Digital signature is used to mean a cryptographically based signature assurance scheme .
Digital signature means an electronic sound, symbol, or process, logically associated with a contract or other record and executed or adopted by a person with the intent to sign the record ; it may also be used as a narrower term encompassing only cryptographic signatures.
There are three common reasons for applying a digital signature to communications:- authentication , non-repudiation and integrity.
See: http://en.wikipedia.org/wiki/Digital_signatures
See: http://www.youdzone.com/signature.html (An introduction to Digital Signatures, by David Youd)
Digital Signature Guidelines - Free Download : http://www.abanet.org/scitech/ec/isc/dsgfree.html
Here is one of many utilities to generate the Digital Signature:
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