A Digital Signature validates the authenticity of a digital message or document as coming from the original sender ususally through the use of a public/private key encryption system. The private key is used to encrypt the hash of a message and only the corresponding public key can decrypt it. If the hashes match the message is validated. Likewise only the private key can decrypt the a message encrypted by the corresponding public key. Digital signatures are considered an equivalent of a hand signature and anyone who uses a digital signature with a message can not repudiate the fact that they sent the message.