On Fri, 27 Jun 2008, Erik Ostermueller wrote:

Hello all,

If I exchange messages with a system and the messages are encrypted with a 
symmetric key, what further benefit would we get by using a MAC (Message 
Authentication Code) along with the message encryption?
Being new to all this, using the encrytpion and MAC together seem redundant.


--Erik Ostermueller

As the other posters have already commented, encryption alone does not
(in general) provide integrity. Furthermore, care must be taken in how
the encryption scheme and the MAC are combined, with
encryption-followed-by-MACing-the-ciphertext being the best choice
unless you know what you are doing. For further discussion, see the
textbook by Katz-Lindell (Section 4.9), and/or the following paper:

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