Encryption and Requirements

Using encryption
Confidentiality
obvious - only meaningless data visible
Integrity
if data modified
will decrypt to junk
attacker does not know key
cannot create correct encoding
Authentication
Message correctly encrptyed
Sender knew key
Only you and I know key
I did not send the message
So I know it must have really come from you
Non-repudiation
A problem ...
I know message came from you
But I cannot prove it
I might have sent the message