Vigenère cipher decoder and encoder

You can find a great write-up about how this cipher works on Wikipedia.

How to use this script:

Flags

• -i: The input text file to encode/decode
• -o: The text file to dump the encoded/decoded message (optional, defaults to output.txt)
• -k: The alphabetic secret key used to encrypt the message
• -e: Encode the message from the input text file (incompatible with -d flag)
• -d: Decode the message from the input text file (incompatible with -e flag)

Please note that the secret key (-k) must only contain alphabetic characters A-Z.

Additionally, your input text file must only contain alphabetic characters A-Z, spaces, newlines, and tabs.

Encoding Example:

./main -e -i example_input.txt -o encoded_output.txt -k CIPHERKEY

You can find an example input file that you can use to encode in example_input.txt.

Decoding Example:

./main -d -i encoded_input.txt -o decoded_output.txt -k CIPHERKEY