Obfuscate WHIP ids.

If the WHIP session is not authenticated, then the only thing
preventing an attacker from DELETEing the session is the session
URL.  Since client ids are known, obfuscate the id before using
it in the session URL.

Author: jech

webserver/webserver_test.go

@@ -136,3 +136,23 @@ func TestFormatICEServer(t *testing.T) {
 		})
 	}
 }
+
+func TestObfuscate(t *testing.T) {
+	id := newId()
+	obfuscated, err := obfuscate(id)
+	if err != nil {
+		t.Fatalf("obfuscate: %v", err)
+	}
+	id2, err := deobfuscate(obfuscated)
+	if err != nil {
+		t.Fatalf("deobfuscate: %v", err)
+	}
+	if id != id2 {
+		t.Errorf("not equal: %v, %v", id, id2)
+	}
+
+	_, err = obfuscate("toto")
+	if err == nil {
+		t.Errorf("obfuscate: no errror")
+	}
+}

webserver/whip.go

@@ -2,8 +2,11 @@ package webserver
 
 import (
 	"bytes"
+	"crypto/aes"
+	"crypto/cipher"
 	crand "crypto/rand"
 	"encoding/base64"
+	"errors"
 	"fmt"
 	"io"
 	"log"
@@ -37,12 +40,58 @@ func parseWhip(pth string) (string, string) {
 	return "", ""
 }
 
+var idSecret []byte
+var idCipher cipher.Block
+
+func init() {
+	idSecret = make([]byte, 16)
+	_, err := crand.Read(idSecret)
+	if err != nil {
+		log.Fatalf("crand.Read: %v", err)
+	}
+	idCipher, err = aes.NewCipher(idSecret)
+	if err != nil {
+		log.Fatalf("NewCipher: %v", err)
+	}
+}
+
 func newId() string {
-	b := make([]byte, 16)
+	b := make([]byte, idCipher.BlockSize())
 	crand.Read(b)
 	return base64.RawURLEncoding.EncodeToString(b)
 }
 
+// we obfuscate ids to avoid exposing the WHIP session URL
+func obfuscate(id string) (string, error) {
+	v, err := base64.RawURLEncoding.DecodeString(id)
+	if err != nil {
+		return "", err
+	}
+
+	if len(v) != idCipher.BlockSize() {
+		return "", errors.New("bad length")
+	}
+
+	idCipher.Encrypt(v, v)
+
+	return base64.RawURLEncoding.EncodeToString(v), nil
+}
+
+func deobfuscate(id string) (string, error) {
+	v, err := base64.RawURLEncoding.DecodeString(id)
+	if err != nil {
+		return "", err
+	}
+
+	if len(v) != idCipher.BlockSize() {
+		return "", errors.New("bad length")
+	}
+
+	idCipher.Decrypt(v, v)
+
+	return base64.RawURLEncoding.EncodeToString(v), nil
+}
+
 func canPresent(perms []string) bool {
 	for _, p := range perms {
 		if p == "present" {
@@ -186,6 +235,13 @@ func whipEndpointHandler(w http.ResponseWriter, r *http.Request) {
 	}
 
 	id := newId()
+	obfuscated, err := obfuscate(id)
+	if err != nil {
+		http.Error(w, "Internal Server Error",
+			http.StatusInternalServerError)
+		return
+	}
+
 	c := rtpconn.NewWhipClient(g, id, token)
 
 	_, err = group.AddClient(g.Name(), c, creds)
@@ -214,7 +270,7 @@ func whipEndpointHandler(w http.ResponseWriter, r *http.Request) {
 			http.StatusInternalServerError)
 	}
 
-	w.Header().Set("Location", path.Join(r.URL.Path, id))
+	w.Header().Set("Location", path.Join(r.URL.Path, obfuscated))
 	w.Header().Set("Access-Control-Expose-Headers",
 		"Location, Content-Type, Link")
 	whipICEServers(w)
@@ -226,8 +282,14 @@ func whipEndpointHandler(w http.ResponseWriter, r *http.Request) {
 }
 
 func whipResourceHandler(w http.ResponseWriter, r *http.Request) {
-	pth, id := parseWhip(r.URL.Path)
-	if pth == "" || id == "" {
+	pth, obfuscated := parseWhip(r.URL.Path)
+	if pth == "" || obfuscated == "" {
+		http.Error(w, "Internal server error",
+			http.StatusInternalServerError)
+		return
+	}
+	id, err := deobfuscate(obfuscated)
+	if err != nil {
 		http.Error(w, "Internal server error",
 			http.StatusInternalServerError)
 		return