Encryption of Polybius

The Greek historian Polybius (203-120 B. C.), created a system of sending messages by means of torches

The method consisted essentially in the creation of a square matrix of 5 \times 5 such as the following

\begin{pmatrix}&1&2&3&4&5\\1&A&B&C&D&E\\2&F&G&H&I/J&K\\3&L&M&N&O&P\\4&Q&R&S&T&U\\5&V&W&X&Y&Z\end{pmatrix}

The message is represented by numbers that form the row and column whose intersection gives as a result the letter you want to send

While the method of Polybius does not initially had a purpose cryptographic, yes that is the base of later systems, and the first known case of replacement monoalfabética multiliteral

A variant of the encryption of Polybius, used by the communists in the Spanish civil war consisted of generating a table with three rows of ten columns

The first row had no numbering, and the second and third rows are ultimately respectively with two of the unused numbers in the columns of the first row

The columns are ultimately with a permutation of the digits from zero to nine

The encryption process consisted in putting a word of eight or fewer different letters in the first row

In this word were removed the letters repeated and the rest, until you complete the alphabet, arranged in two rows

Encryption is similar to that of Polybius, but here the letters can be encoded as one or two numbers

Example of the alternative communist of the encryption of Polybius

The Spanish communists had to send the following message, which would not that franco's troops interceptasen

C=EN PIE FAMELICA LEGION

Taking as a key:

K=FUSIL

Using the following table

\tiny\begin{pmatrix}&8&3&0&2&4&6&1&7&5&9\\&F&U&S&I&L\\5&A&B&C&D&E&G&H&J&K&M\\1&N/\widetilde{N}&O&P&Q&R&T&V&X&Y&Z \end{pmatrix}

So we is that the encrypted message is:

M=54 18 10 2 54 8 58 59 54 4 2 50 58 4 54 56 2 13 18

We can decipher the M previous

We go to the table of encryption, if we have two numbers, check which is your row and the second corresponds to the column

The row of the intersection of both will be the letter that will be used in the message decryption

If we have a figure, is the one corresponding to the column and the row is the one corresponding to the key

There may be confusion in the case of 18, because they share a common position of the N and the Ñ, all depends on the context of the message (in this case the N)

We will repeat the process until you get the message clear

Getting the original C:

C=ENPIEFAMELICALEGION