45 lines
1.8 KiB
TeX
45 lines
1.8 KiB
TeX
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\subsection{CAPTCHA methods analysed}
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We analysed at the following CAPTCHA methods using the above mentioned
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parameters. These are popular methods are currently in deployment.
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%TODO add images
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\subsubsection{Align object}
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Objects in various degrees of misalignments are displayed to the user and are
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asked to chose the one that is perfectly aligned.
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% Example GitHub/Kik inverted Hipop
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\subsubsection{Blurred Text}
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A sequence of randomly generated letters and digits are
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presented to the user with added noise, scattered distribution and
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rotations. Sometimes, they are also presented in 3D form.
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\subsubsection{Context based}
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This method is personalised to the platforms they are displayed on. They usually
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pose challenges which can only be solved if the user is familiar with the
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platforms. Some examples are:
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\begin{itemize}
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\item What is the name of the website's mascot?
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\item Who owns this website?
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\item What are our members collectively called? (example: Reddit users are
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called Redditors)
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\end{itemize}
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\subsubsection{Audio based}
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A audio recording with added noise is presented to the user who is asked to
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transcribe the content of the recording.
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\subsubsection{IP tracking}
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IP address is used to blacklist misbehaving users. Strictly speaking, this isn't
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a CAPTCHA method but is frequently used in conjunction with other methods.
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\subsubsection{Image identification}
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A blurred image with added noise or unusual cropping is presented to the user
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who is requested to identify the object in it. Sometimes, the users are also
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asked to pick images that match a certain description from a collection of
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images.
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\subsubsection{Proof of Work based}
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This is an alternative to CAPTCHA method that has been used for rate-limiting.
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The user agent is presented with a challenge and is tasked generate a
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cryptographic proof which computationally expensive.
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