The Alberti cipher, created by Leon Battista Alberti in the 15th century, is recognized as one of the earliest examples of a polyalphabetic substitution cipher. Alberti, an Italian Renaissance polymath, developed this cipher as a response to the need for stronger, more secure encryption methods that could withstand frequency analysis, a technique that had become effective against simpler monoalphabetic ciphers. The Alberti cipher represents a significant advancement in cryptography, as it was among the first to use multiple alphabets to encode a single message.
Alberti invented a cipher disk, known as the Alberti Cipher Disk, which consisted of two rotating rings: an outer ring containing the plaintext alphabet in uppercase letters and an inner ring with a shifted or scrambled alphabet in lowercase. The outer disk would remain stationary, while the inner disk could rotate to change the substitution relationship between plaintext and ciphertext characters. This enabled the use of multiple alphabets throughout the message, significantly increasing encryption strength.
The key innovation of the Alberti cipher was the ability to change the substitution alphabet during encryption. This process is known as polyalphabetic substitution, where a keyword or special signal letter in the message would prompt the encoder to rotate the inner disk, thereby switching the alphabet used for substitution. By frequently changing the alphabet, the alberti cipher made it challenging for unauthorized parties to decipher the text by frequency analysis, as the same plaintext character would not always correspond to the same ciphertext character.
Suppose we want to encrypt the message "SECRET" using an Alberti cipher disk. We can set the inner disk so that it initially aligns with the letter A on the outer disk corresponding to a lowercase letter on the inner disk. Each time we encounter a special signal, we rotate the inner disk by a predetermined increment.
- Initial Alignment: Outer disk A aligns with inner disk d.
- First Letter: S (outer disk) → encrypted as d.
- Rotate Inner Disk by One.
- Second Letter: E (outer disk) → encrypted as w.
Repeat this for each letter in "SECRET," rotating the inner disk based on a predefined rule or keyword.
Since the Alberti cipher employs a polyalphabetic approach, a full substitution table would not apply as each rotation modifies the substitution dynamically.