Reply To: Extracurricular Challenges
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26th October 2024 at 2:05 pm
#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.