Use more secure exponent for generating keys

Bug: 7246090
Change-Id: Ic6cbac975fcb96b436e27a87b1f81b5e3dac6321
diff --git a/tools/make_key b/tools/make_key
index ac02d17..209d824 100755
--- a/tools/make_key
+++ b/tools/make_key
@@ -49,7 +49,7 @@
 read -p "Enter password for '$1' (blank for none; password will be visible): " \
   password
 
-( openssl genrsa -3 2048 | tee ${one} > ${two} ) &
+( openssl genrsa -f4 2048 | tee ${one} > ${two} ) &
 
 openssl req -new -x509 -sha1 -key ${two} -out $1.x509.pem \
   -days 10000 -subj "$2" &