Digital signature

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 for laws around the world.