Dr Hooves and XHRPPAXLFSVZB cracked!

Hey everypony! Those of you out in ponyland might be aware of this card from the My Little Pony trading card game:

The meaning of the letters “XHRPPAXLFSVZB” has been a mystery to everypony, as far as I have read or heard. But I believe I’ve cracked it! Spoilers after the cut, naturally.

At first I noticed that there are thirteen letters, which might correspond to the Doctor’s thirteen regenerations. But comparing the letters to the names of the actors to play the Doctor didn’t get me anywhere. Next I tried putting the letters through a simple Caesar shift, but none of the 25 shift intervals gave me anything but gibberish.

Then I got creative, and also lucky. Not long ago, I watched an episode of Sleepy Hollow that used a Vigenère cipher. This is like a Caesar cipher, but it uses a keyword to apply a different shift to each letter, repeating over an interval equal to the length of the keyword. The Wikipedia link explains it pretty well. Having watched the episode, and then done some reading about the Vigenère cipher, the idea was fresh in my mind.

I then went to Sharky’s Vigenère cipher, a website that lets the user encode or decode text using a Vigenère cipher and a keyword of their choice. I entered “XHRPPAXLFSVZB” as the ciphertext and tried the most obvious keyword that came to mind: “WHO”. The result was:

BADTIMBERWOLF

which is, of course, a reference to the “Bad Wolf” theme of the Ninth Doctor’s adventures.

So I think that’s solved! It’s not exactly some deep secret, just a cute Doctor Who reference behind a moderately tricky cipher. Good job Enterplay, for making a nice little puzzle for us!

(In more detail: The plaintext “BADTIMBERWOLF” was encoded by shifting the first letter forward 22 places in the alphabet (since W corresponds to 22 if A corresponds to zero), shifting the second letter forward 7 places (H=7), shifting the third letter forward 14 places (O=14), then shifting the fourth letter forward 22 places, the fifth by 7 places, and so on. In other words, it “added” the following two pieces of text:

BADTIMBERWOLF

WHOWHOWHOWHOW

_____________

XHRPPAXLFSVZB

using A=0, B=1… Z=25, and wrapping around modulo 26.)

P.S.: I’ve just realised that there’s another clue to the keyword in the text. Note that the scroll was delivered by an owl.

“Hey, this thing’s in code. What’s the keyword?”

“Who!”

“Not who, what! That’s what I want to know!”

“Who!”

“Me! The Doctor!”

“Who!”

“No, it’s Hooves. Oh, forget it.”