Improve instance wide ssh commit signing

custom/conf/app.example.ini

@@ -1190,17 +1190,24 @@ LEVEL = Info
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;
-;; GPG key to use to sign commits, Defaults to the default - that is the value of git config --get user.signingkey
+;; GPG or SSH key to use to sign commits, Defaults to the default - that is the value of git config --get user.signingkey
+;; Depending on the value of SIGNING_FORMAT this is either:
+;; - openpgp: the GPG key ID
+;; - ssh: the path to the ssh public key "/path/to/key.pub": where "/path/to/key" is the private key, use ssh-keygen -t ed25519 to generate a new key pair without password
 ;; run in the context of the RUN_USER
 ;; Switch to none to stop signing completely
 ;SIGNING_KEY = default
 ;;
-;; If a SIGNING_KEY ID is provided and is not set to default, use the provided Name and Email address as the signer.
+;; If a SIGNING_KEY ID is provided and is not set to default, use the provided Name and Email address as the signer and the signing format.
 ;; These should match a publicized name and email address for the key. (When SIGNING_KEY is default these are set to
-;; the results of git config --get user.name and git config --get user.email respectively and can only be overridden
+;; the results of git config --get user.name, git config --get user.email and git config --default openpgp --get gpg.format respectively and can only be overridden
 ;; by setting the SIGNING_KEY ID to the correct ID.)
 ;SIGNING_NAME =
 ;SIGNING_EMAIL =
+;; SIGNING_FORMAT can be one of:
+;; - openpgp (default): use GPG to sign commits
+;; - ssh: use SSH to sign commits
+;SIGNING_FORMAT = openpgp
 ;;
 ;; Sets the default trust model for repositories. Options are: collaborator, committer, collaboratorcommitter
 ;DEFAULT_TRUST_MODEL = collaborator
@@ -1227,6 +1234,12 @@ LEVEL = Info
 ;; - commitssigned: require that all the commits in the head branch are signed.
 ;; - approved: only sign when merging an approved pr to a protected branch
 ;MERGES = pubkey, twofa, basesigned, commitssigned
+;;
+;; Determines which additional ssh keys are trusted for all signed commits regardless of the user
+;; This is useful for ssh signing key rotation.
+;; Multiple keys should be comma separated.
+;; E.g."ssh-<algorithm> <key>". or "ssh-<algorithm> <key1>, ssh-<algorithm> <key2>".
+;TRUSTED_SSH_KEYS =
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

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 +1,17 @@
+// 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 (
+	// KeyTypeOpenPGP is the key type for GPG keys, expected default of git cli
+	KeyTypeOpenPGP = "openpgp"
+	// KeyTypeSSH is the key type for SSH keys
+	KeyTypeSSH = "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

@@ -6,13 +6,22 @@ package git
 
 import (
 	"fmt"
+	"os"
 	"strings"
 
 	"code.gitea.io/gitea/modules/process"
 )
 
 // LoadPublicKeyContent will load the key from gpg
 func (gpgSettings *GPGSettings) LoadPublicKeyContent() error {
+	if gpgSettings.Format == KeyTypeSSH {
+		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", KeyTypeOpenPGP, "--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 +43,11 @@ 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.KeyID != "" || opts.AlwaysSign {
+		if opts.Key.Format != "" {
+			cmd.AddConfig("gpg.format", opts.Key.Format)
+		}
+		cmd.AddOptionFormat("-S%s", opts.Key.KeyID)
 	}
 
 	if opts.NoGPGSign {

modules/setting/repository.go

@@ -100,11 +100,13 @@ var (
 			SigningKey        string
 			SigningName       string
 			SigningEmail      string
+			SigningFormat     string
 			InitialCommit     []string
 			CRUDActions       []string `ini:"CRUD_ACTIONS"`
 			Merges            []string
 			Wiki              []string
 			DefaultTrustModel string
+			TrustedSSHKeys    []string `ini:"TRUSTED_SSH_KEYS"`
 		} `ini:"repository.signing"`
 	}{
 		DetectedCharsetsOrder: []string{
@@ -242,20 +244,24 @@ var (
 			SigningKey        string
 			SigningName       string
 			SigningEmail      string
+			SigningFormat     string
 			InitialCommit     []string
 			CRUDActions       []string `ini:"CRUD_ACTIONS"`
 			Merges            []string
 			Wiki              []string
 			DefaultTrustModel string
+			TrustedSSHKeys    []string `ini:"TRUSTED_SSH_KEYS"`
 		}{
 			SigningKey:        "default",
 			SigningName:       "",
 			SigningEmail:      "",
+			SigningFormat:     "openpgp", // git.KeyTypeOpenPGP
 			InitialCommit:     []string{"always"},
 			CRUDActions:       []string{"pubkey", "twofa", "parentsigned"},
 			Merges:            []string{"pubkey", "twofa", "basesigned", "commitssigned"},
 			Wiki:              []string{"never"},
 			DefaultTrustModel: "collaborator",
+			TrustedSSHKeys:    []string{},
 		},
 	}
 	RepoRootPath string

