Generate secure token using crypto rand

server/handlers.go

@@ -39,6 +39,7 @@ import (
 	"html"
 	htmlTemplate "html/template"
 	"io"
+	"log"
 	"mime"
 	"net"
 	"net/http"
@@ -399,8 +400,8 @@ func (s *Server) viewHandler(w http.ResponseWriter, r *http.Request) {
 		s.userVoiceKey,
 		purgeTime,
 		maxUploadSize,
-		token(s.randomTokenLength),
-		token(s.randomTokenLength),
+		token(s.randomTokenLength, s.logger),
+		token(s.randomTokenLength, s.logger),
 	}
 
 	w.Header().Set("Vary", "Accept")
@@ -449,7 +450,7 @@ func (s *Server) postHandler(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	token := token(s.randomTokenLength)
+	token := token(s.randomTokenLength, s.logger)
 
 	w.Header().Set("Content-Type", "text/plain")
 
@@ -514,7 +515,7 @@ func (s *Server) postHandler(w http.ResponseWriter, r *http.Request) {
 				}
 			}
 
-			metadata := metadataForRequest(contentType, contentLength, s.randomTokenLength, r)
+			metadata := metadataForRequest(contentType, contentLength, s.randomTokenLength, s.logger, r)
 
 			buffer := &bytes.Buffer{}
 			if err := json.NewEncoder(buffer).Encode(metadata); err != nil {
@@ -591,14 +592,14 @@ type metadata struct {
 	DecryptedContentType string
 }
 
-func metadataForRequest(contentType string, contentLength int64, randomTokenLength int, r *http.Request) metadata {
+func metadataForRequest(contentType string, contentLength int64, randomTokenLength int, logger *log.Logger, r *http.Request) metadata {
 	metadata := metadata{
 		ContentType:   strings.ToLower(contentType),
 		ContentLength: contentLength,
 		MaxDate:       time.Time{},
 		Downloads:     0,
 		MaxDownloads:  -1,
-		DeletionToken: token(randomTokenLength) + token(randomTokenLength),
+		DeletionToken: token(randomTokenLength, logger) + token(randomTokenLength, logger),
 	}
 
 	if v := r.Header.Get("Max-Downloads"); v == "" {
@@ -696,9 +697,9 @@ func (s *Server) putHandler(w http.ResponseWriter, r *http.Request) {
 
 	contentType := mime.TypeByExtension(filepath.Ext(vars["filename"]))
 
-	token := token(s.randomTokenLength)
+	token := token(s.randomTokenLength, s.logger)
 
-	metadata := metadataForRequest(contentType, contentLength, s.randomTokenLength, r)
+	metadata := metadataForRequest(contentType, contentLength, s.randomTokenLength, s.logger, r)
 
 	buffer := &bytes.Buffer{}
 	if err := json.NewEncoder(buffer).Encode(metadata); err != nil {

server/token.go

@@ -25,7 +25,10 @@ THE SOFTWARE.
 package server
 
 import (
-	"math/rand"
+	"crypto/rand"
+	"log"
+	"math/big"
+	mathrand "math/rand"
 )
 
 const (
@@ -34,12 +37,23 @@ const (
 )
 
 // generate a token
-func token(length int) string {
-	result := ""
+func token(length int, logger *log.Logger) string {
+	result := make([]byte, length)
+	var err error
 	for i := 0; i < length; i++ {
-		x := rand.Intn(len(SYMBOLS) - 1)
-		result = string(SYMBOLS[x]) + result
+		if err == nil {
+			var x *big.Int
+			x, err = rand.Int(rand.Reader, big.NewInt(int64(len(SYMBOLS))))
+			if err != nil {
+				logger.Printf("Fallback to math/rand instead of crypto/rand due error %s", err.Error())
+				x = big.NewInt(int64(mathrand.Intn(len(SYMBOLS) - 1)))
+			}
+			result[i] = SYMBOLS[x.Int64()]
+		} else { // fallback to math rand
+			x := int64(mathrand.Intn(len(SYMBOLS) - 1))
+			result[i] = SYMBOLS[x]
+		}
 	}
 
-	return result
+	return string(result)
 }

server/token_test.go

@@ -1,15 +1,21 @@
 package server
 
-import "testing"
+import (
+	"io"
+	"log"
+	"testing"
+)
+
+var logger = log.New(io.Discard, "", log.LstdFlags)
 
 func BenchmarkTokenConcat(b *testing.B) {
 	for i := 0; i < b.N; i++ {
-		_ = token(5) + token(5)
+		_ = token(5, logger) + token(5, logger)
 	}
 }
 
 func BenchmarkTokenLonger(b *testing.B) {
 	for i := 0; i < b.N; i++ {
-		_ = token(10)
+		_ = token(10, logger)
 	}
 }