def auth_str_equal(provided, known):
    """Constant-time string comparison.

    :params provided: the first string
    :params known: the second string

    :returns: True if the strings are equal.

    This function takes two strings and compares them.  It is intended to be
    used when doing a comparison for authentication purposes to help guard
    against timing attacks.  When using the function for this purpose, always
    provide the user-provided password as the first argument.  The time this
    function will take is always a factor of the length of this string.
    """
    result = 0
    p_len = len(provided)
    k_len = len(known)
    for i in range(p_len):
        a = ord(provided[i]) if i < p_len else 0
        b = ord(known[i]) if i < k_len else 0
        result |= a ^ b
    return (p_len == k_len) & (result == 0)