Secret vs Public Key Encryption

1. Last time, we saw how to use secret keys and a KDS to ensure private communications.

2. We also discussed how public keys could be used to provide privacy and irrefutably signed digital messages if we could ensure that one could determine everyone else's public key reliably.

3. With this background on the use of public key encryption to produce signed digital documents, we can sketch out how a certified authority can allow one to determine another's public key.

   • As with a KDS, to get connected an individual or organization has to register with a certified authority by offering:

     • proof of identity

     • your public key

   • In a secret key system, each time Alice wants to talk to a new Bob, she has to make up a private key and ask the KDS to pass it on to Bob.

   • By contrast, when anyone wants to talk to Bob in a public key system they will use the same public key which Bob presented when he registered.

   • As long as Alice receives a message from the CA stating that the CA has verified that Bob is Bob and including Bob's public key she can send messages to Bob.

   • As long as the message is signed by the CA, Alice will know it came from the CA no matter how it is actually delivered. So, the CA can leave that up to Bob by handing him a "signed certificate" (what used to be called a letter of introduction) including his public key and details of his identity.

   • This means the CA is spared from becoming a bottleneck in the system. In fact, people can establish communications even while the CA is down.