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What Are Cryptograms?
They are secret codes, and were first used for secure wartime
communications. One of the oldest cryptogramss known was a strip
of paper wrapped around a stick. This was used by the Spartan
Army over two thousand years ago.
A strip of paper was wrapped around the stick or staff, edge-to-edge
without overlapping, and the message was written vertically.
To read it, the receiver had to wrap the paper strip around a
stick of the exact same diameter as the one used to create the
message, so the letters would line up correctly. The receiver
knew what diameter stick to use, of course. Meanwhile, any messages
intercepted would take some time to be decoded, because even
if the enemy knew to use a stick, he had to find one of the right
diameter.
Cryptograms are used primarily for entertainment now. They
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. These secret codes have been used as entertaining
puzzles for over a thousand years.
Solving one of these puzzles or codes is usually done using
"frequency analysis." This involves looking for the
coded letters which are most frequent in a message, and substituting
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".
(See the page, "Code Breaking"
for more on breaking a code and for a complete letter-frequency
table.)
An Example
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, then 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.
There are many other ways to make secret codes more difficult
to break.
All the pages on codes, ciphers and cryptograms are listed
on the page: Secret Codes.
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