Function decrypt 

pub fn decrypt(
    ctext: Vec<u8>,
    private_keys_for_decryption: &[SignedSecretKey],
    shared_secrets: &[String],
) -> Result<Message<'static>>

Decrypts the message:

• with keys from the private key keyring (passed in private_keys_for_decryption) if the message was asymmetrically encrypted,
• with a shared secret/password (passed in shared_secrets), if the message was symmetrically encrypted.

Returns the decrypted and decompressed message.