Mary Queen of Scots and Secret Letters

A Queen in a Locked Room

Mary Stuart, Queen of Scots, spent the last nineteen years of her life as a prisoner in England. She was moved from house to house, her servants vetted, her visitors watched, her outgoing post read before it went anywhere. And yet she ran a busy correspondence with supporters in France, Spain and the English Catholic underground, carrying politics, money and plots across Europe in handwriting that looked like harmless squiggles to anyone who opened the packet.

Those squiggles are the reason she died. They are also a perfect place for a curious beginner to start learning cryptography, because Mary's letters show the whole game in miniature: how ciphers are made, how they are broken, and how a decoded message can change the fate of a nation.

How Mary's Cipher Actually Worked

Her system was a nomenclator — a hybrid between a substitution cipher and a tiny codebook. It wasn't simply "A becomes a triangle". It had layers:

• Roughly 23 symbols standing for letters of the alphabet, with a few letters sharing a symbol because i/j and u/v/w were not reliably distinguished at the time.
• Around 35 further symbols standing for whole common words — and, the, that, my, for, to — which saved the writer time and made the text much harder to crack.
• Dedicated symbols for names and places: Elizabeth, Walsingham, Babington, France, Spain, Scotland.
• Nulls: meaningless symbols sprinkled through the message purely to confuse anyone trying to count how often each character appeared.

That last point matters. A single symbol might mean the letter b, or the word and, or nothing whatever. Both correspondents held an identical key sheet, so they could read each other perfectly — while an interceptor saw an alphabet of ghosts.

The Codebreaker in the Next Room

Francis Walsingham, Elizabeth I's principal secretary, ran something close to a professional intelligence service. His star was Thomas Phelippes, a linguist and forger who is arguably the first great English cryptanalyst. Phelippes rarely had the key. He worked from the ciphertext alone, using what we now call frequency analysis.

The method is beautifully simple. In any long piece of English, some letters appear far more often than others: e leads, then t, a, o, i, n. Count the symbols in a captured letter, and the most frequent ones are probably those letters. Then look for structure: a symbol standing alone between two gaps is almost certainly a or I. Common three-symbol clusters are usually the, and, you. Once you crack one word, you know several letters, and the whole thing starts to peel open like a fruit.

Phelippes also had help that no algorithm can supply: context. He knew who was writing, to whom, and roughly what about. A guessed phrase like "the French ambassador" can unlock an entire cipher.

The Babington Plot: When an Intercept Becomes Evidence

In 1586, a young Catholic gentleman named Anthony Babington was corresponding with Mary about a plan to free her and put her on the English throne. Messages were smuggled in and out of Chartley Hall hidden in beer barrels. Walsingham's men intercepted them, copied them, and passed the originals on — so both sides thought the channel was secure.

Phelippes deciphered the traffic. In July 1586 a letter reached Mary asking her to approve the assassination of Elizabeth. Her reply gave the approval the plotters wanted and asked for details of how it would be carried out. That reply is the document that condemned her. It was deciphered, copied, and kept.

There is a further twist. According to the surviving papers, a postscript was later added in the same cipher, asking for the names of the six gentlemen who would do the deed — a deliberate hook to draw out more conspirators. Mary was tried at Fotheringhay in October 1586, convicted of treason, and executed on 8 February 1587. A cipher she trusted had been turned into a prosecutor's exhibit.

Try This at Your Kitchen Table

You can practise the same craft tonight with nothing but paper and a pencil. Take any paragraph of ordinary prose, swap each letter for a symbol of your own invention, and hand it to a friend. Then try to break theirs. A few practical habits will take you a long way:

• Count everything first. Tally each symbol before you guess a single letter. Patterns beat intuition.
• Hunt for one-symbol words. In English these are almost always a or I.
• Look for doubled symbols — ll, ee, ss, oo, tt. They are rarer than you think and very distinctive.
• Use a crib. If you know the message mentions a name or a date, search for that pattern in the ciphertext.
• Write your guesses in pencil underneath and keep a tally sheet. You will revise letters several times before it clicks.

Add a few nulls of your own and you will quickly discover how much harder a simple substitution becomes — and why Mary's nomenclator felt so safe.

Why the Letters Still Matter

Cryptography is usually taught as mathematics. Mary's letters show it is also politics with the volume turned up. Two lessons stand out. First, secrecy is only ever as strong as the people handling it: the cipher was decent, but the channel leaked. Second, codebreaking does not merely satisfy curiosity — it produces evidence. Walsingham did not just learn that a plot existed; he acquired a document he could read aloud in a court.

That pattern has never gone away. Encryption protects the private, interception exposes it, and the argument is always about what can be proved rather than merely suspected. For a beginner, that is the real thrill of the subject: puzzles are good fun, but every so often a puzzle decides the fate of a queen.