Challenge 6B foxed a few of you! It was indeed, as described in the forums early this morning by JohnatCCO:
"two monoalphabetic substitutions, mixed:
- upper-case letters with the keyword 'MONHIER';
- lower-case letters a special affine shift called 'Atbash', which effectively reverses the alphabet."
Some very ingenious ideas about how to crack it out there. I particularly liked this from hkbm:
"In the 19th and 20th 5-letter groups of 6B, you will find OzOyz. It occurred to me that if the capitals and lowercase were each each encrypted by a different cipher, then "OzOyz" might be "babba", i.e. the start of Babbage. I found this by realising that "Babbage" could be a crib, and searching for possible occurrences of it. Indeed, I was correct with my assumption that "OzOyz" was "babba", and that helped me to crack 6B by 16:48:02 on the day it came out."
Congratulations to the winners of the prizes for Challenge 6A:
Throat Nanny Who from Harrow Way
Nick, Jacob, Louis, Harry, Dobbo, Heather, Christine, Camel. Ciera, Elloise and Ivor from Bartholomew School
Laura, Holly and Sophie from Kingsway
Helen Wickins from Alcester Grammar School
Brittany Ulm from Cedars Upper School
Engimatic2 from Walton High
George Williams and Alice Williams from Redruth School
Tereasa Thomas from Christopher Whitehead Language College