Mary Queen of Scots cipher

The Mary Queen of Scots cipher is a hybrid system that combines:

1. A homophonic substitution — several distinct symbols per plain letter, to flatten frequency distribution.
2. A nomenclator — a table of whole words or proper nouns coded by a single symbol (a built-in code dictionary).

It entered history because it was the cipher used by Mary Stuart, Queen of Scots, in her secret correspondence with Anthony Babington in 1586. The decryption of that correspondence provided the written evidence that Mary was plotting Elizabeth I’s assassination — evidence that sent her to the scaffold in February 1587.

What is the historical context of Mary Queen of Scots cipher?

Mary Stuart, the deposed Queen of Scots, has been imprisoned in England since 1568 on Elizabeth I’s orders. Her correspondence is constantly surveilled by Secretary of State Sir Francis Walsingham, chief of the English Secret Service.

In 1586, Anthony Babington, a young English Catholic, contacts Mary through what he believes is a secret channel: letters slipped into the bung of beer barrels delivered to her prison. He proposes a plot: assassinate Elizabeth, free Mary, restore Catholicism in England.

Unbeknownst to them, Mary and Babington use a channel entirely controlled by Walsingham: the messenger (Gilbert Gifford) is a double agent, and every letter is intercepted, copied and decrypted before being passed on.

The cipher

Structure

The table contains:

• 23 symbols for the 23 letters of late-16th-century English — no J, no V, no W (J barely exists in the period’s hand, V and U are two graphic variants of the same phoneme, W is a digraph of UU). On encryption, J → I, V → U, W → U.
• 5 space variants, picked at random at every insertion. Since the space is by far the most frequent character of an English text (~17%), having five interchangeable glyphs flattens the signature that would otherwise leak word boundaries.
• 35 nomenclator symbols for the most common words and formulae of a Tudor letter: and, for, with, that, if, but, where, as, of, the, from, by, so, not, when, there, this, in, which, is, what, say, me, my, send, receive, bearer, pray, you. Plus the standalone pronoun “I” (distinct from the letter I), and a few period-specific abbreviations from the Babington correspondence: WYRT, LRE, MTE, MYNE.
• One two-word ligature: YOUR NAME — a single glyph for the entire phrase, which let the writer address a recipient without ever naming them in plaintext.

Sample encryption

Plaintext: “I PRAY YOU SEND BEARER WITH THE LETTER”

Token splitting:

I       → "I"-pronoun glyph (distinct from the letter I)
PRAY    → "PRAY" glyph
YOU     → "YOU" glyph
SEND    → "SEND" glyph
BEARER  → "BEARER" glyph
WITH    → "WITH" glyph
THE     → "THE" glyph
LETTER  → not a ligature → L-E-T-T-E-R, letter by letter

Between every pair of tokens, one of the 5 space glyphs is picked at random. The result is a mosaic of unique symbols where the most frequent words collapse into a single character, making frequency analysis on the remaining letters even more laborious.

Phelippes’s cryptanalysis

Thomas Phelippes (1556–1625) is Walsingham’s personal cryptanalyst. A polyglot (English, French, Italian, Latin, Greek), he is one of the earliest specialists of frequency analysis enriched with cribs.

His method on the Mary Stuart cipher:

1. Salutation guess — every letter starts with a variation of “Right honourable” or a direct reference to Mary. That gives 10–15 initial symbols with probable values.
2. Filler identification — some symbols show no correlation with their neighbours; they are dismissed.
3. Homophone detection — by cumulative frequency, the 3–4 symbols matching the same plain letter become identifiable.
4. Nomenclator reading — clumps of unique symbols between regular letters encode words; they are deduced from context (“the”, “Queen”, “death”).

Phelippes broke the cipher in a few weeks.

The forgery that sealed her fate

Walsingham, having read the exchanges in plaintext, asks Phelippes for something extraordinary: forge a letter from Mary to push Babington into revealing his co-conspirators. Phelippes, now master of the cipher, drafts a fake letter in cipher that is perfectly credible, asking Babington for the list of plotters.

Babington replies — the list is intercepted, and arrests begin. Mary is tried for high treason at Fotheringhay (1586), sentenced to death, executed on 8 February 1587.

Cryptanalytic legacy

The Mary Stuart cipher marks:

• The rise of state cryptanalysis as a political weapon.
• The end of the illusion that homophonic + nomenclator is inherently safe.
• The appearance of ciphertext forgery as a psychological-operations technique.
• The need for authenticity in encrypted communication — Babington accepted the forged letter because no authentication mechanism existed.

It is also a lesson for modern cryptography: confidentiality without authenticity is not enough. Modern protocols (TLS, Signal) encrypt AND sign — Walsingham could no longer mount exactly the same attack today.

The 23 letter glyphs

The cipher’s historical alphabet — 23 letters, no J, V, W. On encryption these three missing letters are folded: J → I, V → U, W → U.

The 5 homophonic spaces

Picked at random between every pair of tokens.

The 35 word ligatures

Each word or phrase below collapses into a single glyph.

In CipherChronicle

The cipher is playable in the workshop: type an English message — YOU SEND ME THE LETTER WITH BEARER — and each recognised word collapses into a single glyph, the rest fall letter by letter, and every space is replaced by one of the 5 random variants. Each refresh changes the visual ciphertext (the space glyphs rotate) while remaining decryptable.