Oliver Cromwell was the most famous dictator in British history and as such had no shortage of enemies. Conspiracy plots to kill him emerged not only form his native England but also from neighboring countries.
For this reason, although he was a miser, he set aside enviable sums of money for a special security force tasked with his safety. This security organization was called the "Black Room" and was comprised of agents who were close to Cromwell.
In 1653, the secret spy network was headed by secretariat John Thurloe. He was also Postmaster General of the Royal Mail and was tasked by Cromwell to check any letters coming from suspicious sources.
Letters received weren't opened with scissors or a knife but using a special machine designed specifically for that purpose. It opened letters carefully without breaking the wax seal. They were then able to be closed back up, without any sign of tampering.
Black Room agents worked under severe pressure, for not only did they have to open suspicious letters, in some cases they had to make copies of them as well. The entire process had to be executed as quickly as possible as any delays between the correspondence would raise suspicion.
Working in the Black Room was also geometry professor John Wallis, who decoded complex ciphers which were not uncommon when sending letters.
Thurloe faced various challenges when opening and distributing these letters, until the day a black-cloaked foreigner entered his cabinet, who surprisingly enough, knew exactly what Cromwell's secret organization was up to.
The stranger announced that he could read even illegible handwriting and make a copy in about 2 min. This kind of claim seemed unheard of in the 17th century.
To test these claims, Thurloe gave the stranger a document that wasn't particularly important and in 2 min. he returned with an exact copy. The stranger added that within a few hours all the letters on the copy would disappear, while the paper would once again turn white.
He was immediately assigned to Cromwell's Black Room. Thanks to his work several plots against the dictator were uncovered.
Even while he was working for the secret organization he never did reveal his real name. After Charles II of the House of Stuart took the throne, the agents of the Black Room were relieved from duty, the stranger was given a sack of gold coins and no one ever saw him again.
Centuries later, British experts are still unable to solve the letter copying mystery. They believe that the copies were made thanks to a photographic trick but this sounds impossible for that period.
The copies were made in a dark room with a burning candle and some experts theorize that the stranger used a special substance that could transfer the writing after the sheets were made to touch.