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Published online by Cambridge University Press: 06 July 2010
Introduction
The need to authenticate both the contents and origin of a message is crucial in any communications network. Consider the following problematic situations in which Alice and Bob face the forger Fred. In each case we suppose that Bob is Alice's banker.
(1) Suppose Fred sends Bob a message claiming to come from Alice asking him to transfer $1000 into Fred's account. If Bob has no way of verifying the origin of this message then Alice is in trouble.
(2) Suppose Fred intercepts a message from Alice to Bob asking him to transfer $1000 into Carol's account. If Fred can alter the message so that ‘Carol’ is replaced by ‘Fred’ then again there is trouble.
(3) Suppose Fred intercepts a message from Alice to Bob asking him to transfer $1000 into Fred's account. Fred stores the message and resends it to Bob whenever he is short of cash!
In each case Fred can succeed if no proper system of message authentication is in place.
Historically the handwritten signature has been the preferred method for authentication of messages. A digital signature is a method for achieving this based on cryptography.
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