Bazeries cipher

The Bazeries cipher was published in 1898 by Étienne Bazeries, a legendary French cryptanalyst. He is best remembered for breaking Louis XIV’s Grand Cipher in 1893 — a nomenclator that had resisted decryption for two centuries — and for spending his career designing cipher devices, several of which directly anticipated the cryptographic machines of the 20th century.

How does Bazeries cipher work?

The Bazeries cipher combines two operations driven by a numeric key:

1. Reversal of the plaintext in blocks whose lengths depend on the key.
2. Substitution through a 5×5 square whose alphabet is scrambled according to the number spelled out in words.

Step 1 — The key

The key is an integer (e.g. 1702). You write it out in words — here in English:

1702 → ONE THOUSAND SEVEN HUNDRED TWO
     → ONETHOUSANDSEVENHUNDREDTWO

Extract the distinct letters in order of appearance, dropping duplicates:

O N E T H U S A D V R W

Then fill in the remaining letters of the alphabet (J merged with I):

O N E T H U S A D V R W B C F G I K L M P Q X Y Z

This scrambled alphabet is written into a 5×5 square row by row:

O N E T H
U S A D V
R W B C F
G I K L M
P Q X Y Z

Step 2 — Block reversal

The plaintext is split into blocks whose lengths cycle through the digits of the key: with 1702, blocks of 1, 7, 0 (skip), 2 letters, repeating. Each block is reversed and the blocks are reconcatenated.

Step 3 — Substitution

Every letter is then replaced by the one sitting at its transposed position in the keyed square. This substitution combines a key effect (the alphabet in the square) with a geometric effect (transposition inside the grid).

What is the legacy of Bazeries cipher?

Bazeries also designed:

• A cipher cylinder with 20 discs, directly in line with the Jefferson disk — and thus with the later M-94 of the US Army.
• A two-alphabet disc combining Alberti with a multi-key system.
• The Four Square method (1901), a precursor to Delastelle’s Four-Square cipher.

His career is also marked by his controversial reading of the Man in the Iron Mask ciphers: he argued, without definitive proof, for a specific identification of the prisoner.

What are the weaknesses of Bazeries cipher?

Strengths

• Three layers (reversal + keyed square + positional substitution) resist naive monoalphabetic analysis.
• The numeric key is easy to memorise (4 to 8 digits suffice) and produces a highly scrambled alphabet.

Weaknesses

• As soon as the spelled-out key is guessed, the whole structure collapses.
• The linguistic dependency (the way numbers are spelled) exposes the cipher to dictionary attacks on common round numbers.
• Block reversal leaves a detectable fingerprint: certain positions retain their frequency signatures.

Against a modern cryptanalyst, it breaks quickly — but it still makes for a pleasing hand-solve challenge.

In CipherChronicle

Bazeries is a hybrid cipher: it stitches together the two great families of classical cryptography — substitution and transposition — into a single operation. Companion grids can ask the player to identify the numeric key from historical hints (dates, altitudes, distances), then apply all three steps.