An authentication code used in email and other documents sent over networks the same as a traditional written signature or name stamp to verify to a recipient that the signed file came from the person who sent it, and that it was not altered since it was signed. Digital signatures are based on public key cryptosystem: a "private key" is used to sign the message, and the recipient of the digital document can use the signer's "public key" to verify whether or not the digital signature is valid and whether the document has been altered since it was signed.
