Why encryption? Why signature?

First of all, what is encryption and signature using GPG or PGP? GPG and PGP are encryption sytems relying on asymmetrical keys. We use two keys. Something closed with the first one can only be opened with the second one. A key can only open what has been closed with it. What a surprise! That's the reason why those keys are asymmetrical. One of the key is called public key. The other one is the private key. The public key is not a secret and can be given to anybody. The private one is only yours, and musn't be sent.

It's personnal, it's private. And nobody's concerned by it. You could be preparing you politic strategy for the next election. You could be talking about the taxes you have to pay. You could secretly fall in love and send confidential messages. Whatever the reason why, you may want to protect your privacy and prevent your confidential documents and messages to be read. There is no shame protecting your privacy, as you do using enveloppes when you send a letter.

But pay attention, this is true for your company as well: would you like to see your industrial secrets to travel all around the world on the internet without any protection? Would you like to see your competitors gather your informations just like if they were fruits?

How do I use those keys?

For example, I would like to send an encrypted document to Bob. I use Bob's public key for encryption. This encrypted document can only be unencrypted using Bob's private key! Bob will be the only person to be able to read this document! All what I need to do this is Bob's public key. If Bob wishes to send me an encrypted document, he's got to use my pulic key, and I will be able to read the document using my private key.

Signature is another common use of encryption. I would like to certify that I am the anthentic author of a message. I therefore need to sign it. I use my public key to sign the document. The person that will receive my message will be able to check my identity using my public key. Moreover, if my message can be opened with my public key, it's because it has been closed with my private key, I am therefore the authentic author of this message!