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You are correct: generally it is required to synchronize or otherwise synchronize information if you want to use a password based key derivation function (PBKDF) for sending messages. That doesn't mean that using a password on the client side a derived key on the server side doesn't have advantages: the password isn't stored on the server side, so an ...


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Sending symmetric key is equivalent to sending password (used for PBKDF) - they are both secret information that require secret channel. Sending session symmetric key encrypted with asymmetric cryptosystem doesn't require secret channel - you use public key of recipient (for instance taking it from certificate signed by root certificate that you trust, say ...


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Trying to use a PBKDF for transferring data over a network is not appropriate. The place to use a PBKDF is when you're protecting data at rest, e.g. some sort of archive file like a zip file or a backup. In works well in those contexts.


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KDFs are better used for offline protocols. What is often used for your second case are Key-agreement protocols. As far I know, there's no secure way of using KDFs in such interactive context where both parties need to know the key a priori. Key-agreement protocols make it feasible to compute a shared-secret (the encryption/decryption key) on demand - both ...


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PBKDF is intended to derive a key (specific length, hard to guess) from a password. That's it. Indeed, when exchanging messages encrypted only with a symetric cipher, at the end all involved parties need to have the same key. The key needs to be shared somehow at some point. If what I understood is correct, then I don't see the goal of using a PBKDF ...


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