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'