.NET/Java PDF, Tiff, Barcode SDK Library

The code in this section of the chapter is purely exploratory. Most of it contains hacks and other workarounds that go against the intended design of an MVC web application. The intent of this section is to see how far we can bend the framework without breaking it. We wouldn t recommend using these methods in a production application unless absolutely necessary. Furthermore, server-side controls are changing considerably from .NET 3.5 SP1 to .NET 4.0. These changes are outside the scope of the book, but expect changes in the rendered HTML as well as the generated client ID. Going forward, all ASP.NET MVC view helpers will work with Web Forms, and many more Web Forms controls will render fine with MVC views.

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We re going to write our plain text to a CryptoStream, using the standard Stream Writer adapter. This works just as well over a CryptoStream as any other, but instead of coming out as plain text, it will be enciphered for us. How does that work

The first control we ll examine is the <asp:TextBox />, which renders as an <input /> HTML element. It requires a <form runat="server"> tag to function, and will be given a generated ID (if it s placed in a container control such as a MasterPage). This is what we re trying to avoid! Because it s a form field, and the form is required to be runat="server", its function is crippled. Figure 7.1 shows it in action, while figure 7.2 shows the resulting HTML.

CryptoStream is quite different from the other streams we ve met so far. It doesn t have any underlying storage of its own. Instead, it wraps around another Stream, and then uses an ICryptoTransform either to transform the data written to it from plain text into cipher text before writing it to that output stream (if we put it into CryptoStream Mode.Write), or to transform what it has read from the underlying stream and turning it back into plain text before passing it on to the reader (if we put it into CryptoStream Mode.Read).

So, how do we get hold of a suitable ICryptoTransform We re making use of a factory class called TripleDESCryptoServiceProvider. This has a method called CreateEncryp tor which will create an instance of an ICryptoTransform that uses the TripleDES algorithm to encrypt our plain text, with the specified key and IV.

variable delete (vdel) variable get (vget) variables set (vset)

A number of different algorithms are available in the framework, with various strengths and weaknesses. In general, they also have a number of different configuration options, the defaults for which can vary between versions of the .NET Framework and even versions of the operating system on which the framework is deployed. To be successful, you re going to have to ensure that you match not just the key and the IV, but also the choice of algorithm and all its options. In general, you should carefully set everything up by hand, and avoid relying on the defaults (unlike this example, which, remember, is here to illustrate streams).

We can see that the rendered HTML contains much we didn t ask for. In addition, notice that the form tag has an action attribute that we didn t specify. This will prevent the form from submitting to an action that we request. We can apply a quick trick to avoid the server-side form requirement. In the Page class, there s a method you can override called VerifyRenderingInServerForm(Control control). If we override this method, we can prevent the error that results when using a control outside of the server form. Because there s no code-behind, the only way to accomplish this is to add a server-side script block in your view directly, like this:

We provide all of those parameters to its constructor, and then we can use it (almost) like any other stream. In fact, there is a proviso about CryptoStream. Because of the way that most cryptographic algorithms work on blocks of plain text, it has to buffer up what is being written (or read) until it has a full block, before encrypting it and writing it to the underlying stream. This means that, when you finish writing to it, you might not have filled up the final block, and it might not have been flushed out to the destination stream. There are two ways of ensuring that this happens: Dispose the CryptoStream. Call FlushFinalBlock on the CryptoStream. In many cases, the first solution is the simplest. However, when you call Dispose on the CryptoStream it will also Close the underlying stream, which is not always what you want to do. In this case, we re going to use the underlying stream some more, so we don t want to close it just yet. Instead, we call Flush on the StreamWriter to ensure that it has flushed all of its data to the CryptoStream, and then FlushFinalBlock on the

stream. We can use any sort of stream for that underlying stream. We could use a file stream on disk, or one of the isolated storage file streams we saw earlier in this chapter, for example. We could even use one of the network streams we re going to see in 13. However, for this example we d like to do everything in memory, and the framework has just the class for us: the MemoryStream.