If you use the setup wizard for Cisco ASA appliances to allow SSH access it doesn’t auto-generate a key.  It will create the access-rules, but you still won’t be able to SSH to the firewall until the key is generated.  The quickest way to generate the key is via the command:

generate crypto key rsa modulus-size” [more]

Note: The modulus-size can be 512, 768, 1024, or 2048.  The value of 1024 is recommended.