L10. What Is A Digital Signature - ZyXEL Communications ZyWALL 5 Support Notes

ZyWALL 5 Support Notes

L10. What is a digital signature?

Not to be confused with a digitized signature (a scan of a hand-written signature), a digital signature
can be used with either encrypted or unencrypted messages to confirm the sender's identity and ensure
the recipient that the message content has not been changed in transmission. Digital signatures
incorporate the characteristics of hand-written signatures in that they can only be generated by the
signer, are verifiable, and cannot easily be imitated or repudiated.
L11. How does a digital signature work?
Suppose that the famous Bob and Alice wish to correspond electronically. Bob wants to assure Alice
that he originated the electronic message, and that its contents have not been tampered with. He does
so by signing the message with a digital signature.
When Bob clicks on the digital signature option on his e-mail application, special software applies a
mathematical formula known as a hash function to the message, converting it to a fixed-length string
of characters called a message digest. The digest acts as a "digital fingerprint" of the original message.
If the original message is changed in any way, it will not produce the same message digest when the
hash function is applied. Bob's software then encrypts the message digest with his private key,
producing a digital signature of the message. He transmits the message and digital signature to Alice.
Alice uses Bob's public key to decrypt the digital signature, revealing the message digest. Since only
Bob's public key can decrypt the digital signature, she is able to verify that Bob was the sender of the
message. This verification process also tells Alice's software which hash function was used to create
the message digest of Bob's original message. To verify the message content, Alice's software applies
the hash function to the message she received from Bob. The message digests should be identical. If
they are, Alice knows the message has not been changed and she is assured of its integrity. (If Bob had
wanted to ensure the confidentiality of his message, he could have encrypted it with Alice's public key
before applying the hash function to the message.)
The best thing about all these encryption, decryption, verifying and authenticating processes is that
special software does them all transparently, so that Bob and Alice receive the assurances they need
without having actually to engage in computations themselves.
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