add prerequisite section and captcha farms info

analysis/align-obj.tex

@@ -8,7 +8,10 @@ Good\\
 The method relies on Optical Character Recognition (OCR) capabilities of human
 users. OCR technology is becoming increasingly sophisticated which would render
 this method ineffective in the future.
-Object  Recognition Technology is becoming
+
+Without OCR, this method can be bypassed using human-powered CAPTCHA farms with
+% TODO cite CAPTCHA farm cost analysis paper
+ease. On an average, farms generate one CAPTCHA solution every 40 seconds
 \subsubsection{Accessibility}
 \subsubsection{Accuracy}
 \subsubsection{Privacy}

paper.tex

@@ -10,6 +10,7 @@
 \maketitle
 
 \input{intro/intro.tex}
+\input{pre-req/main.tex}
 \input{analysis/main.tex}
 
 \end{document}

pre-req/captcha-farms.tex

@@ -0,0 +1,12 @@
+\subsection{CAPTCHA farms}
+CAPTCHA farms are run using cheap labor available in third-world countries. When
+an attacker seeks the services of a CAPTCHA farm, they are provided access to an
+web API to forward CAPTCHA challenges. A labourer working in the farm solves the
+CAPTCHA and the API responds with the solution. The whole process takes less
+then a minute to complete and costs only a fraction of what premium services
+like reCAPTCHA charge.
+
+Overall, this attack is very feasible and cheap and is frequently used to bypass
+traditional CAPTCHA systems.
+
+%% TODO insert price charts

pre-req/main.tex

@@ -0,0 +1,3 @@
+\section{Prerequisite}
+
+\input{pre-req/captcha-farms.tex}