How does Bacon's cipher work?

Each letter is assigned a unique 5-symbol sequence of two characters, traditionally A and B. For example A=AAAAA and B=AAAAB. A message is encoded by replacing every letter with its 5-symbol group.

What is the difference between the 24 and 26 letter versions?

The original 24-letter scheme gives I and J the same code and U and V the same code, because those pairs were not distinguished in Bacon's time. A later 26-letter variant gives every modern letter its own unique code. This tool uses the classic 24-letter scheme.

Why is it called a steganographic cipher?

Bacon hid the A and B values not as visible letters but as two subtly different typefaces, so the message looked like ordinary text. The hidden binary pattern revealed the real message to anyone who knew which font meant A and which meant B.

Can I decode binary 0 and 1 input?

Yes. The decoder treats 0 as A and 1 as B, so you can paste a binary string and it will be grouped into fives and converted to letters.

Is Bacon's cipher secure?

No. Once someone knows the two symbols and the 5-bit table, decoding is immediate. Its strength was always concealment, not cryptographic difficulty.

Bacon's Cipher Encoder & Decoder

Bacon’s cipher, devised by Francis Bacon around 1605, is really a method of steganography: it encodes a message as a binary pattern and then hides that pattern inside ordinary-looking text. Each letter of the secret message becomes a group of five symbols drawn from just two values, traditionally written as A and B. This tool encodes text into those 5-symbol groups and decodes them back, in your browser.

How it works

Assign every letter a fixed 5-symbol code built from two characters. With five positions and two values there are 32 possible patterns, more than enough for the alphabet. The classic 24-letter table starts at A=AAAAA, B=AAAAB, C=AAABA and continues in order, with I and J sharing a code and U and V sharing a code.

To encode, replace each letter with its group and discard anything that is not a letter. To decode, normalise the input into A and B symbols (treating 0 as A and 1 as B), split it into groups of five, and look each group up in the table. Bacon’s true trick was then to hide the A/B values as two slightly different fonts inside an innocent cover text.

Example

Encode the word GERA in the 24-letter scheme:

G → AABBA
E → AABAA
R → BAAAA
A → AAAAA

So GERA becomes AABBA AABAA BAAAA AAAAA. Decoding those groups returns the original letters.

Notes

Because I/J and U/V share codes, decoded text may show an I where a J was meant, or a U where a V was meant — resolve these from context. The binary nature of the cipher makes it a neat introduction to encoding information in 5-bit units, the same idea behind Baudot teleprinter codes.