Improve instance wide ssh commit signing (#34341)

* Signed SSH commits can look in the UI like on GitHub, just like gpg keys today in Gitea
* SSH format can be added in gitea config
* SSH Signing worked before with DEFAULT_TRUST_MODEL=committer

`TRUSTED_SSH_KEYS` can be a list of additional ssh public key contents
to trust for every user of this instance

Closes #34329
Related #31392

---------

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/git/command.go

@@ -47,6 +47,7 @@ type Command struct {
 	globalArgsLength int
 	brokenArgs       []string
 	cmd              *exec.Cmd // for debug purpose only
+	configArgs       []string
 }
 
 func logArgSanitize(arg string) string {
@@ -196,6 +197,16 @@ func (c *Command) AddDashesAndList(list ...string) *Command {
 	return c
 }
 
+func (c *Command) AddConfig(key, value string) *Command {
+	kv := key + "=" + value
+	if !isSafeArgumentValue(kv) {
+		c.brokenArgs = append(c.brokenArgs, key)
+	} else {
+		c.configArgs = append(c.configArgs, "-c", kv)
+	}
+	return c
+}
+
 // ToTrustedCmdArgs converts a list of strings (trusted as argument) to TrustedCmdArgs
 // In most cases, it shouldn't be used. Use NewCommand().AddXxx() function instead
 func ToTrustedCmdArgs(args []string) TrustedCmdArgs {
@@ -321,7 +332,7 @@ func (c *Command) run(ctx context.Context, skip int, opts *RunOpts) error {
 
 	startTime := time.Now()
 
-	cmd := exec.CommandContext(ctx, c.prog, c.args...)
+	cmd := exec.CommandContext(ctx, c.prog, append(c.configArgs, c.args...)...)
 	c.cmd = cmd // for debug purpose only
 	if opts.Env == nil {
 		cmd.Env = os.Environ()

modules/git/key.go

@@ -0,0 +1,15 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package git
+
+// Based on https://git-scm.com/docs/git-config#Documentation/git-config.txt-gpgformat
+const (
+	SigningKeyFormatOpenPGP = "openpgp" // for GPG keys, the expected default of git cli
+	SigningKeyFormatSSH     = "ssh"
+)
+
+type SigningKey struct {
+	KeyID  string
+	Format string
+}

modules/git/repo.go

@@ -28,6 +28,7 @@ type GPGSettings struct {
 	Email            string
 	Name             string
 	PublicKeyContent string
+	Format           string
 }
 
 const prettyLogFormat = `--pretty=format:%H`

modules/git/repo_gpg.go

@@ -6,6 +6,7 @@ package git
 
 import (
 	"fmt"
+	"os"
 	"strings"
 
 	"code.gitea.io/gitea/modules/process"
@@ -13,6 +14,14 @@ import (
 
 // LoadPublicKeyContent will load the key from gpg
 func (gpgSettings *GPGSettings) LoadPublicKeyContent() error {
+	if gpgSettings.Format == SigningKeyFormatSSH {
+		content, err := os.ReadFile(gpgSettings.KeyID)
+		if err != nil {
+			return fmt.Errorf("unable to read SSH public key file: %s, %w", gpgSettings.KeyID, err)
+		}
+		gpgSettings.PublicKeyContent = string(content)
+		return nil
+	}
 	content, stderr, err := process.GetManager().Exec(
 		"gpg -a --export",
 		"gpg", "-a", "--export", gpgSettings.KeyID)
@@ -44,6 +53,9 @@ func (repo *Repository) GetDefaultPublicGPGKey(forceUpdate bool) (*GPGSettings,
 	signingKey, _, _ := NewCommand("config", "--get", "user.signingkey").RunStdString(repo.Ctx, &RunOpts{Dir: repo.Path})
 	gpgSettings.KeyID = strings.TrimSpace(signingKey)
 
+	format, _, _ := NewCommand("config", "--default", SigningKeyFormatOpenPGP, "--get", "gpg.format").RunStdString(repo.Ctx, &RunOpts{Dir: repo.Path})
+	gpgSettings.Format = strings.TrimSpace(format)
+
 	defaultEmail, _, _ := NewCommand("config", "--get", "user.email").RunStdString(repo.Ctx, &RunOpts{Dir: repo.Path})
 	gpgSettings.Email = strings.TrimSpace(defaultEmail)
 

modules/git/repo_tree.go

@@ -15,7 +15,7 @@ import (
 type CommitTreeOpts struct {
 	Parents    []string
 	Message    string
-	KeyID      string
+	Key        *SigningKey
 	NoGPGSign  bool
 	AlwaysSign bool
 }
@@ -43,8 +43,13 @@ func (repo *Repository) CommitTree(author, committer *Signature, tree *Tree, opt
 	_, _ = messageBytes.WriteString(opts.Message)
 	_, _ = messageBytes.WriteString("\n")
 
-	if opts.KeyID != "" || opts.AlwaysSign {
-		cmd.AddOptionFormat("-S%s", opts.KeyID)
+	if opts.Key != nil {
+		if opts.Key.Format != "" {
+			cmd.AddConfig("gpg.format", opts.Key.Format)
+		}
+		cmd.AddOptionFormat("-S%s", opts.Key.KeyID)
+	} else if opts.AlwaysSign {
+		cmd.AddOptionFormat("-S")
 	}
 
 	if opts.NoGPGSign {