The backup repository internals
This chapter describes what the Obnam backup repository looks like. Unless you're interested in that, you can skip that entirely.
For now, look at the Obnam website at http://obnam.org/development/.
Repository file permissions
Obnam sets the permissions of all files it creates in the repository such that only the owner of the files can read or write them. (Technically, 0600 for files, 0700 for directories.)
This is to prevent backups from leaking because someone else has read access to the repository. There is no setting in Obnam to control this.