How end-to-end encryption works: why even WhatsApp cannot read your messages
Key exchange: how two phones build one key while one is switched off
A key exchange lets two phones make the same secret key without ever sending it. In an end-to-end encrypted app, it works even when your friend's phone is switched off. How is that possible?
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1
Your phone sets Miva's public key beside your own private key. The maths of key pairs combines the two into a brand-new key.
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2
That new key scrambles your message. Off it goes through the server, with your public key clipped to it.
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3
That evening, Miva switches her phone on. It combines your public key with her private key, and out comes the very same key. Your message opens.
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4
Two different pairs of keys, one result. Each phone uses its own private key and the other's public key, and both land on exactly the same key.
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5
Rono holds both public keys as well. But every mix that works needs one private key, and those never left your phones. This is a key exchange, done through a server while one phone sleeps.
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One step still rests on trust. Your phone took the key the server handed over and believed it was Miva's. But the server is the one Rono looks after. What if he hands you a key of his own?
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Go further
Two names to keep.