diff --git a/lectures/lec04.tex b/lectures/lec04.tex
index 098508b..064ebc9 100644
--- a/lectures/lec04.tex
+++ b/lectures/lec04.tex
@@ -109,7 +109,7 @@ \subsection{MACs require pseudorandomness}
 \section{Pseudorandom Functions}
 
 A pseudorandom function is defined over a keyspace $\calK$,
-and input spacei $\calX$ and output space $\calY$.
+and input space $\calX$ and output space $\calY$.
 To be useful a pseudorandom function must satisfy the following
 security definition: