networking/security question!HELP!

(30 points) Suppose Alice needs to send a large file F of her to Bob securely and efficiently, i.e., the content of F should be encrypted with some symmetric key known by both ofthem, the content of F cannot be changed by anyone but Alice, and Alice cannot deny the content of F that she owns. Alice and Bob do not know each other, but they both trust an authority certification authority (CA) and have the public key of the CA; also, each of them has already a certificate for her/his own public key that is signed by the CA. Assume the public (asymmetric) key system used is RSA. Describe a protocol (i.e., a sequence of steps) for them to get F delivered securely and efficiently. [Hint: your protocol should include certificate verification, secure sending of symmetric key, file encryption, digital signature generation and verification, etc.)