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Update 2: This post is about this edit to this question


Recently there was a suggested-edit asking for removal of one of the many "how to do cryptanalysis on a 1 time pad if used twice" questions?

The edit deleted all the content of the message, replacing it with a message saying something like

Remove this question, an honour code has been broken. This is from an online course.

I rejected the edit on the grounds that it wasn't a viable edit (it completely destroyed the question), and recommended two possible options: 1) Provide an edit that kept the spirit and point of the question whilst removing any content from the online course 2) If you feel such an edit is impossible, ask for the question to be deleted by flagging for deletion.


I was wondering:

  1. The Asking homework questions thread discusses this, and I presume that the stance discussed there still holds with online courses?
  2. What happened to this question in the end?
  3. If an official representative of a course asked for a question to be removed (the edit in question did not claim to be official), would we comply?
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  • $\begingroup$ Good question. I'll be interested to hear people's thoughts. As for #2, I deleted it. After reviewing the question it was obvious that it was a copy and paste from homework (the OP said they already submitted the assignment). Whether or not that was the right action, I guess we'll see what the community thinks. $\endgroup$
    – mikeazo
    Jan 9 '14 at 16:42
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    $\begingroup$ Just as a thought, if I remember correctly it had a good answer on it? If so, is there a way of putting that answer on one of the [almost]-duplicates? (probably I've not remembered correctly, there was no good answer, and this point is moot!) $\endgroup$ Jan 9 '14 at 16:44
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    $\begingroup$ I'll take a look. $\endgroup$
    – mikeazo
    Jan 9 '14 at 17:01
  • $\begingroup$ It was about this edit to the question Many time pad attack $\endgroup$ Jan 12 '14 at 21:28
  • $\begingroup$ (except of course not all of us can see deleted questions :P ) $\endgroup$ Jan 12 '14 at 23:31
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    $\begingroup$ All, I appreciate all the comments, discussion, etc. I went ahead and undeleted the question and answers. Thanks! $\endgroup$
    – mikeazo
    Jan 13 '14 at 14:38
  • $\begingroup$ @mikeazo et al: As Gilles and e-sushi mention, the proper channel for copyright holders to challenge material submitted to the site by users is a notice of copyright infringement, AKA a DCMA takedown request. Feel free to treat the question on its own merits. $\endgroup$ Jan 13 '14 at 17:42
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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.

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You were correct in rejecting this edit. Whether or not you agree with the request, it would have vandalized content on this site. Even if the original poster had made such an edit, the correct course of action would have been to roll it back.

Whether any honor code has been broken or not, this is between the homework-setter and the poster. We have no way to know the facts. We should not remove content on someone's mere say-so.

I haven't seen that question (or I've seen it but I don't remember or recognize it). It may be that the text that was posted is covered by copyright, in which case the author may file an infringement notice. (Note that moderators are explicitly instructed not to perform any action based on copyright considerations, and refer to the legal procedure instead.)

If copyright does not apply, whether to remove a question that contains an exercise that the asker has requested not to reveal is a matter of ethics. This has been debated on other Stack Exchange sites, including Mathematics and Computer Science.

I consider it unethical to enforce a monopoly on ideas, be they exercises or otherwise. Academic standards may of course require acknowledgement. Therefore I oppose the deletion of this question on this basis (and if this was happening on CS.SE, I would immediately undelete it, unless there was some other reason to keep it deleted such as low quality).

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  • $\begingroup$ I appreciate your comments. Thanks. I am more than willing to undelete. I will wait and see what other responses we get on this question though. $\endgroup$
    – mikeazo
    Jan 9 '14 at 21:29
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I would like to chime in with Gilles' answer.

From my point of view, the edit was "vandalism" and it was correct to reject that edit. The provided reason "an honour code has been broken" practically does not differ much from a personal opinion; especially since we have no means to verify that the "editor" in question has any legal rights related to the posted question's content. Fact is: "honour code" violations do not provide any legal base of action.

If the related question would have infringed any rights or if it would have been legally problematic in any way, the legally correct way to handle something like that would be to issue a DMCA notice. If that DMCA notice would have been verified to be correctly issued and there's indeed reason for removal, the question would then be deleted by the StackExchange team.

But simply editing a question and completely replacing it's content, without providing any kind of legally sound proof of claims (or anything alike), is what it is: the destruction of a potentially valuable question... aka "vandalism".

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I have a different perspective from others here. I don't think the author has the right to demand that the question be removed.

However.... in this case, I think this particular question is bad and should be closed. I think it is a broken window. I'm embarrassed to have it on our site. I think it makes us look bad. I think when we have blatant homework questions that ask for someone to answer their question, it makes the site look sketchy.

Frankly, that question is just a homework dump. The author just dumps the assignment and asks for someone to the author what to do. There's no sign of effort; no sign that the original author tried some stuff and got stuck; no specific question about their attempt -- it's just a copy of the exercise and a request for someone to explain to them how to solve it. That's not the kind of question we want.

Therefore, I think our site would be improved by deleting that question and any others that we know to be a homework dump from Coursera.


To be clear: I am not saying that we must delete every question that has any relationship to a homework. I am saying we should close and delete "homework dump" questions, where the author basically just shares the exercise and is looking for someone to give them the answer or give them a hint. Such questions are bad. The right way to ask a homework-related question is to ask a more narrowly focused question: e.g., try doing the homework exercise, then ask a specific question about a specific issue with one's attempt (e.g., where you got stuck). Yes, that takes more effort; good questions often do.

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