From d80aed71a965c3bf456f41e38a1a92b6ca13a56b Mon Sep 17 00:00:00 2001
From: Tobias Eidelpes <tobias@eidelpes.info>
Date: Wed, 15 Jun 2022 15:33:47 +0200
Subject: [PATCH] Add solution for 4d

---
 exam/ex.tex | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/exam/ex.tex b/exam/ex.tex
index 45bacfd..8e88781 100644
--- a/exam/ex.tex
+++ b/exam/ex.tex
@@ -186,7 +186,12 @@
     \item IND-CCA2 is proven for both the KEM and the PKC. IND-CPA is proven for
       the KEM.
 
-    \item \TODO
+    \item LEDAcrypt is based on the hardness of the decoding problem for linear
+      codes. Given a parity-check matrix $H$ and a received codeword $y$, the
+      syndrome is $s=yH$. Find a minimum-weight solution $z_0$ for the equation
+      $s=zH$. The best estimate for the received codeword is $x=y+z_0$. Finding
+      a minimum-weight solution to $s=zH$ given $s$ and $H$ is
+      $\mathsf{NP}$-hard.
 
     \item \TODO