SSRS keys can be backed up and restored in 2 easier ways. One, offcourse using the GUI panel of Reporting Configuration Manager and the other one is command line tool by executing rskeymgmt utility.
The rskeymgmt utility can be found in the binn sub-directory of your SQL Server install directory. Opening the command prompt and navigating to this directory, we can run rskeymgmt -? to get a list of arguments and additionally some example commands.
To backup the key, issue this command within cmd prompt: rskeymgmt.exe -e -f D:\TestBackup\SSRSKeybackup -p Yourownpwd -i MSSQLServer
While, to restore, use this command: rskeymgmt.exe -a -f D:\TestBackup\SSRSKeybackup -p Yourownpwd -i MSSQLServer