Port "Use general token signing secret"

Port of https://github.com/go-gitea/gitea/pull/29205

Use a clearly defined "signing secret" for token signing.

(cherry picked from commit 8be198cdef0a486f417663b1fd6878458d7e5d92)

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 TestDecodeJwtSecret(t *testing.T) {
+	_, err := DecodeJwtSecret("abcd")
+	assert.ErrorContains(t, err, "invalid base64 decoded length")
+	_, err = DecodeJwtSecret(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 := DecodeJwtSecret(encoded32)
+	assert.NoError(t, err)
+	assert.Equal(t, str32, string(decoded32))
+}
+
+func TestNewJwtSecret(t *testing.T) {
+	secret, encoded, err := NewJwtSecret()
+	assert.NoError(t, err)
+	assert.Len(t, secret, 32)
+	decoded, err := DecodeJwtSecret(encoded)
+	assert.NoError(t, err)
+	assert.Equal(t, secret, decoded)
+}