On the Subject of Playfair Cipher

Encrypting secrets since 1854, exploding bombs since 2018.

Your purpose is to decrypt an encrypted message. Find the key, decrypt the message and press the correct button sequence. Easy, right?

Obtaining the Key

Find the two key parts using the following two tables. In each table, stop at the first rule that applies.

Part 1

It’s Monday* PLAY
It’s Tuesday HIDDEN
It’s Wednesday* SECRET
It’s Thursday CIPHER
It’s Friday* FAIL
It’s Saturday PARTYHARD
It’s Sunday BECOZY
* If a lit BOB indicator is present on the bomb, use the following day. BOB hates odd-numbered days (except Sunday).

Part 2

Screen Text Color
Magenta Blue Orange Yellow
Bomb has serial and parallel port SAFE EFAS MESSAGE GROOVE
Sum of serial number digits > 10 CODE EDOC QUIET ETIUQ
D batteries > AA batteries GROOVE EVOORG TEIUQ QUITE
Otherwise MESSAGE EGASSEM SAFE EDOC

Modifying the key

  • If the serial number doesn’t contain a vowel, swap the parts.
  • If you have one or more strikes, add a third part:
    • For one strike, add "ONE".
    • For two strikes, add "TWO".
    • For more strikes, add "MANY".
  • If the sum of digits in serial number is a prime number, reverse the complete key. (Prime numbers: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47)

Decrypting the message

  • Create a a 5×5 matrix of letters. Start with your keyword and fill the rest with the unused letters of the alphabet. Each letter must occur only once in the matrix, so only add the first occurrence. ‘J’ and ‘I’ are interchangeable.
  • Split the message into character pairs. For each pair:
    • If the letters appear on the same row of your matrix, replace them with the letters to their immediate left respectively, wrapping around to the right side of the row if necessary.
    • If the letters are on the same column of your matrix, replace them with the letters immediately above, wrapping to the bottom if necessary.
    • If the letters are on different rows and columns, replace each of them with the letter on the same row but in the column of the other letter in the original pair.
  • Drop any Xs that don’t make sense and locate any Is that should be Js.
Screen Text Color
Magenta Blue Orange Yellow
Decrypted Message STRIKE ABCD CDAB BADC DABC
STRIK BCDA DACB ADCB ABCD
STRYKE CDAB ACBD DCBA BCDA
STRYK DABC CBDA CBAD CDAB
ZTRIKE ABDC BDAC BACD DACB
ZTRIK BDCA DBCA ACDB ACBD
ZTRYKE CABD BCAD CDBA CBDA
ZTRYK DCAB CADB DBAC BDAC