9
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What method has been used to encode this message?

I s Txz KhxIiHM s q nivRs OafToHB hduiTplssz gil s a q aT mWphl ens di gmVUJbgVzanbm pCleepLsxM smshyao rSgeREq jezknqcVxs s aNvSEge

Okay just one clue:

The message can be decoded, even without knowing the key number in no more than (message length)/2 attempts, using pencil, paper and mathematics no more complex than counting and comparing. No computer knowledge is required.

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    $\begingroup$ If you wan't to ask for more clues do so. I will happily answer yes or no questions. $\endgroup$ – Bob Jul 6 '15 at 10:43
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    $\begingroup$ I view this on my iphone. Is the linebreak between gmVUJbgVzanbm and pCleepLsxM deliberately chosen or simply for formatting on the website? $\endgroup$ – LeppyR64 Jul 6 '15 at 12:40
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    $\begingroup$ @LeppyR64 Just the website. The code doesn't contain any line breaks. $\endgroup$ – Bob Jul 6 '15 at 12:42
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    $\begingroup$ Yes there is a single space character. Using block quote has fiddled with formatting slightly. $\endgroup$ – Bob Jul 6 '15 at 12:50
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    $\begingroup$ It is most likely a permutation of the given string and probably obtained by swapping S[i] and S[i+key]. $\endgroup$ – Aravind Dec 8 '15 at 14:13
9
+100
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The encoded message is:

This is a sample message

Explanation:

The last hint from question_asker was my starting point. He quoted a clue which I couldn't find anywhere. I looked at the revision history of the question and there it was, together with many more. The one which revealed the "encoding" to me was:

This puzzle's message starts with the word This.

Lets take a look at the beginning of the encoded text and mark T with 1, h with 2, i with 3 and s with 4.

I s Txz KhxIiHM s q nivRs OafToHB hduiTplssz gil s
...4.1....2..3...4....3..4....1....2..31..44...3..4

We can see the sequence 1234 near the beginning with some irregular spacing between the letters. The interesting thing is, the same sequence appears a little further to the right with exact the same spacing (ignoring some of the numbers which are just there as fillers). I copied the encoded text to a second line below the first and rotated it so that both sequences are one above other (25 characters to the left). Taking the characters which are equal in both lines reveals the message.

I s Txz KhxIiHM s q nivRs OafToHB hduiTplssz gil s a q aT mWphl e
s OafToHB hduiTplssz gil s a q aT mWphl ens di gmVUJbgVzanbm pClee
.....T....h..i...s.. .i..s .a..... ........s.............a..m.p.l.e

ns di gmVUJbgVzanbm pCleepLsxM smshyao rSgeREq jezknqcVxs s aNvSEge
pLsxM smshyao rSgeREq jezknqcVxs s aNvSEgeI s Txz KhxIiHM s q nivR
..... .m...............e.......s.s..a....ge.........................

Now most of the other clues make sense as well.

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    $\begingroup$ @question_asker One of the (now deleted) clues says the message starts with the word "This". Therefore I marked the letters in the encoded text. To find the message take a text editor where line wrapping can be disabled, copy the message in 2 lines one above other, and remove 25 chars at the beginning of the 2nd line. Then mark all chars which are equal in both lines. $\endgroup$ – Sleafar Dec 11 '15 at 16:20

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