OpenSSH require different keys depending if you use SSH1 or SSH2 protocol. All keys are generated by “ssh-keygen” commad, that one should be available on your system with the ssh package. Default key lengths are also appropriate (2048 bits for rsa and 1024 bits for dsa)
For SSH1 protocol, you need a rsa1 key generated has follow:
# ssh-keygen -q -f /etc/ssh/ssh_host_key -N '' -t rsa1
For SSH2 protocol, you need two keys, one rsa key and one dsa key generated has follow:
# ssh-keygen -f /etc/ssh/ssh_host_rsa_key -N '' -t rsa
# ssh-keygen -f /etc/ssh/ssh_host_dsa_key -N '' -t dsa