As the author of the highest-voted answer to that question, and as someone with enough rep to view deleted questions, let me throw in my two bits.
First, the question did indeed include a verbatim copy of the first two paragraphs of this homework question from Dan Boneh's Introduction to Cryptography on Coursera. In the interests of facilitating discussion, let me quote these paragraphs here:
Let us see what goes wrong when a stream cipher key is used more than once. Below are eleven hex-encoded ciphertexts that are the result of encrypting eleven plaintexts with a stream cipher, all with the same stream cipher key. Your goal is to decrypt the last ciphertext, and submit the secret message within it as solution.
Hint: XOR the ciphertexts together, and consider what happens when a space is XORed with a character in [a-zA-Z].
The paragraphs were formatted as a blockquote (like above) in the question, but were only attributed to "an online class", with no backlink or author attribution. If the question is undeleted, this information should definitely be edited in.
The remaining four paragraphs in the question mainly describe the asker's prior attempts to solve the problem. (They say they already solved the problem before asking the question, but using a method other than the one suggested in the hint, and want to learn how to solve it in the "intended" way.) I see no reason to believe that these parts of the question would be anything but original.
Except for the poor attribution noted above, the way the quoted material was used in the question seems perfectly appropriate to me, well within both fair use under copyright law as well as academic ethics. Indeed, the text quoted from the homework exercise seems to me the bare minimum necessary to provide essential context for understanding the question.
As for any claim of facilitating cheating or academic dishonesty, I would like to note that none of the answers to the question actually provided a solution to the homework problem itself. Indeed, the way the question was asked, it would have been hard for any answerers to do so, since the question did not actually include any of the ciphertexts one was supposed to decrypt during the exercise.
Instead, my answer merely expands upon the quoted hint, describing how the ASCII character code points are arranged, what this implies about the effect of XORing a letter with a space, and how this effect could be used to recognize the likely locations of spaces in multiple ASCII text strings XORed with the same keystream. This is the kind of information that, in a non-virtual course, a student might obtain e.g. by asking a teaching assistant. It is not something that would allow the asker, or anyone else reading the answer, to complete the exercise without actually understanding the topic.
The other answer (by D.W.) is considerably shorter, and merely provides two additional short hints. In fact, let me quote it in its entirety:
Hint: Are you familiar with frequency analysis (for breaking classical ciphers)?
Hint: If you have a guess/hypothesis that there is a space at a particular position in one of the ciphertexts, can you think of any way to test whether your guess/hypothesis seems likely to be correct or not?
Thus, whether or not the user who originally asked the question technically violated some kind of honor code in doing so, I see no reason for us to keep either the question or its answers deleted.
