Alberti cipher

The Alberti cipher is considered by historians to be the true birth of modern Western cryptography. Described by Leon Battista Alberti in De cifris (around 1467), it introduces the polyalphabetic principle for the first time — using a mechanical device: the cipher disk.

The Alberti disk

Two concentric rings, a fixed outer and a rotating inner, each carry an alphabet — the outer in plain Latin, the inner shuffled in an order known only to the correspondents. In use, the small disk turns to change the mapping mid-message, triggered by a signal letter (e.g. a capital or a digit).

Alberti also describes adding a numeric supercipher: inserting numbers in the ciphertext to deepen confusion. In the 15th century, this was extraordinarily ahead of its time.

Principle

1. Both parties agree on the inner disk permutation (the key).
2. The message starts at an initial disk position.
3. At agreed intervals (every 3-5 letters, say) or upon a signal, the disk rotates one position.
4. Each new position defines a new monoalphabetic substitution.

Formally, if d(i) is the inner disk’s position at letter i, the cipher letter is given by a table T:

C_i = T[d(i)][P_i]

Simplified example

Imagine a disk that applies a simple shift and rotates every 5 characters. On CIPHERCHRONICLE with successive shifts +3, +7, +11:

Segment 1 (pos 0-4, +3)   : C I P H E → F L S K H
Segment 2 (pos 5-9, +7)   : R C H R O → Y J O Y V
Segment 3 (pos 10-14, +11): N I C L E → Y T N W P

Result: FLSKHYJOYVYTNWP.

The real Alberti disk used shuffled alphabets rather than simple shifts — but the principle is the same.

Historical importance

• 1467: Alberti invents the first polyalphabetic cipher with a physical device.
• 1508: Trithemius systematizes the idea into the tabula recta.
• 1553: Bellaso adds the cyclically-repeated text key.
• 1586: Vigenère publishes the version that will carry his name.

Alberti is thus the direct ancestor of three centuries of cryptography.

Weaknesses

• Signaling the disk turn (the signal letter) is a fragile point: it leaks the structure.
• If the disk rotates at regular intervals, Kasiski’s attack becomes possible again.
• The inner disk has at most 26 positions — each segment can be brute-forced.

In CipherChronicle

Alberti is an opportunity for visual staging: the cipher disk makes a tangible game object, easily rendered on screen. Grids built on it can let the player rotate a virtual disk to experiment with the transformation.