Encryption and Decryption are two cryptographic techniques. Encryption is used to transform text to meaningless characters, and decryption is used to transform meaningless characters into meaningful text. The algorithm that does the encryption is called a cipher.

A simple encryption algorithm is Caesar cipher, which works as follows: replace each clear text letter by a letter chosen to be n places later in the alphabet. The number of places, n, is called the cipher key.

For example, if the cipher key is 3, the clear text “HELLO THERE” becomes “KHOOR2WKHUH”.

 

Here, we restrict ourselves to encrypting/decrypting digits (0…9), lowercase, uppercase alphabetic characters and space. Assume that these characters have the decimal representation from 0 to 62 as follows:

0 1 ………........…….9	10  11  12 ………….35	36   37  38 ………..….61    62
0 1 2 3 4 5 6 7 8 9	a     b    c ………..…....z	A     B    C ………....…..Z   space

 

Hint: The following formula can be used to encrypt a character C using the Caesar cipher algorithm:

E = (C + k) % 63  [where k is the cipher key]

You need to figure out the formula for decrypting text on your own.

NOTE: the user may enter any key value.

Write a Python/C++ program for encrypting and decrypting a given string. Since this program performs two different functionalities (encryption and decryption), prompt the user to select the type of cryptographic technique as shown below:

Welcome to Cryptographic Techniques Program

Please enter your selection:

1. Encrypt
2. Decrypt

Please enter the key K

 

When the user selects 1 or 2, s/he will be asked to specify an input and output message.

Here is an example:

Assume that the user enters the following message:

HOW ARE YOU DOING?

If the user selects to encrypt the message (i.e. option 1), the program will encrypt the original message and outputs an encrypted text.

If the user selects to decrypt a message (i.e. option 2), the program will decrypt the encrypted text and outputs the decrypted text on the screen.

The program should give the user the option whether to continue with encrypting/decrypting messages (i.e. entering letter ‘C’) or exit the program (i.e. entering the letter ‘E’).