A digital signature is prepared by first passing the message through a cryptographic function to calculate the message digest. The digest is then encrypted with the private key to produce a signature which is then added to the original message. The recipient of the digital signature can be sure that the message really came from the sender. And, because changing even one character in the message changes the message digest in an unpredictable way, the recipient can be sure that the message was not changed after the message digest was generated. Digital certificates provide the basis for secure electronic transactions as they enable all participants in a transaction to quickly and easily verify the identity of the other participants.
Privacy is guaranteed through encryption.
Although data can still be intercepted by a third party they will be unable to read it as they have no access to the encryption key Integrity is also ensured through encryption. If a message is received that will not decrypt properly then the recipient knows that the information has been tampered with during transmission. Authentication is provided through digital certificates. Digital certificates provide the basis for secure electronic transactions as they enable all participants in a transaction to quickly and easily verify the identity of the other participants. Here you can get a more detailed explanation of SSL .