public static byte[] Decrypt(byte[] encodedCms, X509Certificate2 certificate)
{
    EnvelopedCms envelopedCms = new EnvelopedCms();

    envelopedCms.Decode(encodedCms);
    ...
}

public unsafe static void ProcessMessage(SafeMsgHandle hMsg, FileStream file)
{
	// Variables
	BinaryReader stream = null;
	byte[] pbData = null;
	long dwRemaining = 0;
	bool bFinal = false;
	bool bResult = false;

	// Process message
	dwRemaining = file.Length;
	stream = new BinaryReader(file);
	do
	{
		// Read one chunk of data
		pbData = stream.ReadBytes(1024 * 1000 * 10);
		if (pbData.Length == 0)
		{
			break;
		}

		// Update message piece by piece    
		bFinal = (dwRemaining <= 1024 * 1000 * 10);
		fixed (byte* pAux = &pbData[0])
		{
			bResult = CryptMsgUpdate(
				hMsg.DangerousGetHandle(),
				new IntPtr(pAux),
				pbData.Length,
				bFinal
			);
			if (!bResult)
			{
				throw new Exception("CryptMsgUpdate error #" + Marshal.GetLastWin32Error().ToString(), new Win32Exception(Marshal.GetLastWin32Error()));
			}
		}
		dwRemaining = dwRemaining - pbData.Length;

	} while (!bFinal);
}

while (dwRemaining > 0)
{
	inFile.Read(pbData, 0, dwSize);

	// Вызываем обработку блока
	bResult = CMSWin32.CryptMsgUpdate( hMsg, pbPtr,
		dwSize, (dwRemaining <= dwSize) ? true : false );
	if (!bResult)
	{
		throw new CryptographicException(
			"CryptMsgUpdate error #" + 
			Marshal.GetLastWin32Error().ToString(), 
			new Win32Exception(Marshal.GetLastWin32Error()));
	}

	// Переходим к следующему блоку
	dwRemaining -= dwSize;
	if (dwRemaining < dwSize)
	{
		dwSize = (int)dwRemaining;
	}
	if (progress != null)
		progress(dwFileSize, dwRemaining);
}