routers/api/v1/api.go

@@ -972,6 +972,7 @@ func Routes() *web.Router {
 		m.Group("", func() {
 			m.Get("/version", misc.Version)
 			m.Get("/signing-key.gpg", misc.SigningKey)
+			m.Get("/signing-key.pub", misc.SigningKeySSH)
 			m.Post("/markup", reqToken(), bind(api.MarkupOption{}), misc.Markup)
 			m.Post("/markdown", reqToken(), bind(api.MarkdownOption{}), misc.Markdown)
 			m.Post("/markdown/raw", reqToken(), misc.MarkdownRaw)
@@ -1428,6 +1429,7 @@ func Routes() *web.Router {
 					Get(repo.GetFileContentsGet).
 					Post(bind(api.GetFilesOptions{}), repo.GetFileContentsPost) // POST method requires "write" permission, so we also support "GET" method above
 				m.Get("/signing-key.gpg", misc.SigningKey)
+				m.Get("/signing-key.pub", misc.SigningKeySSH)
 				m.Group("/topics", func() {
 					m.Combo("").Get(repo.ListTopics).
 						Put(reqToken(), reqAdmin(), bind(api.RepoTopicOptions{}), repo.UpdateTopics)

routers/api/v1/misc/signing.go

@@ -4,8 +4,10 @@
 package misc
 
 import (
+	"errors"
 	"fmt"
 
+	"code.gitea.io/gitea/modules/git"
 	asymkey_service "code.gitea.io/gitea/services/asymkey"
 	"code.gitea.io/gitea/services/context"
 )
@@ -50,11 +52,70 @@ func SigningKey(ctx *context.APIContext) {
 		path = ctx.Repo.Repository.RepoPath()
 	}
 
-	content, err := asymkey_service.PublicSigningKey(ctx, path)
+	content, format, err := asymkey_service.PublicSigningKey(ctx, path)
 	if err != nil {
 		ctx.APIErrorInternal(err)
 		return
 	}
+	if format != git.KeyTypeOpenPGP {
+		ctx.APIErrorNotFound(errors.New("SSH keys are used for signing, not GPG"))
+		return
+	}
+	_, err = ctx.Write([]byte(content))
+	if err != nil {
+		ctx.APIErrorInternal(fmt.Errorf("Error writing key content %w", err))
+	}
+}
+
+// SigningKey returns the public key of the default signing key if it exists
+func SigningKeySSH(ctx *context.APIContext) {
+	// swagger:operation GET /signing-key.pub miscellaneous getSigningKeySSH
+	// ---
+	// summary: Get default signing-key.pub
+	// produces:
+	//     - text/plain
+	// responses:
+	//   "200":
+	//     description: "ssh public key"
+	//     schema:
+	//       type: string
+
+	// swagger:operation GET /repos/{owner}/{repo}/signing-key.pub repository repoSigningKeySSH
+	// ---
+	// summary: Get signing-key.pub for given repository
+	// produces:
+	//     - text/plain
+	// parameters:
+	// - name: owner
+	//   in: path
+	//   description: owner of the repo
+	//   type: string
+	//   required: true
+	// - name: repo
+	//   in: path
+	//   description: name of the repo
+	//   type: string
+	//   required: true
+	// responses:
+	//   "200":
+	//     description: "ssh public key"
+	//     schema:
+	//       type: string
+
+	path := ""
+	if ctx.Repo != nil && ctx.Repo.Repository != nil {
+		path = ctx.Repo.Repository.RepoPath()
+	}
+
+	content, format, err := asymkey_service.PublicSigningKey(ctx, path)
+	if err != nil {
+		ctx.APIErrorInternal(err)
+		return
+	}
+	if format != git.KeyTypeSSH {
+		ctx.APIErrorNotFound(errors.New("GPG keys are used for signing, not SSH"))
+		return
+	}
 	_, err = ctx.Write([]byte(content))
 	if err != nil {
 		ctx.APIErrorInternal(fmt.Errorf("Error writing key content %w", err))

routers/web/repo/setting/setting.go

@@ -62,7 +62,7 @@ func SettingsCtxData(ctx *context.Context) {
 	ctx.Data["CanConvertFork"] = ctx.Repo.Repository.IsFork && ctx.Doer.CanCreateRepoIn(ctx.Repo.Repository.Owner)
 
 	signing, _ := asymkey_service.SigningKey(ctx, ctx.Repo.Repository.RepoPath())
-	ctx.Data["SigningKeyAvailable"] = len(signing) > 0
+	ctx.Data["SigningKeyAvailable"] = len(signing.KeyID) > 0
 	ctx.Data["SigningSettings"] = setting.Repository.Signing
 	ctx.Data["IsRepoIndexerEnabled"] = setting.Indexer.RepoIndexerEnabled
 
