Add solution for 2a

exam/ex.tex

@@ -107,7 +107,13 @@
   \item \textbf{(16 points)} 
   \begin{enumerate}
 
-    \item \TODO
+    \item $\mathsf{CrAp}_K^{-1}$ operates by taking the ciphertexts
+      $C_1,\cdots,C_l$ and passing them to the decryption function
+      $\widetilde{E}^{-1}(K,N,\cdot)$. The decryption function takes 128-bit
+      inputs and produces a 128-bit output. The output has to be stripped of the
+      counter (the last 26 bits) to obtain the 102-bit message block
+      $M_1,\cdots,M_l$. Finally, the padding (if any) has to be removed from
+      $M_1,\cdots,M_l$ to obtain the original message block (102 bits).
 
     \item \TODO