6.4 Key Granularity
The most common technique for protecting backups is to encrypt files locally using a key derived from a passphrase. There are several commercial products that do this, as I will discuss shortly. One choice that needs to be made is how many keys to use. If you use one key to encrypt all of the files that are backed up, then loss or compromise of that key means loss or compromise of the entire archive. Breaking backups down into finer-grained keys is much more complicated and difficult to maintain. You could have a program with a database for controlling all the keys, but you had better back that database up very carefully. In the end, the problem reduces to protecting and backing up keys securely.