Variable Alphabet Mapping Code

This code uses a numeric string as the key, with each letter in the message to be encoded being incremented by the number of letters indicated by a digit in the numeric key. The first letter is incremented using the first digit, the second letter is incremented using the second digit, and so on. Once the last digit of the key has been reached, we return to the first digit of the key. Non alphabetic characters are left unchanged. The numeric key could typically be something easy to remember such as a date (e.g. '14031957').