Crypto in the classroom: digital signatures for homework

If you don’t know, I’m a graduate student at the University of Utah, which means I make a living my teaching classes. Recently a student charged that I lost a good deal of her homework. We wound up in a “he-said/she-said” situation where ultimately the dean concluded that we need to raise her grade by a letter under the assumption that I really was up to shenanigans (we ultimately gave her 100% credit in the “homework” column in the grade book, raising her grade from F to D). Not a pleasant situation: aside from a track record of strong teaching evaluations, there was nothing to defend my reputation.

Experienced teachers know that claims of “lost” work are frequent. If we want to be objective about this (and we do), the claims need to be taken seriously, since lost things rarely leave a trail. All we have when analyzing such claims is the following:

  • The missing work never seems to turn up. Not after a week, a month, a semester, a year, or ever.
  • If a person rarely finds that they misplace his own belongings, it’s hard to accept that he is misplacing student work (assuming they treat student work with a reasonable amount of care, as we typically do, given how terrible it would be to lose it!)
  • These claims never seem to come from students who are doing well on exams; they tend to come from students who are backed into a corner, grade-wise.

Of course, it is entirely conceivable that these claims are occasionally correct, and it would be terrible to allow such mistakes — our mistakes — to adversely effect our students.

Last Spring was the only time a student has accused me of losing their work. It was a lousy situation that I have no intention of ever repeating. So when I was assigned to teach a half-term class this summer, I decided it was time to try something new. I’ve recently finished teaching that class; here’s what I did.