Bacon cipher

The Bacon cipher (or biliteral cipher) was invented by Francis Bacon in 1605. It uses a two-symbol alphabet — A and B — where every letter is represented by a sequence of five symbols. Bacon intended it to be hidden inside a carrier text via two slightly different fonts, making the message invisible to unwary readers: an ancestor of modern steganography.

Principle

The original Bacon table merges I/J and U/V (24-letter alphabet). Each plaintext letter is coded on 5 bits:

A = AAAAA   H = AABBB   O = ABBAB   V = BAABB
B = AAAAB   I = ABAAA   P = ABBBA   W = BABAA
C = AAABA   K = ABAAB   Q = ABBBB   X = BABAB
D = AAABB   L = ABABA   R = BAAAA   Y = BABBA
E = AABAA   M = ABABB   S = BAAAB   Z = BABBB
F = AABAB   N = ABBAA   T = BAABA
G = AABBA

With two symbols and 5 positions, you cover 2⁵ = 32 combinations — enough for 24 or 26 letters. Mathematically, this is binary code, three centuries before computing.

Example

CIPHE → AAABA ABAAA ABBBA AABBB AABAA (25 symbols — exactly one 5×5 grid).

Variants

• 26-letter Bacon — modern tables that separate I/J and U/V, still 5 bits.
• Steganographic Bacon — A and B are not written explicitly: they correspond to two typographic styles (roman/italic) in an unremarkable text.
• Baudot, ASCII — direct descendants of the biliteral idea.

Strengths and weaknesses

Strengths

• The hiding inside a carrier text is its historical strength: an innocuous message can contain a second, hidden one.
• The system is intellectually appealing and easy to memorize.

Weaknesses

• Once the two symbols are spotted (often A/B, 0/1, ●/○), the code is trivially readable: group by 5 and consult the table.
• The ciphertext is five times longer than the plaintext, limiting practical use.

In CipherChronicle

Bacon yields an extremely graphical grid — you can play with two colors, two shapes, two fonts. Puzzles can blend aesthetics and decoding, and introduce the notion of binary code in a natural way.