feat(ssh): auto generate additional ssh keys (#33974)

adds capabilities for gitea to generate ecdsa and ed25519 keys by
default
adds cli for built-in ssh key generation helpers


closes: https://github.com/go-gitea/gitea/issues/33783

---------

Co-authored-by: Nicolas <bircni@icloud.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Giteabot <teabot@gitea.io>

modules/consts/asymkey.go

@@ -0,0 +1,12 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package consts
+
+const (
+	AsymKeyMinBitsRsa = 3071 // 3072-1 to tolerate the leading zero
+	AsymKeyMinBitsEC  = 256
+
+	AsymKeyDefaultBitsRsa   = 4096 // ssh-keygen command defaults to 3072
+	AsymKeyDefaultBitsEcdsa = 256
+)

modules/generate/generate.go

@@ -5,15 +5,23 @@
 package generate
 
 import (
+	"crypto"
+	"crypto/ecdsa"
+	"crypto/ed25519"
+	"crypto/elliptic"
 	"crypto/rand"
+	"crypto/rsa"
 	"encoding/base64"
+	"encoding/pem"
 	"fmt"
 	"io"
 	"time"
 
+	"gitea.dev/modules/consts"
 	"gitea.dev/modules/util"
 
 	"github.com/golang-jwt/jwt/v5"
+	"golang.org/x/crypto/ssh"
 )
 
 // NewInternalToken generate a new value intended to be used by INTERNAL_TOKEN.
@@ -67,3 +75,75 @@ func NewJwtSecretWithBase64() ([]byte, string) {
 func NewSecretKey() (string, error) {
 	return util.CryptoRandomString(64), nil
 }
+
+type SSHKeyType string
+
+const (
+	SSHKeyRSA     SSHKeyType = "rsa"
+	SSHKeyECDSA   SSHKeyType = "ecdsa"
+	SSHKeyED25519 SSHKeyType = "ed25519"
+)
+
+func NewSSHKey(keyType SSHKeyType, bits int) (ssh.PublicKey, *pem.Block, error) {
+	pub, priv, err := commonKeyGen(keyType, bits)
+	if err != nil {
+		return nil, nil, err
+	}
+	pemPriv, err := ssh.MarshalPrivateKey(priv, "")
+	if err != nil {
+		return nil, nil, err
+	}
+	sshPub, err := ssh.NewPublicKey(pub)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	return sshPub, pemPriv, nil
+}
+
+// commonKeyGen is an abstraction over rsa, ecdsa, and ed25519 generating functions
+func commonKeyGen(keyType SSHKeyType, bits int) (crypto.PublicKey, crypto.PrivateKey, error) {
+	switch keyType {
+	case SSHKeyRSA:
+		bits = util.IfZero(bits, consts.AsymKeyDefaultBitsRsa)
+		if bits < consts.AsymKeyMinBitsRsa {
+			return nil, nil, util.NewInvalidArgumentErrorf("invalid rsa bits: %d", bits)
+		}
+		privateKey, err := rsa.GenerateKey(rand.Reader, bits)
+		if err != nil {
+			return nil, nil, err
+		}
+		return &privateKey.PublicKey, privateKey, nil
+	case SSHKeyED25519:
+		return ed25519.GenerateKey(rand.Reader)
+	case SSHKeyECDSA:
+		bits = util.IfZero(bits, consts.AsymKeyDefaultBitsEcdsa)
+		if bits < consts.AsymKeyMinBitsEC {
+			return nil, nil, util.NewInvalidArgumentErrorf("invalid elliptic-curve bits: %d", bits)
+		}
+		curve, err := getEllipticCurve(bits)
+		if err != nil {
+			return nil, nil, err
+		}
+		privateKey, err := ecdsa.GenerateKey(curve, rand.Reader)
+		if err != nil {
+			return nil, nil, err
+		}
+		return &privateKey.PublicKey, privateKey, nil
+	default:
+		return nil, nil, util.NewInvalidArgumentErrorf("unknown key type: %s", keyType)
+	}
+}
+
+func getEllipticCurve(bits int) (elliptic.Curve, error) {
+	switch bits {
+	case 256:
+		return elliptic.P256(), nil
+	case 384:
+		return elliptic.P384(), nil
+	case 521:
+		return elliptic.P521(), nil
+	default:
+		return nil, util.NewInvalidArgumentErrorf("unsupported elliptic-curve bits: %d", bits)
+	}
+}

modules/setting/ssh.go

@@ -9,6 +9,7 @@ import (
 	"text/template"
 	"time"
 
