USE THIS COMMAND TO START SERVER: $ python server.py -pr private_key.der -pu public_key.der

AND THIS TO START CLIENT: $ python client.py -pus public_key.der -pr u1private_key.der -pu u1public_key.der -u bob $ python client.py -pus public_key.der -pr u2private_key.der -pu u2public_key.der -u alice