Bacon’s cipher or the Baconian cipher is a method of steganography (a method of hiding a secret message as opposed to just a cipher) devised by Francis Bacon in 1605. A message is concealed in the presentation of text, rather than its content.
The Baconian cipher is a substitution cipher in which each letter is replaced by a sequence of 5 characters. In the original cipher, these were sequences of ‘A’s and ‘B’s e.g. the letter ‘D’ was replaced by ‘aaabb’, the letter ‘O’ was replaced by ‘abbab’ etc. Each letter is assigned to a string of five binary digits. These could be the letters ‘A’ and ‘B’, the numbers 0 and 1 or whatever else you may desire.
There are 2 kinds of Baconian ciphers –
- The 24 letter cipher: In which 2 pairs of letters (I, J) & (U, V) have same ciphertexts.
Letter Code Binary A aaaaa 00000 B aaaab 00001 C aaaba 00010 D aaabb 00011 E aabaa 00100 F aabab 00101 G aabba 00110 H aabbb 00111 I, J abaaa 01000 K abaab 01001 L ababa 01010 M ababb 01011 Letter Code Binary N abbaa 01100 O abbab 01101 P abbba 01110 Q abbbb 01111 R baaaa 10000 S baaab 10001 T baaba 10010 U, V baabb 10011 W babaa 10100 X babab 10101 Y babba 10110 Z babbb 10111
The 26 letter cipher: In which all letters have unique ciphertexts.
Letter Code Binary A aaaaa 00000 B aaaab 00001 C aaaba 00010 D aaabb 00011 E aabaa 00100 F aabab 00101 G aabba 00110 H aabbb 00111 I abaaa 01000 J abaab 01001 K ababa 01010 L ababb 01011 M abbaa 01100 Letter Code Binary N abbab 01101 O abbba 01110 P abbbb 01111 Q baaaa 10000 R baaab 10001 S baaba 10010 T baabb 10011 U babaa 10100 V babab 10101 W babba 10110 X babbb 10111 Y bbaaa 11000 Z bbaab 11001
We will extract a single character from the string and if its not a space then we will replace it with its corresponding ciphertext according to the cipher we are using else we will add a space and repeat it until we reach the end of the string. For example ‘A’ is replaced with ‘aaaaa’
We will extract every set of 5 characters from the encrypted string and check if the first character in that set of 5 characters is a space. If not we will lookup its corresponding plaintext letter from the cipher, replace it and increment the index of character by 5 (to get the set of next 5 characters) else if its a space we add a space and repeat a process by incrementing the current index of character by 1
In Python, we can map key-value pairs using a data structure called a dictionary. We are going to use just one dictionary in which we will map the plaintext-ciphertext pairs as key-value pairs.
For encryption we will simply lookup the corresponding ciphertext by accessing the value using the corresponding plaintext character as key.
In decryption we will extract every 5 set of ciphertext characters and retrieve their keys from the dictionary using them as the corresponding value. For an accurate decryption we will use the 26 letter cipher. If you are not coding in python then you can come up with your own approach.
Output aabbaaabaaaabaaabababaaba aabababbbabaaab aabbaaabaaaabaaabababaaba ENJOY
Analysis: This cipher offers very little communication security, as it is a substitution cipher. As such all the methods used to cryptanalyse substitution ciphers can be used to break Baconian ciphers. The main advantage of the cipher is that it allows hiding the fact that a secret message has been sent at all.
References: Practical Cryptography
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