Is there ever a good reason to say yes to web canvass image data?

[joey marino]

Is there ever a good reason to say yes to web canvass image data?

I often get this warning with the tor browser:
This website attempted to extract HTML5 canvass image data,
which may be used to uniquely identify your computer.
Should Tor allow this website to extract HTML5 canvass image data?

Is there ever a need or reason to say yes to canvass data?

Can we just automatically have the browser also say no instead of having
to manually select "Never for this site" each time?

[VanguardLH]

Get the prompt if you disable images in Firefox?

I'm sure there are settings in about:config but that requires you go
into the advanced editor and search for the settings (provided you know
them). Better is to use an add-on. I have QuickJava installed which
lets me disable Java, Javascript, images, animated images, style sheets,
and so on. I don't enable all that in its favorites button, just
javascript, images, animated images, and style. I can click on the
favorites button to disable/enable them all at once, or click its down
chevron to pick which one to disable/enable, or customize the toolbar to
add one of the direct buttons (JS, I, A, CS) if I don't want to use its
favorites button (the one that you can customize what it handles and
shows). I suspect the PrefBar add-on also lets you disable/enable
images. There's even a simple image block add-on if that's all you
want.

Of course, you have to disable the image (and other) features of Firefox
before visiting a site since most likely you want those features when
visiting other web sites. With QuickJava, you can configure what is the
default state when Firefox loads or opens a new window, like having it
disable Javascript by default. If you visit a site that needs
Javascript then it's up to you to decide to enable it. These add-ons
affect settings within Firefox so they don't anything more than what
Firefox can do. Disabling images in Firefox does not guarantee that
images are always blocked in a web page. Perhaps Firefox is just
ignoring the <I> image HTML tag in a page but there might be other
methods of delivering images.

https://developer.mozilla.org/en-US/docs/Web/API/Canvas_API

Apparently canvas is part of the Javascript API so disabling Javascript
will progrably disable use of canvas. If so, you could just disable
Javascript (QuickJava, Prefbar, and other add-ons that block Javascript,
or you could disable it in about:config for javascript.enabled). I see
there is a CanvasBlocker add-on that disables just the Javascript API
dealing with canvas. Like Firefox's image setting that can ignore <I>
tags, looks like this add-on ignores <canvas> tags. If canvas is part
of Javacript's API then it seems you could also use NoScript to disable
Javascript by default when visiting a web site and decide for yourself
whether or not you let NoScript enable Javascript for that site
(whitelist it).

I have NoScript and QuickJava but you gave no example web site where I
could test if disabling Javascript. I don't have a Tor-modified web
browser so I cannot say how easy or difficult it is to disable
Javascript in what you are using.

[B00ze]

On 2015-11-10 11:28, joey marino <joeym...@nospam.example.com> wrote:

> Is there ever a good reason to say yes to web canvass image data?

No idea; I have the CanvasBlocker addOn install and set to push fake
image data on all websites, and I've never had a problem. I have
whitelisted only Google and YouTube, just in case...

Regards,

--
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