The Babington Plot was a famous conspiracy, in 1586, to assassinate Queen Elizabeth. It led to the execution of Mary Queen of Scots, whose weak cipher implicated her. Check out that cipher today, cryptography fans!
In 1586, Europe was in a state of turmoil. Religious differences between largely Catholic France and Spain, and Protestant England, compounded the political aggression between the countries. One person who could sway the balance was Mary Queen of Scots. The woman had a claim to the throne of both Scotland and France, and would be well-placed to reclaim England for Catholicism.
Mary had a personal, as well as political, grudge against the reigning Queen Elizabeth. Elizabeth had imprisoned Mary, and separated her from her son. When Marywas contacted by Sir Anthony Babington, an Englishman who represented French and Spanish conspirators, she jumped at the chance to kill Elizabeth and be rescued from her prison. The plot was foiled but Mary clung to hope that her letters were indecipherable. She'd written them in code.
Unfortunately, Mary had never had a chance. Elizabeth's officials had tracked them from the start, and handed them over the Thomas Phelippes. Phelippes was a linguist and an early analyzer of codes. He was willing to put in the time to do frequency analysis - noting which characters in Mary's cipher came up most frequently, and proposing values for them. He'd plug the values in, and see if they came up with anything reasonable. Mary's code was rather simple, substitution of symbols for letters, and a few more symbols for frequently used words. Phelippes knew of the plot to assassinate Elizabeth before Mary did - the letters went to him. But he allowed the letter describing the plot to go to Mary, so she could approve it and condemn herself. When Phelippes sent that letter on to Elizabeth's spymaster, he sketched a gallows on the envelope that contained it.
The picture shows Mary's code, her letters and the code for the most frequently used words. A cleaner copy of the code can be found here.
Top Image: © Ad Meskens / Wikimedia Commons
[Via The Code Book.]