Once a message is received with a public key, it is necessary to verify the source of the key to ensure that the person sending it is who you expect it to be. This can be performed using certificates and the trust network.

⁴Certificates are a digital assurance that a public key belongs to the person who claims to own it. As well as containing your public key, it will also hold the issuer of the certificate and its validity period which are provided by a Certification Authority (CA), the most well known being Verisign.

Revocation lists

A security check built into certificates is the revocation list, this is a list constantly updated by the CA of the certificate which the recipient can reference to make sure that the certificate has not been revoked. Revocation occurs when a CA loses trust that the certificate belongs to the person/business who claim to own it. This can be triggered by the person whose certificate it is reporting it stolen and therefore raising doubts about the validity of it any more thus stopping the person who stole the key being able to fake being the person who the certificate really belongs to.

Trust Network

Although you could just believe that every certificate you receive comes from the source that you believe sent it, this would leave a large flaw in the system. The fact that the certificate could be intercepted and changed if sent by email poses an obvious threat to the security of the proposes system. Therefore unless you know the person with the certificate in real life and they transfer it to you, for example by floppy disk, you can never be completely sure that it is the correct one. To combat this, the trust network was established. The trust network works by a method of “transferring trust”, for example Bob gets a certificate from an authority, his certificate and public key are stored by that CA. If Bob would like to send a message to Alice he can encrypt it with his private key and Alice can then decode it with a copy of Bob’s public key downloaded from that CA.
If Alice knows Bob well enough, she can check his key is valid e.g. by ringing him up and asking. She can then vouch for his key by signing it with hers. If another friend Charlie knows and trusts Alice and verifies her key then she can sign Alice’s key with hers and so a network it established. As Charlie trusts Alice and Alice trusts Bob, Charlie can infer that Bob is trustworthy and believe that the key stored by the CA is indeed his and not that of an impostor. The more people who sign a key, the more certain others can be that it is trustworthy.

Example of using a certificate and signature

A message is written, and when encrypted with your private key, a digital signature is produced. This is then sent. When it is received, the recipient uses your public key to decrypt the message. After checking the certificate with a CA to verify the validity of the key, and checking the certificate hasn’t been revoked, the receiver can be sure the message has been sent by you and not been tampered with since it was sent.

If for example the transfer had been intercepted and altered, the public key would not decode the message (there would be an insignificantly low chance that the changed message somehow matched what the public key would have produced) and therefore the message can be re-requested and hopefully arrive without being tampered with.