Add solution for 2b

exam/ex.tex

@@ -2,6 +2,7 @@
 \usepackage[english]{babel}
 \usepackage{amsmath,amssymb,amsthm}
 \usepackage{color}
+\usepackage{units}
 
 \newcommand{\TODO}{\textcolor{red}{TO DO}}
 
@@ -115,7 +116,11 @@
       $M_1,\cdots,M_l$. 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
+    \item The length of the message $M$ is limited by the counter, which is at
+      most 26 bits long. Since the very first counter ($\langle 0\rangle_{26}$)
+      is reserved for the tag, $2^{26}-2$ message blocks remain. Every block
+      (without the counter) is at most 102 bits long which gives a maximum
+      message length of $102\cdot (2^{26}-2) = \unit[6845103924]{bits}$.
 
     \item \TODO