Question

Difference between monoalphabetic and polyalphabetic cipher with example

Answer 1

A monoalphabetic cipher mixes up the letters of the alphabet and keeps that same arrangement for the entire message. This is the type of cipher normally seen in newspaper cryptograms and is very easy to break. A polyalphabetic cipher, by contrast, uses more than one monoalphabetic cipher within the same message.

For example, if you use a Vigenere disk and leave the two disks stationary while encoding a message, that is a monoalphabetic cipher. If you rotate the disks while encoding the message, using a keyword, then that is a polyalphabetic cipher.

The reason the monoalphabetic cipher is easy to break is because of a decryption method called letter frequency analysis, which was invented in the 1300s by Ibn ad-Duraihim. By studying text in the language of the cipher, the frequency of each letter can be determined. In English the E is the most frequent letter, followed by the T. The entire frequency order is: ETAOINSHRDLCUMWFGYPBVKJXQZ.

By substituting the most frequent letter in the ciphertext with an E and the second most frequent letter with a T, etc., the plaintext words will tend to form. For a short message, this is not exact, but it shouldn’t take long to try out a few possibilities and figure out the cipher used.

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