How a Digital Signature Works
The trick of public key encryption -- the best known approach is called RSA for the initials of its inventors -- is that one key can be used to scramble the data while a different, mathematically related, key is used to unscramble it. When you download a digitally signed program, the first thing your computer does is check the Web site's digital certificate. It then queries the CA that issues the certificate to make sure it's still valid and to obtain the public key.
When the download is complete, your computer uses the public key to decrypt the message digest. It also runs the same one-way hash procedure on the downloaded software. If everything is as it should be, the decrypted message digest and the one just created should be identical. If they differ by a single bit, so
