CentOS 7 no longer supports MD5, which is a good thing.
But not so great if your still using certificates with MD5 encryption for openVPN as it will fail to connect. Here's a workaround to get it working.
yum install openvpn cd /etc/openvpn mv client.conf service.conf systemctl enable email@example.com
[Service] Environment="OPENSSL_ENABLE_MD5_VERIFY=1 NSS_HASH_ALG_SUPPORT=+MD5"
systemctl daemon-reload systemctl restart firstname.lastname@example.org