Certificate Authority - Example

Example

Public-key cryptography can be used to encrypt data communicated between two parties. This can typically happen when a user logs on to any site that implements the HTTP Secure protocol. In this example let us suppose that the user logs on to his bank's homepage www.bank.example to do online banking. When the user opens www.bank.example homepage, he receives a public key along with all the data that his web-browser displays. The public key could be used to encrypt data from the client the server but the safe procedure is to use it in a protocol that determines a shared symmetric encryption key; messages in such protocol are ciphered with the public key and only the bank server has the private key to read them. The rest of the communication proceeds using the new (disposable) symmetric key, so when the user enters some information to the bank's page and submits the page (sends the information back to the bank) then the data the user has entered to the page will be encrypted by his web browser. Therefore, even if someone can access the (encrypted) data that was communicated from the user to www.bank.example, such eavesdropper cannot read or decipher it.

This mechanism is only safe if the user can be sure that it is the bank that he sees in his web browser. If the user types in www.bank.example, but his communication is hi-jacked and a fake web-site (that pretends to be the bank web-site) sends the page information back to the user's browser, the fake web-page can send a fake public key to the user (for which the fake site owns a matching private key). The user will fill the form with his personal data and will submit the page. The fake web-page will get access to the user's data.

A certificate authority (CA) is an organization that stores public keys and their owners and every party in a communication trusts this organization (and knows its public key). When the user's web browser receives the public key from www.bank.example it also receives a digital signature of the key (with some more information, in a so-called X509 certificate). The browser already possesses the public key of the CA and can consequently can verify the signature, trust the certificate and the public key in it: since www.bank.example uses a public key that the certification authority certifies, a fake www.bank.example can only use the same public key. Since the fake www.bank.example does not know the corresponding private key, it cannot perform the steps to generate a symmetric key for communicating with the browser.