Can You Keep a Secret?

In his personal notebooks, see figure , Leonardo Da Vinci, a lefty, wrote all his notes in a form of ``mirror writing.'' He wrote everything in Italian from right to left, opposite the normal order of things in that language. In addition, he wrote in a special ``short-hand'' in which he used various unconventional abbreviations and spellings. There have been various hypothesis as to why he decided to write in such a perverse manner. One of these speculations holds that he did so to protect the secrets of his research from the Roman Catholic church. No one knows if this is true. But, strangely enough, later in his life a a German mirror maker named Giovanni degli Specchi, who could have used his wares to read Leonardo's notebooks, accused Leonardo of blasphemy, and as a result, Leonardo was banned from doing anatomical investigations by Pope Leo X. So, the ability to keep secrets is a integral part of security; it may just save you from excommunication.

Though Leonardo Da Vinci was doubtlessly one of the most brilliant people ever to grace the face of this Earth, he was not so hot at keeping a secret. Lets look in bit more detail on one way of keeping a secret which is similar to Leonardo's, but usually bit more secure.

Encryption is the process of taking what is called plaintext, normal text that even Pope Leo X could read if he were alive, and converting it in to an indecipherable muddle that no-one can read, so called ciphertext. So, what Leonardo was doing above, in writing his notebooks in such a perverse manner, was converting the plaintext of his mind to the ciphertext of the page. This process of converting plaintext into ciphertext is called encryption; the opposite process of converting ciphertext into plaintext is called decryption, and the actual transformation which converts plaintext into ciphertext or ciphertext into plaintext is called a cipher. In addition, in the vast majority of cases a cipher will use a key, a sequence of bytes, in the process of encryption or decryption. A key is like a password that a cipher requires in the process of decryption or encryption. For example, if a cipher encodes a some of plaintext with two different keys, then it will result in two different ciphertexts. Also, a cipher can only decrypt ciphertext into the original plaintext using the proper key.

So what use is all of this? Consider if you were in Leonardo's shoes and wanted to protect your anatomical research from the prying eyes of Pope Leo X and all his minions. What you might do is do exactly what Leonardo did write in his ``mirror script.'' However, after a bit more reflection, you might decide that that Giovanni character down the street, who had been giving the eye to your wife, might decide to out you to the Pope. As this Giovanni degli Specchi is, as the name suggests, a man of the mirror, you would know that ``mirror script'' is the first thing he would figure out. So, you'd think of something a little more secure.

With the mirrors Giovanni was matchless, but with the maths, well, he sat at the back of the class. So you might decide to do a little trick with the letters in you missives. For example, you might try to switch each letter in you notebook with a letter 4 letters down in the alphabet. So, for example, ``A'' would be switched with ``E'', ``B'' with ``F'', ``C'' with ``G'', .... You'd make one cipher to encrypt things and a a second to decrypt. But then lightning strikes, and you realize that with 26 letters in the alphabet you can get away with a single cipher to encrypt things and decrypt things by shifting 13 yrggref qbja va gur nycunorg vafgrnq bs 4. Sbe rknzcyr, guvf cnentencu pbhyq or rapelcgrq hfvat guvf fpurzr.

Jvgu gur zveebef Tvbinaav jnf zngpuyrff, ohg jvgu gur znguf, jryy, ur fng ng gur onpx bs gur pynff. Fb lbh zvtug qrpvqr gb qb n yvggyr gevpx jvgu gur yrggref va lbh zvffvirf. Sbe rknzcyr, lbh zvtug gel gb fjvgpu rnpu yrggre va lbh abgrobbx jvgu n yrggre 4 yrggref qbja va gur nycunorg. Fb, sbe rknzcyr, ``N'' jbhyq or fjvgpurq jvgu ``R'', ``O'' jvgu ``S'', ``P'' jvgu ``T'', .... Lbh'q znxr bar pvcure gb rapelcg guvatf naq n n frpbaq gb qrpelcg. Ohg gura yvtugavat fgevxrf, naq lbh ernyvmr gung jvgu 26 yrggref va gur nycunorg lbh pna trg njnl jvgu n fvatyr pvcure gb rapelcg guvatf naq qrpelcg guvatf ol fuvsgvat 13 letters down in the alphabet instead of 4. For example, this paragraph could be encrypted using this scheme.

This might be good enough to keep this from the prying eyes of Giovanni ( Tvbinaav ), but in the 485 years since Leonardo's death things have changed a bit, and this ain't gonna cut it no more. Now even the most inept bloke would, after a bit of thought, be able to crack this cipher. We need to work a little more these days. What has evolved over the ensuing 485 years is an entire branch of science devoted to this type of thing, cryptography. For this particular type of problem there have evolved two clases of ciphers ``symmetric ciphers'' and ``asymmetric ciphers.''

• Symmetric Ciphers
• Asymmetric Ciphers