I have a private key file used to log into remote servers. As the file resides only on a desktop computer, key file was prepared without a passphrase (or at least that's what I remember and I am never asked to enter a passphrase when I use ssh, which makes me think I remember right). However, now I am going on a trip and I want to be able to use the same key on my laptop - so to be safe I wanted to add a passphrase. Judging from the man page to change a passphrase I should use something like ssh-keygen -p -f id_rsa_priv.ppk but it doesn't work - that is, I am asked to enter the old passphrase, but when I just press Enter I am told it is a bad one. Any ideas what should I do? I already tried using -P "" to no avail.