According to GQ, individuals receiving Secret Service protection are allowed to select their own names, if they wish. However, they may also elect to have the White House Communications Agency do it for them, as in the case of John "Phoenix" McCain.
Ryan, who appears on the cover of the latest issue of Deer & Deer Hunting Magazine, likely chose his name as a nod to his unending quest to procure the perfect hair bows for his American Girl dolls, Felicity (whom he has renamed "Jacinta") and Samantha.
Ryan's wife, Janna, an adult woman of 43, whose life is based on a pony, will henceforth be known as "Buttercup." This name gives 11-year old Sasha "Rosebud" Obama (who was 7 when she received her code name) a real run in the cuteness department.)
So you can pretend you are a member of the Secret Service, here is a list of the code names (allegedly) used for the candidates and their families:
Paul Ryan: Bowhunter
Janna Ryan: Buttercup aka Li'l Buttercup QT
Mitt Romney: Javelin
Ann Romney: [not reported, probably because Ann Romney does not permit the frivolity of "eke names"]
Joe Biden: Celtic (because bOOOOOObs proved difficult to render in speech in a way satisfactory to the Vice President)
Jill Biden: Capri
Barack Obama: Renegade
Michelle Obama: Renaissance
Malia Obama: Radiance
Sasha Obama: Rosebud
GQ reports that the practice of using secret nicknames to refer to the candidates originated in an era before Secret Service and White House Communications Agency communications were encrypted. Today, it is mostly maintained for the sake of brevity, clarity, and tradition, and because Janna Ryan refuses to answer to anything but "Buttercup."