I did unique assignments for each student a different way. I created an assignment called "Author Assigned" and filled in the "Feedback" field with each author for each student (actually, I imported it from a spreadsheet, but you can import into Feedback for each grade instead of the actual grade).
They can look at their user grade report and see the Feedback, and it is the author they were assigned. Not sure how many characters it takes, but it worked for my purpose.