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Here’s an encrypted string which I submit to you for deciphering. I do know the answer.

m"eqpI!#ttfytp"wJ"/vvq!vj"egbpd‛fo!gucwksrs"pmnufo!ujju"!wsq!"tuswhkg"echwtcmctgpkucfc!kp"oqhcepoqmxfgg{ffpgh"tiDwbtn"!v!m!gp.scuj!"vijjjv/w!{pw!qeqowppf"u"iqx"upzkrrhnQnfctg!uf"e"f"mkoi!of"iqxcvhgqsybte"uq!tfqvnm{-"ucmm!vp"z

Good luck!

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    $\begingroup$ Is this an original puzzle? $\endgroup$
    – bobble
    Sep 6, 2021 at 22:35
  • $\begingroup$ As far as I know, yes. It’s an encryption system I devised, although I can’t guarantee it has never been used before (I can’t know everything). $\endgroup$ Sep 6, 2021 at 23:23
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    $\begingroup$ Thanks for clarifying. I'll just leave a link to Code Puzzles: What (Not) To Do? $\endgroup$
    – bobble
    Sep 6, 2021 at 23:32
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    $\begingroup$ The string is 225=5x5x3x3 long, character frequencies unbalanced, leading to some combination of classic substitution and transposition ciphers. $\endgroup$
    – z100
    Sep 7, 2021 at 16:16
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    $\begingroup$ I never said there’s no substitution, no transposition… I’ll give you two hints: The decrypted text is 100% valid English, and your grid size is wrong. $\endgroup$ Sep 9, 2021 at 22:51

1 Answer 1

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Ciphertext:

Congratulations! You have figured out how to read this message. Please send me a private message telling me how you figured it out. I'm looking forward to reading your answers. Good luck and hopefully, talk to you quite soon!

Cipher:

The first cipher applied is a Vigenere-like cipher with a two-letter key. It alternates between Caesar shifts of 1 and 2, like the Vigenere key BC. It also uses a custom alphabet which is made of the standard alphabet followed by the punctuation { !"./#'?‛,- (presumably... I'm not sure exactly what happened with the apostrophes). The second cipher applied is a spiral route transposition on a 15x15 grid, read counter-clockwise starting from the center and finishing upwards along the rightmost column of the grid.

Solve path:

Comments suggested a substitution+transposition combo. The message size can be factored into a few rectangles, plus a 15x15 square which is good for first attempts. I searched for repetitions and found a few 4-letter ones on the rows plus a few more 4- and 5-letter repeats on the columns. I recognized the positions of those repetitions (which stayed away from the center of the grid) as characteristic of a spiral route transposition. I read along each of the outer rows and columns and noticed that two punctuation marks behaved a lot like word spaces. The column zqv"rwjvf"twpp" had a decisive result when I looked at all of the Caesar cipher solutions ("running down the alphabets" in the old days) - alternating letters between the first two solutions spelled out the words you quite soon. The rest of the solution was working out the exact spiral path and arranging the punctuation in the alphabet.

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    $\begingroup$ Thanks for the puzzle! I hope you don't mind that I've posted the answer here instead of privately... it has been a couple of years, after all. I enjoyed solving it using pure analysis, without having to deal with riddles or cryptic clues. $\endgroup$ Feb 23, 2023 at 19:52
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    $\begingroup$ I don’t mind at all that you didn’t answer privately! I had given up all hope of someone ever replying, actually, because like you said, it was a few years ago, and the first few comments were basically just to tell me it didn’t seem like fair play on my part. I’d congratulate you, but it was already in the message! ;-) Heck, I had even forgotten how I had encrypted it! Bravo! $\endgroup$ Feb 25, 2023 at 2:43

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