+	"gitea.dev/modules/consts"
 	"gitea.dev/modules/log"
 	"gitea.dev/modules/util"
 
@@ -52,8 +53,8 @@ var SSH = struct {
 	Domain:                        "",
 	Port:                          22,
 	MinimumKeySizeCheck:           true,
-	MinimumKeySizes:               map[string]int{"ed25519": 256, "ed25519-sk": 256, "ecdsa": 256, "ecdsa-sk": 256, "rsa": 3071},
-	ServerHostKeys:                []string{"ssh/gitea.rsa", "ssh/gogs.rsa"},
+	MinimumKeySizes:               map[string]int{"ed25519": consts.AsymKeyMinBitsEC, "ed25519-sk": consts.AsymKeyMinBitsEC, "ecdsa": consts.AsymKeyMinBitsEC, "ecdsa-sk": consts.AsymKeyMinBitsEC, "rsa": consts.AsymKeyMinBitsRsa},
+	ServerHostKeys:                []string{"ssh/gitea.rsa", "ssh/gitea.ed25519", "ssh/gitea.ecdsa", "ssh/gogs.rsa"},
 	AuthorizedKeysCommandTemplate: "{{.AppPath}} --config={{.CustomConf}} serv key-{{.Key.ID}}",
 	PerWriteTimeout:               PerWriteTimeout,
 	PerWritePerKbTimeout:          PerWritePerKbTimeout,

modules/ssh/ssh.go

@@ -6,9 +6,6 @@ package ssh
 import (
 	"bytes"
 	"context"
-	"crypto/rand"
-	"crypto/rsa"
-	"crypto/x509"
 	"encoding/pem"
 	"errors"
 	"io"
@@ -23,11 +20,11 @@ import (
 	"syscall"
 
 	asymkey_model "gitea.dev/models/asymkey"
+	"gitea.dev/modules/generate"
 	"gitea.dev/modules/graceful"
 	"gitea.dev/modules/log"
 	"gitea.dev/modules/process"
 	"gitea.dev/modules/setting"
-	"gitea.dev/modules/util"
 
 	"github.com/gliderlabs/ssh"
 	gossh "golang.org/x/crypto/ssh"
@@ -59,7 +56,7 @@ func getExitStatusFromError(err error) int {
 		return 0
 	}
 
-	exitErr, ok := err.(*exec.ExitError)
+	exitErr, ok := errors.AsType[*exec.ExitError](err)
 	if !ok {
 		return 1
 	}
@@ -322,7 +319,7 @@ func publicKeyHandler(ctx ssh.Context, key ssh.PublicKey) bool {
 }
 
 // sshConnectionFailed logs a failed connection
-// -  this mainly exists to give a nice function name in logging
+// - this mainly exists to give a nice function name in logging
 func sshConnectionFailed(conn net.Conn, err error) {
 	// Log the underlying error with a specific message
 	log.Warn("Failed connection from %s with error: %v", conn.RemoteAddr(), err)
@@ -351,40 +348,37 @@ func Listen(host string, port int, ciphers, keyExchanges, macs []string) {
 		},
 	}
 
-	keys := make([]string, 0, len(setting.SSH.ServerHostKeys))
+	hostKeyFiles := make([]string, 0, len(setting.SSH.ServerHostKeys))
 	for _, key := range setting.SSH.ServerHostKeys {
-		isExist, err := util.IsExist(key)
+		_, err := os.Stat(key)
 		if err != nil {
-			log.Fatal("Unable to check if %s exists. Error: %v", setting.SSH.ServerHostKeys, err)
-		}
-		if isExist {
-			keys = append(keys, key)
+			if !errors.Is(err, os.ErrNotExist) {
+				log.Fatal("Unable to check if %s exists. Error: %v", setting.SSH.ServerHostKeys, err)
+			}
+			continue
 		}
+		hostKeyFiles = append(hostKeyFiles, key)
 	}
 
