code: func (c *Challenge) SignRequest(regs []Registration) *WebSignRequest {
  	var sr WebSignRequest
  	sr.AppID = c.AppID
  	sr.Challenge = encodeBase64(c.Challenge)
  	for _, r := range regs {
  		rk := getRegisteredKey(c.AppID, r)
  		sr.RegisteredKeys = append(sr.RegisteredKeys, rk)
  	}
  	return &sr
  }
docs: // SignRequest creates a request to initiate an authentication.