Refactor JWT secret generating & decoding code (#29172)

Old code is not consistent for generating & decoding the JWT secrets.

Now, the callers only need to use 2 consistent functions:
NewJwtSecretWithBase64 and DecodeJwtSecretBase64

And remove a non-common function Base64FixedDecode from util.go

9 files changed, 57 insertions, 46 deletions

diff --git a/cmd/generate.go b/cmd/generate.go
index 4ab10da22a..90b32ecaf0 100644
--- a/cmd/generate.go
+++ b/cmd/generate.go
@@ -70,7 +70,7 @@ func runGenerateInternalToken(c *cli.Context) error {
 }
 
 func runGenerateLfsJwtSecret(c *cli.Context) error {
-	_, jwtSecretBase64, err := generate.NewJwtSecretBase64()
+	_, jwtSecretBase64, err := generate.NewJwtSecretWithBase64()
 	if err != nil {
 		return err
 	}
diff --git a/modules/generate/generate.go b/modules/generate/generate.go
index ee3c76059b..2d9a3dd902 100644
--- a/modules/generate/generate.go
+++ b/modules/generate/generate.go
@@ -7,6 +7,7 @@ package generate
 import (
 	"crypto/rand"
 	"encoding/base64"
+	"fmt"
 	"io"
 	"time"
 
@@ -38,19 +39,24 @@ func NewInternalToken() (string, error) {
 	return internalToken, nil
 }
 
-// NewJwtSecret generates a new value intended to be used for JWT secrets.
-func NewJwtSecret() ([]byte, error) {
-	bytes := make([]byte, 32)
-	_, err := io.ReadFull(rand.Reader, bytes)
-	if err != nil {
+const defaultJwtSecretLen = 32
+
+// DecodeJwtSecretBase64 decodes a base64 encoded jwt secret into bytes, and check its length
+func DecodeJwtSecretBase64(src string) ([]byte, error) {
+	encoding := base64.RawURLEncoding
+	decoded := make([]byte, encoding.DecodedLen(len(src))+3)
+	if n, err := encoding.Decode(decoded, []byte(src)); err != nil {
 		return nil, err
+	} else if n != defaultJwtSecretLen {
+		return nil, fmt.Errorf("invalid base64 decoded length: %d, expects: %d", n, defaultJwtSecretLen)
 	}
-	return bytes, nil
+	return decoded[:defaultJwtSecretLen], nil
 }
 
-// NewJwtSecretBase64 generates a new base64 encoded value intended to be used for JWT secrets.
-func NewJwtSecretBase64() ([]byte, string, error) {
-	bytes, err := NewJwtSecret()
+// NewJwtSecretWithBase64 generates a jwt secret with its base64 encoded value intended to be used for saving into config file
+func NewJwtSecretWithBase64() ([]byte, string, error) {
+	bytes := make([]byte, defaultJwtSecretLen)
+	_, err := io.ReadFull(rand.Reader, bytes)
 	if err != nil {
 		return nil, "", err
 	}
diff --git a/modules/generate/generate_test.go b/modules/generate/generate_test.go
new file mode 100644
index 0000000000..af640a60c1
--- /dev/null
+++ b/modules/generate/generate_test.go
@@ -0,0 +1,34 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package generate
+
+import (
+	"encoding/base64"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestDecodeJwtSecretBase64(t *testing.T) {
+	_, err := DecodeJwtSecretBase64("abcd")
+	assert.ErrorContains(t, err, "invalid base64 decoded length")
+	_, err = DecodeJwtSecretBase64(strings.Repeat("a", 64))
+	assert.ErrorContains(t, err, "invalid base64 decoded length")
+
+	str32 := strings.Repeat("x", 32)
+	encoded32 := base64.RawURLEncoding.EncodeToString([]byte(str32))
+	decoded32, err := DecodeJwtSecretBase64(encoded32)
+	assert.NoError(t, err)
+	assert.Equal(t, str32, string(decoded32))
+}
+
+func TestNewJwtSecretWithBase64(t *testing.T) {
+	secret, encoded, err := NewJwtSecretWithBase64()
+	assert.NoError(t, err)
+	assert.Len(t, secret, 32)
+	decoded, err := DecodeJwtSecretBase64(encoded)
+	assert.NoError(t, err)
+	assert.Equal(t, secret, decoded)
+}
diff --git a/modules/setting/lfs.go b/modules/setting/lfs.go
index a5ea537cef..22a75f6008 100644
--- a/modules/setting/lfs.go
+++ b/modules/setting/lfs.go
@@ -4,12 +4,10 @@
 package setting
 
 import (
-	"encoding/base64"
 	"fmt"
 	"time"
 
 	"code.gitea.io/gitea/modules/generate"
-	"code.gitea.io/gitea/modules/util"
 )
 
 // LFS represents the configuration for Git LFS
@@ -62,9 +60,9 @@ func loadLFSFrom(rootCfg ConfigProvider) error {
 	}
 
 	LFS.JWTSecretBase64 = loadSecret(rootCfg.Section("server"), "LFS_JWT_SECRET_URI", "LFS_JWT_SECRET")
-	LFS.JWTSecretBytes, err = util.Base64FixedDecode(base64.RawURLEncoding, []byte(LFS.JWTSecretBase64), 32)
+	LFS.JWTSecretBytes, err = generate.DecodeJwtSecretBase64(LFS.JWTSecretBase64)
 	if err != nil {
-		LFS.JWTSecretBytes, LFS.JWTSecretBase64, err = generate.NewJwtSecretBase64()
+		LFS.JWTSecretBytes, LFS.JWTSecretBase64, err = generate.NewJwtSecretWithBase64()
 		if err != nil {
 			return fmt.Errorf("error generating JWT Secret for custom config: %v", err)
 		}
diff --git a/modules/setting/oauth2.go b/modules/setting/oauth2.go
index 0d15e91ef0..e16e167024 100644
--- a/modules/setting/oauth2.go
+++ b/modules/setting/oauth2.go
@@ -4,13 +4,11 @@
 package setting
 
 import (
-	"encoding/base64"
 	"math"
 	"path/filepath"
 
