[PATCH v8 5/9] ssh signing: provide a textual signing_key_id

From: "Fabian Stelzer via GitGitGadget" <gitgitgadget@gmail.com>
To: git@vger.kernel.org
Cc: "Han-Wen Nienhuys" <hanwen@google.com>,
	"Fabian Stelzer" <fs@gigacodes.de>,
	"brian m. carlson" <sandals@crustytoothpaste.net>,
	"Randall S. Becker" <rsbecker@nexbridge.com>,
	"Bagas Sanjaya" <bagasdotme@gmail.com>,
	"Hans Jerry Illikainen" <hji@dyntopia.com>,
	"Ævar Arnfjörð Bjarmason" <avarab@gmail.com>,
	"Felipe Contreras" <felipe.contreras@gmail.com>,
	"Eric Sunshine" <sunshine@sunshineco.com>,
	"Gwyneth Morgan" <gwymor@tilde.club>,
	"Jonathan Tan" <jonathantanmy@google.com>,
	"Josh Steadmon" <steadmon@google.com>,
	"Fabian Stelzer" <fabian@stelzer.cloud>,
	"Fabian Stelzer" <fs@gigacodes.de>
Subject: [PATCH v8 5/9] ssh signing: provide a textual signing_key_id
Date: Fri, 10 Sep 2021 20:07:38 +0000	[thread overview]
Message-ID: <0864ed04670c271c3e686cc5301bebd45eae2242.1631304462.git.gitgitgadget@gmail.com> (raw)
In-Reply-To: <pull.1041.v8.git.git.1631304462.gitgitgadget@gmail.com>

From: Fabian Stelzer <fs@gigacodes.de>

For ssh the user.signingkey can be a filename/path or even a literal ssh pubkey.
In push certs and textual output we prefer the ssh fingerprint instead.

Signed-off-by: Fabian Stelzer <fs@gigacodes.de>
---
 gpg-interface.c | 56 +++++++++++++++++++++++++++++++++++++++++++++++++
 gpg-interface.h |  6 ++++++
 send-pack.c     |  8 +++----
 3 files changed, 66 insertions(+), 4 deletions(-)

diff --git a/gpg-interface.c b/gpg-interface.c
index 3a0cca1b1d2..0f1c6a02e53 100644
--- a/gpg-interface.c
+++ b/gpg-interface.c
@@ -24,6 +24,7 @@ struct gpg_format {
 	int (*sign_buffer)(struct strbuf *buffer, struct strbuf *signature,
 			   const char *signing_key);
 	const char *(*get_default_key)(void);
+	const char *(*get_key_id)(void);
 };
 
 static const char *openpgp_verify_args[] = {
@@ -61,6 +62,8 @@ static int sign_buffer_ssh(struct strbuf *buffer, struct strbuf *signature,
 
 static const char *get_default_ssh_signing_key(void);
 
+static const char *get_ssh_key_id(void);
+
 static struct gpg_format gpg_format[] = {
 	{
 		.name = "openpgp",
@@ -70,6 +73,7 @@ static struct gpg_format gpg_format[] = {
 		.verify_signed_buffer = verify_gpg_signed_buffer,
 		.sign_buffer = sign_buffer_gpg,
 		.get_default_key = NULL,
+		.get_key_id = NULL,
 	},
 	{
 		.name = "x509",
@@ -79,6 +83,7 @@ static struct gpg_format gpg_format[] = {
 		.verify_signed_buffer = verify_gpg_signed_buffer,
 		.sign_buffer = sign_buffer_gpg,
 		.get_default_key = NULL,
+		.get_key_id = NULL,
 	},
 	{
 		.name = "ssh",
@@ -88,6 +93,7 @@ static struct gpg_format gpg_format[] = {
 		.verify_signed_buffer = NULL, /* TODO */
 		.sign_buffer = sign_buffer_ssh,
 		.get_default_key = get_default_ssh_signing_key,
+		.get_key_id = get_ssh_key_id,
 	},
 };
 
@@ -484,6 +490,41 @@ int git_gpg_config(const char *var, const char *value, void *cb)
 	return 0;
 }
 
