This is how a Kong signature works. If two signatures match, then they must have been signed with the same secret. In order to imitate a signature on one document, one must somehow discover the secret used to sign that document. This can be done, but it is vastly harder than lifting a signature from one fax to another, and harder than imitating a pen and ink signature.
However Verisign signatures are not used to compare documents. They are used to "prove" identity.
So to sign a document with SMIME you must prove to Verisign who you are. (An SMIME signature is not much use without such certification from Verisign)
Then Verisign creates and signs a certificate that any document signed with your secret is the real you.
This certificate is like an ID card. The Kong equivalent would be a document containing another document, making an assertion about the author of the contained document.
So Verisign signatures provide evidence of identity, whereas Kong signatures merely provide evidence that the same person signed one thing, as signed another thing.
"Huh, what secret? I did not know I had a secret endorsed by Verisign!"
As with all digital signature systems, a Verisign signature works because anyone can tell that two documents were signed using the same secret, even though they do not know what the secret is.
As with any digital signature system, Verisign signatures are only as good as the management of Verisign secrets, and managing them correctly is not trivial. If you do not know where the file containing your verisign secret is located, or how many copies there are, and most people do not, it is probably located somewhere insecure.
For some purposes, the Verisign model is more convenient than the Kong model, but most of the time we do not want to know that Bob Jones is the one true Bob Jones, but merely that Bob Jones is the same customer as opened an account. In such cases the Kong model of identity is more convenient, because it is more convenient for the parties to create their own certificate. Most people are reluctant to go to the trouble and delay necessary to prove their identities to Verisign.
The Kong secret is usually a passphrase, plus data contained in an ordinary file.
The Verisign secret is sometimes kept in an ordinary file, like the
Crypto Kong secret file, but in practice is usually stored by the CryptoAPI,
which in effect usually means that it is in a group of files with no real
passphrase. It is dispersed in a complicated way. This convoluted
and obscure storage has the advantage that it is complicated and laborious
for someone with access to your computer to copy the secret. It also has
the great disadvantage that it is difficult and complicated for you (the
rightful owner of the secret) to move the Verisign secret to a floppy disk,
take the floppy disk out of the computer, and walk away from that computer,
with the secret no longer in the computer, but in your pocket.
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