You should be careful when using encryption not to lose important keys.

If you are making KDMs from a DCP-o-matic film you must ensure that you have a backup of the metadata.xml file from the project, as well as the DCP itself.

If you are using a DKDM you must ensure that you have a backup of DCP-o-matic's config.xml file, since it contains the only key which can decrypt the DKDM. The config.xml file location depends on your operating system; possible locations are listed in Chapter 22, Configuration files.