bug: correctly generate oauth2 jwt signing key

- When RS256, RS384, ES384, ES512 was specified as the JWT signing
algorithm they would generate RS512 and ES256 respectively.
- Added unit test.

(cherry picked from commit 7d59060dc6)

services/auth/source/oauth2/jwtsigningkey.go

@@ -347,12 +347,30 @@ func loadOrCreateAsymmetricKey() (any, error) {
 			key, err := func() (any, error) {
 				switch {
 				case strings.HasPrefix(setting.OAuth2.JWTSigningAlgorithm, "RS"):
-					return rsa.GenerateKey(rand.Reader, 4096)
+					var bits int
+					switch setting.OAuth2.JWTSigningAlgorithm {
+					case "RS256":
+						bits = 2048
+					case "RS384":
+						bits = 3072
+					case "RS512":
+						bits = 4096
+					}
+					return rsa.GenerateKey(rand.Reader, bits)
 				case setting.OAuth2.JWTSigningAlgorithm == "EdDSA":
 					_, pk, err := ed25519.GenerateKey(rand.Reader)
 					return pk, err
 				default:
-					return ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
+					var curve elliptic.Curve
+					switch setting.OAuth2.JWTSigningAlgorithm {
+					case "ES256":
+						curve = elliptic.P256()
+					case "ES384":
+						curve = elliptic.P384()
+					case "ES512":
+						curve = elliptic.P521()
+					}
+					return ecdsa.GenerateKey(curve, rand.Reader)
 				}
 			}()
 			if err != nil {

services/auth/source/oauth2/jwtsigningkey_test.go

@@ -0,0 +1,116 @@
+// Copyright 2024 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package oauth2
+
+import (
+	"crypto/ecdsa"
+	"crypto/ed25519"
+	"crypto/rsa"
+	"crypto/x509"
+	"encoding/pem"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/test"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestLoadOrCreateAsymmetricKey(t *testing.T) {
+	loadKey := func(t *testing.T) any {
+		t.Helper()
+		loadOrCreateAsymmetricKey()
+
+		fileContent, err := os.ReadFile(setting.OAuth2.JWTSigningPrivateKeyFile)
+		require.NoError(t, err)
+
+		block, _ := pem.Decode(fileContent)
+		assert.NotNil(t, block)
+		assert.EqualValues(t, "PRIVATE KEY", block.Type)
+
+		parsedKey, err := x509.ParsePKCS8PrivateKey(block.Bytes)
+		require.NoError(t, err)
+
+		return parsedKey
+	}
+	t.Run("RSA-2048", func(t *testing.T) {
+		defer test.MockVariableValue(&setting.OAuth2.JWTSigningPrivateKeyFile, filepath.Join(t.TempDir(), "jwt-rsa-2048.priv"))()
+		defer test.MockVariableValue(&setting.OAuth2.JWTSigningAlgorithm, "RS256")()
+
+		parsedKey := loadKey(t)
+
+		rsaPrivateKey := parsedKey.(*rsa.PrivateKey)
+		assert.EqualValues(t, 2048, rsaPrivateKey.N.BitLen())
+
+		t.Run("Load key with differ specified algorithm", func(t *testing.T) {
+			defer test.MockVariableValue(&setting.OAuth2.JWTSigningAlgorithm, "EdDSA")()
+
+			parsedKey := loadKey(t)
+			rsaPrivateKey := parsedKey.(*rsa.PrivateKey)
+			assert.EqualValues(t, 2048, rsaPrivateKey.N.BitLen())
+		})
+	})
+
+	t.Run("RSA-3072", func(t *testing.T) {
+		defer test.MockVariableValue(&setting.OAuth2.JWTSigningPrivateKeyFile, filepath.Join(t.TempDir(), "jwt-rsa-3072.priv"))()
+		defer test.MockVariableValue(&setting.OAuth2.JWTSigningAlgorithm, "RS384")()
+
+		parsedKey := loadKey(t)
+
+		rsaPrivateKey := parsedKey.(*rsa.PrivateKey)
+		assert.EqualValues(t, 3072, rsaPrivateKey.N.BitLen())
+	})
+
+	t.Run("RSA-4096", func(t *testing.T) {
+		defer test.MockVariableValue(&setting.OAuth2.JWTSigningPrivateKeyFile, filepath.Join(t.TempDir(), "jwt-rsa-4096.priv"))()
+		defer test.MockVariableValue(&setting.OAuth2.JWTSigningAlgorithm, "RS512")()
+
+		parsedKey := loadKey(t)
+
+		rsaPrivateKey := parsedKey.(*rsa.PrivateKey)
+		assert.EqualValues(t, 4096, rsaPrivateKey.N.BitLen())
+	})
+
+	t.Run("ECDSA-256", func(t *testing.T) {
+		defer test.MockVariableValue(&setting.OAuth2.JWTSigningPrivateKeyFile, filepath.Join(t.TempDir(), "jwt-ecdsa-256.priv"))()
+		defer test.MockVariableValue(&setting.OAuth2.JWTSigningAlgorithm, "ES256")()
+
+		parsedKey := loadKey(t)
+
+		ecdsaPrivateKey := parsedKey.(*ecdsa.PrivateKey)
+		assert.EqualValues(t, 256, ecdsaPrivateKey.Params().BitSize)
+	})
+
+	t.Run("ECDSA-384", func(t *testing.T) {
+		defer test.MockVariableValue(&setting.OAuth2.JWTSigningPrivateKeyFile, filepath.Join(t.TempDir(), "jwt-ecdsa-384.priv"))()
+		defer test.MockVariableValue(&setting.OAuth2.JWTSigningAlgorithm, "ES384")()
+
+		parsedKey := loadKey(t)
+
+		ecdsaPrivateKey := parsedKey.(*ecdsa.PrivateKey)
+		assert.EqualValues(t, 384, ecdsaPrivateKey.Params().BitSize)
+	})
+
+	t.Run("ECDSA-512", func(t *testing.T) {
+		defer test.MockVariableValue(&setting.OAuth2.JWTSigningPrivateKeyFile, filepath.Join(t.TempDir(), "jwt-ecdsa-512.priv"))()
+		defer test.MockVariableValue(&setting.OAuth2.JWTSigningAlgorithm, "ES512")()
+
+		parsedKey := loadKey(t)
+
+		ecdsaPrivateKey := parsedKey.(*ecdsa.PrivateKey)
+		assert.EqualValues(t, 521, ecdsaPrivateKey.Params().BitSize)
+	})
+
+	t.Run("EdDSA", func(t *testing.T) {
+		defer test.MockVariableValue(&setting.OAuth2.JWTSigningPrivateKeyFile, filepath.Join(t.TempDir(), "jwt-eddsa.priv"))()
+		defer test.MockVariableValue(&setting.OAuth2.JWTSigningAlgorithm, "EdDSA")()
+
+		parsedKey := loadKey(t)
+
+		assert.NotNil(t, parsedKey.(ed25519.PrivateKey))
+	})
+}