gpg-interface: introduce new signature format "x509" using gpgsm

This commit allows git to create and check x509 type signatures using
gpgsm.

Signed-off-by: Henning Schild <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: henning-schild

Documentation/config.txt

@@ -1830,12 +1830,13 @@ gpg.program::
 
 gpg.format::
 	Specifies which key format to use when signing with `--gpg-sign`.
-	Default is "openpgp", that is also the only supported value.
+	Default is "openpgp" and another possible value is "x509".
 
 gpg.<format>.program::
 	Use this to customize the program used for the signing format you
 	chose. (see gpg.program) gpg.openpgp.program is a synonym for the
-	legacy gpg.program.
+	legacy gpg.program, while the default gpg.x509.program is "gpgsm".
+
 
 gui.commitMsgWidth::
 	Defines how wide the commit message window is in the

gpg-interface.c

@@ -18,12 +18,18 @@ static const char *openpgp_verify_args[] = { "--keyid-format=long", NULL };
 static const char *openpgp_sigs[] = {
 	"-----BEGIN PGP SIGNATURE-----",
 	"-----BEGIN PGP MESSAGE-----", NULL };
+static const char *x509_verify_args[] = { NULL };
+static const char *x509_sigs[] = { "-----BEGIN SIGNED MESSAGE-----", NULL };
 
 static struct gpg_format gpg_formats[] = {
 	{ .name = "openpgp", .program = "gpg",
 	  .extra_args_verify = openpgp_verify_args,
 	  .sigs = openpgp_sigs
 	},
+	{ .name = "x509", .program = "gpgsm",
+	  .extra_args_verify = x509_verify_args,
+	  .sigs = x509_sigs
+	},
 };
 static struct gpg_format *current_format = &gpg_formats[0];
 
@@ -185,6 +191,9 @@ int git_gpg_config(const char *var, const char *value, void *cb)
 	if (!strcmp(var, "gpg.program") || !strcmp(var, "gpg.openpgp.program"))
 		fmtname = "openpgp";
 
+	if (!strcmp(var, "gpg.x509.program"))
+		fmtname = "x509";
+
 	if (fmtname) {
 		fmt = get_format_by_name(fmtname);
 		return git_config_string(&fmt->program, var, value);