A System for Detecting Software Similarity

UPDATES

Nov 13, 2022, Due to persistent use of robots to submit tens of thousands of Moss jobs per day, the submission limit of 100 submissions/day/user is now being enforced.
April 26, 2021, If you are having trouble registering, please try using Gmail to send the registration message.
April 10, 2020, The dramatic increase in online courses due to the coronavirus pandemic seems to have led to a corresponding dramatic increase in the use of Moss over the last few weeks. (It's not clear why these two things should be related, but it appears they are.) The public server has been moved to a larger machine (more cores and more memory), which should help meet the increased demand. Difficulties connecting to the server or receiving results are a sign that the server is overloaded at that time; please try again later. All users are asked to limit submissions to no more than 100/day.
Dec 14, 2018, Added UTF8 conversion script to community contributions
Feb 1, 2018 And even more community contributions have been added!
Nov 9, 2017 More community contributions have been added below ...
Aug 31, 2017 Thanks to Chrstophe Troestler for an OCaml client for Moss.
May 18, 2014 Community contributions (incuding a Windows submission GUI from Shane May, thanks!) are now in their own section on this page.
May 14, 2014 And here is a Java version of the submission script. Thanks to Bjoern Zielke!
May 2, 2014 Here is a PHP version of the submission script. Many thanks to Phillip Rehs!
June 9, 2011 There were two outages over the last couple of days that lasted no more than a hour each (I think). I've made some changes to the disk management software that should prevent these problems from recurring.
April 29, 2011 There was an outage lasting a few hours today, the first since last summer, but everything is back up.
August 1, 2010 Everything is back to normal.
July 27, 2010 The Moss server is back on line. There may be some more tuning and possibly downtime in the coming weeks, but any outages should be brief. New registrations are not yet working, but people with existing accounts can submit jobs.
July 25, 2010 As many (many!) people have noticed, the Moss server has been down for all of July. Unfortunately the hardware failed while I was away on a trip. I am hopeful it will be back up within a few days.

What is Moss?

Moss (for a Measure Of Software Similarity) is an automatic system for determining the similarity of programs. To date, the main application of Moss has been in detecting plagiarism in programming classes. Since its development in 1994, Moss has been very effective in this role. The algorithm behind moss is a significant improvement over other cheating detection algorithms (at least, over those known to us).

What is Moss Not?

Moss is not a system for completely automatically detecting plagiarism. Plagiarism is a statement that someone copied code deliberately without attribution, and while Moss automatically detects program similarity, it has no way of knowing why codes are similar. It is still up to a human to go and look at the parts of the code that Moss highlights and make a decision about whether there is plagiarism or not. One way of thinking about what Moss provides is that it saves teachers and teaching staff a lot of time by pointing out the parts of programs that are worth a more detailed examination. But once someone has looked at those portions of the programs, it shouldn't matter whether the suspect code was first discovered by Moss or by a human; the case that there was plagiarism should stand on its own.

In particular, it is a misuse of Moss to rely solely on the similarity scores. These scores are useful for judging the relative amount of matching between different pairs of programs and for more easily seeing which pairs of programs stick out with unusual amounts of matching. But the scores are certainly not a proof of plagiarism. Someone must still look at the code.

Languages

Moss can currently analyze code written in the following languages:

C, C++, Java, C#, Python, Visual Basic, Javascript, FORTRAN, ML, Haskell, Lisp, Scheme, Pascal, Modula2, Ada, Perl, TCL, Matlab, VHDL, Verilog, Spice, MIPS assembly, a8086 assembly, a8086 assembly, HCL2.

An Internet Service

Moss is being provided as an Internet service. The service has been designed to be very easy to use--you supply a list of files to compare and Moss does the rest.

The current Moss submission script is for Linux.

In response to a query the Moss server produces HTML pages listing pairs of programs with similar code. Moss also highlights individual passages in programs that appear the same, making it easy to quickly compare the files. Finally, Moss can automatically eliminate matches to code that one expects to be shared (e.g., libraries or instructor-supplied code), thereby eliminating false positives that arise from legitimate sharing of code.

Registering for Moss

Moss is being provided in the hope that it will benefit the educational community. Moss is fast, easy to use, and free. In the past, access has been restricted to instructors and staff of programming courses. This is no longer the case, and anyone may obtain a Moss account.

However, Moss is for non-commercial use. If you are interested in commercial uses of Moss, contact Similix Corporation.

To obtain a Moss account, send a mail message to moss@moss.stanford.edu. The body of the message should appear exactly as follows:

registeruser
mail username@domain

where the last bit in italics is your email address.

If you already have an account, the latest submission script can be downloaded here.

Security and Legal Stuff

If you use Moss, the results will contain copies of the code you submitted and these will be accessible to anyone with the result URL. Stanford can assume no liability whatsoever for your submissions.

Reasonable precautions have been taken to protect the confidentiality of the code you submit. These measures include: only you receive the result URL and it contains a random integer, so it is not easy to guess a result URL. Furthermore, the directory with the results cannot be broswed or indexed by robots. Finally, submissions are not retained indefinitely on the server; typically results are deleted after 14 days, though they may be deleted earlier to free up disk space when the server is particularly busy. If some results you need have been deleted, you can simply resubmit your job.

Community Contributions

A number of Moss users have contributed versions of the submission script:

AutoMoss provides a web-based UI to Moss with a number of additional features, all documented in the accompanying manual. Thanks to Daniel Lochner. Joshua Lochner, and Carl Combrinck.
A version of the Moss submission script in Nim contributed by David Edwin.
A script to convert files to UTF8. Moss currently doesn't like UTF16! (from Ian Scott-Fleming).
A Swing GUI and the corresponding jar file (from Muhammet Alkan).
A Python GUI (from Jeremiah Blanchard).
A Python client (from Syed Chishti)
An OCaml client (from Christophe Troestler).
A Common Lisp submission utility (from Wojciech Gac).
A Java version (from Bjoern Zielke).
A PHP version (from Phillip Rehs).
A GUI for Windows (from Shane May).

Andrew Cain has writtten a Ruby gem for Moss.

Hjalti Magnussion has written a summarization/visualization script.

How Does it Work?

A paper on the ideas behind Moss can be found here.

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