For running an encryption based backup, you might have created an Encryption Key which was associated to a passphrase. When you re-install Backup Exec 2012 and if you start from scratch(a blank database/catalogs), then at a point where you perform an inventory/catalog, you'll be asked to recreate the Encryption Key using the passphrase. The passphrase is the only thing that you need to remember.
of if you backup the BEDB before you iuninstall and re-instal the same versuon of BE, you can reload your existing BEDB and get at the keys that way
We recommend keep records outside of Backup Exec of the details of your encryption keys (which you also do not keep next to the backup media) but take regular backusp of the BEDB as well.
Note: for anyone using BE 15 or later that reads this - you need the exported detabase encryption key (DEK) to go with the backup of the BEDB for those versions as the recovered BEDB on its own will no longer give access to the backup set encyption keys without the DEK