def get_encoding_table(letters):
    """Defines the mapping between plain and cipher text"""
    if letters: 
        return {
            '1': 'nine',
            '2': 'eight',
            '3': 'seven',
            '4': 'six',
            '5': 'zero',
            '6': 'four',
            '7': 'three',
            '8': 'two',
            '9': 'one',
            '0': 'five'
        }

    return {
        '1': '9',
        '2': '8',
        '3': '7',
        '4': '6',
        '5': '0',
        '6': '4',
        '7': '3',
        '8': '2',
        '9': '1',
        '0': '5'
    }