15 files changed, 1341 insertions, 66 deletions

diff --git a/Documentation/config/gpg.txt b/Documentation/config/gpg.txt
index d94025cb36..51a756b2f1 100644
--- a/Documentation/config/gpg.txt
+++ b/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
@@ -33,3 +33,44 @@ gpg.minTrustLevel::
 * `marginal`
 * `fully`
 * `ultimate`
+
+gpg.ssh.defaultKeyCommand:
+	This command that will be run when user.signingkey is not set and a ssh
+	signature is requested. On successful exit a valid ssh public key is
+	expected in the	first line of its output. To automatically use the first
+	available key from your ssh-agent set this to "ssh-add -L".
+
+gpg.ssh.allowedSignersFile::
+	A file containing ssh public keys which you are willing to trust.
+	The file consists of one or more lines of principals followed by an ssh
+	public key.
+	e.g.: user1@example.com,user2@example.com ssh-rsa AAAAX1...
+	See ssh-keygen(1) "ALLOWED SIGNERS" for details.
+	The principal is only used to identify the key and is available when
+	verifying a signature.
++
+SSH has no concept of trust levels like gpg does. To be able to differentiate
+between valid signatures and trusted signatures the trust level of a signature
+verification is set to `fully` when the public key is present in the allowedSignersFile.
+Therefore to only mark fully trusted keys as verified set gpg.minTrustLevel to `fully`.
+Otherwise valid but untrusted signatures will still verify but show no principal
+name of the signer.
++
+This file can be set to a location outside of the repository and every developer
+maintains their own trust store. A central repository server could generate this
+file automatically from ssh keys with push access to verify the code against.
+In a corporate setting this file is probably generated at a global location
+from automation that already handles developer ssh keys.
++
+A repository that only allows signed commits can store the file
+in the repository itself using a path relative to the top-level of the working tree.
+This way only committers with an already valid key can add or change keys in the keyring.
++
+Using a SSH CA key with the cert-authority option
+(see ssh-keygen(1) "CERTIFICATES") is also valid.
+
+gpg.ssh.revocationFile::
+	Either a SSH KRL or a list of revoked public keys (without the principal prefix).
+	See ssh-keygen(1) for details.
+	If a public key is found in this file then it will always be treated
+	as having trust level "never" and signatures will show as invalid.
diff --git a/Documentation/config/user.txt b/Documentation/config/user.txt
index 59aec7c3ae..ad78dce9ec 100644
--- a/Documentation/config/user.txt
+++ b/Documentation/config/user.txt
@@ -36,3 +36,10 @@ 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. If not set git will call
+	gpg.ssh.defaultKeyCommand (e.g.: "ssh-add -L") and try to use the first
+	key available.
diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index 48960a9575..46ddeea0cf 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -134,6 +134,10 @@ static int receive_pack_config(const char *var, const char *value, void *cb)
 	if (status)
 		return status;
 
+	status = git_gpg_config(var, value, NULL);
+	if (status)
+		return status;
+
 	if (strcmp(var, "receive.denydeletes") == 0) {
 		deny_deletes = git_config_bool(var, value);
 		return 0;
diff --git a/fmt-merge-msg.c b/fmt-merge-msg.c
index b969dc6ebb..2901c5e4f8 100644
--- a/fmt-merge-msg.c
+++ b/fmt-merge-msg.c
@@ -528,11 +528,11 @@ static void fmt_merge_msg_sigs(struct strbuf *out)
 			buf = payload.buf;
 			len = payload.len;
 			if (check_signature(payload.buf, payload.len, sig.buf,
-					 sig.len, &sigc) &&
-				!sigc.gpg_output)
+					    sig.len, &sigc) &&
+			    !sigc.output)
 				strbuf_addstr(&sig, "gpg verification failed.\n");
 			else
-				strbuf_addstr(&sig, sigc.gpg_output);
+				strbuf_addstr(&sig, sigc.output);
 		}
 		signature_check_clear(&sigc);
 
diff --git a/gpg-interface.c b/gpg-interface.c
index 127aecfc2b..433482307c 100644
--- a/gpg-interface.c
+++ b/gpg-interface.c
@@ -3,11 +3,14 @@
 #include "config.h"
 #include "run-command.h"
 #include "strbuf.h"
+#include "dir.h"
 #include "gpg-interface.h"
 #include "sigchain.h"
 #include "tempfile.h"
+#include "alias.h"
 
 static char *configured_signing_key;
+static const char *ssh_default_key_command, *ssh_allowed_signers, *ssh_revocation_file;
 static enum signature_trust_level configured_min_trust_level = TRUST_UNDEFINED;
 
