5
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Just a short but really scary looking code breaking challenge.

It breaks down into a couple of sentences once cracked.

24141325121914011720242320110720150920201224161720111012190613080310160806252412191417050526212017171013

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13
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First step:

Split the digits into pairs:
Result is 24, 14, 13, 25, 12, 19, 14, 01, 17, 20, 24, 23, 20, 11, 07, 20, 15, 09, 20, 20, 12, 24, 16, 17, 20, 11, 10, 12, 19, 06, 13, 08, 03, 10, 16, 08, 06, 25, 24, 12, 19, 14, 17, 05, 05, 26, 21, 20, 17, 17, 10, 13.

Second step:

Convert 01 to a, 02 to b, 03 to c, ... 26 to z
Result is xnmylsnaqtxwtkgtoittlxpqtkjlsfmhcjphfyxlsnqeezutqqjm.

Third step:

Rotate 5 letters in the backward direction (or equivalently 21 in the forward direction).
This changes f to a, g to b, h to c, i to d, ..., y to t, z to u, a to v, b to w, c to x, d to y and e to z.
Result is sihtgnivlosrofbojdoogsklofegnahcxekcatsgnilzzupolleh.

Fourth step:

Reverse the string.
Result is hellopuzzlingstackexchangefolksgoodjobforsolvingthis.

Fifth (and final) step:

Add some spaces.
Result is hello puzzling stack exchange folks good job for solving this, which is the plain-text message.

A note about the message:

The original message had two typos in the plain-text message. Originally it was "24141325121914011720242320 16 0720150920201224161720111012190613080310160806252412191417050526 22 2017171013",
instead of
"24141325121914011720242320 11 0720150920201224161720111012190613080310160806252412191417050526 21 2017171013"
which used to produce the plain-text message hello quzzling stack exchange folks good job kor solving this, but the OP already fixed that.

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  • $\begingroup$ Step 6. Type correct "hello quzzling stack exchange folks good job kor solving this" -> "hello puzzling stack exchange folks good job for solving this" $\endgroup$ – warspyking Dec 14 '14 at 13:05
  • $\begingroup$ Thanks for letting me know about the errors. I have fixed the errors in the post. $\endgroup$ – Colorfully Monochrome Dec 14 '14 at 20:27
  • $\begingroup$ @ColorfullyMonochrome Ok, I have changed the answer to reflect that. $\endgroup$ – Victor Stafusa Dec 14 '14 at 20:37
4
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Splitting the digits into groups of two and taking letters

241413... = 24,14,13,... = XNM...

gives the length-52 string

'XNMYLSNAQTXWTPGTOITTLXPQTKJLSFMHCJPHFYXLSNQEEZVTQQJM'

This method is implicitly confirmed in that all the resulting indices are 1 through 26, which is unlikely in a random string of numbers.

I don't care to go further though because now it's just a random-looking string of letters, too short to do any meaningful cryptanalysis, and I see nothing better to do then throw methods at it until one works. I did try Caesar cipher though; none of the shifts produced anything meaningful.

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  • $\begingroup$ Actually, caesar shift was correct (ROT5) - then the result is backwards. $\endgroup$ – Colorfully Monochrome Dec 14 '14 at 5:17
  • $\begingroup$ +1 for reasoning the 'split' is correct even without finding a clear text. $\endgroup$ – BmyGuest Dec 14 '14 at 8:31

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