I had just arrived for my lunchtime meet with Megan in Pizza Espresso, when up walks the woman herself, resplendent in a long velvet skirt and matching jacket.
"Gosh, you look beautiful."
"Thanks. Thought I'd wear something a bit special for Gwyl Canol Hydref." Seeing my blank look, she went on, "The Welsh name."
"I see." I made a mental note to look that name up later.
Megan ordered calzone, and I ordered pizza. "Nice choice of venue," I said, "I love a pizza."
"So do I, but I picked this place as it does other Italian food, too. Anyway, something in an e-mail the OH sent me the other day made me think it might be appropriate." She took a folded A4 sheet from her shoulder-bag and handed it to me.
I unfolded it, to be greeted by a tangled mess of words, right-way-up and upside-down. "Double-sided printing gone wrong, Megan?"
"The other side!" she said.
I turned the sheet over.
From: RD_Jones <firstname.lastname@example.org>
To: Megan <email@example.com>
Subject: Another cipher puzzle
I'm impressed at how your friends managed to crack my cipher puzzle. I wonder how they'll get on with this one. Same deal as last time. English text which can be found on the web. And I've enciphered the spaces.
Like a calzone? ;-)
"I see he's using letters now. And spaces -- I take it those underscores mean spaces. No mention of a key, though, so I guess we don't need one." I smiled in recognition at the last line. "So that's why you picked here -- you like calzone?"
"Never had it before. Ordered it to see what it was."
Let's see if inspiration strikes. With or without eating any Italian food, can you identify the plaintext and find out the encoding method?
To make a calzone, you take a pizza base, and put fillings on one half. Then what do you do with the second half? Jasen has the right idea.
Steve Mangiameli is onto something with his comment about the day on which I posted this puzzle. Might a pertinent English word appear somewhere in the plaintext? And the spaces have been enciphered. So if, for instance, you suspected that plaintext contains the word "
crib", you may look for the 6-character sequence "
_crib_" (where _ stands for a space).
The second half of the plaintext has been combined with the first half like a calzone -- the second half is folded back over the first half. As a result, it lies upside-down. What does that correspond to in text terms? Addition -- or rather subtraction, because the second half is upside-down. Modulo what base? The set of characters seen in the ciphertext should make that clear -- remember that underscore stands for space.
The plaintext turned out to be of odd length. The resulting ciphertext is the same as it would be if the plaintext had an extra space at the end to make its length even.