Why use symmetric encryption with Public Key? A. From what I heard (and I am beginner at a security/cryptography), if you have a payload that is very large, asymmetric is a no-go.

B. Also, if you do have secret symmetric (e.g. AES) keys, you might not want to share them with others, if they have high importance to you.

I am trying to understand things and I cannot find good enough internet resources explaining this use case.

Here is the reasons I have thought of, for doing such an encryption:


asymmetric encryption is not available; e.g. due to large data size
symmetric keys are available but you do not want them to be shared with other parties
sender/encryptor needs to send data to the receiver securely (encrypted)


Please validate/criticise my ideas or add others.
I think you're looking for Hybrid cryptosystems.

As you correctly noted it's not only unsafe to encrypt large data with a asymmetric system it's also very slow compared to symmetric systems. That's why we usually use both of these systems together (hence hybrid) to "fill each other's gaps".

We share a symmetric key using asymmetric cryptography. The advantage is that we only have to share the symmetric key once at the beginning of a session and all succeeding messages will be encrypted using the symmetric key. The asymmetric system provides safe key-sharing and the symmetric system guarantees fast encryption / decryption.


  B. Also, if you do have secret symmetric (e.g. AES) keys, you might not want to share them with others, if they have high importance to you.


That's true, but we always create new keys for each communication channel, we don't use a single symmetric key for all communication.