Using a MAC in addition to symmetric encryption

[Greg Rose]

Erik Ostermueller wrote:
> 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.

One of my favourite papers, by Steve Bellovin, is at 
http://www.usenix.org/publications/library/proceedings/sec96/bellovin.html

It shows a number of ways in which IPsec with encryption but no 
integrity can fail.

Abstract:
The Internet Engineering Task Force (IETF) is in the process of adopting 
standards for IP-layer encryption and authentication (IPSEC). We 
describe a number of attacks against various versions of these 
protocols, including confidentiality failures and authentication 
failures. The implications of these attacks are troubling for the 
utility of this entire effort.

Greg.

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