I have about 1TB of data I'd like to make an encrypted backup of over a period of a few sessions. I'm guessing under the hood deja-dup uses something like rsync and only modifies a backup when necessary (files have been added or changed). However, when making an encrypted backup does each file have to be reencrypted before it can be compared to its backed up version? That is, can an encrypted backup resume where it left off, or must it start from the beginning, doing each encryption along the way?
Deja-dup is a front-end for duplicity. An interrupted backup(encrypted or not) can be resumed without starting all over again, you only lose the progress of the last chunk/slice of the backup-chain. By default in deja-dup slices are of about 50M, so not much of a loss. Anyway if you've that much data to backup I'd suggest using a command-line tool with more options and flexibility (dar, rdiff-backup, or maybe duplicity itself)