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Caesar Cipher / Rot-13

 

Background

The Caesar cipher is named after the Roman military and political leader Gaius Julius Caesar (100 BC – 44 BC).1 Caesar used this relatively simple form of ciphering to encode military messages.

 

Principle

The classic version uses the capital letters A-Z, but, in principle, an arbitrary alphabet can be used. The first step is to write the alphabet down two times.
A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z
A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z

Now, the bottom alphabet is shifted by an arbitrary number of positions. The number of positions is the key-value. Shifting the bottom alphabet 3 positions to the right yields the following result:
A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z
D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z  A  B  C

The letter A becomes the letter D. B is replaced by E and C replaced by F, etc. The word "example" would be encoded by: "hadpsoh".

 

Security

The key length is identical to the size of the given alphabet. Using the capital letters A-Z as alphabet allows 26 different keys, with the 26th key rendered meaningless because it would map each letter to itself. With only 25 meaningful keys, it would be quite easy to test for all possible keys until the correct one is found (brute-force analysis). The Caesar cipher can also easily be cracked with a frequency-analysis.

 

Details

The Caesar cipher replaces the characters of the plaintext-alphabet with other characters. This is called monoalphabetic substitution.
When the alphabet A-Z is used and the key is 13, this cipher is also called Rot-13. This way of encoding is often used to make a text incomprehensible for the reader at first, but easy to decode, for example in case of solutions for riddles.

A  B  C  D  E  F  G  H  I  J  K  L  M
N  O  P  Q  R  S  T  U  V  W  X  Y  Z

 

Weblinks

http://en.wikipedia.org/wiki/Caesar_cipher
http://en.wikipedia.org/wiki/Rot13

 

References

1 "Gaius Julius Cäsar", http://de.wikipedia.org/wiki/Julius_Caesar, 2009-02-20

 

 
   
     
 
 
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