CAPTCHAs: Or How Computers Say Gotcha to Other Computers

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CAPTCHAs Or How Computers Say Gotcha! to
Other Computers

• Arturo Sanchez, PhD
• Associate Professor
• University of North Florida

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Want to see Hilary Duff?

• Need to buy the ticket

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Why the Funny Image?

• They are called CAPTCHAs, which stands for
• Completely
• Automated
• Public
• Turing Test To Tell
• Computers and
• Humans
• Apart

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All Interesting Terms

• and well get to them in a second, but first
• What are CAPTCHAs good for?
• We can let the creators tell us about it

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Examples of CAPTCHAs

• We can try some of them

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Lets Talk About the Terms

• Completely Automated is at the heart of how
  CAPTCHAs work.
• What is needed
• Collection of words (e.g. a dictionary).
• Algorithm
• Pick seven words randomly
• Produce an image containing the words in a
  distorted way
• Pose the question to the user

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Lets Talk About the Terms

• Public means the algorithm, and the data are
  known nothing hidden all in the open
• Why?

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Lets Talk About the Terms

• The rationale is like this
• in order for somebody to break the CAPTCHA
  they would need to be able to solve a hard
  problem.
• Intuitively, a problem is considered to be hard
  if finding a solution to it would take a
  ridiculous amount of time

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Lets Talk About the Terms

• In this case, the problem considered hard is
  that of recognizing words in a cluttered
  environment with a high degree of precision
  well get to this later.

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Running out of Time

• Useful links
• Breaking CAPTCHAs Mori and Malik
• Turing Test Wikipedia
• Turing Test 50 years later
• Loebner Prize
• Meet Jabberwock the winner of the 2003
  competition

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Running out of Time

• The ACM paper on CAPTCHAs
• Article on breaking CAPTCHAs
• Page on RSA

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Ticketmasters
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BONGO