-	if len(keys) == 0 {
-		filePath := filepath.Dir(setting.SSH.ServerHostKeys[0])
-
-		if err := os.MkdirAll(filePath, os.ModePerm); err != nil {
-			log.Error("Failed to create dir %s: %v", filePath, err)
+	if len(hostKeyFiles) == 0 {
+		hostKeyDir := filepath.Dir(setting.SSH.ServerHostKeys[0])
+		err := os.MkdirAll(hostKeyDir, os.ModePerm)
+		if err != nil {
+			log.Error("Failed to create dir %s: %v", hostKeyDir, err)
 		}
-
-		err := GenKeyPair(setting.SSH.ServerHostKeys[0])
+		hostKeyFiles, err = InitDefaultHostKeys(hostKeyDir)
 		if err != nil {
 			log.Fatal("Failed to generate private key: %v", err)
 		}
-		log.Trace("New private key is generated: %s", setting.SSH.ServerHostKeys[0])
-		keys = append(keys, setting.SSH.ServerHostKeys[0])
 	}
 
-	for _, key := range keys {
-		log.Info("Adding SSH host key: %s", key)
-		err := srv.SetOption(ssh.HostKeyFile(key))
+	for _, keyFile := range hostKeyFiles {
+		log.Info("Adding SSH host key: %s", keyFile)
+		err := srv.SetOption(ssh.HostKeyFile(keyFile))
 		if err != nil {
 			log.Error("Failed to set Host Key. %s", err)
 		}
 	}
-
 	go func() {
 		_, _, finished := process.GetManager().AddTypedContext(graceful.GetManager().HammerContext(), "Service: Built-in SSH server", process.SystemProcessType, true)
 		defer finished()
@@ -395,43 +389,44 @@ func Listen(host string, port int, ciphers, keyExchanges, macs []string) {
 // GenKeyPair make a pair of public and private keys for SSH access.
 // Public key is encoded in the format for inclusion in an OpenSSH authorized_keys file.
 // Private Key generated is PEM encoded
-func GenKeyPair(keyPath string) error {
-	privateKey, err := rsa.GenerateKey(rand.Reader, 4096)
+func GenKeyPair(keyPath string, keyType generate.SSHKeyType, bits int) error {
+	publicKey, privateKeyPEM, err := generate.NewSSHKey(keyType, bits)
 	if err != nil {
 		return err
 	}
 
-	privateKeyPEM := &pem.Block{Type: "RSA PRIVATE KEY", Bytes: x509.MarshalPKCS1PrivateKey(privateKey)}
-	f, err := os.OpenFile(keyPath, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0o600)
-	if err != nil {
-		return err
-	}
-	defer func() {
-		if err = f.Close(); err != nil {
-			log.Error("Close: %v", err)
-		}
-	}()
-
-	if err := pem.Encode(f, privateKeyPEM); err != nil {
-		return err
-	}
-
-	// generate public key
-	pub, err := gossh.NewPublicKey(&privateKey.PublicKey)
+	public := gossh.MarshalAuthorizedKey(publicKey)
+	privateKeyBuf := &bytes.Buffer{}
+	err = pem.Encode(privateKeyBuf, privateKeyPEM)
 	if err != nil {
 		return err
 	}
 
-	public := gossh.MarshalAuthorizedKey(pub)
-	p, err := os.OpenFile(keyPath+".pub", os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0o600)
+	err = os.WriteFile(keyPath, privateKeyBuf.Bytes(), 0o600)
 	if err != nil {
 		return err
 	}
-	defer func() {
-		if err = p.Close(); err != nil {
-			log.Error("Close: %v", err)
-		}
-	}()
-	_, err = p.Write(public)
-	return err
+
+	return os.WriteFile(keyPath+".pub", public, 0o644)
+}
+
+// InitDefaultHostKeys mirrors how ssh-keygen -A operates
+// it runs checks if public and private keys are already defined and creates new ones if not present
+// key naming does not follow the OpenSSH convention due to existing settings being gitea.{KeyType} so generation follows gitea convention
+func InitDefaultHostKeys(path string) (keyFiles []string, _ error) {
+	var errs []error
+	keyTypes := []generate.SSHKeyType{generate.SSHKeyRSA, generate.SSHKeyECDSA, generate.SSHKeyED25519}
+	for _, keyType := range keyTypes {
+		keyPath := filepath.Join(path, "gitea."+string(keyType))
+		_, errStatPriv := os.Stat(keyPath)
+		if errStatPriv != nil {
+			err := GenKeyPair(keyPath, keyType, 0)
+			if err != nil {
+				errs = append(errs, err)
+				continue
+			}
+		}
+		keyFiles = append(keyFiles, keyPath)
+	}
+	return keyFiles, errors.Join(errs...)
 }

