It's common knowledge that if somebody has physical access to your machine they can do whatever they want with it1.

So why do we always lock our computers? If somebody has physical access to my computer, it doesn't really matter if it's locked or not. They can either boot a live CD and reset my password or read my files (if it's not encrypted), or perform a cold boot attack to get my encryption keys from memory (if it is encrypted).

What's the point of locking a computer besides keeping the average coworker from messing with your stuff? Does it provide any real security benefit, or is it just a convenience to deter untrained people?



1. unless the computer has been off for a while and you're using full-disk encryption

In some places they have a saying: "opportunity makes the thief". All you're doing by screen-locking a computer is making the cost of hacking it just a little bit harder.

Security is an economic good, with a price and a value. The value of locking is somewhat larger than the price of locking it.  Sort of like how in good neighborhoods, you don't need to lock your front door. In most neighborhoods, you do lock your front door, but anyone with a hammer, a large rock or a brick could get in through the windows.  In some neighborhoods, not only do you lock the door, you have a solid-core door with a deadbolt, and you have steel gratings over the windows.  In the best neighborhoods, the value of the steel gratings isn't worth the price, but in bad neighborhoods, the value does exceed the price.
