream sourceStream, Stream destinationStream, bool autoSeekStart = true, bool autoSeekEnd = true)
    {
        //await Process(InitCipher(CipherMode.Encrypt), sourceStream, destinationStream, autoSeekStart, autoSeekEnd);
        await ProcessBlocks(InitCipher(CipherMode.Encrypt), sourceStream, destinationStream, autoSeekStart, autoSeekEnd);
    }


    public static async Task Decrypt(Stream sourceStream, Stream destinationStream, bool autoSeekStart = true, bool autoSeekEnd = true)
    {
        //await Process(InitCipher(CipherMode.Decrypt), sourceStream, destinationStream, autoSeekStart, autoSeekEnd);
        await ProcessBlocks(InitCipher(CipherMode.Decrypt), sourceStream, destinationStream, autoSeekStart, autoSeekEnd);
    }

    private static async Task Process(PaddedBufferedBlockCipher cipher, Stream sourceStream, Stream destinationStream, bool autoSeekStart, bool autoSeekEnd)
    {
        if (autoSeekStart)
        {
            sourceStream.ToBegin();
            destinationStream.ToBegin();
        }

        var size = Convert.ToInt16(sourceStream.Length);
        byte[] inBuffer = new byte[size];
        byte[] outBuffer = new byte[cipher.GetOutputSize(size)];
        int inCount = 0;
        int outCount = 0;

        try
        {
            inCount = await sourceStream.ReadAsync(inBuffer, 0, inBuffer.Length);

            outCount = cipher.ProcessBytes(inBuffer, 0, inCount, outBuffer, 0);
            outCount += cipher.DoFinal(outBuffer, outCount);

            await destinationStream.WriteAsync();

            await destinationStream.FlushAsync();
        }
        catch { }

        if (autoSeekEnd)
        {
            sourceStream.ToBegin();
            destinationStream.ToBegin();
        }
    }

    private static async Task ProcessBlocks(PaddedBufferedBlockCipher cipher, Stream sourceStream, Stream destinationStream, bool autoSeekStart, bool autoSeekEnd)
    {
        if (autoSeekStart)
        {
            sourceStream.ToBegin();
            destinationStream.ToBegin();
        }

        byte[] inBuffer = new byte[SIZE];
        byte[] outBuffer = new byte[cipher.GetOutputSize(SIZE)];
        int inCount = 0;
        int outCount = 0;

        try
        {
            while ((inCount = await sourceStream.ReadAsync(inBuffer, 0, inBuffer.Length)) > 0)
            {
                outCount += cipher.ProcessBytes(inBuffer, 0, inCount, outBuffer, 0);
                await destinationStream.WriteAsync(outBuffer, 0, outBuffer.Length);
            }

            outBuffer = ?
            outCount += cipher.DoFinal(outBuffer, outCount);

            await destinationStream.WriteAsync(outBuffer, 0, outCount);

            await destinationStream.FlushAsync();
        }
        catch { }

        if (autoSeekEnd)
        {
            sourceStream.ToBegin();
            destinationStream.ToBegin();
        }
    }
}

My Process() method works fine, but when on the instruction

inCount = await sourceStream.ReadAsync(inBuffer, 0, inBuffer.Length);

I'm afraid it may occurr an OutOfMemoryException if the stream has too much data. So, I was trying to build the ProcessBlocks() method, which should read from the stream progressively, one block per time, without overcharging the RAM. I have some doubts on how to behave with outBuffer: it should be overwritten in every cycle in which cipher.ProcessBytes() gets executed, but of which size should it be just before the cipher.DoFinal() invocation?

Thank you

UPDATE 30/07/2015

I modified the Main in the answer to handle a zip file and the outcoming zip file is no more a valid ZIP, could someone explain me why?

 public static void Main(string[] args)
    {
            var plainPath = @"C:\Users\Federico\Desktop\0abed72d-defc-4c9a-a8ae-3fec43f01224.zip";
            var decryptPath = @"C:\Users\Federico\Desktop\0abed72d-defc-4c9a-a8ae-3fec43f01224 - decrypted.zip";

            var plainStream = new FileStream(plainPath, FileMode.Open, FileAccess.Read);
            var cipherStream = new MemoryStream();
            EncryptHelper.Encrypt(plainStream, cipherStream);
            cipherStream.Seek(0, SeekOrigin.Begin);
            FileStream fs = new FileStream(decryptPath, FileMode.Create);
            EncryptHelper.Decrypt(cipherStream, fs);

            fs.Flush();
            fs.Close();
        }