How can the chats between two devices be encrypted without them sharing the same key through the server.
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imagine you want to send a package to friend without anyone being able to know what it is. if you just send it the mail man could open it.
to avoid that you put it in a box and lock it. but its not a normal lock its a special lock. it has a keypad and two codes. the special part is, that if you use one of those codes to lock it the other code has to be used to open it. both can be used to lock and unlock it, but you always need the other one to do the oposite.
now. you and your friend have thoses locks. in order for the other person to send you stuff you need to publish one of these codes. so you find a thrusted person and tell them one of your codes. this code is then called the public key. the other code you keep private. therfore its called the private key.
now everything is set up for the secure pakage. you get the public key from you friend and use it to lock the box shut. because everyone only knows the public key, noone is able to unlock it again. when your friend recieves the box he can unlock it with his private key.
this is the basic idea of asymetric encryption. there are a few more things you can do. for exaple if you want to prove to your friend that you send the box, you can add a second lock on it that you close with your private key. if your friend can open it with your public key, he knows that you send it.
the only big problem with this tecnology is that its slow. imagine the lock taking a few minutes to open or close. for a fast comunication its not good. so usally you only do the first comunication with this and send a key to a normal lock
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