@@ -105,7 +105,7 @@ func SettingsPost(ctx *context.Context) {
 	ctx.Data["MinimumMirrorInterval"] = setting.Mirror.MinInterval
 
 	signing, _ := asymkey_service.SigningKey(ctx, ctx.Repo.Repository.RepoPath())
-	ctx.Data["SigningKeyAvailable"] = len(signing) > 0
+	ctx.Data["SigningKeyAvailable"] = len(signing.KeyID) > 0
 	ctx.Data["SigningSettings"] = setting.Repository.Signing
 	ctx.Data["IsRepoIndexerEnabled"] = setting.Indexer.RepoIndexerEnabled
 

services/asymkey/commit.go

@@ -398,11 +398,85 @@ func ParseCommitWithSSHSignature(ctx context.Context, c *git.Commit, committer *
 			}
 		}
 	}
+	// Trust more than one key for every User
+	for _, k := range setting.Repository.Signing.TrustedSSHKeys {
+		fingerprint, _ := asymkey_model.CalcFingerprint(k)
+		commitVerification := verifySSHCommitVerification(c.Signature.Signature, c.Signature.Payload, &asymkey_model.PublicKey{
+			Verified:    true,
+			Content:     k,
+			Fingerprint: fingerprint,
+			HasUsed:     true,
+		}, committer, committer, c.Committer.Email)
+		if commitVerification != nil {
+			return commitVerification
+		}
+	}
+
+	defaultReason := asymkey_model.NoKeyFound
+
+	if setting.Repository.Signing.SigningFormat == git.KeyTypeSSH && setting.Repository.Signing.SigningKey != "" && setting.Repository.Signing.SigningKey != "default" && setting.Repository.Signing.SigningKey != "none" {
+		// OK we should try the default key
+		gpgSettings := git.GPGSettings{
+			Sign:   true,
+			KeyID:  setting.Repository.Signing.SigningKey,
+			Name:   setting.Repository.Signing.SigningName,
+			Email:  setting.Repository.Signing.SigningEmail,
+			Format: setting.Repository.Signing.SigningFormat,
+		}
+		if err := gpgSettings.LoadPublicKeyContent(); err != nil {
+			log.Error("Error getting default signing key: %s %v", gpgSettings.KeyID, err)
+		}
+		fingerprint, _ := asymkey_model.CalcFingerprint(gpgSettings.PublicKeyContent)
+		if commitVerification := verifySSHCommitVerification(c.Signature.Signature, c.Signature.Payload, &asymkey_model.PublicKey{
+			Verified:    true,
+			Content:     gpgSettings.PublicKeyContent,
+			Fingerprint: fingerprint,
+			HasUsed:     true,
+		}, committer, &user_model.User{
+			Name:  gpgSettings.Name,
+			Email: gpgSettings.Email,
+		}, gpgSettings.Email); commitVerification != nil {
+			if commitVerification.Reason == asymkey_model.BadSignature {
+				defaultReason = asymkey_model.BadSignature
+			} else {
+				return commitVerification
+			}
+		}
+	}
+
+	defaultGPGSettings, err := c.GetRepositoryDefaultPublicGPGKey(false)
+	if defaultGPGSettings.Format == git.KeyTypeSSH {
+		if err != nil {
+			log.Error("Error getting default public gpg key: %v", err)
+		} else if defaultGPGSettings == nil {
+			log.Warn("Unable to get defaultGPGSettings for unattached commit: %s", c.ID.String())
+		} else if defaultGPGSettings.Sign {
+			if err := defaultGPGSettings.LoadPublicKeyContent(); err != nil {
+				log.Error("Error getting default signing key: %s %v", defaultGPGSettings.KeyID, err)
+			}
+			fingerprint, _ := asymkey_model.CalcFingerprint(defaultGPGSettings.PublicKeyContent)
+			if commitVerification := verifySSHCommitVerification(c.Signature.Signature, c.Signature.Payload, &asymkey_model.PublicKey{
+				Verified:    true,
+				Content:     defaultGPGSettings.PublicKeyContent,
+				Fingerprint: fingerprint,
+				HasUsed:     true,
+			}, committer, &user_model.User{
+				Name:  defaultGPGSettings.Name,
+				Email: defaultGPGSettings.Email,
+			}, defaultGPGSettings.Email); commitVerification != nil {
+				if commitVerification.Reason == asymkey_model.BadSignature {
+					defaultReason = asymkey_model.BadSignature
+				} else {
+					return commitVerification
+				}
+			}
+		}
+	}
 
 	return &asymkey_model.CommitVerification{
 		CommittingUser: committer,
 		Verified:       false,
-		Reason:         asymkey_model.NoKeyFound,
+		Reason:         defaultReason,
 	}
 }