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Problems Personified

Recently I was in a meeting about computer security where the presenter concluded an explanation with "…basically, it's an example of the Confused Deputy problem." For me, the term confused deputy vividly invoked a kind of genial befuddlement that might indeed result in a security problem.

In the world of math and computers, certain problems are typically described using personifications. The Confused Deputy is an example; this is a security breach in which the bad guy gets a trusted entity to (inadvertently) perform a task on the bad guy's behalf. I can't withdraw money from your bank account directly, but if I can get you to withdraw your own money (the bank trusts you with your own account, after all) and give it to me, then you are the confused deputy. Social engineering scams and browser-based security breaches often are examples of the Confused Deputy problem.

Another personification in computer security is the man-in-the-middle attack, where the bad guy puts himself between two parties. Unlike simple eavesdropping, the bad guy forges messages to both parties, who believe that the messages are from a trusted source and therefore willingly disclose sensitive information. MiTM attacks can be a problem anywhere there's public WiFi, like in coffee shops or libraries.

Not all personified problems are about security. Many computer students know about the Dining Philosophers Problem. A group of philosophers is around a table; each has a fork to the right and to the left. Per the problem, to eat dinner, a philosopher needs two forks, one in each hand — meaning there aren't enough forks to go around. Devising a way for each philosopher to get dinner in turn is an exercise in designing processes that depend on shared resources.

In the Byzantine Generals Problem (PDF), information comes from several sources, some of which might be incorrect or even deliberately misleading. The personification helps clarify the problem of computer systems that must work reliably in spite of imperfect input. (One of the authors actually recounted where the name came from.) A problem that can particularly affect those who work in cloud computing is that your process is slowed down because someone else's process on the same computer is using too much memory or processing power. This is known as the Noisy Neighbor Problem.

The programmer, entrepreneur, and blogger (also, former Israeli paratrooper) Joel Spolsky once coined his own personified problem, perhaps the only one based on Yiddish humor: Shlemiel the Painter's Algorithm. Shlemiel is hired to paint lines down a road. Each day he gets less and less done. When his boss asks him why, he says, "I can't help it. Every day I get farther and farther away from the paint can!" Spolsky's blog post (linked above) shows the programming problem that's illustrated by the hapless painter.

And then there's an infamous problem in probability studies. Let's say you're on a game show where you can choose between three doors. Two of the doors conceal goats; one conceals a new car. You choose door #1. To help you, the host now opens door #3 and reveals one of the goats. The question is this: should you change your choice from door #1 to door #2? This problem, which has vexed people for decades, is known as the Monty Hall Problem, named after a famous game-show host on American television.


The famous (infamous) Monty Hall Problem. (Source: Wikipedia)

A very well-known problem in math (specifically, in computational mathematics) is the Traveling Salesman Problem. Given a number of cities interconnected by roads, what's the most efficient route by which a salesman can visit each city once? (Seasonally speaking, there's a Santa Claus version of the problem also.)


The Traveling Salesman problem. (Source: Wikipedia)

There are other personifications as well. The Sleeping Barber Problem personifies a problem with synchronizing multiple processes. Probably many people have heard of the Prisoner's Dilemma, which is a personified version of a problem in game theory.

Personifying problems by giving them names is not just a way to keep undergraduates amused in computer or math classes. Illustrating problems using a scenario is an effective way to help people conceptualize abstract ideas. For example, the Monty Hall Problem has a formula;  the formula is compact and lends itself to mathematical manipulation, but it doesn't help people — even mathematicians — visualize a problem. Similarly, you might not know what it means for a problem to have an O(n2) solution, but imagining poor Shlemiel walking back and forth to his paint bucket sure can give you a feel for the issue. And of course, putting a name on a problem — Monty Hall, Dining Philosophers, Confused Deputy — adds a mnemonic that makes it easy to recall.

In fact, when we eventually got around to documenting the feature that mitigates the Confused Deputy problem, we described the problem in the abstract, but then added a parenthetical note: "(… known as the ‘Confused Deputy' problem.)" We know that adding this little note would help at least some of our readers instantly understand what problem we were talking about. Fortunately, I doubt we'll ever have to write about optimizing routes or about the probabilities of choosing between three doors. However, if we must, we'll have a shorthand way to describe those problems as well.


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Mike Pope has been a technical writer and editor for nearly 30 years. He has worked at Microsoft and Amazon, and currently works at Tableau Software. You can read more at Mike's Web Log and Evolving English II. Click here to read more articles by Mike Pope.

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Comments from our users:

Wednesday December 12th 2012, 12:08 PM
Comment by: Craig J. (Mundelein, IL)
You give me hope for solution of a "problem" which has proved surprisingly difficult. I have been looking for "The Missionaries' Dilemma" for some time to complete a writing project. The "dilemma" is what I have been told is called a crossing problem: a group of missionaries accompanied by a smaller group of cannibals is trying to cross a river with a small boat without leaving missionaries outnumbered by cannibals on either side of the river or in the boat, the idea being that the cannibals would dine on outnumbered missionaries. If you are in a position to provide a link to a complete write-up of the problem, without undue, effort, would you do so? Previous internet searches and assistance from a noted puzzle expert have drawn a blank; hopefully you, or someone in your reading audience has "special knowledge" that will make retrieval possible. Thank you.
Wednesday December 12th 2012, 1:05 PM
Comment by: Mike P. (Seattle, WA)Visual Thesaurus Contributor
Craig, your problem is known as the River Crossing Problem, and your description is a variant on what's known as the Fox and Geese problem (sometimes the Fox, Goose, and Bag of Beans problem -- see http://en.wikipedia.org/wiki/Fox,_goose_and_bag_of_beans_puzzle). Is that what you're looking for?
Wednesday December 12th 2012, 1:07 PM
Comment by: Mike P. (Seattle, WA)Visual Thesaurus Contributor
Wait, I see that there's an even more specific writeup of the missionaries and cannibals problem:

http://en.wikipedia.org/wiki/Missionaries_and_cannibals_problem
Thursday December 13th 2012, 12:30 PM
Comment by: Craig J. (Mundelein, IL)
God bless you, Mike. Apparently since my last search (some time ago) Wikipedia has expanded or search engines have improved, or your special knowledge made it look easy. Now I have absolutely no excuse for not finishing the project, other than constraints of time. Thanks.

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