ssh signing: add ssh key format and signing code

Implements the actual sign_buffer_ssh operation and move some shared
cleanup code into a strbuf function

Set gpg.format = ssh and user.signingkey to either a ssh public key
string (like from an authorized_keys file), or a ssh key file.
If the key file or the config value itself contains only a public key
then the private key needs to be available via ssh-agent.

gpg.ssh.program can be set to an alternative location of ssh-keygen.
A somewhat recent openssh version (8.2p1+) of ssh-keygen is needed for
this feature. Since only ssh-keygen is needed it can this way be
installed seperately without upgrading your system openssh packages.

Signed-off-by: Fabian Stelzer <fs@gigacodes.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/gpg.txt

@@ -11,13 +11,13 @@ gpg.program::
 
 gpg.format::
 	Specifies which key format to use when signing with `--gpg-sign`.
-	Default is "openpgp" and another possible value is "x509".
+	Default is "openpgp". Other possible values are "x509", "ssh".
 
 gpg.<format>.program::
 	Use this to customize the program used for the signing format you
 	chose. (see `gpg.program` and `gpg.format`) `gpg.program` can still
 	be used as a legacy synonym for `gpg.openpgp.program`. The default
-	value for `gpg.x509.program` is "gpgsm".
+	value for `gpg.x509.program` is "gpgsm" and `gpg.ssh.program` is "ssh-keygen".
 
 gpg.minTrustLevel::
 	Specifies a minimum trust level for signature verification.  If

Documentation/config/user.txt

@@ -36,3 +36,8 @@ user.signingKey::
 	commit, you can override the default selection with this variable.
 	This option is passed unchanged to gpg's --local-user parameter,
 	so you may specify a key using any method that gpg supports.
+	If gpg.format is set to "ssh" this can contain the literal ssh public
+	key (e.g.: "ssh-rsa XXXXXX identifier") or a file which contains it and
+	corresponds to the private key used for signing. The private key
+	needs to be available via ssh-agent. Alternatively it can be set to
+	a file containing a private key directly.

gpg-interface.c

@@ -41,12 +41,20 @@ static const char *x509_sigs[] = {
 	NULL
 };
 
+static const char *ssh_verify_args[] = { NULL };
+static const char *ssh_sigs[] = {
+	"-----BEGIN SSH SIGNATURE-----",
+	NULL
+};
+
 static int verify_gpg_signed_buffer(struct signature_check *sigc,
 				    struct gpg_format *fmt, const char *payload,
 				    size_t payload_size, const char *signature,
 				    size_t signature_size);
 static int sign_buffer_gpg(struct strbuf *buffer, struct strbuf *signature,
 			   const char *signing_key);
+static int sign_buffer_ssh(struct strbuf *buffer, struct strbuf *signature,
+			   const char *signing_key);
 
 static struct gpg_format gpg_format[] = {
 	{
@@ -65,6 +73,14 @@ static struct gpg_format gpg_format[] = {
 		.verify_signed_buffer = verify_gpg_signed_buffer,
 		.sign_buffer = sign_buffer_gpg,
 	},
+	{
+		.name = "ssh",
+		.program = "ssh-keygen",
+		.verify_args = ssh_verify_args,
+		.sigs = ssh_sigs,
+		.verify_signed_buffer = NULL, /* TODO */
+		.sign_buffer = sign_buffer_ssh
+	},
 };
 
 static struct gpg_format *use_format = &gpg_format[0];
@@ -443,6 +459,9 @@ int git_gpg_config(const char *var, const char *value, void *cb)
 	if (!strcmp(var, "gpg.x509.program"))
 		fmtname = "x509";
 
+	if (!strcmp(var, "gpg.ssh.program"))
+		fmtname = "ssh";
+
 	if (fmtname) {
 		fmt = get_format_by_name(fmtname);
 		return git_config_string(&fmt->program, var, value);
@@ -463,12 +482,30 @@ int sign_buffer(struct strbuf *buffer, struct strbuf *signature, const char *sig
 	return use_format->sign_buffer(buffer, signature, signing_key);
 }
 
