Protect TTF Font files by making them unopenable, but still work on as webfonts

Question

I'm a font maker and I want to protect my web-fonts.

I know about the HTACCESS way to protect font files, but I found that there is another way to protect web-fonts: making the TTF font files intentionaly corrupted when you try to open them with Windows Font Viewer so they won't be installable, while the the same exact file will still work as webfont in the browser.

Here's a working example I have found (how they did it is the mystery): http://fontface.co.il/fonts/demo_s.asp?id=131

As you can see, you are able to access the font files if they are cached in your browser, but when you download the TTF file and open it via "Windows Font Viewer" in order to install it, it will give you this error: "The requested file _____.ttf is not a valid font file".

You can test it yourself in your computer and you'll see that the web font will still work only with the TTF. (without including the woff / eot files in @font-face)

That's exactly what I want to do to my own fonts. That way they would still work as web fonts, and will not work as desktop fonts, making it harder for untrained people to install them without a license.

However, I can't figure out how they did it. Any ideas?

Answer 1

On the webpage indicated as an example, the font is served in Embedded OpenType format.

Embedded OpenType (EOT) fonts are a compact form of OpenType fonts designed by Microsoft for use as embedded fonts on web pages. [...] These font files can be created from existing TrueType font files using Microsoft's Web Embedding Fonts Tool (WEFT), and other proprietary and open source software. [...] The font files are made small in size by use of subsetting (only including the needed characters), and by data compression (LZ compression, part of Agfa's MicroType Express). (Wikipedia)

Answer 2

The OS has different requirements for what constitutes as a valid font than a web browser. A web browser is more lenient when the font misses certain pieces of data. The trick is to figure out how much you can "mangle" a font so that it's not installable, but still passes OTS and is usable in a browser.