Contents

Introduction
Background
Using the Utility
Reviewing the Code

Introduction

File hashes are used to verify the authenticity and integrity of files - especially those transmitted over the internet. When downloading a file from MSDN for instance, you are presented with the SHA-1 Hash - a list of seemingly random characters and numbers that are generated using the SHA-1 encryption algorithm to uniquely identify that file. If even a single bit of that file is changed, the hash itself will change. Most importantly, it is nearly impossible to create a different file that will produce the same hash, making spoofing unfeasable. But the question remains, how can I easily verify that hash against the file that I actually downloaded? This article will present a window shell extension that gives you easy access to the hash of any file quickly and easily.

Background

There are two coding concepts covered in this article. The first is cryptographic hash and the second is windows shell extensions. The cryptographic hash, of which there are many different algorithms, produces the digital fingerprint of a file. The .NET Framework provides 6 System.Security.Cryptography.HashAlgorithm implementations that can be used for generating the digital fingerprint: MD5, SHA-1,SHA-256,SHA-384,SHA-512 and RIPEMD-160. The two most commonly used when downloading a file are MD5 and SHA-1. Windows shell extensions take us beyond the comfortable realm of managed code and force us to implement COM to integrate into the unmanaged world of Win32. For this element of the utility,code is pulled directly from Dino Esposito's article Manage with the Windows Shell: Write Shell Extensions with C#. Dino's explanation of the COM interfaces and sample code were invaluable to this project. To really understand the implemention of a shell extension, I highly recommend his article.

Using the Utility

With the integration into the Windows Explorer context menu, generating the hash as simple as selecting one or more files then right-clicking to display the context menu. From this menu, you can select which type of file hash you want to generate by selecting the sub-menu option. You will then be presented with a window containing the file name, the hash that you selected, plus the full path to the file(s) that you selected. This DataGridView table allows for selecting of the desired values and copying to the clipboard if you need to publish the hash value. It's that easy!

Reviewing the Code

Generating a file hash is a very straighforward task. In the code sample below, the specific implementation of the HashAlgorithm abstract type is created based on a user selected HashType enumeration value. Next, using a System.IO.FileStream object, the file is opened and streamed into the HashAlgorithm object to let it do its magic. The resulting Byte array is then put into a StringBuilder so that the hash string can be returned to the calling object.

    public static string GetFileHash(string filePath, HashType type)
        {
            if (!File.Exists(filePath))
                return string.Empty;

            System.Security.Cryptography.HashAlgorithm hasher;
            switch(type)
            {
                case HashType.SHA1:
                default:
                    hasher = new SHA1CryptoServiceProvider();
                    break;
                case HashType.SHA256:
                    hasher = new SHA256Managed();
                    break;
                case HashType.SHA384:
                    hasher = new SHA384Managed();
                    break;
                case HashType.SHA512:
                    hasher = new SHA512Managed();
                    break;
                case HashType.MD5:
                    hasher = new MD5CryptoServiceProvider();
                    break;
                case HashType.RIPEMD160:
                    hasher = new RIPEMD160Managed();
                    break;
            }
            StringBuilder buff = new StringBuilder();
            try
            {
                using (FileStream f = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.Read, 8192))
                {
                    hasher.ComputeHash(f);
                    Byte[] hash = hasher.Hash;
                    foreach (Byte hashByte in hash)
                    {
                        buff.Append(string.Format("{0:x2}", hashByte));
                    }
                }
            }
            catch
            {
                return "Error reading file." + new System.Random(DateTime.Now.Second * DateTime.Now.Millisecond).Next().ToString();
            }
            return buff.ToString();
    }
     public enum HashType
        {
            [Description("SHA-1")]
            SHA1,
            [Description("SHA-256")]
            SHA256,
            [Description("SHA-384")]
            SHA384,
            [Description("SHA-512")]
            SHA512,
            [Description("MD5")]
            MD5,
            [Description("RIPEMD-160")]
            RIPEMD160
           
        }

The trick comes in when you want to be able to right-click on the file in Windows explorer and generate the hash. This is where Dino Esposito's expertise comes in. By modifying the code found in his article, I was able to hook into the shell context menu and add my menu option along with the sub-menu selections for the various hash types. This code can be found in the FileHashShellExt.cs file in the sample project.The key element for this integration are the IContextMenu interface. The first method, QueryContextMenu, in conjunction with a shell32 DragQueryFile method allows you to interrogate the number and type of files that were selected to determine if the custom menu option should be added. The second method, InvokeCommand provides back the information on the sub-menu item selected so that you can execute the proper hash type.

Now that you have your code ready to go, you need to make Windows aware of your utility. This is handled via the registry. It's easy enough to register an assembly for COM interop, but how do you let Windows know what exactly it's supposed to do with it? By implementing the System.Runtime.InteropServices.ComRegisterFunctionAttribute attribute along with the RegisterServer method. This method will get executed when you register the assembly with regasm.exe to add the needed registry entries that tell the Windows Shell to accept the new extension and present the context menu option for all files.

        
    [System.Runtime.InteropServices.ComRegisterFunctionAttribute()]
    static void RegisterServer(String str1)
    {
        try
        {
            // For Winnt set me as an approved shellex
            RegistryKey root;
            RegistryKey rk;
            root = Registry.LocalMachine;
            rk = root.OpenSubKey("Software\\Microsoft\\Windows\\CurrentVersion\\Shell Extensions\\Approved", true);
            rk.SetValue(guid.ToString(), "FileHash shell extension");
            rk.Flush();
            rk.Close();

            // Set "*\\shellex\\ContextMenuHandlers\\BatchResults" regkey to my guid
            root = Registry.ClassesRoot;
            rk = root.CreateSubKey("*\\shellex\\ContextMenuHandlers\\FileHash");
            rk.SetValue("", guid.ToString());
            rk.Flush();
            rk.Close();
        }
        catch(Exception e)
        {
            System.Console.WriteLine(e.ToString());
        }
    }

There is an associated method UnregisterServer that is called when you unregister the assemply with regasm.exe and the "/u" flag to remove the registry entries. If you install the Shell extension via the demo project installer, all of this is taken care of for you. If you choose to download the code and install it manually, you can use the register.bat and unregister.bat files respectively (you'll want to check that the paths to regasm.exe and gacutil.exe are correct for your environment.

File Hash Generator Shell Extension