Read the last :bytelen bytes of
    fd and interpret it as a base64-encoded
    string

def snapshot_peek_sigb64( fd, off, bytelen ):
    """
    Read the last :bytelen bytes of
    fd and interpret it as a base64-encoded
    string
    """
    fd.seek( off - bytelen, os.SEEK_SET )
    sigb64 = fd.read(bytelen)
    if len(sigb64) != bytelen:
        return None

    try:
        base64.b64decode(sigb64)
    except:
        return None

    return sigb64