code: func (wfe *WebFrontEndImpl) Certificate(ctx context.Context, logEvent *web.RequestEvent, response http.ResponseWriter, request *http.Request) {
  
  	serial := request.URL.Path
  	// Certificate paths consist of the CertBase path, plus exactly sixteen hex
  	// digits.
  	if !core.ValidSerial(serial) {
  		wfe.sendError(response, logEvent, probs.NotFound("Certificate not found"), nil)
  		return
  	}
  	logEvent.Extra["RequestedSerial"] = serial
  
  	cert, err := wfe.SA.GetCertificate(ctx, serial)
  	// TODO(#991): handle db errors
  	if err != nil {
  		ierr := fmt.Errorf("unable to get certificate by serial id %#v: %s", serial, err)
  		if strings.HasPrefix(err.Error(), "gorp: multiple rows returned") {
  			wfe.sendError(response, logEvent, probs.Conflict("Multiple certificates with same short serial"), ierr)
  		} else {
  			wfe.sendError(response, logEvent, probs.NotFound("Certificate not found"), ierr)
  		}
  		return
  	}
  
  	// TODO Content negotiation
  	response.Header().Set("Content-Type", "application/pkix-cert")
  	relativeIssuerPath := web.RelativeEndpoint(request, issuerPath)
  	response.Header().Add("Link", link(relativeIssuerPath, "up"))
  	response.WriteHeader(http.StatusOK)
  	if _, err = response.Write(cert.DER); err != nil {
  		wfe.log.Warningf("Could not write response: %s", err)
  	}
  	return
  }
docs: // Certificate is used by clients to request a copy of their current certificate, or to
  // request a reissuance of the certificate.