Subject: [PATCH v5 4/9] ssh signing: provide a textual representation of the signing key

From: "Fabian Stelzer via GitGitGadget" <[email-masked]>
Date: Tue, 27 Jul 2021 13:15:39 +0000

From: Fabian Stelzer <[email-masked]>

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 <[email-masked]>
---
 gpg-interface.c | 46 ++++++++++++++++++++++++++++++++++++++++++++++
 gpg-interface.h |  6 ++++++
 send-pack.c     |  8 ++++----
 3 files changed, 56 insertions(+), 4 deletions(-)

diff --git a/gpg-interface.c b/gpg-interface.c
index 3afacb48900..ec48a37b6cc 100644
--- a/gpg-interface.c
+++ b/gpg-interface.c
@@ -470,6 +470,41 @@ 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 (istarts_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 char *get_default_ssh_signing_key(void)
 {
@@ -490,6 +525,17 @@ static char *get_default_ssh_signing_key(void)
 	return "";
 }
 
+/* Returns a textual but unique representation ot the signing key */
+const char *get_signing_key_id(void)
+{
+	if (!strcmp(use_format->name, "ssh")) {
+		return get_ssh_key_fingerprint(get_signing_key());
+	} else {
+		/* 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 5a79e0e7110..50cca7e439b 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;
 }
-- 
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