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In a forgotten corner of an old library, hidden among many dusty volumes, I stumbled upon a puzzle in a book that has me utterly intrigued. The artifact I found contains a cipher text, and it seems that uncovering its true meaning is not as straightforward as it first appeared. Here's what I found:

ioteceael fi slhbx rso ofs atnh uui n talrog tere pssgnbO

Alongside this cipher text was a dusty old paper with a clue, which itself appears to be encoded.

Clue #1 Puzzle Clue

Clue #2 enter image description here

To decode the text, you'll first need to decode the clue to discover the cipher used in the encryption.

Can you decipher what it means?

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  • $\begingroup$ Shouldn't the first word of the dusty old paper be four letters long, and not three? $\endgroup$ Commented Aug 21, 2024 at 6:24
  • $\begingroup$ @codewarrior0 Haha no, but nice try! $\endgroup$ Commented Aug 21, 2024 at 15:47
  • $\begingroup$ Can you double check that the cipher can be decrypted with the intended key? It seems to have an extra letter "x". $\endgroup$ Commented Aug 24, 2024 at 9:10
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    $\begingroup$ @codewarrior0 could you post your presumed answer if it does indeed turn out that the cipher is wrong? This one has been bugging me for days! (Since it doesn't seem like OP is coming back to answer your question...) $\endgroup$ Commented Aug 26, 2024 at 15:45
  • $\begingroup$ @codewarrior0 In the clue(s), 3 of the words are hints to give you the name and symmetric key of the cipher. The others are just used as distractions to draw away from its discovery. $\endgroup$ Commented Aug 26, 2024 at 17:20

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The dusty old paper appears to read,

??? COLUMN DEUTSCH TWO KEY CIPHER

This probably refers to the double columnar transposition cipher, a hand cipher favored by Americans, Brits, and Germans during both world wars. The letter distribution in the ciphertext also indicates a transposition cipher. Using an online decryption tool with the key "cipher" produces nonsense. Assuming the ciphertext is damaged, I decrypted the message with all possible pairs of keys of up to six letters each. This gave a suggestive decryption:
AIGNORBNCEISOLISSFERTHESALFBUTEPLAGUTONTOOXHERS
The words "Ignorance is bliss" have almost surfaced, and match the word lengths given in the cryptogram. I separated out the letters that seem to break up the phrase and found they occur every five letters exactly. This is a characteristic of a misaligned column during encipherment:

A B O E A E T X .IGNOR.NCEIS.LISSF.RTHES.LFBUT.PLAGU.ONTOO.HERS

I deciphered the crypogram with only a single columnar transposition using the equivalent key 034215 found during the search, and then broke the result into columns by hand:

INLRLPOHRSFSTUOOISEUGOSGCITFLNENESHBATRABOEAETX 1 INLRLPOH 2 RSFSTUO 3 OISEUGOS 4 GCITFLNE 5 NESHBATR 6 ABOEAETX

Note that this text is transposed - the original columns from the encryption process are written horizontally, and the rows vertically. Arranging the columns such that the plaintext can be read vertically reveals an extra "X" in the bottom-right corner of the transposition matrix. An ordinary columnar transposition will not leave a gap in the last row like this.

1 INLRLPOH 4 GCITFLNE 5 NESHBATR 3 OISEUGOS 2 RSFSTUO 6 ABOEAETX

The most likely explanation is that the plaintext Ignorance is bliss for the self but a plague onto others was encrypted once with columnar transposition with the key CIPHER, then the letter X was appended, and then the result was encrypted a second time with columnar transposition with the key CIPHER.

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  • $\begingroup$ Could you explain how you decoded the clues on the dusty old paper? I'd worked out the "TWO KEY" section based on the appearance of the letters, and that gave a few other characters scattered throughout, but not enough to decode it. Looking at it now I know the answer, the words on the beginnings of the two clues are ?IT and C?? where the ? could be A B F G J Q V X Z. So I still don't know what words those were meant to represent. $\endgroup$ Commented Aug 27, 2024 at 20:42
  • $\begingroup$ Astonishing work! This is the correct answer and you did it without even targeting the original cipher variation it was encrypted with! For a little more on this, the cipher used was Ubchi --a very old double columnar cipher that uses the same key, but adds a number of padding characters to the message. It was used by the Germans in WW1. $\endgroup$ Commented Aug 28, 2024 at 3:48

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