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Capture the Flag

Six messages to crack. Nothing is sent anywhere and there is nothing to sign up for.

Capture the flag

Real code-breakers compete in contests called capture the flag. Crack the message, then type the answer as a flag, like this: SOC{THE ANSWER}. Your score is kept on this device only — nothing is sent anywhere and there is nothing to sign up for.

0 points
Level 110 points
A message left in four pens.
Cipher: Pigpen
Level 110 points
Caesar would have read this in a second.
Cipher: Shift cipher
Level 220 points
Two rails, and three nulls on the end.
Cipher: Rail fence
Level 220 points
Nothing but numbers.
Cipher: Polybius squares
Level 330 points
The key word is a girl’s name with five letters.
Cipher: Key word columns
Level 330 points
One key word built a whole new alphabet.
Cipher: Key word alphabet
Level 110 points
No key at all. The alphabet is simply the wrong way round.
Cipher: Atbash
Level 220 points
Eight switches for every letter.
Cipher: Binary
Level 220 points
Somebody knocked this on the wall of the next room.
Cipher: Tap code
Level 330 points
The letters travel in pairs.
Cipher: Playfair
Level 440 points
No key word, no pattern, and far too many keys to try. Count the letters instead.
Cipher: Simple substitution
Level 440 points
Every letter was cut in half and mixed with its neighbours.
Cipher: Bifid
Cross-branch40 points
A lamp flashed this from the hill. The dots and dashes are not the message.
Two steps: Morse, then a shift of 5
Cross-branch40 points
An ordinary note. Read down the left-hand edge, then reverse the alphabet.
Two steps: An acrostic, then atbash
Cross-branch40 points
Three long flashes, then two short. It is not a word. It tells you where to look.
Two steps: A signal that points at a hiding place
Cross-branch50 points
The whole job. Read the signal, find the hidden line, then break the cipher on it.
Two steps: Signal, then find, then break
Cross-branch40 points
A signaller on the far bank. Read the flags, then reverse the alphabet.
Two steps: Semaphore, then atbash
Cross-branch40 points
Carved on a wall. Each picture stands for the first letter of the thing it shows.
Two steps: Hieroglyphs, then a rail fence
Mix and match60 points
Every letter arrives a different way. Morse, then semaphore, then braille, then a hieroglyph.
Two steps: Four systems, one word

Why these four are different

Twelve of the challenges are one cipher each, climbing from ten points to forty. The other seven need two steps and two branches: read the signal, or find the hiding place, and only then break the cipher. That is how it was really done, and it is why a code-breaker’s first question is never which cipher, but what am I even looking at. The last one is worth sixty points and arrives in four different systems at once.