The Secret Code Solution Revealed! · 11:22pm Jan 31st, 2016
Well, the new chapter is up and unfortunately nobody found the correct solution to the secret code I've been posting. 
I intended it to be difficult, but I hadn't counted on it being so tough that nobody would get it. Anyway, if you haven't read the latest chapter of The Iron Horse, "Technologic, Part 4," then the rest of this post is a spoiler. Proceed at your own risk!
Okay, so here, once again, is the code:
13 22 18 2 7 12 2 13 3 12 15 10
Now, in my previous post, I gave some hints:
1. Guess the first letter. That one should be easier than the others, and it is key to figuring out the rest.
2. This arc is a bit of an homage to Alan Turing, and one of the major ways he decoded things was by taking guesses at likely words.
3. What is a major element introduced in this arc? Using the previous two hints, you might be able to guess the first word. From there, the rest will fall into place.
Hint #1 was the first key and the one person to hazard a guess did get this right: 13 = M because it's the 13th letter of the alphabet.
However, the other letters wouldn't make sense using that same technique. Even so, hints #2 and 3 could have helped to guess the first word. In that case, you might have figured out that the significant element was one of the new characters in this arc whose name starts with "M": Mint.
The code, now partially deciphered, reads:
M I N T 7 12 2 13 3 12 15 10
With that, it would now be possible to determine how the letters were encoded. And here it is:
Step 1: Take the letter's corresponding number (Ex. A=1, B=2, C=3, etc.) and subtract the previous number. The second letter is I, so its number is 9. The number before it was 13. But that number is -4 and the original code is 22! Why? Well, there was an additional step, which you might have noticed a few hints for...
If everything's coming up negative, just stay positive and keep trying! Good luck!
Step 2: If the resulting number is a negative number, then add 26. The second letter codes as -4, so add 26 and you get 22. Keep following that same formula with the guesses narrowed down and the end result is the solution to the code, which I will spoiler here just in case you've read this far and you really didn't want to get spoiled: M I N T | I S | N O | P O N Y
Thus, a major plot point was revealed a few weeks early to anyone astute enough to guess it. Too bad nobody did, but I suppose at least you got a surprise twist in the end. 
See you next time, folks!
Are you sure you're not a cryptographer? Cause you should be.
Huh... welp, I didn't crack the code, but I figured that twist out (or was pretty sure of it at least) back in the second part of the arc. Does that count?
I did not, however, expect him and Dr. Turing to be a couple, so... props on that.
....That is brilliant...
And here I was using Enigma machines and Bombe simulators to try and crack the thing... 
Drat, went in the completely wrong direction. I tried the standard A=1, B=2, C=3, and then tried all manner of Caesarian and Substitution shifts, and then plugged all of those outputs into an online Engima Machine Simulator. Oh well, at least I tried.
3725596 Nope. I'm an English teacher. I think I'd go nuts if I had to figure out math stuff all day long.
3725669 If you correctly guessed, then I consider that more impressive than figuring out the code anyway. It was just a little bonus to let some astute viewers be "in the know" since I figured the hints of Mint's true nature were hidden well enough to where few would guess it.
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I actually based the code off one of the Alien language ciphers from Futurama. I figured it was juuust tough enough to be challenging, but not utterly impossible to solve with good math skills and/or a calculator. Still, I'm genuinely pleased that people took the effort to try and decode it and I applaud you taking on the challenge!
Just one question to those who tried it... Why did you waste their time and effort trying to work out the puzzle? What reward is there but mere personal vanity? Just step around it and continue on with your life, as I did.
To be fair, I think most of us already guessed that particular plot twist without the help of a secret code. I mean, I know I predicted the whole "Choco Mint is actually a Changeling" reveal ever since he was introduced.
I really need to start on this fic,..
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I'll admit, it was mainly the "I'm good at reading ponies (imagine sinister laughter here)" and the fact that his cutie mark seemed to have very little to do with anything he did in the stories. And of course the strange readings Turing said she got from him. Maybe the unabashed flirting, too.
What I didn't know was whether the other scientists knew what he was, and what his motivations were.
Though I didn't really know he was a changeling, I just had a strong feeling that he would turn out to be. Maybe I'm just good at predicting plot twists because those make about 30% of my stories.
3725912 Much as RPGs have bonus quests and life is full of diversions, the reward is sometimes found not in the destination, but in the journey.
I'm a little sad nobody figured it out, but I'm actually kind of glad people gave it a shot just for fun anyway.
3726034 I was willing to bet a few did guess about Mint, but I suspect it's lower than you might think. Most reader comments seemed to just think he was a closed-minded jerk. If you did figure it out, though, kudos for picking up on the hints!
3726232 When you do start it, I hope you'll enjoy it!
Ah, sorry. I did try, but my attempts didn't produce anything that looked like it might be the plaintext.
...Ohhh. The different positions have different systems within the larger cypher? Clever. :)While I did know that that sort of thing was a possibility, I didn't actually try it on this one.And I was wrong about that too, it looks like. :)
Very neat. Thanks for the explanation. :)
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Hm. Interesting. I started to get suspicions pretty early too, based on the evidence in the chapter, though I too didn't guess that Mint was so involved with Dr. Turing.