If your Cisco router supports encryption, you can use the following commands to create a RSA keys so you can connect to the router with SSH instead of with an insecure telnet connection:
ip domain-name <domain name>
crypto key generate rsa
Note that the "ip domain-name" command is required before issuing "crypto key generate rsa" or else the router will give an error such as "Please define a domain-name first."
In comparison, "ip http secure-server" generates keys for accessing the router through SSL without having to specify the domain, and in theory these keys can also be used for SSH access. The "ip http secure-server" command enables the HTTPS server, so if you don't plan to connect to the router in that way, you may want to issue the "no ip http secure-server" command.