Original authorization for SC in May 2017. Apparently Mueller asked Rosenstein to whip up something so he could go after Manafort for Ukraine money. Rosenstein obliged, in the form of a memo, in August 2017 (supposedly). Not reported, that I know of, until April 2018.Copy of the memo at link.
Sounds fishy to me.
But Rosenstein was only authorized to permit the investigation on the Russian-Trump connection. Any other investigation beyond Russia/Trump would put Sessions back in charge, he only recused himself from Russia/Trump, Sessions is still the boss on any other investigation. So any secret mandate for broadening the mandate of the SC beyond that are null and void. Rosenstein didn’t have that authority.