+/*
+ * Strip CR from the line endings, in case we are on Windows.
+ * NEEDSWORK: make it trim only CRs before LFs and rename
+ */
+static void remove_cr_after(struct strbuf *buffer, size_t offset)
+{
+	size_t i, j;
+
+	for (i = j = offset; i < buffer->len; i++) {
+		if (buffer->buf[i] != '\r') {
+			if (i != j)
+				buffer->buf[j] = buffer->buf[i];
+			j++;
+		}
+	}
+	strbuf_setlen(buffer, j);
+}
+
 static int sign_buffer_gpg(struct strbuf *buffer, struct strbuf *signature,
 			  const char *signing_key)
 {
 	struct child_process gpg = CHILD_PROCESS_INIT;
 	int ret;
-	size_t i, j, bottom;
+	size_t bottom;
 	struct strbuf gpg_status = STRBUF_INIT;
 
 	strvec_pushl(&gpg.args,
@@ -494,13 +531,98 @@ static int sign_buffer_gpg(struct strbuf *buffer, struct strbuf *signature,
 		return error(_("gpg failed to sign the data"));
 
 	/* Strip CR from the line endings, in case we are on Windows. */
-	for (i = j = bottom; i < signature->len; i++)
-		if (signature->buf[i] != '\r') {
-			if (i != j)
-				signature->buf[j] = signature->buf[i];
-			j++;
-		}
-	strbuf_setlen(signature, j);
+	remove_cr_after(signature, bottom);
 
 	return 0;
 }
+
+static int sign_buffer_ssh(struct strbuf *buffer, struct strbuf *signature,
+			   const char *signing_key)
+{
+	struct child_process signer = CHILD_PROCESS_INIT;
+	int ret = -1;
+	size_t bottom, keylen;
+	struct strbuf signer_stderr = STRBUF_INIT;
+	struct tempfile *key_file = NULL, *buffer_file = NULL;
+	char *ssh_signing_key_file = NULL;
+	struct strbuf ssh_signature_filename = STRBUF_INIT;
+
+	if (!signing_key || signing_key[0] == '\0')
+		return error(
+			_("user.signingkey needs to be set for ssh signing"));
+
+	if (starts_with(signing_key, "ssh-")) {
+		/* A literal ssh key */
+		key_file = mks_tempfile_t(".git_signing_key_tmpXXXXXX");
+		if (!key_file)
+			return error_errno(
+				_("could not create temporary file"));
+		keylen = strlen(signing_key);
+		if (write_in_full(key_file->fd, signing_key, keylen) < 0 ||
+		    close_tempfile_gently(key_file) < 0) {
+			error_errno(_("failed writing ssh signing key to '%s'"),
+				    key_file->filename.buf);
+			goto out;
+		}
+		ssh_signing_key_file = strbuf_detach(&key_file->filename, NULL);
+	} else {
+		/* We assume a file */
+		ssh_signing_key_file = expand_user_path(signing_key, 1);
+	}
+
+	buffer_file = mks_tempfile_t(".git_signing_buffer_tmpXXXXXX");
+	if (!buffer_file) {
+		error_errno(_("could not create temporary file"));
+		goto out;
+	}
+
+	if (write_in_full(buffer_file->fd, buffer->buf, buffer->len) < 0 ||
+	    close_tempfile_gently(buffer_file) < 0) {
+		error_errno(_("failed writing ssh signing key buffer to '%s'"),
+			    buffer_file->filename.buf);
+		goto out;
+	}
+
+	strvec_pushl(&signer.args, use_format->program,
+		     "-Y", "sign",
+		     "-n", "git",
+		     "-f", ssh_signing_key_file,
+		     buffer_file->filename.buf,
+		     NULL);
+
+	sigchain_push(SIGPIPE, SIG_IGN);
+	ret = pipe_command(&signer, NULL, 0, NULL, 0, &signer_stderr, 0);
+	sigchain_pop(SIGPIPE);
+
+	if (ret) {
+		if (strstr(signer_stderr.buf, "usage:"))
+			error(_("ssh-keygen -Y sign is needed for ssh signing (available in openssh version 8.2p1+)"));
+
+		error("%s", signer_stderr.buf);
+		goto out;
+	}
+
+	bottom = signature->len;
+
+	strbuf_addbuf(&ssh_signature_filename, &buffer_file->filename);
+	strbuf_addstr(&ssh_signature_filename, ".sig");
+	if (strbuf_read_file(signature, ssh_signature_filename.buf, 0) < 0) {
+		error_errno(
+			_("failed reading ssh signing data buffer from '%s'"),
+			ssh_signature_filename.buf);
+	}
+	unlink_or_warn(ssh_signature_filename.buf);
+
+	/* Strip CR from the line endings, in case we are on Windows. */
+	remove_cr_after(signature, bottom);
+
+out:
+	if (key_file)
+		delete_tempfile(&key_file);
+	if (buffer_file)
+		delete_tempfile(&buffer_file);
+	strbuf_release(&signer_stderr);
+	strbuf_release(&ssh_signature_filename);
+	FREE_AND_NULL(ssh_signing_key_file);
+	return ret;
+}