Ssh – Duplicity – how to put passphrase in command

To change the passphrase on your default key:

$ ssh-keygen -p

If you need to specify a key, pass the -f option:

$ ssh-keygen -p -f ~/.ssh/id_dsa

then provide your old and new passphrase (twice) at the prompts. (Use ~/.ssh/id_rsa if you have an RSA key.)

More details from man ssh-keygen:

[...]
SYNOPSIS
    ssh-keygen [-q] [-b bits] -t type [-N new_passphrase] [-C comment]
               [-f output_keyfile]
    ssh-keygen -p [-P old_passphrase] [-N new_passphrase] [-f keyfile]
[...]
     -f filename
             Specifies the filename of the key file.
[...]
     -N new_passphrase
             Provides the new passphrase.

     -P passphrase
             Provides the (old) passphrase.

     -p      Requests changing the passphrase of a private key file instead of
             creating a new private key.  The program will prompt for the file
             containing the private key, for the old passphrase, and twice for
             the new passphrase.
[...]

The default mode of duplicity is to use a symmetric key which consists of a simple passphrase. There's no way I would use that though: if you have to type the key, you can't run an unattended backup!

If you want to run an unattended backup, you have to pass duplicity a public key somehow. The only kind of public key that duplicity supports is GPG, and that requires a key pair. If you don't want more security than the passphrase provides, keep plenty of copies of the private key around (e.g. store it on every backup media, and print it).

Note that you need to choose a really good passphrase (as in long and having high entropy) to get reasonable security from offline attacks (which is the threat here).