def check_credentials(user, password):
    """Checks a given user and password against /etc/shadow (or /etc/passwd if /etc/shadow doesn't exist).
    Returns None on success and an error message on failure."""
    from crypt import crypt
    shadow_hash = ''
    salt_regex = re.compile(r'\$.*\$.*\$')
    if os.path.exists('/etc/shadow'):
        password_file = '/etc/shadow'
    else:
        password_file = '/etc/passwd'
    shadow = open(password_file, 'r').readlines()
    for line in shadow:
        cols = line.split(':')
        if cols[0] == user:
            shadow_hash = cols[1]
    if salt_regex.match(shadow_hash):
        salt = salt_regex.match(shadow_hash).group()
        hashed_pass = crypt(password, salt)
        if hashed_pass == shadow_hash:
            return True
    # Now check the case where OpenWRT was just started up for the first time and there's no root password set yet...
    # If the root password hasn't been set the telnet daemon will be running with /bin/login.sh (i.e. no-password login)
    retcode = getstatusoutput('ps aux | grep -v grep | grep "telnetd -l /bin/login.sh"')[0]
    re.purge()
    if retcode == 0: # A return code of 0 means grep actually grepped something (as opposed to no output)
        return True # Let them through
    else:
        return _("Invalid user and/or password")