Let's establish the objectives for digital signatures.
Before we do so, here are the objectives for a message authentication.
Message authentication is a procedure to verify that
received messages come from the alleged source and have not been altered.
Message authentication protects two parties who exchange messages from any third party.
So the attacker is not between the two parties.
However, message authentication does not protect the two parties against each other,
where when either of the party fraudulently create or deny creation of a message.
A digital signature is an authentication technique that
also includes measures to counter repudiation by the source,
which is when the source denies sending the packet after it has sent it.
A digital signature is comparable to
handwriting signature and provides a unique set of security capabilities.
More specifically, it prevents another user spoofing the sender.
It also prevents the receiver forging packets as if they originated from a sender.
Further more, digital signature provides non-repudiation property so that the sender
cannot deny sending messages after he has done so.
You can envision that this property is important in digital business transactions.
For example, in a stock market context,
against an attacker who purchased a stock but
later re-signs the order after he realizes that the stock price went down.
To support such objectives,
digital signature must have the following properties.
It must verify the author and the date and time of the signature.
It must authenticate the contents at the time of the signature.
It must also be verifiable by third parties to resolve disputes.
Therefore, the digital signature function includes the authentication function.
Let's farther describe about how to achieve these properties.