Skip to main content
The National Cipher Challenge

Reply To: Extracurricular Challenges

#97998
upsidedown
Participant

I think this is a variation on the Caesar cipher, with:

  • A 25-letter alphabet (Q removed)
  • The cipher alphabet proceeds through all odd letters then all even letters
  • A “shift” of 7 (ie. alphabet starts with H)

This is my substitution key:

ABCDEFGHIJKLMNOPRSTUVWXYZ (plain)
H I J K L M N O P R S T U (cipher, even)
 V W X Y Z A B C D E F G  (cipher, odd)

It is also similar to the Affine cipher, but with a & b coming from the plaintext letter rather than the key:

a = plain mod 2
b = plain div 2
cipher = a * 13 + b + 7    (mod 25)

This reminds me of one of the GCHQ puzzles (I think it was a Bletchley park interview test) where you had to work out the scheme through which a substitution key was generated.

Report a problem