objective-c code to right pad a NSString?

Can someone give a code example of how to right pad an NSString in objective-c please?

For example want these strings:

Testing 123 Long String
Hello World
Short 

if right padded to a column width of say 12: and then a sting "XXX" is added to the end of each, it would give:

Testing 123 xxx
Hello World xxx
Short       xxx

That is a 2nd column would like up.