What the HTTPS padlock means: how your browser talks in secret on a Wi-Fi anyone can read
Diffie-Hellman key exchange: how your phone and a website build the same key without sending it
Your phone and a website want to talk in secret. For that, they need the same secret key. But anyone can listen to every message they send to agree on the key. So they cannot just send the key.
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1
The trick fits in pots of paint. Both ends start from the same yellow, which anyone may know. Each one stirs in a secret color of its own and keeps that color to itself.
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2
They send each other their mixes, in the open. Rono copies both, and they don't help him: paint is easy to mix and very hard to un-mix, so no copy gives back a secret color.
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3
Now each end stirs its own secret into the mix it received. Both pots hold yellow, coral and teal: the same color. That color is the key, and it never crossed the Wi-Fi.
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4
Rono pours his two copies together, but that pot holds the yellow twice, and the color is wrong. With numbers in place of paint, this is a key exchange, published by Whitfield Diffie and Martin Hellman in 1976. No known method undoes its arithmetic in a useful time.
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5
Miva's phone now shares a secret key with whoever sent the other mix. It took that mix on trust. What stops Rono from sending one of his own?
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Quick check
Rono copied the yellow and both mixes. What is he missing to build the key?
Show the answer
One of the two secret colors. They never left Miva's phone or the shop's server.
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Go further
The real names behind the paint.