A simple command-line tool written in Python to perform encryption and decryption using the Caesar cipher algorithm.

The Caesar cipher is one of the simplest and oldest encryption techniques. Named after Julius Caesar, who used it to encode his private messages, it works by shifting each letter in the message a fixed number of places down or up the alphabet.

• Encrypt messages using a Caesar cipher with a specified shift value.
• Decrypt messages that were encrypted using a Caesar cipher.

Select operation:
1. Encrypt
2. Decrypt
3. Exit
Enter choice (1-3): 1
Enter text: Hello World
Enter shift value: 3
Encrypted text: Khoor Zruog

Select operation:
1. Encrypt
2. Decrypt
3. Exit
Enter choice (1-3): 2
Enter text: Khoor Zruog
Enter shift value: 3
Decrypted text: Hello World

You choose a number, called the shift value or key. This number determines how many positions each letter in the message will be shifted.

To encrypt a message, you shift each letter in the message forward by the shift value. For example, if the shift value is 3:

• The letter 'A' becomes 'D'
• The letter 'B' becomes 'E'
• The letter 'Z' becomes 'C' (since it wraps around from the end of the alphabet)

To decrypt a message, you shift each letter backward by the same shift value. This reverses the encryption and retrieves the original message.

Original Message: HELLO
Shift Value: 3
Encrypted Message: KHOOR

Here's how it works with the shift value of 3:

• 'H' (the 8th letter) becomes 'K' (the 11th letter)
• 'E' (the 5th letter) becomes 'H' (the 8th letter)
• 'L' (the 12th letter) becomes 'O' (the 15th letter)
• 'O' (the 15th letter) becomes 'R' (the 18th letter)

Encrypted Message: KHOOR
Shift Value: 3
Decrypted Message: HELLO

Here, each letter is shifted back by 3 positions:

• 'K' becomes 'H'
• 'H' becomes 'E'
• 'O' becomes 'L'
• 'R' becomes 'O'