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Curious Kids: How do keys open locks?

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The depths of the valleys on a key act like a code that must match the lock. Robin Smith/The Image Bank via Getty Images

Scott Craver, Binghamton University, State University of New York


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How are keys made, and how do they open locks? – Noli, age 12, Wisconsin

Have you ever wondered how keys work? I teach a course in computer security where we learn how locks function – and also how they can be broken or bypassed. We do this because locks teach important principles about security in general.

A ruler is next to a key. Red arrows show how the key's intendations are evenly spaced.
The spacing of the valleys is key. Scott Craver, CC BY-ND

If you look closely at a key, you’ll see its top edge has a bunch of V-shaped valleys. If you inspect the key more closely, perhaps with a ruler, you’ll notice the bottoms of these valleys are equally spaced. The depth of the valleys encodes a sequence that is accepted by the lock, with each valley contributing one value to the combination.

Inside the lock is a cylinder – the part that moves when you stick your key in and turn it. The key can turn only if all its valleys are the right depth for your particular lock.

But how does your lock detect whether your key’s valleys have the right sequence of depths?

A lock with its inner-workings exposed. Labeled are the shafts, pins and cylinder.
A peek at the parts inside a lock. Scott Craver, CC BY-ND

Inside the lock are vertical shafts, one over each valley of the key. In each shaft is a pair of metal pins that can freely slide up and down. Depending on where the pins are, they can block the cylinder from turning and prevent the lock from opening. This happens whenever a pin is partially sticking into or out of the cylinder.

Side by side photos showing the inside of a lock. The left image shows pins that are too high and too low. The right image shows the pins aligned.
For a lock to open, all the pins must be aligned. Scott Carver, CC BY-ND

When you stick a key in the lock, the pins fall into the valleys. If a valley is too high, it causes a pin to stick out and jam the cylinder. If a valley is too low, the pin sinks too low and the pin above it will sink into the cylinder and jam it. However, if the right key is inserted with the valleys at just the right depths, none of the pins get in the way.

Keys are made by placing a blank key into a grinding machine that is programmed to carve out the exact valleys that are needed. A locksmith can also change a lock by removing its pins and fitting it with new ones to match a chosen key.

In computer security, we say that security relies on “something you know, something you have or something you are.” A password is an example of something you know. A key is an example of something you have. A fingerprint would be an example of something you are. But as you can see, a key is also very much like a password, except it is encoded by grinding a piece of metal.

For this reason, you shouldn’t ever post a picture of your house key on the internet. That would be like posting a picture of a credit card or a password – someone could use the photo to duplicate the key.

It is also possible to unlock or “pick” locks without a key. By sliding a thin piece of metal into the cylinder and gently pushing the pins to the correct height one by one, locks can be opened. However, it takes a great deal of skill and practice to do this.

What does this teach us about security? First, we must make keys secret by making a very large number of possible keys, so that the right one is hard to guess or build. It’s the same for passwords. Second, it’s important to engineer a lock or computer program that requires every bit of the key or password to be exactly correct.

It’s important to study the inner workings of locks and computer programs to understand how their design might allow someone to break them.

Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to Please tell us your name, age and the city where you live.

And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.The Conversation

Scott Craver, Associate Professor of Electrical and Computer Engineering, Binghamton University, State University of New York

This article is republished from The Conversation under a Creative Commons license. Read the original article.