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Digital signatures provide source authentication for online documents, messages or records, in a manner analogous to what a signature provides for a paper document.
Two cryptographic techniques are used together to produce a digital signature, a cryptographic hash and a public key cryptosystem.
The steps for the sender are as follows:
- calculate a hash or message digest from the message
- encrypt that hash with the sender's private key
- combine the encrypted hash with information identifying the signer
- append the combination to the message as a signature
Steps for the receiver are:
- obtain the sender's public key and authenticate its integrity and source
- decrypt the signature, using the sender's public key, to get the hash value; call it H1
- hash the message body yourself to get another hash value, H2
- compare H1 and H2
- if they are identical, then you know with overwhelming probability (though not with absolute certainty):
- the documents signed (hash H1) and the document received (hash H2) are identical (from properties of a cryptographic hash)
- whoever generated the signature knew the signer's private key (which only the signer should know in a public key system)
- so you can accept the signature as valid
If both the hash and the public key system used are secure, and no-one except the sender knows his private key, then the signatures are trustworthy.
The use of digital signatures raises legal issues. There is an online reference [1] for laws around the world.
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