Caesar Cipher
The Caesar Cipher, also known as a shift cipher, is one of the oldest and simplest forms of encrypting a message. It is a type of substitution cipher where each letter in the original message (which in cryptography is called the plaintext) is replaced with a letter corresponding to a certain number of letters shifted up or down in the alphabet.
For each letter of the alphabet, you would take its position in the alphabet, say 3 for the letter 'C', and shift it by the key number. If we had a key of +3, that 'C' would be shifted down to an 'F' - and that same process would be applied to every letter in the plaintext.
In this way, a message that initially was quite readable, ends up in a form that cannot be understood at a simple glance.
The Caesar Cipher, also known as a shift cipher, is one of the oldest and simplest forms of encrypting a message. It is a type of substitution cipher where each letter in the original message (which in cryptography is called the plaintext) is replaced with a letter corresponding to a certain number of letters shifted up or down in the alphabet.
For each letter of the alphabet, you would take its position in the alphabet, say 3 for the letter 'C', and shift it by the key number. If we had a key of +3, that 'C' would be shifted down to an 'F' - and that same process would be applied to every letter in the plaintext.
In this way, a message that initially was quite readable, ends up in a form that cannot be understood at a simple glance.
Introduction
The Caesar cipher is one of the earliest known and simplest ciphers. It is a type of substitution cipher in which each letter in the plaintext is 'shifted' a certain number of places down the alphabet. For example, with a shift of 1, A would be replaced by B, B would become C, and so on. The method is named after Julius Caesar, who apparently used it to communicate with his generals.
More complex encryption schemes such as the Vigenère cipher employ the Caesar cipher as one element of the encryption process. The widely known ROT13 'encryption' is simply a Caesar cipher with an offset of 13. The Caesar cipher offers essentially no communication security, and it will be shown that it can be easily broken even by hand.
History and usage
The Caesar cipher is named after Julius Caesar, who, according to Suetonius, used it with a shift of three to protect messages of military significance. While Caesar's was the first recorded use of this scheme, other substitution ciphers are known to have been used earlier.
"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."
His nephew, Augustus, also used the cipher, but with a right shift of one, and it did not wrap around to the beginning of the alphabet:
"Whenever he wrote in cipher, he wrote B for A, C for B, and the rest of the letters on the same principle, using AA for Z."
Evidence exists that Julius Caesar also used more complicated systems,and one writer, Aulus Gellius, refers to a (now lost) treatise on his ciphers:
Example
To pass an encrypted message from one person to another, it is first necessary that both parties have the 'key' for the cipher, so that the sender may encrypt it and the receiver may decrypt it. For the caesar cipher, the key is the number of characters to shift the cipher alphabet.
Here is a quick example of the encryption and decryption steps involved with the caesar cipher. The text we will encrypt is 'defend the east wall of the castle', with a shift (key) of 1.
plaintext: defend the east wall of the castle ciphertext: efgfoe uif fbtu xbmm pg uif dbtumf
It is easy to see how each character in the plaintext is shifted up the alphabet. Decryption is just as easy, by using an offset of -1.
plain: abcdefghijklmnopqrstuvwxyz cipher: bcdefghijklmnopqrstuvwxyza
Examples :
Text : ABCDEFGHIJKLMNOPQRSTUVWXYZ Shift: 23 Cipher: XYZABCDEFGHIJKLMNOPQRSTUVW Text : ATTACKATONCE Shift: 4 Cipher: EXXEGOEXSRGI
Algorithm for Caesar Cipher:
Input:
Input:
- A String of lower case letters, called Text.
- An Integer between 0-25 denoting the required shift.
Procedure:
- Traverse the given text one character at a time .
- For each character, transform the given character as per the rule, depending on whether we’re encrypting or decrypting the text.
- Return the new string generated.
How to encrypt a message
Very simple steps:
- Open one of the sample files provided in the CD.
- Enter the encryption key.
- Click on the Encrypt button.
- Save the encrypted message to you hard disk.
Conclusion
I hope that I gave a good idea on how to crack a cipher text with the frequency letters. I know may be no body using this encryption technique nowadays, but I wanted to show how to do this.
