ZyXEL Communications 2WG - Manual (Page 258)

ZyWALL 2WG Support Notes
All contents copyright (c) 2006 ZyXEL Communications Corporation.
258
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.