code: func (c *Client) GetU2FSignRequest(user string, password []byte) (*u2f.SignRequest, error) {
  	out, err := c.PostJSON(
  		c.Endpoint("u2f", "users", user, "sign"),
  		signInReq{
  			Password: string(password),
  		},
  	)
  	if err != nil {
  		return nil, trace.Wrap(err)
  	}
  	var signRequest *u2f.SignRequest
  	if err := json.Unmarshal(out.Bytes(), &signRequest); err != nil {
  		return nil, err
  	}
  	return signRequest, nil
  }
docs: // GetU2FSignRequest generates request for user trying to authenticate with U2F token