[bct] Fw: CAPTCHA the Internet

  • From: "Ray Foret Jr." <rforetjr@xxxxxxxxxxxxx>
  • To: <blindcooltech@xxxxxxxxxxxxx>
  • Date: Mon, 27 Feb 2006 18:04:34 -0600

----- Original Message ----- 
From: Barb O'connor 
To: broconnor1972@xxxxxxxxxxxxx 
Sent: Monday, February 27, 2006 2:25 PM
Subject: CAPTCHA the Internet

I thought you might find this interesting.


Tag-strategia.com (Blog)
Tuesday, February 21, 2006

CAPTCHA the Internet

CAPTCHA (an acronym for "Completely Automated Public Turing test to tell
Computers and Humans Apart") has been on my mind ever since Phil Windley
suggested a graphical CAPTCHA would make a good web service. I thought there
might be those willing to pay to use it. Well, it's been done.

There is a need for this type of test. Yahoo! and Hotmail use a CAPTCHA to
stave off spammers when a user requests an email account. I suspect the most
common use is on other sites is an attempt block automated comment spam in

CAPTCHA excludes legitimate users

As the W3C points out graphical CAPTCHAs are a significant barrier to
low-vision and blind users. Those with learning disabilities, such as
dyslexia, may also be adversely affected. As visual CAPTCHAs become more
sophisticated, busy, patterned background becomes more of an issue for
color-blind users.

The U.S. Census Bureau estimated that in 1997 about 7.7 million Americans
had difficulty seeing the words and letters in an ordinary newspaper. The
American Foundation for the blind reported about 5 in 1,000 Americans are
legally blind, and gives a low estimate of 1.5 million visually impaired
computer users. That's a fairly significant potential market to ignore.

Requiring users to interpret a visual CAPTCHA may lead to legal challenges.
Earlier this month, the National Federation for the Blind filed suit against
Target, claiming target.com discriminates by not being accessible to
visually impaired users.


Some companies are experimenting with audio CAPTCHAs, spelling out random
letters with random noise in the background. However, aural disabilities are
more common than visual ones, so the approach isn't really more accessible.
Speech recognition software is more advanced than character recognition, so
the purported purpose of differentiating between humans and computers is not
filled anyway.

CAPTCHA is broken

Several projects to crack common visual CAPTCHA algorithms, particularly The
CAPTCHA Project (by the Carnegie Mellon School of Computer Science), the UC
Berkeley Computer Vision Group, and Sam Hocevar's PWNtcha, have had good
success. Howard Yeend demonstrated a vulnerability in several public
algorithms where he could reuse a solution several thousand times after
manually solving it once.

Social engineering is often easier than fancy programming. The first widely
recognized social engineering solution was "borrowing" CAPTCHAs from target
sites and showing them at entry points to porn sites. Visitors to porn sites
would solve the CAPTCHAs, allowing spammers to get essentially free labor.
Amazon's Mechanical Turk (tagline: "Artificial Artificial Intelligence"),
which gives micro-payments for simple tasks is an example of another way
CAPTCHAs could be defeated. Even at a few cents per image, the cost may
still be too high for spammers, but it is a demonstration that the process
can be outsourced. After all, the world is flat.

What is the underlying purpose?

The real reason for CAPTCHA is to screen undesirables. For low traffic
sites, it means preventing automated access. This can be accomplished in a
relatively simple way: add a single required question to the comment submit
form. Something like "What color was George Washington's white horse?" or
"Enter the fourth word in this sentence." This is enough to make the form
non-standard, thus unusable by generic bots. Bypassing this added security
would be very easy for spammers, the advantage is the relative obscurity of
most blogs. To target multiple blogs, a spammer would need to address each
one individually; individual attention is unlikely, so I suggest this method
is the easiest for bloggers with a knowledge of web programming, and is as
accessible as a comment form without a CAPTCHA.

Major sites like Yahoo! and Google have a bigger problem. After all, they
are targets both because of the value of their services, and their size.
When it first launched Gmail, Google limited accounts to those who had been
invited by other active users. Initially there was a good bit of commotion
in the tech community as gmail.com addresses became a sign of prestige. The
invitation system allows Google to track which users may be abusing the
service, and which users invited the abusers. Google has gone a step
further, and now allows potential users to have an invitation code sent to
their mobile phones. The number of accounts requested per phone number can
be tracked. The potential gain from a limited handful of throw-away email
accounts, and the cost of mobile phones (even disposable ones) is enough to
deter spammers, because less troublesome alternatives exist.

If you look at Google's account request page, you'll see a CAPTCHA there.
Google responsibly offers a way for users with disabilities to bypass the
CAPTCHA, although it involves human-to-human interaction (and quite a bit
more time) to complete-a costly alternative.

Real solutions

Several solutions to the problems with CAPTCHA have been proposed and
debated. Most have major cost or accessibility problems.

It would seem the only good solution is some sort of federated identity
system, which is really just offloading the trouble of user validation to
someone else.


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