Caesar Cipher Script
1B|08 Avril, Lorenzo, Tze Hern
INTRODUCTION
A very good morning to one and all, we are group 3 and our presentation for today in on a
classical cipher: Caesar’s Cipher. I’m Avril, he’s Tze Hern and that’s Lorenzo. Here is an
overview of what we’ll be covering today. I’ll be sharing about the history and development
of the Caesar’s Cipher, while Tze Hern will describe how it is used. Lorenzo will round up our
presentation with the pros and cons of this cipher and how to break it.
HISTORY & DEVELOPMENT
 Named after Julius Caesar. It was used during 50BC by notable Romans including Julius
Caesar
 In cryptography, the Caesar cipher is also known as Caesar’s shift or the Shift cipher
 Julius Caesar used the Caesar cipher to communicate with his generals during military
campaigns to protect/ encrypt messages important to the military and the government.
 It is a type of substitution cipher. Each letter in the plaintext is replaced by a letter some
fixed number of positions further down the alphabet.
 The Caesar cipher is a “monoalphabetic substitution cipher”, meaning only one letter is
assigned to the alphabet it is supposed to represent
 According to Suetonius, who wrote the book Life of Julius Caesar, Julius Caesar used the
substitution cipher to a shift of three. This means that he shifted each letter 3 places further
through the alphabet. a(plaintext) becomes D(cipher)
 “If he had anything confidential to say, he wrote it in cipher, that is, by so changing the
order of the letters of the alphabet, that not a word could be made out. If anyone wishes
to decipher these, and get at their meaning, he must substitute the fourth letter of the
alphabet, namely D, for A, and so with the others”
- Suetonius, Life of Julius Caesar
56
 Julius Caesar’s nephew, Augustus, also used the Caesar cipher. However, he used it to the
shift of one
 According to David Kahn’s book, The Code breakers, lovers used the Caesar code to
communicate secretly.
 The plaintext is usually in lower case but the cipher text is in upper case.
 The ROT13 is an application of the Caesar cipher. ROT13 replaces each letter by its
partner 13 characters further down the alphabet. As the alphabet consist of 26 letters, the
“ROT13 function is its own inverse”, meaning C becomes P and P becomes C.
HOW TO USE
The Caesar Cipher replaces each letter in the plain text (the alphabet) with a letter that has
a fixed number of places down the alphabet. For example, the diagram below has defined
its parameters with a shift of 3 also known as the Caesar shift. As such, the letter B in the
plaintext becomes E in the ciphertext. Here is a sample of the revolvable cipher which makes
encryption much more convenient by turning the inner and outer wheel. The outer wheel is
the original alphabet or plaintext and the inner wheel is the ciphertext which can be
adjusted accordingly.
PROS & CONS
The good points of the Caesar cipher is that it is very simple to use. All a person has to do is to
write out his message and then referring to his cipher, rewrite his message again, now
encrypted. To give the recipient the key, one can just tell them for example “move back 3″
so ‘c’ would be represented by ‘z’, and ‘d’ would be represented by ‘a’. No machines or
devices are needed to decode it. A paper and pen for convenience may be all that you
need.
The bad points are that due to the nature of the cipher, an encrypted text has only 26
possibilities, 25 not including the given text. Within 5 minutes, an experienced cryptobreaker
Caesar Cipher Script
1B|08 Avril, Lorenzo, Tze Hern
could crack the code. Anyone else could crack the code in 10 minutes. With the revolvable
cipher, anyone could crack the code in 4-5 minutes. The key could be obtained through trial
and error. This is called a Brute Force Attack.
To save time a series of strips could be prepared. Each with 52 letters, the letters would be in
alphabetical order twice over. The strips would be aligned so that the ciphertext read in one
position. They would then be scanned to reveal the plaintext.
An alternative and quicker way to guessing/trial and error is through observing the frequency
of the letters. Here is a diagram representing the frequency of the letters in the alphabet. For
example, if a certain letter ‘z’ for example is noticed to be repeated very often in the
ciphertext, you could start off by guessing that ’z’ represents e in the plaintext, which is the
most commonly used letter. Subsequently you can use this method to determine the rest of
the letters.
We have now come to the end of our presentation; here is a list of the web-based material
we have referenced. Thank you and have a nice day.
BIBLIOGRAPHY
 http://en.wikipedia.org/wiki/Caesar_cipher
 http://www.murky.org/blg/the-caesar-shift/
 Chris Savarese and Brian Hart, The Caesar Cipher, 1999
 F.L. Bauer, Decrypted Secrets, 2nd edition, 2000, Springer
 David Kahn, The Codebreakers — The Story of Secret Writing
 http://www.spiritus-temporis.com/caesar-cipher/history-and-usage.html
 http://en.wikipedia.org/wiki/ROT13
 http://all.net/BOOKS/IP/cHAP2-1.HTML