Why would it even be a secret? I can't imagine the Rebels keeping quiet about it for one moment because of the enormous PR win they'd have;
"This is the Underground Rebel News Network - The corrupt and despotic Empires new and much vaunted planet destroying space station was destroyed today mere days after it's inauguration, and senseless slaughter of the planet Alderaan. And it's destruction is all thanks to the heroic effort of one small-planet farmboy who always dreamed of flying and his trusty X-wing. We salute young Luke Skywalker of Red Squadron for his extraordinary achievement. Next up, Tatootine Tammy with messages for the Stormtroopers in the field...."