Media hash and resizing

Sitecore introduced a new security feature in Sitecore version 7.5 regarding handling image requests in Sitecore. The feature restricts media URLs containing dynamic image-scaling parameters so only server-generated request are processed. Meaning, if a media request is processed, Sitecore skips the image scaling, if any of the parameters in the image URL have been altered or appended to the URL client side. Sitecore has added a new processor to the pipeline (added to a new include file, placed: /App_Config/Include/ Sitecore.Media.RequestProtection.config):

<pipelines>
      <renderField>
        <processor patch:before="processor[@type='Sitecore.Pipelines.RenderField.RenderWebEditing, Sitecore.Kernel']" type="Sitecore.Pipelines.RenderField.ProtectedImageLinkRenderer, Sitecore.Kernel" />
      </renderField>
    </pipelines>

What Sitecore do is, Sitecore adds a hash value to the image URL query string, when images requests contains scaling parameters. This means, in older versions than Sitecore versions 7.5 a media request may look something like this:

http://[yourDomain]/~/media/Default Website/sc_logo.ashx?w=100&h=75

scaling the original image from:

to:

(for image scaling parameters [link: http://www.sitecore.net/learn/blogs/technical-blogs/john-west-sitecore-blog/posts/2011/05/media-options-and-query-string-parameters-in-the-sitecore-aspnet-cms.aspx])

In Sitecore version 7.5 and newer versions, the media request may look something like this:
http://[yourDomain]/~/media/Default Website/sc_logo.ashx?w=100&h=75&hash=D02739F8C8E02B64D5D2AD008175975D28123C17

Therefore, if you are manually setting up the image URL containing scaling parameters, you need to generate the correct hash value and append it to the image URL, or else Sitecore will only return the original unaltered image. You can generate the hash value by your C# code or by Sitecores new mediaHash.aspx tool.

By your C# code:
If you need the full media URL containing scaling parameters and the hash value in your C# code, you can use the HashingUtils.ProtectAssertUrl(url):

Sitecore.Resources.Media.HashingUtils.ProtectAssetUrl(
                               Sitecore.Resources.Media.MediaManager.GetMediaUrl(
                                   myMediaItem,
                                   new MediaUrlOptions()
                                   {
                                       Language = Context.Language,
                                       Width = 100,
                                       Height = 75
                                   }))

If you only need the hash value for the provided image URL, you could use the GetAssertUrlHash(url).

By MediaHash.aspx tool
You find the new mediaHash.aspx tool here: http://[yourDomain]/sitecore/admin/mediaHash.aspx. Add the media URL containing the scaling parameters into the textfield:

You can disable the security feature by setting the Media.RequestProtection.Enabled to false, but it is strongly recommended not to disable this feature.

<settings>
<!-- MEDIA - REQUEST PROTECTION - ENABLED
Specifies whether media request protection is enabled or not.
Default value: true-->
      <setting name="Media.RequestProtection.Enabled" value="false">
</setting>

...
</settings>