Bacon's cipher or the Baconian cipher is a method of steganographic message encoding devised by Francis Bacon in 1605. A message is concealed in the presentation of text, rather than its content. Bacon cipher is categorized as both a substitution cipher (in plain code) and a concealment cipher (using the two typefaces).
To encode a message, each letter of the plaintext is replaced by a group of five of the letters 'A' or 'B'. This replacement is a 5-bit binary encoding and is done according to the alphabet of the Baconian cipher (from the Latin Alphabet), shown below:
A second version of Bacon's cipher uses a unique code for each letter. In other words, I, J, U and V each have their own pattern in this variant:
The writer must make use of two different typefaces for this cipher. After preparing a false message with the same number of letters as all of the As and Bs in the real, secret message, two typefaces are chosen, one to represent As and the other Bs. Then each letter of the false message must be presented in the appropriate typeface, according to whether it stands for an A or a B.
To decode the message, the reverse method is applied. Each "typeface 1" letter in the false message is replaced with an A and each "typeface 2" letter is replaced with a B. The Baconian alphabet is then used to recover the original message.
Any method of writing the message that allows two distinct representations for each character can be used for the Bacon Cipher. Bacon himself prepared a Biliteral Alphabet for handwritten capital and small letters with each having two alternative forms, one to be used as A and the other as B. This was published as an illustrated plate in his De Augmentis Scientiarum (The Advancement of Learning).
Because any message of the right length can be used to carry the encoding, the secret message is effectively hidden in plain sight. The false message can be on any topic and thus can distract a person seeking to find the real message.
Baconian cipher example
The word 'steganography', encoded with quotation marks, where standard text represents "typeface 1" and text in boldface represents "typeface 2":
To encode a message each letter of the plaintext is replaced by a group of five of the letters 'A' or 'B'.
The pattern of standard and boldface letters is:
ba aabbaa b aaabaaa abba aaaaaa bb aaa bbabaabba ba aaaaaaaa ab b baaab bb babb ab baa abbaabb 'b' bb 'b'.
This decodes in groups of five as
baaab(S) baaba(T) aabaa(E) aabba(G) aaaaa(A) abbaa(N) abbab(O) aabba(G) baaaa(R) aaaaa(A) abbba(P) aabbb(H) babba(Y) bbaaa bbaab bbbbb
where the last three groups, being unintelligible, are assumed not to form part of the message.
Bacon and Shakespeare
Some proponents of the Baconian theory of Shakespeare authorship, such as Elizabeth Wells Gallup, have claimed that Bacon used the cipher to encode messages revealing his authorship in the First Folio. However, American cryptologists William and Elizebeth Friedman refuted the claims that the works of Shakespeare contain hidden ciphers that disclose Bacon's or any other candidate's secret authorship in their The Shakespeare Ciphers Examined (1957). Typographical analysis of the First Folio shows that a large number of typefaces were used, instead of the two required for the cipher, and that printing practices of the time would have made it impossible to transmit a message accurately.
- Baudot, a set of 5-bit codes for the English alphabet, used world-wide for teleprinter communications during most of the 20th century.
- Null Cipher, a related cipher.
- Bacon, Francis (1605). The Proficience and Advancement of Learning Divine and Humane.
- Bacon, Francis (1640). Of the Advancement and Proficience of Learning. Translated by Wats, Gilbert. Oxford University. pp. 257–271.
- Dupuy, Jr., Paul J. "The Advancement of Learning". An Authorship Analysis – Francis Bacon as Shake-speare. London: Shake-n-Bacon. Chapter 1. Archived from the original on 2017-03-18. Retrieved 2017-03-18.
- Helen Fouché Gaines, Cryptanalysis: a Study of Ciphers and Their Solutions (1989), page 6]
- Biliteral can mean: "written in two different scripts", Oxford English Dictionary
- Kahn, David (1996). The Code-breakers (2nd ed.). New York: Scribner. pp. 882–888. ISBN 0-684-83130-9.
- Dunin, Elonka (17 April 2017). "Cipher on the William and Elizebeth Friedman tombstone at Arlington National Cemetery is solved" (PDF). Elonka.com.
- William Friedman and Elizebeth Friedman, The Shakespearean Ciphers Examined, Cambridge University Press, 1957