Compares two strings in a cryptographically save way:
        Runtime is not affected by length of common prefix.

def _lscmp(a, b):
    ''' Compares two strings in a cryptographically save way:
        Runtime is not affected by length of common prefix. '''
    return not sum(0 if x==y else 1 for x, y in zip(a, b)) and len(a) == len(b)