Use crypto/rand instead of math/rand

math/rand isn't suitable for security-sensitive contexts. This PR
replaces it with crypto/rand (which uses the somewhat inconvenient
big.Int type).

internal/server/turn.go

@@ -106,7 +106,10 @@ func handleAllocateRequest(r Request, m *stun.Message) error {
 			return buildAndSendErr(r.Conn, r.SrcAddr, err, insufficientCapacityMsg...)
 		}
 		requestedPort = randomPort
-		reservationToken = randSeq(8)
+		reservationToken, err = randSeq(8)
+		if err != nil {
+			return err
+		}
 	}
 
 	// 7. At any point, the server MAY choose to reject the request with a

internal/server/util.go

@@ -5,10 +5,11 @@ package server
 
 import (
 	"crypto/md5" //nolint:gosec,gci
+	"crypto/rand"
 	"errors"
 	"fmt"
 	"io"
-	"math/rand"
+	"math/big"
 	"net"
 	"strconv"
 	"time"
@@ -22,13 +23,17 @@ const (
 	nonceLifetime             = time.Hour // See: https://tools.ietf.org/html/rfc5766#section-4
 )
 
-func randSeq(n int) string {
+func randSeq(n int) (string, error) {
 	letters := []rune("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ")
 	b := make([]rune, n)
 	for i := range b {
-		b[i] = letters[rand.Intn(len(letters))] //nolint:gosec
+		j, err := rand.Int(rand.Reader, big.NewInt(int64(len(letters))))
+		if err != nil {
+			return "", err
+		}
+		b[i] = letters[int(j.Int64())] //nolint:gosec
 	}
-	return string(b)
+	return string(b), nil
 }
 
 func buildNonce() (string, error) {
@@ -37,7 +42,15 @@ func buildNonce() (string, error) {
 	if _, err := io.WriteString(h, strconv.FormatInt(time.Now().Unix(), 10)); err != nil {
 		return "", fmt.Errorf("%w: %v", errFailedToGenerateNonce, err) //nolint:errorlint
 	}
-	if _, err := io.WriteString(h, strconv.FormatInt(rand.Int63(), 10)); err != nil { //nolint:gosec
+
+	maxInt63 := big.NewInt(1<<63 - 1)
+	maxInt63.Add(maxInt63, big.NewInt(1))
+	randInt63, err := rand.Int(rand.Reader, maxInt63)
+	if err != nil {
+		return "", fmt.Errorf("%w: %v", errFailedToGenerateNonce, err)
+	}
+
+	if _, err := io.WriteString(h, randInt63.String()); err != nil { //nolint:gosec
 		return "", fmt.Errorf("%w: %v", errFailedToGenerateNonce, err) //nolint:errorlint
 	}
 	return fmt.Sprintf("%x", h.Sum(nil)), nil