Transcript
NARRATOR:
We’ve already seen that exchanging encrypted documents using public key means that Alice and Bob each have to generate their own key pairs, comprised of a public key and a private key. Before they can exchange documents, they first need to send one another copies of their public keys. Then, Alice can send secrets to Bob by encrypting documents using Bob’s public key, and Bob can share secrets with Alice using her public key. But there’s more you can do with public key cryptography than just hiding secrets. It’s also possible to encrypt data using the private key, which might sound like a pointless thing to do.
After all, a file encrypted using Bob’s private key can be decrypted by anyone who has a copy of his public key. And Bob gives that away to anyone who asks, including evil Eve. So, if encrypting using the private key isn’t going to protect any secrets, what’s it for? Whilst the encrypted file can be decrypted by any copy of Bob’s public key, it can only have been encrypted by the corresponding private key. If Bob has obeyed the rules and not shared his private key, then the documents can only have come from Bob. Encrypting using the private key is therefore a way of authenticating data.
Now, anyone wanting data from Bob can download a copy of the encrypted document and a copy of his public key. They decrypt the file using the public key and can satisfy themselves the data is genuine. But it’s not quite as simple as that. Bob’s public key is only authenticated by his email address. If Eve can steal Bob’s email address, there is nothing to stop her generating new keys under Bob’s identity. Eve can now send false documents or malware in Bob’s name. Alice will open them, because she trusts Bob. Oh dear. Bob can prevent Eve impersonating him by certifying his public key.
Here, a so-called trusted third party, which can be another individual, a government, or a private company, will confirm that Bob’s key is genuine. To do this, Bob must prove his identity to them using personal information that isn’t readily available to Eve, such as his passport, business registration, or birth certificate. The certification body can either certify the public key itself or provide Bob with a digital certificate containing his public key.
As well as the holder’s public key, a certificate contains a unique serial number, the name of the certificate’s owner, the name of the agency that issued the certificate, the agency’s digital signature, proving it is authentic, the issue date of the certificate and the date it will expire, after which it can no longer be considered valid, and a hash value used to check that the certificate has not been altered since it was issued. As well as individual use, certificates are used to authenticate software downloads, such as those from app stores. Certificates are also used by websites who presents copies of their certificates to web browsers. The browser checks that the certificate is authentic, proving that the site is genuine.
If the certificate is invalid, the browser will warn the user they may be navigating to a page that has been hijacked, and it will offer them an opportunity to stop. Certificate holders have to be careful to renew their certificates before they expire. Otherwise, they might find users avoiding their websites or that their software downloads are not valid. This happened to Apple in November 2015, when millions of users could not update apps on their Macs. Fortunately, a new certificate was quickly issued, and everything worked again.