With the invention of Public Key Cryptography, another process known as a digital signature is possible. A digital signature is much like a hand signature in that it provides proof that you are the originator of the message (Authentication). If you want to sign the message which you sent to an addressee, you pass the message through a mathematical function (known as a hash function) which provides a summary (hash code) of the message. This summary is unique for every message and is much like a fingerprint. You then encrypt this hash code with your Private Key and attach the code to the end of your message. This attached code is known as a digital signature. The addressee can then verify that the message was sent by you by decrypting the digital signature, using your public key, to get the hash code. The addressee then passes the received message through the same hash function. If the two hash codes are the same, then the message was sent from you (Non-repudiation) and was not altered (Integrity). All this sounds complicated but, in practice, selecting an icon on your computer screen is all that it takes to make it happen.