Playfair Cipher

This is a super hard cipher to break.  It’s encoded by taking pairs of letters and numbers from a matrix. There are three rules to follow.

  1. If both letters are in the same row, then use the letters immediately to the right of each other. (Think of the rows as wrapping from the right end back around to that same row’s left end).
  2. If both letters are in the same column, then use the letters immediately below them. If necessary, the bottom letter wraps back around to the top of the same row.
  3. If the two letters or numbers are in different rows and in different columns, then each letter is replaced by the letter in the same row that’s also in the same column of the other letter. Basically, you find each intersection of the pair. Use the letter or number below the pair and then the one above the pair.

Play Fair sounds really complicated, but that also makes it a tough code to crack! Watch the video and I’ll explain it all for you.


Download Student Worksheet & Exercises

Isn’t this cool? It makes a lot more sense when you see it in action, right? Remember: when decoding the Playfair cipher, you have to shift up instead of down and left instead of right. And it’s easy to make a mistake by encoding in the incorrect order. So always  double check your cipher before sending it on to the recipient. Mistakes make messages much harder for the decoder to interpret!

Exercises

  1. What is the name given to the following table?
A H M V L 3 Y D
X K B 5 P Z E O
N 7 W U F T 6 J
G R 2 Q C A I S

Use the table in 1 above to answer question 2 – 10.

What will be the cipher for the following?

  1. KB
  2. HR
  3. AR
  4. EU
  5. COME TO SCHOOL
  6. GO HOME THEN

Decode the following messages

  1. 73 3N SG YZ 6X
  2. SG MN YK A7 JO HD
  3. Why is it important the number of letters in the message to be encoded be even?