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Today I Learned What Entropy Is

I’ve heard the term entropy in the past, and it was a vague concept to me.

I recall an XKCD cartoon a few years ago that talked about passwords having “28 bits of entropy”. What does that even mean? Well, I took it to be a synonym of the word complexity. In general, you want your passwords to be harder to guess, so the higher number of bits of entropy, the better.

But what is entropy?

This TedEd video takes a relatively decent attempt at explaining it.

Let me attempt to explain it back.

Some people say entropy is a measure of disorder or randomness. Like a clean office desk will gradually become messy over time unless you invest the energy in cleaning it. So one definition is clearly this move from order to disorder.

This TedEd video explains it as having two systems next to each other. One is a high energy system and one is a low energy system. Like you have a burning candle wick being a high energy system and the wax underneath it being a low energy one. Energy will always move from the high energy system (the fire) to the low energy one (by heating the wax and melting it). Energy never moves from the low energy system to the high one. It COULD do that. It’s entirely possible that the energy of the wax could be transferred to the fire. But it never works like that in practice. Energy always travels from high energy to low.

So low energy systems always increase their energy (heat up or melt), and high energy systems always lose their energy (cool down or harden). Always.

It’s like a law of thermodynamics or something.

I don’t know what that has to do with passwords still.


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