 struct gpg_format {
@@ -15,6 +18,14 @@ struct gpg_format {
 	const char *program;
 	const char **verify_args;
 	const char **sigs;
+	int (*verify_signed_buffer)(struct signature_check *sigc,
+				    struct gpg_format *fmt, const char *payload,
+				    size_t payload_size, const char *signature,
+				    size_t signature_size);
+	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[] = {
@@ -35,14 +46,59 @@ 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 verify_ssh_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 const char *get_default_ssh_signing_key(void);
+
+static const char *get_ssh_key_id(void);
+
 static struct gpg_format gpg_format[] = {
-	{ .name = "openpgp", .program = "gpg",
-	  .verify_args = openpgp_verify_args,
-	  .sigs = openpgp_sigs
+	{
+		.name = "openpgp",
+		.program = "gpg",
+		.verify_args = openpgp_verify_args,
+		.sigs = openpgp_sigs,
+		.verify_signed_buffer = verify_gpg_signed_buffer,
+		.sign_buffer = sign_buffer_gpg,
+		.get_default_key = NULL,
+		.get_key_id = NULL,
+	},
+	{
+		.name = "x509",
+		.program = "gpgsm",
+		.verify_args = x509_verify_args,
+		.sigs = x509_sigs,
+		.verify_signed_buffer = verify_gpg_signed_buffer,
+		.sign_buffer = sign_buffer_gpg,
+		.get_default_key = NULL,
+		.get_key_id = NULL,
 	},
-	{ .name = "x509", .program = "gpgsm",
-	  .verify_args = x509_verify_args,
-	  .sigs = x509_sigs
+	{
+		.name = "ssh",
+		.program = "ssh-keygen",
+		.verify_args = ssh_verify_args,
+		.sigs = ssh_sigs,
+		.verify_signed_buffer = verify_ssh_signed_buffer,
+		.sign_buffer = sign_buffer_ssh,
+		.get_default_key = get_default_ssh_signing_key,
+		.get_key_id = get_ssh_key_id,
 	},
 };
 
@@ -72,7 +128,7 @@ static struct gpg_format *get_format_by_sig(const char *sig)
 void signature_check_clear(struct signature_check *sigc)
 {
 	FREE_AND_NULL(sigc->payload);
-	FREE_AND_NULL(sigc->gpg_output);
+	FREE_AND_NULL(sigc->output);
 	FREE_AND_NULL(sigc->gpg_status);
 	FREE_AND_NULL(sigc->signer);
 	FREE_AND_NULL(sigc->key);
@@ -257,16 +313,16 @@ error:
 	FREE_AND_NULL(sigc->key);
 }
 
-static int verify_signed_buffer(const char *payload, size_t payload_size,
-				const char *signature, size_t signature_size,
-				struct strbuf *gpg_output,
-				struct strbuf *gpg_status)
+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)
 {
 	struct child_process gpg = CHILD_PROCESS_INIT;
-	struct gpg_format *fmt;
 	struct tempfile *temp;
 	int ret;
-	struct strbuf buf = STRBUF_INIT;
+	struct strbuf gpg_stdout = STRBUF_INIT;
+	struct strbuf gpg_stderr = STRBUF_INIT;
 
 	temp = mks_tempfile_t(".git_vtag_tmpXXXXXX");
 	if (!temp)
@@ -279,10 +335,6 @@ static int verify_signed_buffer(const char *payload, size_t payload_size,
 		return -1;
 	}
 
-	fmt = get_format_by_sig(signature);
-	if (!fmt)
-		BUG("bad signature '%s'", signature);
-
 	strvec_push(&gpg.args, fmt->program);
 	strvec_pushv(&gpg.args, fmt->verify_args);
 	strvec_pushl(&gpg.args,
@@ -290,18 +342,216 @@ static int verify_signed_buffer(const char *payload, size_t payload_size,
 		     "--verify", temp->filename.buf, "-",
 		     NULL);
 
-	if (!gpg_status)
-		gpg_status = &buf;
-
 	sigchain_push(SIGPIPE, SIG_IGN);
-	ret = pipe_command(&gpg, payload, payload_size,
-			   gpg_status, 0, gpg_output, 0);
+	ret = pipe_command(&gpg, payload, payload_size, &gpg_stdout, 0,
+			   &gpg_stderr, 0);
 	sigchain_pop(SIGPIPE);
 
 	delete_tempfile(&temp);
 