modules/ssh/ssh_test.go

@@ -0,0 +1,123 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package ssh
+
+import (
+	"crypto/ecdsa"
+	"crypto/ed25519"
+	"crypto/rsa"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"gitea.dev/modules/generate"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	gossh "golang.org/x/crypto/ssh"
+)
+
+func TestGenKeyPair(t *testing.T) {
+	testCases := []struct {
+		keyType      generate.SSHKeyType
+		expectedType any
+	}{
+		{
+			keyType:      generate.SSHKeyRSA,
+			expectedType: &rsa.PrivateKey{},
+		},
+		{
+			keyType:      generate.SSHKeyED25519,
+			expectedType: &ed25519.PrivateKey{},
+		},
+		{
+			keyType:      generate.SSHKeyECDSA,
+			expectedType: &ecdsa.PrivateKey{},
+		},
+	}
+	tmpDir := t.TempDir()
+	for _, tc := range testCases {
+		name := "gitea." + string(tc.keyType)
+		fn := filepath.Join(tmpDir, name)
+		t.Run("Generate "+name, func(t *testing.T) {
+			require.NoError(t, GenKeyPair(fn, tc.keyType, 0))
+
+			bytes, err := os.ReadFile(fn)
+			require.NoError(t, err)
+
+			privateKey, err := gossh.ParseRawPrivateKey(bytes)
+			require.NoError(t, err)
+			assert.IsType(t, tc.expectedType, privateKey)
+		})
+	}
+	t.Run("Generate unknown key type", func(t *testing.T) {
+		err := GenKeyPair(t.TempDir()+"gitea.badkey", "badkey", 0)
+		require.Error(t, err)
+	})
+}
+
+func TestInitKeys(t *testing.T) {
+	tempDir := t.TempDir()
+
+	keyTypes := []string{"rsa", "ecdsa", "ed25519"}
+	for _, keyType := range keyTypes {
+		privKeyPath := filepath.Join(tempDir, "gitea."+keyType)
+		pubKeyPath := filepath.Join(tempDir, "gitea."+keyType+".pub")
+		assert.NoFileExists(t, privKeyPath)
+		assert.NoFileExists(t, pubKeyPath)
+	}
+
+	// Test basic creation
+	keyFiles, err := InitDefaultHostKeys(tempDir)
+	require.NoError(t, err)
+	assert.Len(t, keyFiles, len(keyTypes))
+
+	metadata := map[string]os.FileInfo{}
+	for _, keyType := range keyTypes {
+		privKeyPath := filepath.Join(tempDir, "gitea."+keyType)
+		pubKeyPath := filepath.Join(tempDir, "gitea."+keyType+".pub")
+		info, err := os.Stat(privKeyPath)
+		require.NoError(t, err)
+		metadata[privKeyPath] = info
+
+		info, err = os.Stat(pubKeyPath)
+		require.NoError(t, err)
+		metadata[pubKeyPath] = info
+	}
+
+	// Test recreation on missing private key and noop for missing pub key
+	require.NoError(t, os.Remove(filepath.Join(tempDir, "gitea.ecdsa.pub")))
+	require.NoError(t, os.Remove(filepath.Join(tempDir, "gitea.ed25519")))
+
+	keyFiles, err = InitDefaultHostKeys(tempDir)
+	require.NoError(t, err)
+	assert.Len(t, keyFiles, len(keyTypes))
+
+	for _, keyType := range keyTypes {
+		privKeyPath := filepath.Join(tempDir, "gitea."+keyType)
+		pubKeyPath := filepath.Join(tempDir, "gitea."+keyType+".pub")
+
+		infoPriv, err := os.Stat(privKeyPath)
+		require.NoError(t, err)
+
+		switch keyType {
+		case "rsa":
+			// No modification to RSA key
+			infoPub, err := os.Stat(pubKeyPath)
+			require.NoError(t, err)
+			assert.Equal(t, metadata[privKeyPath], infoPriv)
+			assert.Equal(t, metadata[pubKeyPath], infoPub)
+		case "ecdsa":
+			// ECDSA public key should be missing, private unchanged
+			assert.Equal(t, metadata[privKeyPath], infoPriv)
+			assert.NoFileExists(t, pubKeyPath)
+		case "ed25519":
+			// ed25519 private key was removed, so both keys regenerated
+			infoPub, err := os.Stat(pubKeyPath)
+			require.NoError(t, err)
+			assert.NotEqual(t, metadata[privKeyPath], infoPriv)
+			assert.NotEqual(t, metadata[pubKeyPath], infoPub)
+		}
+	}
+}