Wednesday, June 13, 2007

Cryptograms - Code Making And Breaking

Cryptograms are essentially secret codes. They were first used for secret wartime communications. One of the oldest versions recorded was a strip of paper wrapped around a stick. This type of cryptogram was used by the Spartan Army over two thousand years ago.

The paper was wrapped around the stick or staff, edge-to-edge without overlapping, and the message was written vertically. To be read, the paper strip had to be wrapped around a stick of the exact same diameter as the original one used to create the message, so the letters would line up correctly. The receiver knew what diameter stick to use, of course, while any messages intercepted would take some time to be decoded. Even if the enemy knew to use a stick, he had to find one of the right diameter.

Cryptograms are primarily for entertainment now, and are usually created using a simple substitution cipher, in which each letter is replaced by another letter or number. The Caesar Cipher, invented by Julius Caesar, may have been the first of this type. They have been used as simple puzzles for entertainment for over a thousand years now.

Solving a cryptogram is most often done using "frequency analysis," meaning that you look for the coded letters which are most frequent in a message, and then substitute the real letters which occur most often in common usage. In English, the most common letter used is "e," followed by "t" and "a". You also look for one-letter words, since these typically can only be "a" or "i".

Example Of A Cryptogram

A Caesar Cipher is a simple "shift cipher." You simply substitute for each letter another letter that is a fixed number of positions away in the alphabet. For example, if you were to use a "shift" of five letters, the letter "a" would be represented by "f", "b" would be represented by "g", and so on. Here is the complete code:

a=f, b=g, c=h, d=i, e=j, f=k, g=l, h=m, i=n, j=o, k=p, l=q, m=r, n=s, o=t, p=u, q=v, r=w, s=x, t=y, u=z, v=a, w=b, x=c, y=d, z=e

A short coded message:

Ymnx xnruqj rjxxflj nx bwnyyjs zxnsl f Hfjxfw Hnumjw.

Of course, if the code breaker suspects that this cryptogram is a simple shift-cipher, she could start with the the single-letter word "f", which would almost certainly be "a". Counting the five letters from "a" to "f", the code would be broken. The message could be decoded in minutes and read as follows:

"This simple message is written using a Caesar Cipher."

As you can imagine, any cryptogram as simple as this can be easily broken. Since there are only 26 "shifts" possible in English, you could break such a code quickly by trial and error. A computer program could try all 26 in seconds, display the 26 versions, and the viewer (or computer) would immediately recognize which was readable.

This is why simple substitution ciphers, while used for entertaining puzzles, are not used by themselves for truly secret messages. They may be used as a start, however. The Vigenère cipher, for example, uses a shift, but shifts again at different points in a message, the shift value determined by a repeating keyword. Of course there are many other ways to make a cryptogram or secret code more difficult to break.

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