Digital signature and signed a written document similar

Use of digital signatures, but also to confirm the following two points: first, the message is sent by the signer; Second, the information received from the issuance date has not been any changes. This digital signature can be used to prevent electronic information easily modified as a result of which was fake, or fraudulent use of the name of other people send a message. Or sent (received) a letter and then be denied to happen, and so on.