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Puzzlehunt 2015

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author: Will Earley

  1. Search for cyrene sieve on google to work out that the title refers to the Sieve of Eratosthenes (who was from Cyrene). This is an algorithm to determine prime numbers.
  2. The linked text file appears to be in octal, but this is deceiving. In the range [0,8), there are four primes and four non-primes. 'Sieving' the file by primes and non-primes, we get two equal-length lists.
  3. The beginnings of both these lists have the same pattern, suggesting that they may encode a file header, perhaps a BMP as per the title. Replacing each of [2,3,5,7] with [00,01,10,11] and similarly for the non-primes, we indeed obtain two BMP files.
  4. The files look like white noise, but have the same dimensions, if you flick between them you will notice the white noise is the same except for some points in the middle. If you have a keen eye, you can solve the puzzle now, else take the xor of the pixels to obtain the message, 'WAN SHI TONG'.

the XOR of the two images