The Baconian Cipher, also known as Francis Bacon’s Substitution Cipher, was developed by Francis Bacon around 1605 and described in his work De Augmentis Scientiarum (1623). It is a steganographic substitution system that encodes each letter of the alphabet into a unique five-character pattern composed of two symbols, traditionally “A” and “B”.

Unlike many classical ciphers that substitute one letter for another, the Baconian system converts each plaintext letter into a five-bit binary-like sequence. These sequences can then be hidden inside seemingly ordinary text by using two typefaces, two fonts, bold vs. regular styling, or any other pair of visual variations.

Baconian Cipher: The 5-Bit Alphabet

Bacon’s original version used a 24-letter alphabet, combining I/J and U/V. Each letter is represented by a five-character sequence of A’s and B’s:

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

Baconian Cipher: Encoding

To encode a message, replace each plaintext letter with its corresponding five-character A/B sequence. For example, encrypting “HELLO”:

Plaintext: H E L L O

H = AABBB
E = AABAA
L = ABABA
L = ABABA
O = ABBAB

Ciphertext:
AABBB AABAA ABABA ABABA ABBAB

In practice, these A and B symbols are often disguised. For example, A could be represented by normal text and B by italic text, allowing the message to be hidden inside an innocent-looking paragraph.

Baconian Cipher: Decoding

To decode, group the ciphertext into five-character blocks and translate each block back into its corresponding letter.

Ciphertext:
AABBB AABAA ABABA ABABA ABBAB

AABBB → H
AABAA → E
ABABA → L
ABABA → L
ABBAB → O

Plaintext: HELLO

Baconian Cipher: Notes

The brilliance of the Baconian Cipher lies in its early use of binary encoding principles — long before modern computing formalized binary systems. Each letter is effectively reduced to a five-bit pattern. While the cipher itself is simple to break if discovered, its true strength lies in concealment. It represents one of the earliest structured attempts to hide information inside other information — a foundational idea in steganography.

Baconian Cipher

Substitution
Symmetric
Bitwise