MEXICO CITY — He may have just taken on the toughest job in Cuba. Rivals at home will try to take him down. Enemies abroad will discredit him. Almost anyone with an interest in Cuba — including American spies and Cuban intelligence officers — will dig through his public and private lives, rummaging for secrets or clues about his plans. Connect With Us on Twitter Follow @nytimesworld for international breaking news and headlines. Twitter List: Reporters and Editors All the while, Miguel Díaz-Canel Bermúdez, 52, the top contender to succeed the Castros after more than half a century of their rule, will need to display the authority of a future president while…

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