 	"code.gitea.io/gitea/modules/generate"
 	"code.gitea.io/gitea/modules/log"
-	"code.gitea.io/gitea/modules/util"
 )
 
 // OAuth2UsernameType is enum describing the way gitea 'name' should be generated from oauth2 data
@@ -137,13 +135,12 @@ func loadOAuth2From(rootCfg ConfigProvider) {
 	}
 
 	if InstallLock {
-		if _, err := util.Base64FixedDecode(base64.RawURLEncoding, []byte(OAuth2.JWTSecretBase64), 32); err != nil {
-			key, err := generate.NewJwtSecret()
+		if _, err := generate.DecodeJwtSecretBase64(OAuth2.JWTSecretBase64); err != nil {
+			_, OAuth2.JWTSecretBase64, err = generate.NewJwtSecretWithBase64()
 			if err != nil {
 				log.Fatal("error generating JWT secret: %v", err)
 			}
 
-			OAuth2.JWTSecretBase64 = base64.RawURLEncoding.EncodeToString(key)
 			saveCfg, err := rootCfg.PrepareSaving()
 			if err != nil {
 				log.Fatal("save oauth2.JWT_SECRET failed: %v", err)
diff --git a/modules/util/util.go b/modules/util/util.go
index c47931f6c9..0e5c6a4e64 100644
--- a/modules/util/util.go
+++ b/modules/util/util.go
@@ -6,7 +6,6 @@ package util
 import (
 	"bytes"
 	"crypto/rand"
-	"encoding/base64"
 	"fmt"
 	"math/big"
 	"strconv"
@@ -246,13 +245,3 @@ func ToFloat64(number any) (float64, error) {
 func ToPointer[T any](val T) *T {
 	return &val
 }
-
-func Base64FixedDecode(encoding *base64.Encoding, src []byte, length int) ([]byte, error) {
-	decoded := make([]byte, encoding.DecodedLen(len(src))+3)
-	if n, err := encoding.Decode(decoded, src); err != nil {
-		return nil, err
-	} else if n != length {
-		return nil, fmt.Errorf("invalid base64 decoded length: %d, expects: %d", n, length)
-	}
-	return decoded[:length], nil
-}
diff --git a/modules/util/util_test.go b/modules/util/util_test.go
index 8509d8aced..c5830ce01c 100644
--- a/modules/util/util_test.go
+++ b/modules/util/util_test.go
@@ -4,7 +4,6 @@
 package util
 
 import (
-	"encoding/base64"
 	"regexp"
 	"strings"
 	"testing"
@@ -234,16 +233,3 @@ func TestToPointer(t *testing.T) {
 	val123 := 123
 	assert.False(t, &val123 == ToPointer(val123))
 }
-
-func TestBase64FixedDecode(t *testing.T) {
-	_, err := Base64FixedDecode(base64.RawURLEncoding, []byte("abcd"), 32)
-	assert.ErrorContains(t, err, "invalid base64 decoded length")
-	_, err = Base64FixedDecode(base64.RawURLEncoding, []byte(strings.Repeat("a", 64)), 32)
-	assert.ErrorContains(t, err, "invalid base64 decoded length")
-
-	str32 := strings.Repeat("x", 32)
-	encoded32 := base64.RawURLEncoding.EncodeToString([]byte(str32))
-	decoded32, err := Base64FixedDecode(base64.RawURLEncoding, []byte(encoded32), 32)
-	assert.NoError(t, err)
-	assert.Equal(t, str32, string(decoded32))
-}
diff --git a/routers/install/install.go b/routers/install/install.go
index 5c0290d2cc..064575d34c 100644
--- a/routers/install/install.go
+++ b/routers/install/install.go
@@ -409,7 +409,7 @@ func SubmitInstall(ctx *context.Context) {
 		cfg.Section("server").Key("LFS_START_SERVER").SetValue("true")
 		cfg.Section("lfs").Key("PATH").SetValue(form.LFSRootPath)
 		var lfsJwtSecret string
-		if _, lfsJwtSecret, err = generate.NewJwtSecretBase64(); err != nil {
+		if _, lfsJwtSecret, err = generate.NewJwtSecretWithBase64(); err != nil {
 			ctx.RenderWithErr(ctx.Tr("install.lfs_jwt_secret_failed", err), tplInstall, &form)
 			return
 		}
diff --git a/services/auth/source/oauth2/jwtsigningkey.go b/services/auth/source/oauth2/jwtsigningkey.go
index eca0b8b7e1..2afe557b0d 100644
--- a/services/auth/source/oauth2/jwtsigningkey.go
+++ b/services/auth/source/oauth2/jwtsigningkey.go
@@ -18,6 +18,7 @@ import (
 	"path/filepath"
 	"strings"
 
+	"code.gitea.io/gitea/modules/generate"
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/util"
@@ -336,7 +337,7 @@ func InitSigningKey() error {
 // loadSymmetricKey checks if the configured secret is valid.
 // If it is not valid, it will return an error.
 func loadSymmetricKey() (any, error) {
-	return util.Base64FixedDecode(base64.RawURLEncoding, []byte(setting.OAuth2.JWTSecretBase64), 32)
+	return generate.DecodeJwtSecretBase64(setting.OAuth2.JWTSecretBase64)
 }
 
 // loadOrCreateAsymmetricKey checks if the configured private key exists.