Ciphers have been used since the begging of the written word to aid in the recording and transferring secret messages and information. There are many different ciphers and most require a code or a key to decode or solve the message. This topic will discuss in depth the Playfair Cipher and how they work.

Although this page does not discus amateur radio related items, it does come into play with historical facts and for future activities you can work on with your kids. It is important to note to your kids that it is illegal to transmit coded messages over amateur radio frequencies according to the FCC Part 97 document; however, we feel that it is equally important that the students are aware of the historical significance that these ciphers play in military operations and how CW is actually used for military communications.

History

The Playfair cipher or Playfair square or Wheatstone-Playfair cipher or Wheatstone cipher is a manual symmetric encryption technique and was the first literal diagram substitution cipher. The scheme was invented in 1854 by Charles Wheatstone (photo on left), but bears the name of Lord Playfair who promoted the use of the cipher.

The technique encrypts pairs of letters (bigrams or diagrams), instead of single letters as in the simple substitution cipher and rather more complex Vigenère cipher systems then in use. The Playfair is thus significantly harder to break since the frequency analysis used for simple substitution ciphers does not work with it. The frequency analysis of bigrams is possible, but considerably more difficult. With 600 possible bigrams rather than the 26 possible monograms (single symbols, usually letters in this context), a considerably larger cipher text is required in order to be useful.

It became known as the Playfair cipher after Lord Playfair (photo right), who heavily promoted its use, despite its invention by Wheatstone. The first recorded description of the Playfair cipher was in a document signed by Wheatstone on 26 March 1854.

It was rejected by the British Foreign Office when it was developed because of its perceived complexity. Wheatstone offered to demonstrate that three out of four boys in a nearby school could learn to use it in 15 minutes, but the Under Secretary of the Foreign Office responded, "That is very possible, but you could never teach it to attachés."

It was used for tactical purposes by British forces in the Second Boer War and in World War I and for the same purpose by the British and Australians during World War II. This was because Playfair is reasonably fast to use and requires no special equipment - just a pencil and some paper. A typical scenario for Playfair use was to protect important but non-critical secrets during actual combat e.g. the fact that an artillery barrage of smoke shells would commence within 30 minutes to cover soldiers' advance towards the next objective. By the time enemy cryptanalysts could decode such messages hours later, such information would be useless to them because it was no longer relevant.

During World War II, the Government of New Zealand used it for communication among New Zealand, the Chatham Islands, and the coastwatchers in the Pacific Islands

For more information...

How it Works

We know that the video was rather quick with an explanation and overview on how the cipher works for encoding and decoding, so we have put together a few things for you to help learn more about it and how you can actually teach this to your kids.

First, we have 2 handouts that you can use as a teacher's aid and handouts for the students. ONLY PROVIDE THE STUDENTS with the first three pages of the "Playfiar Cipher" handout, and the second handout can be passed out during your presentation when it talks about the message and President Kennedy (see notes in the presentation.)

Links to Handouts (Can also be found on the activities page)

• The Playfair Cipher Handout
• The Kennedy Playfair Cipher Practice Handout

Classroom Presentation

Next we have put together and provided a presentation on the Plaifair Cipher that we used with our students. Please feel free to use the same presentation with your students or to learn more about the cipher yourself.

We hope that you and your students find this topic as exciting as we and our students have. This is a great way to demonstrate the use of coding and how they can be incorporated with hidden or secretive messages. Again, we do not allow our students to transmit the actual encoded message over the air waves, but there is nothing to stop the transmission as to clues for the key to the cipher or to use this in conjunction with a fox hunt (or ARDF) type activity. So, 73 and have fun with this topic.

Do you have questions or more information to share?