If you are lazy bum network administrator or fresh network administrator, you could create RSA Key wrongly on your Cisco ASA Firewall sometimes I believe.
In this kind of situation, most of network administrator (especially fresher) are don't know how to delete wronlgy created RSA Key as they used ASDM GUI Interface to create RSA Key.
Here, I'd like to share a way how to delete this wrongly created RSA Key by using Command Line Interface for the one who still don't know how to.
It's simple if you know that.
First, login to your Cisco ASA and go to Global Configuration Mode and follow as below,
ciscoasa(config)#crypto key zeroize rsa label key-name
Insert the name of RSA Key that you wrongly created previously in "key-name". Now, you're done in deleting wrongly created RSA Key and you can create new one correctly.
But do not forget to put "label" while you are deleting and if not you are deleting Default RSA Key that you've created previously for other usage.
I've done The above commands in Cisco ASA 5520.
May you all be happy.
(Be knowledgeable, pass it on then)