Cryptography is science that uses mathematics to encrypt and decrypt data. The most elementary
idea in cryptology is the idea of a cryptosystem. This is a system in which information can be made unintelligible
to all but the
intended reader. The first component of a cryptosystem is the original set
of information, called the plaintext. The next element of a cryptosystem is the algorithm, commonly known
cipher. This is the process that makes the information unreadable to the average person. The next part of
a cryptosystem is the information that has been altered, which we call the ciphertext. This is the information
that is not recognizable,
and therefore can be sent out over public channels without fear of anybody understanding it. Decoding a message is
the reverse process of encoding.
One way to encode messages is to replace each character in the message with another character. Such an
encoding rule is called a substitution cipher. The first substitution
rule we introduce is a shift transformation. The first known shift transformation is
attributed to Julius Caesar, and is now called the Caesar cipher. The cipher works in
the following way. Each letter in the alphabet is replaced by another letter using a
predefined rule which shifts the alphabet a uniform amount to the right.
A Caesar cipher can consist of any size of shift, as long as the sender and receiver agree on this size. Observe
that there are 25 different shifts making this cipher easily broken; just try out all the different shift values until
the ciphertext makes sense.
To decrease the likely hood of being broken a substitution cipher can include a stretch value which
adds a value between each letter.
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