On Saturday 04 May 2002 22:18, Hugh Bothwell wrote:
> "Jason Wong" <[EMAIL PROTECTED]> wrote in message
> > On Saturday 04 May 2002 04:58, Hugh Bothwell wrote:
> > > NOTE that for someone sufficiently persistent, they could still
> > > pattern-match the generated image to retrieve the number and
> > > auto-register that way.  (I could write a PHP file to do that, too :-)
> >
> > The pics generated by services like Yahoo are distorted and thus are
> > extremely difficult to OCR.
> Yahoo apparently uses a mixture of static, marble, grid,
> colored background, ripple, and warp to obfuscate images.
>  They don't seem to vary the typeface, size, spacing, or
> alignment of their font, although they do randomize the
> location. Of these, only ripple and warp seem inherently
> difficult to OCR.  They also weaken it by using only short
> dictionary words; this could be useful in an attack (ie if the
> result is not a word in the dictionary, scrap it and
> start fresh).

There are many variations on the theme. Altavista use random letters and 
numbers and different fonts and at varying degrees of rotation. Something 
like that would probably be the easiest to implement in PHP.

Jason Wong -> Gremlins Associates -> www.gremlins.com.hk
Open Source Software Systems Integrators
* Web Design & Hosting * Internet & Intranet Applications Development *

Give me a fish and I will eat today.

Teach me to fish and I will eat forever.

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