enter single passphrase for multiple encrypted partitions

[Owen Heisler <owenh000@gmail.com>]

I have three partitions encrypted (by d-i), each listed in /etc/crypttab:
/var, /home, and one for backups

At startup, I am prompted to enter three passphrases.  Is there a way to
enter only one?

Using luks, I have included a keyfile in another key slot, and put that
keyfile on /var.  This doesn't work, though, because the latter two
partitions are started before /var is mounted.

What seems to be necessary is to have /var, containing the keyfile,
started with the cryptdisks-early script, like would happen if the root
fs were encrypted.  Then I suppose I can set CRYPTDISKS_MOUNT in
/etc/default/cryptdisks to mount /dev/mapper/var before starting the
other two partitions...?

Everything but the backup partition resides on a raid1 (mirror) array.

Thanks