When attempting to SSH into a PIX/ASA firewall you receive the following error on the client
ssh_exchange_identification: Connection closed by remote host
Investigating the log on the PIX/ASA will yield a corresponding error:
Fail to establish SSH session because RSA host key retrieval failed.
The issue is the result of a corrupt or missing RSA key on the firewall.
You need to generate a new RSA key on the firewall.
Magic juju (from either SDM or a prompt):
conf t ca generate rsa key 1024 wr mem copy run start