Digital signature

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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.


  1. Digital Signature Law Survey