How do I securely automate inputting passwords?

I would like to automate an SSH connection so that I wouldn't have to enter password and I am not allowed to use authorized_keys. I was thinking about making an expect script but I don't know where I could store the password to not store it openly in the script.

Is there any place (/etc/passwd?) in *nix systems to store such passwords? How do I access this place from a expect or Bash script?