Use single shared random string generation function (#15741)

* Use single shared random string generation function

- Replace 3 functions that do the same with 1 shared one
- Use crypto/rand over math/rand for a stronger RNG
- Output only alphanumerical for URL compatibilty

Fixes: #15536

* use const string method

* Update modules/avatar/avatar.go

Co-authored-by: a1012112796 <1012112796@qq.com>

Co-authored-by: a1012112796 <1012112796@qq.com>

modules/generate/generate.go

@@ -9,31 +9,12 @@ import (
 	"crypto/rand"
 	"encoding/base64"
 	"io"
-	"math/big"
 	"time"
 
+	"code.gitea.io/gitea/modules/util"
 	"github.com/dgrijalva/jwt-go"
 )
 
-// GetRandomString generate random string by specify chars.
-func GetRandomString(n int) (string, error) {
-	const alphanum = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
-
-	buffer := make([]byte, n)
-	max := big.NewInt(int64(len(alphanum)))
-
-	for i := 0; i < n; i++ {
-		index, err := randomInt(max)
-		if err != nil {
-			return "", err
-		}
-
-		buffer[i] = alphanum[index]
-	}
-
-	return string(buffer), nil
-}
-
 // NewInternalToken generate a new value intended to be used by INTERNAL_TOKEN.
 func NewInternalToken() (string, error) {
 	secretBytes := make([]byte, 32)
@@ -69,19 +50,10 @@ func NewJwtSecret() (string, error) {
 
 // NewSecretKey generate a new value intended to be used by SECRET_KEY.
 func NewSecretKey() (string, error) {
-	secretKey, err := GetRandomString(64)
+	secretKey, err := util.RandomString(64)
 	if err != nil {
 		return "", err
 	}
 
 	return secretKey, nil
 }
-
-func randomInt(max *big.Int) (int, error) {
-	rand, err := rand.Int(rand.Reader, max)
-	if err != nil {
-		return 0, err
-	}
-
-	return int(rand.Int64()), nil
-}