Vigenère cipher

The Vigenère cipher is a form of written communication created in the sixteenth century to encode text to keep its meaning secret or private. It is a form of code known as polyalphabetic, meaning it uses more than the alphabet as its source. The cipher was broken in the nineteenth century and no longer considered an effective tool for messages that require true secrecy. However, the Vigenère cipher is often used by hobbyists called geocachers who solve puzzles, codes, and ciphers as an enjoyable mental exercise. Ciphers such as the Vigenère were also the forerunners of some of the programming used in modern computers.

Background

A cipher is a type of written code that allows two or more people to send encrypted, or secret, messages to each other. Codes and ciphers have existed for at least four thousand years according to ancient written records. The hieroglyphics used by the ancient Egyptians are considered an early form of code because they were intentionally vague in their meaning. This was meant more as a way of making the writing about the Egyptian royalty seem more mysterious instead of a way to keep it completely secret.

The hieroglyphs of the Egyptians were similar to the ideographs used by the Chinese. This type of writing lends itself to ciphering and encryption because the pictures it uses can represent a meaning other than the obvious. While there is evidence that the ancient Chinese used coding and encryption, it does not appear to have been widespread or used for official purposes like military campaigns.

The Indian government was more likely to use ciphers and codes to transmit secret messages among spies. While these codes were more complex than those used by the Chinese or Egyptians, they used a simple form of coding that involved making phonetic substitutions within the alphabet. The ancient Hebrews who recorded the books of the Bible used a code more similar to those used today. It substituted letters by flipping the alphabet; the letter a became z, b changed to y, c was represented by x, etc.

The Greeks developed a number of different ways to send coded messages, including some similar to the one used by the Hebrews and another form that used a long strip of papyrus wrapped around a stick. The message was written on the wrapped papyrus, and the recipient needed to wrap it around a similar stick to decode the message.

Cryptology, or the study and use of codes and ciphers, significantly advanced during the Middle Ages when more sophisticated methods were developed to send secret messages between governments. This continued into the twenty-first century, especially for military organizations. Much of the success of the Allied Forces during World War II depended on the accomplishments of the member nations in developing and using codes and ciphers.

Overview

The Vigenère cipher was devised by Blaise de Vigenère (1523–1596), a French diplomat. Diplomats and other government officials of the time made frequent use of codes and ciphers to send messages they hoped would give their country an advantage in some way in dealing with other countries. Vigenère's cipher used an alphabet to establish the key, or method of decoding the cipher. Many different versions of an alphabet could be used for the code, which is why it is called a polyalphabetic code. Very often, the English alphabet would be used.

The code would be established by means of a Vigenère square. For the twenty-six-character English alphabet, this square would be made up of twenty-six squares across and twenty-six squares down the side. The alphabet would be written across and down, one letter to a square. The alphabet is then repeated over and over until all blocks are filled, but the letters are shifted to the left one position, so the second row starts with the letter b, the third row with the letter c, and so on. Cryptographers call this a Caesar shift; it is said it was developed by Julius Caesar (100 BCE–44 BCE) for military communications.

This square is used by means of a keyword, which all parties using the code know. To use the code, the keyword is written out underneath the text to be encoded, also called the plaintext. The keyword is repeated as many times as necessary to make it as long as the plaintext. To further obscure the meaning, all punctuation, breaks between words, and capitalization is omitted, and the plaintext and keyword are broken into groups of five letters.

The coder now uses the Vigenère square by finding the first letter of the plaintext in the alphabet that goes across the top of the square and the first letter of the code word in the alphabet that goes down the side of the square and following each line to where it intersects. The letter in the block where the top column and the side column meet becomes the first letter of the code. This process is repeated until the entire message is encoded.

The person decoding the message works the process in reverse. The decoder takes the first letter of the coded message and finds where it appears on the square in relation to the keyword down the side. By following where the first letter of the keyword and the first letter of the encoded text align to the alphabet at the top, the decoder can find the first letter of the coded message.

The Vigenère cipher was broken in 1863 by Friedrich Kasiski (1805–1881), a major in the Prussian army. He used a method of determining the length of the keyword to create a series of easier-to-decrypt codes. Charles Babbage (1791–1871), considered the grandfather of the modern computer, created a different method of cracking the code in 1854. Other methods were devised that involved determining or even guessing the code word, much like some hackers attempt to guess a person's password for a computer. This method was used by Union cryptographers during the Civil War to decode Confederate messages.

In contemporary times, the Vigenère cipher is not considered secure enough to use for any real secrets or confidential information. Instead, it is often used by people who enjoy the hobby of geocaching. Geocaching involves hiding small items and leaving clues that will enable others to find them. Different forms of coding can be used, including global positioning satellite (GPS) coordinates, map coordinates, or various forms of ciphers, such as the Vigenère.

Bibliography

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