I got a request from a client asking to "encrypt some images to prevent people from downloading them". This will be within an HTML email from some email sending application that I do not know of at this time, so I will need to use a string for the source such as "http://somewhere.com/images/img.jpg" etc. Does that kind of thing even exist? The only thing I can think of is 'encrypting' an image so instead of the src being "whatever/img.jpg", it would be "http://somewhere.com/img.php?i=12383824724. But I am just spitballing and don't know how that would work. Before you start answering:

1. Client knows we can put a watermark on the images
2. Client knows we can put image as a background (using appropriate methods for cross client compatibility)

Any ideas? Thanks all,

Fil

• php
• html
• image
• email

If the user can see the image, he has already downloaded it. Preventing a user from downloading an image means preventing him from seeing it. And that seems rather pointless.

What you may want is to prevent your user from saving the already downloaded image to a file or elsewhere. That would only be possible if you'd have perfect control over the environment in which the user is viewing the image, including anything in front of the screen like a digital camera. I.e., it is futile.

You may want to consider using PGP with your mail. It's the de-facto standard here, but only if you're doing something informationally tractable.

While you won't be able to, "prevent people from downloading [your images]," you can prevent them from decrypting them unless they're a trusted party. If some finer-grained level of DRM is desired, you may want to consider talking it out with the client and explaining to them precisely why this is infeasible.

In short: if the target party has already seen the image content, there's little you can do about it. The best of watermarks and steganographic markers are doomed to the analog hole, even if you go out of your way to secure your web server.