Signed files are used to authenticate both ends of the transmission. Both recipient and sender
are authenticated this way.
You will need to know the corresponding keys that were used to encrypt and sign the message.
Your public key was used to encrypt the message while the senders private key was used to sign it.
Thus, to properly decrypt it you will need your private key and the senders public key.
We will use defaults to set some other options. Please review code for available treatment.