johnoyegbite/caesar-cipher-encryption

Caesar Cipher Encryption method obscure or encode messages to make them unreadable until they are decrypted and also making encrypted messages readable again by decoding them

Caesar Cipher Encryption

Encryption is the process of obscuring information to make it unreadable without special knowledge. For centuries, people have devised schemes to encrypt messages - some better than others - but the advent of the computer and the Internet revolutionized the field. These days, it's hard not to encounter some sort of encryption, whether you are buying something online or logging into a shared computer system. Encryption lets you share information with other trusted people, without fear of disclosure.

A cipher is an algorithm for performing encryption (and the reverse, decryption). The original information is called plaintext. After it is encrypted, it is called ciphertext. The ciphertext message contains all the information of the plaintext message, but it is not in a format readable by a human or computer without the proper mechanism to decrypt it; it should resemble random gibberish to those for whom it is not intended.

A cipher usually depends on a piece of auxiliary information, called a key. The key is incorporated into the encryption process; the same plaintext encrypted with two different keys should have two different ciphertexts. Without the key, it should be difficult to decrypt the resulting ciphertext into readable plaintext.

Some vocabulary in the encryption method called the Caesar cipher:

• Encryption - the process of obscuring or encoding messages to make them unreadable until they are decrypted
• Decryption - making encrypted messages readable again by decoding them
• Cipher - algorithm for performing encryption and decryption
• Plaintext - the original message
• Ciphertext - the encrypted message. Note: a ciphertext still contains all of the original message information, even if it looks like gibberish.

The Caesar Cipher

The idea of the Caesar Cipher is to pick an integer and shift every letter of your message by that integer. In other words, suppose the shift is k. Then, all instances of the i-th letter of the alphabet that appear in the plaintext should become the (i+k)-th letter of the alphabet in the ciphertext. You will need to be careful with the case in which i + k > 26 (the length of the alphabet).

Here is what the whole alphabet looks like shifted three spots to the right:

Original:  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  
 3-shift:  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

Using the above key, we can quickly translate the message "happy" to "kdssb" (note how the 3-shifted alphabet wraps around at the end, so x -> a, y -> b, and z -> c).

Note!! We are using the English alphabet for this problem - that is, the following letters in the following order:

--- import string  
--- print string.ascii_lowercase  
abcdefghijklmnopqrstuvwxyz  

We will treat uppercase and lowercase letters individually, so that uppercase letters are always mapped to an uppercase letter, and lowercase letters are always mapped to a lowercase letter. If an uppercase letter maps to "A", then the same lowercase letter should map to "a". Punctuation and spaces should be retained and not changed.
For example, a plaintext message with a comma should have a corresponding ciphertext with a comma in the same position.

plaintext	shift	ciphertext
'abcdef'	2	'cdefgh'
'Hello, World!'	5	'Mjqqt, Btwqi!'
''	any value	''

PS: I wrote this encryption method during an MITx course on edX.

• 6.00.1x: Introduction to Computer Science and Programming Using Python