CryptoMessageInABottle/PublicSecretCrypto/README.md

Build a mechanical representaiton of encryption and signing via public/secret key systems

Inspired from https://blog.vrypan.net/2013/08/28/public-key-cryptography-for-non-geeks/ create a lock with 2 keys of which one can only unlock and the other can only lock

There are 2 lock cylinders, as can be found on normal doors, located in a padlock style device. It allows to close a box, envelope or be attached to items. The first trials with a prototype were a success. People are amazed.

How to run this workshop

Asymetric cryptography adresses the problem of key transmission.

You have a public key which everyone knows and a secret key which only the reciever knows.

The keys are generated by the reciever who then publishes the public key in a "phonebook"

Introduction

• show the padlocks with the 2 locks and the public keys. Take the secret key out of a pocket. "never give this anyone"

• show that there are 2 sets of padlocks and keys. The ones for Alice, and the ones for Bob

• IMPORTANT: show on a separate lock that there is a one-way function on the lock. It only turns one way around. This is the whole secret behind asymetric encryption systems. With one key you can only go in one direction, and never back.

• Chose kids (or groups of kids) who play Alice and Bob

• There are 3 Alice padlocks and 3 Bob padlocks

• The exercise can be done 6 times, by sending 3 messages from Alice to Bob and 3 from Bob to Alice. But this may complicate the explanations. In the following, only the direction Alice to Bob will be detailed.

Encryption

• Alice wites a message and puts it in an envelope/box. close the box by using the padlock and public key.

  • but which key to use? Remember, the keys are created by the reciever. So we need to use Bob's public key.
  • insist that the padlocks and public keys are accessible to anyone. Only the secret keys are... secret

• Bob recieves the box, and can unlock the padlock with his secret key

  • he is happy for the message
  • but who wrote it? It says Alice, but anyone could write that, then use Bob's public key and send it to him

Signing

• Alice wants to prove that the message comes from her.
  • Show that the padlocks have a second function. If you use the secret key first
  • Alice writes her message, then attaches a padlock to it and closes it with her secret key. only she has that key. So she is the only one who is able to close the padlock that way.
  • Alice puts the message with the padlock-signature into a box and encrypts it with the reciever's (Bob) public key.
  • Bob recieves the box, and opens it (decrpyt) with his secret key
  • He sees the message with Alice signature
  • He veryfies the signature by opening the lock with alice's public key. It works! So that proves that the message comes from her.
  • One could explain that it's technically possible to sign, making the message unreadable (but easily made readable with the public key) or to sign by leaving the message in clear text.
  • But where does this public key of Alice come from? How do we know it's Alice? Can be anyone...

Certification authorities

• Transform one or two padlocks into Cert padlocks (change cylinders and stickers)
• You can verify a signature, but for that you need a public key. Anyone in the world coud say "I am Alice, here is my key". So we are back to starting square.
  • Some higher instance needs to prove that the public key is the one of Alice. That is called a certification authority.
  • The certification authority signs the public key. This creates a certificate.
  • Demonstrate by attaching the CA's padlock using the CA's secret key (signing) to alice's public key (not to her padlock)
  • But who signs the certification authority's public key?
  • another certification authority, and so on... at some point you need to trust some authority.
  • Demonstrate by attaching another padlock-signature to the CA's public key. => "keychain"
  • Finally at the end, the top secret key is secured in a box/envelope by attaching a combination code padlock. And the code is put in your head.
  • Web browsers come with built-in certification authorities which the browser trusts. That's how the websites prove that they are who they say. Show a webbrowser's CA list.
  • Show how a website's certificate is signed by a row of CAs.