Tag: Alice & Bob

Bob and Alice get Divorced – Meet the Three Headed Beast (and Trent)


bobaliceWe have a problem in modern society … how can we be sure that someone really is who they say they are? On the Internet we normally trusted third party that both parties trust. In many cases when we discuss secret communications we have Bob and Alice, so let’s call our pair Bob and Alice, and we will define their trusted person as Trent, who is the only person in the world that both Bob and Alice trust. Unfortunately Trent’s fees are high, so they want to minimise the things that he has to do, and must thus use Trent to set-up a secret channel for them to communicate. For this Trent will create a secret box, which can be passed between Bob and Alice, who will have secret keys for it. If they loose them, hopefully Trent will have a copy of the key, just in case.

Messy Divorce

Bob and Alice and going through a difficult time, and now they only communicate through their lawyer, who they both trust, but want a way to identify each other and communicate in a secret way (so that Trent doesn’t actually see them messages). Unfortunately they cannot afford any expensive lawyer fees for their communications, so how do they use Trent to create a secret box for their communications? Well they can use a proposal known as Kerberos.

Trent Gets a Box and a Key

So Alice goes to Trent and says that she has to prove her identity to Bob, and vice-versa. For this Trent will make a special key for a box, and will make a copy for Bob and Alice (he might also keep a copy for himself, just in case they lose them – this technique is know as key escrow). Trent will then take a photograph of Alice, and write down the date and time on it, and the amount of time he can verify Alice for. He will then put it into the box, and gives the box to Alice, along with the key. Along with this he will give her a sealed letter for the attention Bob which has his stamp on it. Inside will be a photograph of Alice that he took, and the secret key, along with the date/time that he created the key. With key escrow, Trent will keep a copy of the key, just in case that Bob and Alice loose their keys, but if even Bob and Alice do not want this, then Trust must prove that he does not have any other keys. In this way Trent will not be blamed for any leakage of the information in the box.

Alice Sends the Box

Alice then goes home, and then puts her photograph in the box, and locks it with the secret key. She then passes the box, without the key, along with the sealed letter to Bob. Bob opens the sealed letter, which has a key inside to open up the box, and which has the photograph that Trent took of Alice. Bob then opens the box with the secret key provided by Trent, and takes out the photograph that Alice has provided. If it is the same as the one that Trent put in the sealed letter, Bob thus verifies Alice’s identity.

Now we have a Secret Box … and no lawyer

Bob and Alice now have the same key to open and close the secret box, and can now use it to send secret messages to each other. No-one else will have that unique key, thus any messages in there must have been provided by Bob and Alice. Now they can both define the terms of the divorce without being billed for more solicitor fees for their messages.

The Encryption Protocol

kerIf you are interested, here is the actual encryption proposal based on this example: