Removing OpenVPN 2 Private Key Password

I wanted to remove the password protection for a OpenVPN 2 client private key. The solution is to use the ssh-keygen command. For instance, assuming the key file is client_private_key.key, we run

ssh-keygen -p -N "" -m pem -f client_private_key.key

The key file will be replaced by the key without the password protection.

Before I figured out this solution for OpenVPN 2, I ran into a problem. That is, I ran the following command instead,

ssh-keygen -p -N "" -f client_private_key.key

where there is no "-m" option to specify "pem" as the format of the key file and the key file is in the default PKCS 8 format. Then when I attempted to restart the OpenVPN 2 client, I saw in the log,

Error: private key password verification failed

The message is misleading in this case because the actual problem is that OpenVPN 2 does not recognize the PKCS 8 format. To correct the problem, we just need to run the ssh-keygen again and this time we specify the pem format as the file format, i.e.,

ssh-keygen -p -N "" -m pem -f client_private_key.key

Of course, in retrospective, I should have had run this command from the beginning with.