Time to play fair

You receive a message from your sister back on Earth. Just as a couple of years ago, you still communicate with each other using a private cipher - for a little bit of nerdy fun between siblings. Recently you both agreed to switch to using a Playfair cipher, so that’s what you are going to have to figure out in order to read the message.

Since there are a few ways of implementing playfair, you remember the two of you agreed to the following stipulations:

  • Replace any and all occurrences of j with the letter i.
  • For words of odd length, append an x (unless the last letter is already x, in which case use a q).

Example

Consider this example playfair grid, created using the encryption key of helloworld and the phrase to decode of wp nehslv ewgw.

h e l o w
r d a b c
f g i k m
n p q s t
u v x y z
  • Notice that repeat occurrences of letters have been ignored when created the grid (only the first l and o appear).
  • The rest of the alphabet has been added, ignoring letters used by the cipher key and the letter j.
  • Break each word of the phrase into letter pairs: wp, ne hs lv and ew gw.
  • wp appears on different rows and columns in the grid. That means w is replaced by the letter at the same row as w but the column of p, which is the letter e. Likewise, p is replaced by the letter at the same row as p but the column of w, which is the letter t.
  • The pairs ne, hs, lv, and gw all appear in different rows and columns to their paired letter, so the same method applies.
  • The pair ew appear in the same row to each other, so in that case, the e is replaced by the letter immediately to the left which is h, and the w is also replaced with the letter immediately to the left which is o (wrap around if required. For instance h would be decoded with w in the above example). A similar process applies if a pair of letters appear in the same column, you decode with the letter immediately above each letter.

The above phrase decodes to et phonex home. The x in phonex appears as the x was added to ensure all words have an even number of letters. It is one of the quirks of the playfair algorithm.

Your task

Obtain your decryption key and message to decode from your input data. The decoded message is your answer.

Copyright © Paul Baumgarten.