WASHINGTON — As she asked the U.S. Supreme Court this month to overturn President Donald Trump’s election loss, the attorney Sidney Powell cited testimony from a secret witness presented as a former intelligence contractor with insights on a foreign conspiracy to subvert democracy. Powell told courts that the witness is an expert who could show that overseas corporations helped shift votes to President-elect Joe Biden. The witness’s identity must be concealed from the public, Powell has said, to protect her “reputation, professional career and personal safety.” The Washington Post identified the witness by determining that portions of her affidavit match,...