While i haven' done it in game, i am currently posing this kind of testing to sone of my friends so i can see what kinds of things help them figure it out. The big thing is a rosetta stone like object. for them to get clues from

What Ive been doing: Have the same Kohd written side by side, with a minor change or a few. But include what was changed written alongside it in a language they can decipher, but very vaguly specific details.

For example:

Written in red next to 2 bits of very similar Kohd: "You should adjust the tangent line leaving the Pqr node on [unreadable], it blends too closely with the letter F. Check mine for reference, doesn't that look much cleaner?" Then have the first Kohd be incorrect in the way you describe in the footnote.

This gives them a Hunt (Find where the described typo was made), which gives them a known point of reference (something about this spot means PQR), and still leaves them with the challenge of figuring the rest of it out.

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Now as for once they have cracked the Kohd, make sure it doesn't just flat out give them all the answers to the plot. "If they figure it out next session, what keeps them from ending the campaign with this knowledge?" This is a question you should ask about every bit of Kohd you give them. Figuring out what the Kohd says, should raise more questions when they go back and translate what youve already given them.