There has been much written about what digital signatures are. But more technically, how do digital signatures work?
Digital signatures are based on a specific protocol, called Public Key Infrastructure (PKI). This protocol uses cryptographic algorithms to create two long numbers. These are called keys. One of the keys is public, the other one is private.
As digital signatures are unique to a signer, each time a signer signs the document, the signature is created using the signer’s private key. This private key is always securely kept by the signer and is included in the signature when he signs. Basically, the digital signature securely associates a signer with a document in the form of a coded message.
Next to this key, the signature also contains the certificate of the signer including the public key and other information, like date and time at which the document was signed.
Before signing, a cryptographic function is used to create a message digest (comparable with some data), called a hash. Afterwards this hash is encrypted (signed) with the private key of the signer and included in the digital signature.
When the document arrives at the receiver, another hash will be created. By decrypting the hash that was included in the signature you will be able to compare it with the hash that was created for the document. If they don´t match, the receiver of the document will see that the document is tampered with, resulting in an invalid digital signature.
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