Original Alberti Cipher

• A traditional Alberti cipher consisted of two metal discs connected by a common axle. The outer, immobile disc contained uppercase letters of the Latin alphabet, minus H, K and Y, and the numbers 1 through 4. The Latin alphabet matches the English alphabet, but lacks the letters J, U and W. The numbers were intended for use with a code book that contained phrases and words with corresponding four-digit codes. The inner, mobile disc contained random uppercase letters of the Latin alphabet and "et," which probably meant "and." This type of Alberti cipher was used until the 16th century.
  
  

How It Worked

• The original cipher discs worked by matching a mark on the inner disc with a letter on the outer disc. To create the key, the correspondent only needed to know which letter to match with the mark. As the inner disc rotated to match the mark to any of the letters on the outer disc, users also could change the code during the message. For example, correspondents could use a trigger word to let each other know when to shift the inner disc during the message. As the shift of the disc would create a new pattern in the key, this further complicated the cipher and made it difficult to break the code.
  
  

Why It Worked

• Based on the assumptions of frequency analysis -- a method using the frequency of the letters in a cryptotext in reference to a normal distribution of letters to find the code -- the cipher could have been unbreakable. Frequency analysis deciphers monoalphabetic cryptograms, but proves ineffective in solving polyalphabetic cryptograms such as Alberti's cipher, because the letter distribution isn't even throughout the message.
  
  

Porta's Variation

• In the 16th century, Giovan Battista della Porta developed a variation of the Alberti cipher that used a two-dimensional table to replace the rotating disc. This table used one keyword to form a version of the alphabet and another keyword to signal which line of the table would be used for each letter of the message. On one line, letters were written in a straight line but in non-alphabetical or random order. Another line of the same letters in the same pattern was written directly underneath the first line, but these letters were shifted one position to the right or left. A third line, written under the second, again shifted one position. These lines were numbered down the left side of the table, forming the number cues for the translation.