Are public key and private key interchangeable?

Your public key is used to encrypt a message, your private one to decrypt it. Thus with the public key, which you distribute, anyone can encrypt a message safe in the knowledge that only you (or someone with your private key) can decrypt it. To answer your question directly no they are not interchangeable .

You should never distribute your private key.

Public keys encrypt. Private keys decrypt. They do not switch around.

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