Even before then, the Macron campaign had begun looking for ways to make life a little harder for the Russians, showing a level of skill and ingenuity that was missing in Hillary Clinton’s presidential campaign and at the Democratic National Committee, which had minimal security protections and for months ignored F.B.I. warnings that its computer system had been penetrated.

“We went on a counteroffensive,” said Mr. Mahjoubi. “We couldn’t guarantee 100 percent protection” from the attacks, “so we asked: what can we do?” Mr. Mahjoubi opted for a classic “cyber-blurring” strategy, well known to banks and corporations, creating false email accounts and filled them with phony documents the way a bank teller keeps fake bills in the cash drawer in case of a robbery.

“We created false accounts, with false content, as traps. We did this massively, to create the obligation for them to verify, to determine whether it was a real account,” Mr. Mahjoubi said. “I don’t think we prevented them. We just slowed them down,” he said. “Even if it made them lose one minute, we’re happy,” he said.

Mr. Mahjoubi refused to reveal the nature of the false documents that were created, or to say whether, in the Friday document dump that was the result of the hacking campaign, there were false documents created by the Macron campaign.