Q:

Can zkSNARK or other zero-knowledge proofs be used to proof message authenticity without revealing private key?

The situation:
Alice and Bob are exchanging messages encrypted by some known asymmetric encryption protocol. All their encrypted messages, as well as public keys are intercepted by Charlie.
Can Alice reveal decrypted messages to Charlie and provide him a zero-knowledge proof (for example zkSNARK) that deciphered text of the messages are indeed decrypted messages which he intercepted?
Can she do it without revealing her private key?
If so, can this method be used in any asymmetric cryptographic protocol without any special modifications to that protocol?
What about symmetric encryption, and passphrase revealing?

A:

In a "fair trade" paper at Bitcoin workshop Financial Crypto 2016, a snark-proof was introduced to assure that a message (a Sudoku solution) could be properly decrypted.

