How end-to-end encryption works: why even WhatsApp cannot read your messages
Public and private keys: the key your phone can show to everyone
Your phone has two keys: a private key that it keeps secret, and a public key that it can show to everyone. With these keys, two phones that never met end up with the same secret key, and the server never learns it.
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1
Go back to the day Miva installed the app. Her phone made a pair of keys: first a private key, a long random number, then a public key, worked out from the private one.
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The working out goes one way only. From the private key, the phone gets the public key in a blink. From the public key, no known method finds the private key, even with a crowd of computers working for years.
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3
So the private key never leaves Miva's phone. The public key goes up to the chat server, which keeps it for anyone who wants to write to her.
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4
Your phone did the same when you installed the app. Now the server holds both public keys, and Rono can stare at them all day: on its own, a public key opens nothing.
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5
Miva's phone is still switched off in her bag. Her public key, though, waits on the server, and that is all your phone needs to start a locked chat with her.
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Quick check
A thief copies every public key on the chat server.
Show the answer
No. A public key is made to be seen, and it does not lead back to the private key, which never left your phone.
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Go further
One name to keep.