Polybius Square

The letters of the alphabet are placed in a square table. For the Russian alphabet, a 6×6 table is used (36 cells for 33 letters + Ё). For English — 5×5 (the letters I and J share one cell). Rows and columns are numbered starting from 1.

Alice enters the original message. Each character is checked against the chosen alphabet. Case does not matter — all letters are converted to uppercase. Spaces and punctuation marks are kept without encryption.

For each letter of the message:

• Find the letter in the Polybius table
• Determine the row number (first coordinate)
• Determine the column number (second coordinate)
• Write the pair of numbers as the cipher

Example: The letter "А" in the Russian square is at row 1, column 1 → cipher "11". The letter "Б" → "12".

The ciphertext is transmitted over an open channel. Without knowing the Polybius table, an interceptor sees only a set of numbers. A secret arrangement of letters in the table can be used to strengthen protection.

Bob uses the same Polybius table:

• Read the pair of numbers
• The first number is the row number
• The second number is the column number
• Find the letter at the intersection

Example: Cipher "23" → row 2, column 3 → letter "И" (in the Russian alphabet).

After decryption, the system compares the received text with the original. If they match, the transmission was successful. Visual highlighting of the table helps track every step of the process.