Discussion
Why is public-key cryptography mainly used for key exchange ?
Answer: C
The reason is that public-key methods avoid sharing a secret beforehand.
With symmetric encryption both parties need the same secret key, and getting it to the other side safely is the problem. Public-key cryptography solves it: a sender can encrypt a fresh session key with the recipient's public key, or the two sides can run Diffie-Hellman, establishing a shared secret over an open channel without any prior secret. The bulk data is then encrypted with the fast symmetric key, the hybrid scheme used by TLS.
Public-key methods are much slower than symmetric ones, which is why they are kept to key exchange. Symmetric keys can be distributed in other ways, by courier or pre-shared keys, just less conveniently. Public keys do expire, usually with their certificates.
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