In Russia, said Thomas Rid, a scholar of intelligence history at King’s College London, “there’s an appreciation that information is power, and having information that somebody considers very private information is even more powerful.”

Mr. Skuratov has insisted all along that he was not the man in the tape. But Mr. Chaika, after becoming acting prosecutor general in 1999, endorsed the video’s authenticity, saying it showed a “legal basis” to open an investigation.