diff --git a/exam/ex.tex b/exam/ex.tex
index f7ff722..3b2a9be 100644
--- a/exam/ex.tex
+++ b/exam/ex.tex
@@ -289,7 +289,16 @@
       isomorphic to $\mathcal{G}$. If it is, the verifier accepts, otherwise it
       rejects.
 
-    \item \TODO
+    \item The domain of the commitment scheme is the set of graphs isomorphic to
+      $\mathcal{G}$ and the range is the number ($2^{130}$) of isomorphic
+      graphs. The scheme consists of three phases: setup, commitment and
+      opening. The setup phase consists of choosing an appropriate random
+      permutation $\psi$ from the set of isomorphisms on $\mathcal{G}$. The
+      commitment phase takes the isomorphism $\psi$ and the graph $\mathcal{G}$
+      as input and produces a commitment $\mathcal{G}'$. The opening phase takes
+      an isomorphism $\mathsf{resp}$ and another graph
+      $\mathcal{G}_{\mathsf{ch}}$ isomorphic to $\mathcal{G}$ as input and
+      outputs $\top$ if the result matches $\mathcal{G}'$ and $\bot$ otherwise.
 
     \item \TODO