People who are under the US Secret Service’s protection receive their codenames from the The White House Communications Agency. Originally, the agency selected these names for security purposes, because many sensitive government communications were not encrypted. In recent years, government officials and candidates have continued to receive codenames in keeping with tradition, as well for the convenience of Secret Service agents.
More about Secret Service codenames:
In some cases, a codename might make reference to a characteristic of the person. For example, Ronald Reagan was known as “Rawhide,” perhaps reflecting that he had appeared in cowboy films while he was an actor.
The US president and his or her family typically receive codenames that begin with the same letter.
Karenna Gore, daughter of former Vice-President Al Gore, was permitted to choose her own code name, as long as it started with the letter “S” and had two syllables. Her choice? “Smurfette.”