The Beaufort cipher, created by Sir Francis Beaufort

Francis Beaufort

Rear-Admiral Sir Francis Beaufort, FRS, FRGS was an Irish hydrographer and officer in Britain's Royal Navy...

, is a substitution cipher

Substitution cipher

In cryptography, a substitution cipher is a method of encryption by which units of plaintext are replaced with ciphertext according to a regular system; the "units" may be single letters , pairs of letters, triplets of letters, mixtures of the above, and so forth...

 that is similar to the Vigenère cipher

Vigenère cipher

The Vigenère cipher is a method of encrypting alphabetic text by using a series of different Caesar ciphers based on the letters of a keyword. It is a simple form of polyalphabetic substitution....

 but uses a slightly modified enciphering mechanism and tableau

Cryptographic tableau

A cryptographic tableau is a construct of rows and columns with which one can encrypt and decrypt information. The Vigenère cipher is a well-known example of a cryptographic scheme that uses a tableau.- External links :*...

.
. Its most famous application was in a rotor-based cipher machine, the Hagelin M-209

M-209

In cryptography, the M-209, designated CSP-1500 by the Navy is a portable, mechanical cipher machine used by the US military primarily in World War II, though it remained in active use through the Korean War...

.

Using the cipher

To encrypt (or decrypt for that matter), first choose the plaintext character from the top row of the tableau,call this column P. Secondly, travel down column P to the corresponding keyLetter K. Finally, move directly left from the Key letter to the left edge of the tableau, the CipherText encryption of Plaintext P with Key K will be there. For example if encrypting Plain text character "d" with Key "m" the steps would be: 1) find the column with "d" on the top, 2) travel down that column to find Key "m", 3) travel to the left edge of the tableau to find the CipherText letter ("J" in this case). The Beaufort cipher is a reciprocal cipher

Reciprocal cipher

A reciprocal cipher means, just as one enters the plaintext into the cryptography system to get the ciphertext, one could enter the ciphertext into the same place in the system to get the plaintext. Sometimes also referred as self-reciprocal cipher....

, that is, Decryption and Encryption algorithms are the same.

Distinguished from 'variant Beaufort'

The Beaufort cipher should not be confused with the "variant Beaufort" cipher. In variant Beaufort, encryption is performed by performing the decryption step of the standard Vigenère cipher, and likewise decryption is performed by using Vigenère encryption.