harry potter - Was Voldemort's secret identity always known to the Ministry of Magic? Was it ever secret?

Often called "He-Who-Must-Not-Be-Named" and "You know who", was it common knowledge at the Ministry of Magic that Voldemort was the same person as Tom Riddle?

Answer

I don't think that Voldemort was ever really a "secret" identity. Voldemort was a name that Tom Riddle was using as early as his time at Hogwarts, though at that time only amongst his closest "friends" (to the extent that he had them).

He pulled Harry’s wand from his pocket and began to trace it through the air, writing three shimmering words:

TOM MARVOLO RIDDLE

Then he waved the wand once, and the letters of his name rearranged themselves:

I AM LORD VOLDEMORT

“You see?” he whispered. “It was a name I was already using at Hogwarts, to my most intimate friends only, of course. You think I was going to use my filthy Muggle father’s name forever? I, in whose veins runs the blood of Salazar Slytherin himself, through my mother’s side? I, keep the name of a foul, common Muggle, who abandoned me even before I was born, just because he found out his wife was a witch? No, Harry — I fashioned myself a new name, a name I knew wizards everywhere would one day fear to speak, when I had become the greatest sorcerer in the world!”

_{Chamber of Secrets, Chapter 17 - The Heir of Slytherin}

When Voldemort returned to Hogwarts to ask Dumbledore for the position teaching Defense Against the Dark Arts, he was already using the name Voldemort exclusively and publicly:

“So, Tom… to what do I owe the pleasure?”

Voldemort did not answer at once, but merely sipped his wine.

“They do not call me ‘Tom’ anymore,” he said. “These days, I am known as —”

“I know what you are known as,” said Dumbledore, smiling, pleasantly. “But to me, I’m afraid, you will always be Tom Riddle. It is one of the irritating things about old teachers. I am afraid that they never quite forget their charges’ youthful beginnings.”

_{Half-Blood Prince, Chapter 20 - Lord Voldemort's Request}

I think, if anything, Tom Riddle would be considered the "secret identity", since that was the initial motivation behind the new name. Voldemort, as the Heir of Slytherin and with ambitions to be the most powerful and feared practitioner of the Dark Arts, would have wanted to ignore his Muggle ancestry entirely.