@Techreport{elahmad2011robustness,
  author = {El Ahmad, Ahmad S and Yan, Jeff and Tayara, Mohamad},
  title  = {The Robustness of Google CAPTCHAs},

  year   = {2011},
  number = {},
  url    = {http://homepages.cs.ncl.ac.uk/jeff.yan/google.pdf},

  const  = {\ text}
}