Asymmetric Ciphers

An asymmetric cipher is a cipher which uses two keys instead of a single key, see figure . One of these keys is called a public key, which can be seen by anyone with no loss in privacy. The second key is called a private key and should be kept private, like the key of a symmetric cipher. These keys are always generated and used in pairs. So, each public key will have an associated a private key and each private key will have an associated public key. The use of these keys is a bit different from the use of keys in a symmetric cipher.

When using an asymmetric cipher, data encrypted using the public key can only be decrypted using the associated private key and no other key can decrypt the so encrypted data. Furthermore, in some asymmetric ciphers the reverse of this is also possible, one can encrypt data using the private key and the public key is the only key which can decrypt the so encrypted data. The differences between an asymmetric cipher and a symmetric cipher have very different security implications.

Consider the case we dealt with above. You, the Leonardo fall-guy, have decided to send your encrypted anatomical research to your patron, King Francis I. This time, instead of using the old school symmetric cipher you decide to use one of these new asymmetric cipher that seem all the rage with the kids these days. What you then do is to encrypt your anatomical research using the King's public key, which he publishes in Nove da Firenze, then send the encrypted research to the King. As the King is, at least in theory, the only person who has the private key associated with his public key, he is the only person who can decrypt your anatomical treatise. As you can see this model is extremely different from the old symmetric cipher model. You don't need to worry about how to get a private key from you to the King, the use of a public key obviates the need for such backflips. However, there is another problem with this means of encryption.

Consider if the elite ``Double 0'' agent 006, known to his friends as Alec Trevelyan, was a typesetter for the Nove da Firenze. If the ``Double 0'' Alec Trevelyan replaces the King's public key with the his public key in the Nove da Firenze, then when you, I mean Leonardo, uses the public key in the paper to encrypt your research Alec, the ``Double 0,'' could intercept your dissertation and decrypt it with his private key. However, there is a way around even this.

Your benefactor, the King, has already thought about such rouges; he has met and knows the evil in the eyes of Pope Leo X. (Pope Leo X, even though he was a Medici by blood, was evil! Look at the eyes on that man, figure , pure evil.) To bypass such rouges, the King, instead of publishing his public key only in the Nove da Firenze, actually publishes his public key in all the papers of Italy. So, unless you're sporting an aluminium beanie and looking for black helicopters in the sky, after checking four or five newspapers and finding that the King's public key is the same in all of them, save the Nove da Firenze, you'd feel safe in using this public key.