char_map to store the mapping from the cipher key to the alphabet.current_char to 'a' to keep track of the current alphabet character to map to.key string.
char_map, add it to char_map with the value of current_char.current_char to the next letter in the alphabet.decoded_message to build the decoded message.message string.
decoded_message.char_map to decoded_message.decoded_message as the final decoded message.