+static char *get_ssh_key_fingerprint(const char *signing_key)
+{
+	struct child_process ssh_keygen = CHILD_PROCESS_INIT;
+	int ret = -1;
+	struct strbuf fingerprint_stdout = STRBUF_INIT;
+	struct strbuf **fingerprint;
+
+	/*
+	 * With SSH Signing this can contain a filename or a public key
+	 * For textual representation we usually want a fingerprint
+	 */
+	if (starts_with(signing_key, "ssh-")) {
+		strvec_pushl(&ssh_keygen.args, "ssh-keygen", "-lf", "-", NULL);
+		ret = pipe_command(&ssh_keygen, signing_key,
+				   strlen(signing_key), &fingerprint_stdout, 0,
+				   NULL, 0);
+	} else {
+		strvec_pushl(&ssh_keygen.args, "ssh-keygen", "-lf",
+			     configured_signing_key, NULL);
+		ret = pipe_command(&ssh_keygen, NULL, 0, &fingerprint_stdout, 0,
+				   NULL, 0);
+	}
+
+	if (!!ret)
+		die_errno(_("failed to get the ssh fingerprint for key '%s'"),
+			  signing_key);
+
+	fingerprint = strbuf_split_max(&fingerprint_stdout, ' ', 3);
+	if (!fingerprint[1])
+		die_errno(_("failed to get the ssh fingerprint for key '%s'"),
+			  signing_key);
+
+	return strbuf_detach(fingerprint[1], NULL);
+}
+
 /* Returns the first public key from an ssh-agent to use for signing */
 static const char *get_default_ssh_signing_key(void)
 {
@@ -532,6 +573,21 @@ static const char *get_default_ssh_signing_key(void)
 	return default_key;
 }
 
+static const char *get_ssh_key_id(void) {
+	return get_ssh_key_fingerprint(get_signing_key());
+}
+
+/* Returns a textual but unique representation of the signing key */
+const char *get_signing_key_id(void)
+{
+	if (use_format->get_key_id) {
+		return use_format->get_key_id();
+	}
+
+	/* GPG/GPGSM only store a key id on this variable */
+	return get_signing_key();
+}
+
 const char *get_signing_key(void)
 {
 	if (configured_signing_key)
diff --git a/gpg-interface.h b/gpg-interface.h
index feac4decf8b..beefacbb1e9 100644
--- a/gpg-interface.h
+++ b/gpg-interface.h
@@ -64,6 +64,12 @@ int sign_buffer(struct strbuf *buffer, struct strbuf *signature,
 int git_gpg_config(const char *, const char *, void *);
 void set_signing_key(const char *);
 const char *get_signing_key(void);
+
+/*
+ * Returns a textual unique representation of the signing key in use
+ * Either a GPG KeyID or a SSH Key Fingerprint
+ */
+const char *get_signing_key_id(void);
 int check_signature(const char *payload, size_t plen,
 		    const char *signature, size_t slen,
 		    struct signature_check *sigc);
diff --git a/send-pack.c b/send-pack.c
index b3a495b7b19..bc0fcdbb000 100644
--- a/send-pack.c
+++ b/send-pack.c
@@ -341,13 +341,13 @@ static int generate_push_cert(struct strbuf *req_buf,
 {
 	const struct ref *ref;
 	struct string_list_item *item;
-	char *signing_key = xstrdup(get_signing_key());
+	char *signing_key_id = xstrdup(get_signing_key_id());
 	const char *cp, *np;
 	struct strbuf cert = STRBUF_INIT;
 	int update_seen = 0;
 
 	strbuf_addstr(&cert, "certificate version 0.1\n");
-	strbuf_addf(&cert, "pusher %s ", signing_key);
+	strbuf_addf(&cert, "pusher %s ", signing_key_id);
 	datestamp(&cert);
 	strbuf_addch(&cert, '\n');
 	if (args->url && *args->url) {
@@ -374,7 +374,7 @@ static int generate_push_cert(struct strbuf *req_buf,
 	if (!update_seen)
 		goto free_return;
 
-	if (sign_buffer(&cert, &cert, signing_key))
+	if (sign_buffer(&cert, &cert, get_signing_key()))
 		die(_("failed to sign the push certificate"));
 
 	packet_buf_write(req_buf, "push-cert%c%s", 0, cap_string);
@@ -386,7 +386,7 @@ static int generate_push_cert(struct strbuf *req_buf,
 	packet_buf_write(req_buf, "push-cert-end\n");
 
 free_return:
-	free(signing_key);
+	free(signing_key_id);
 	strbuf_release(&cert);
 	return update_seen;
 }
-- 
gitgitgadget

