If you would like to backup your SQL Server Reporting Services encryption key from a command prompt you can use rskeymgmt.exe to accomplish this. Here is the syntax:

rskeymgmt.exe -e -f “C:\KeyBackup\RSKey.snk” -p “YourStrongPassword”

In the above command:
-e = Extracts a key from a report server instance.
-f = Full path and file name to read/write key.
-p = Password used to encrypt or decrypt key.

RSKeyMgmt has many other functions as well. Here are the other parameters you can pass to it:

-e = extract – Extracts a key from a report server instance
-a = apply – Applies a key to a report server instance
-s = reencrypt – Generates a new key and reencrypts all encrypted content
-d = delete content – Deletes all encrypted content from a report server database
-l = list – Lists the report servers announced in the report server database
-r = installation ID – Remove the key for the specified installation ID
-j = join – Join a remote instance of report server to the scale-out deployment of the local instance
-i = instance – Server instance to which operation is applied. Default is MSSQLSERVER
-f = file – Full path and file name to read/write key.
-p = password – Password used to encrypt or decrypt key.
-m = machine name – Name of the remote machine to join to the scale-out deployment
-n = instance name – Name of the remote machine instance to join to the scale-out deployment
-u = user name – User name of an administrator on the machine to join to the scale-out deployment. If not supplied, the current user is used.
-v = password – Password of an administrator on the machine to join to the scale-out deployment
-t = trace – Include trace information in error message

You can get all of the above in addition to examples by running:
RSKeyMgmt.exe /?