A strong cryptography (secret key and public key systems as well) can assure the confidentiality of the encrypted files. But there are other issues needed to be solved about the security of the communications.
- Secret key exchange. Q. How to distribute one secret key to your party?
- Authentification. Q. How do you know that the message is really from your party?
- Integrity?. Q. How to maker sure that your message has not been altered?
- Nonrepudiation?. Q. How to make sure that your party cannot deny the receiving of the message?