Source: New Horizons eTips
Electronic documents and email messages are a common way to conduct business transactions, but it’s important to be able to verify that the author of a document or message is really the person he or she claims to be.
You can use digital signatures to verify identity. This is easy to do with programs such as Pretty Good Privacy (PGP). PGP is based on a public/private key pair; you sign the document by encrypting it with your private key, to which only you have access. The recipient uses your public key to decrypt it. Note that this doesn’t provide data confidentiality because the public key is available to everyone. It does, however, ensure that it was really you who signed it, because no one but you has the private key that’s paired with that public key.
PGP is available in both freeware and commercial versions. You can get the commercial version at http://www.pgp.com or download the free version for Windows XP at http://www.pgpi.org/products/pgp/versions/freeware/winxp/8.0/.
There are also versions for earlier Windows operating systems, UNIX, Mac, and even MS-DOS, OS/2, and Palm OS.