Symmetric Ciphers

A symmetric cipher is a cipher which uses the same key for the process of encryption and decryption, see figure . A key used for a symmetric cipher is often called a private key or a secret key. This is due to the fact that when using a symmetric cipher one has to keep the key secret or Pope Leo X might read your stuff! It's just like the key for a safe box. If you placed your most prized anatomical research in a safe box and didn't lose the key, then you'd have no worries. No one can get to your papers, assuming they don't break the safe box. In the case of a symmetric cipher, the same is true. If you encrypt your plaintext using a key and don't lose the key, then you'd be without a worry. No one can read you plaintext, assuming they don't break the algorithm you used to encrypt the plaintext. To see the practical implications of using a symmetric cipher, lets take a page from Leonardo's life.

Say in your stand-in role as Leonardo stunt-double you want two transmit some of you anatomical research to your patron King Francis I, but to keep that snoopy Poe Leo X out of your business, you decide to use a symmetric cipher to encode the text of your anatomical research. The problem is that for King Francis I to decode your research has to have a copy of the secret key you used to encrypt your text. So, how do you get this key to the King?

The most obvious solution is to meet the King and give him the key, but both of you are busy people and don't have time to meet for months now. So, the next most obvious thing would be to get someone else to give the king the key. You decide on this course of action as it seems the most practical. But this seemingly practical course of action is actually pregnant with problems. Say, for example, if the messanger was a ``Double 0'' for Pope Leo X and made a copy of the key for ``Uncle Leo.'' Both you and the King wouldn't know if this had happened, and all of your future communications would be comrimised as a result. So, on reflection, you decide to personally deliver the key to the King.

As you have personally delivered the key to the King in this case, you know he has a copy of the key and you know that, at least in the moment you gave it to him, nobody but the two of you had such a copy. However, months later you really can't really be sure that this is still the case. For example, another ``Double 0'' might have infultrated the King's inner circles and copied the key. If this occurred, then any missive from you to your King could be intercepted and read by this ``Double 0.'' One way around some of this is introducting the notion of an ``asymmetric cipher.''