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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def generate_certificate( ctx, slot, management_key, pin, public_key, subject, valid_days):
 """ Generate a self-signed X.509 certificate. A self-signed certificate is generated and written to one of the slots on the YubiKey. A private key need to exist in the slot. \b SLOT PIV slot where private key is stored. PUBLIC-KEY File containing a public key. Use '-' to use stdin. """

    controller = ctx.obj['controller']
    _ensure_authenticated(
        ctx, controller, pin, management_key, require_pin_and_key=True)

    data = public_key.read()
    public_key = serialization.load_pem_public_key(
        data, default_backend())

    now = datetime.datetime.now()
    valid_to = now + datetime.timedelta(days=valid_days)

    try:
        controller.generate_self_signed_certificate(
            slot, public_key, subject, now, valid_to,
            touch_callback=prompt_for_touch)

    except APDUError as e:
        logger.error('Failed to generate certificate for slot %s', slot,
                     exc_info=e)
        ctx.fail('Certificate generation failed.')