ABC-132: sign error page parameters (#8197)

* sign error page parameters

* add comments

utils/api.go

@@ -4,6 +4,9 @@
 package utils
 
 import (
+	"crypto"
+	"crypto/rand"
+	"encoding/base64"
 	"fmt"
 	"html/template"
 	"net/http"
@@ -32,13 +35,25 @@ func OriginChecker(allowedOrigins string) func(*http.Request) bool {
 	}
 }
 
-func RenderWebError(err *model.AppError, w http.ResponseWriter, r *http.Request) {
-	status := http.StatusTemporaryRedirect
-	if err.StatusCode != http.StatusInternalServerError {
-		status = err.StatusCode
-	}
+func RenderWebAppError(w http.ResponseWriter, r *http.Request, err *model.AppError, s crypto.Signer) {
+	RenderWebError(w, r, err.StatusCode, url.Values{
+		"message": []string{err.Message},
+	}, s)
+}
+
+func RenderWebError(w http.ResponseWriter, r *http.Request, status int, params url.Values, s crypto.Signer) {
+	queryString := params.Encode()
+
+	h := crypto.SHA256
+	sum := h.New()
+	sum.Write([]byte("/error?" + queryString))
+	signature, err := s.Sign(rand.Reader, sum.Sum(nil), h)
+	if err != nil {
+		http.Error(w, "", http.StatusInternalServerError)
+		return
+	}
+	destination := strings.TrimRight(GetSiteURL(), "/") + "/error?" + queryString + "&s=" + base64.URLEncoding.EncodeToString(signature)
 
-	destination := strings.TrimRight(GetSiteURL(), "/") + "/error?message=" + url.QueryEscape(err.Message)
 	if status >= 300 && status < 400 {
 		http.Redirect(w, r, destination, status)
 		return