How a digital signature works
A technology called public key cryptography makes it possible for you to make sure that the publisher of any piece of software that claims to be from Microsoft (MSFT) or any other publisher really came from there. It has the added benefit of insuring that the contents weren't maliciously altered or damaged in transmission. Here's how it works.
The publisher first has to obtain a digital certificate from a recognized "certificate authority" or CA [VeriSign (VRSN) is the largest and best known CA in the U.S.]. The publisher receives a private and a public key, each of which is a long number of about 300 digits. These are used to create a digital signature for each program.
By Stephen H. Wildstrom at MSNBC.
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