The definition
A digital signature is an electronic (code) signature that can be used to authenticate the identity of the sender of a message or the signer of a document and to ensure that the original content of the message or document that has been sent is unchanged. Digital signatures are easily transportable, cannot be imitated by someone else, and can be automatically time-stamped. The ability to ensure that the original signed message arrived means that the sender cannot easily repudiate it later.
A digital signature can be used with any kind of message, whether it is encrypted or not, simply so that the receiver can be sure of the sender’s identity and that the message arrived intact. A digital certificate contains the digital signature of the certificate-issuing authority so that anyone can verify that the certificate is real.
A more formal definition: “(I) A value computed with a cryptographic algorithm and appended to a data object in such a way that any recipient of the data can use the signature to verify the data’s origin and integrity.
(II) Data appended to, or a cryptographic transformation of, a data unit that allows a recipient of the data unit to prove the source and integrity of the data unit and protect against forgery, e.g. by the recipient.”
Source: IETF (http://www.ietf.org/rfc/rfc2828.txt).
E-signature
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