-	ret |= !strstr(gpg_status->buf, "\n[GNUPG:] GOODSIG ");
-	strbuf_release(&buf); /* no matter it was used or not */
+	ret |= !strstr(gpg_stdout.buf, "\n[GNUPG:] GOODSIG ");
+	sigc->payload = xmemdupz(payload, payload_size);
+	sigc->output = strbuf_detach(&gpg_stderr, NULL);
+	sigc->gpg_status = strbuf_detach(&gpg_stdout, NULL);
+
+	parse_gpg_output(sigc);
+
+	strbuf_release(&gpg_stdout);
+	strbuf_release(&gpg_stderr);
+
+	return ret;
+}
+
+static void parse_ssh_output(struct signature_check *sigc)
+{
+	const char *line, *principal, *search;
+	char *key = NULL;
+
+	/*
+	 * ssh-keygen output should be:
+	 * Good "git" signature for PRINCIPAL with RSA key SHA256:FINGERPRINT
+	 *
+	 * or for valid but unknown keys:
+	 * Good "git" signature with RSA key SHA256:FINGERPRINT
+	 *
+	 * Note that "PRINCIPAL" can contain whitespace, "RSA" and
+	 * "SHA256" part could be a different token that names of
+	 * the algorithms used, and "FINGERPRINT" is a hexadecimal
+	 * string.  By finding the last occurence of " with ", we can
+	 * reliably parse out the PRINCIPAL.
+	 */
+	sigc->result = 'B';
+	sigc->trust_level = TRUST_NEVER;
+
+	line = xmemdupz(sigc->output, strcspn(sigc->output, "\n"));
+
+	if (skip_prefix(line, "Good \"git\" signature for ", &line)) {
+		/* Valid signature and known principal */
+		sigc->result = 'G';
+		sigc->trust_level = TRUST_FULLY;
+
+		/* Search for the last "with" to get the full principal */
+		principal = line;
+		do {
+			search = strstr(line, " with ");
+			if (search)
+				line = search + 1;
+		} while (search != NULL);
+		sigc->signer = xmemdupz(principal, line - principal - 1);
+	} else if (skip_prefix(line, "Good \"git\" signature with ", &line)) {
+		/* Valid signature, but key unknown */
+		sigc->result = 'G';
+		sigc->trust_level = TRUST_UNDEFINED;
+	} else {
+		return;
+	}
+
+	key = strstr(line, "key");
+	if (key) {
+		sigc->fingerprint = xstrdup(strstr(line, "key") + 4);
+		sigc->key = xstrdup(sigc->fingerprint);
+	} else {
+		/*
+		 * Output did not match what we expected
+		 * Treat the signature as bad
+		 */
+		sigc->result = 'B';
+	}
+}
+
+static int verify_ssh_signed_buffer(struct signature_check *sigc,
+				    struct gpg_format *fmt, const char *payload,
+				    size_t payload_size, const char *signature,
+				    size_t signature_size)
+{
+	struct child_process ssh_keygen = CHILD_PROCESS_INIT;
+	struct tempfile *buffer_file;
+	int ret = -1;
+	const char *line;
+	size_t trust_size;
+	char *principal;
+	struct strbuf ssh_principals_out = STRBUF_INIT;
+	struct strbuf ssh_principals_err = STRBUF_INIT;
+	struct strbuf ssh_keygen_out = STRBUF_INIT;
+	struct strbuf ssh_keygen_err = STRBUF_INIT;
+
+	if (!ssh_allowed_signers) {
+		error(_("gpg.ssh.allowedSignersFile needs to be configured and exist for ssh signature verification"));
+		return -1;
+	}
+
+	buffer_file = mks_tempfile_t(".git_vtag_tmpXXXXXX");
+	if (!buffer_file)
+		return error_errno(_("could not create temporary file"));
+	if (write_in_full(buffer_file->fd, signature, signature_size) < 0 ||
+	    close_tempfile_gently(buffer_file) < 0) {
+		error_errno(_("failed writing detached signature to '%s'"),
+			    buffer_file->filename.buf);
+		delete_tempfile(&buffer_file);
+		return -1;
+	}
+
+	/* Find the principal from the signers */
+	strvec_pushl(&ssh_keygen.args, fmt->program,
+		     "-Y", "find-principals",
+		     "-f", ssh_allowed_signers,
+		     "-s", buffer_file->filename.buf,
+		     NULL);
+	ret = pipe_command(&ssh_keygen, NULL, 0, &ssh_principals_out, 0,
+			   &ssh_principals_err, 0);
+	if (ret && strstr(ssh_principals_err.buf, "usage:")) {
+		error(_("ssh-keygen -Y find-principals/verify is needed for ssh signature verification (available in openssh version 8.2p1+)"));
+		goto out;
+	}
+	if (ret || !ssh_principals_out.len) {
+		/*
+		 * We did not find a matching principal in the allowedSigners
+		 * Check without validation
+		 */
+		child_process_init(&ssh_keygen);
+		strvec_pushl(&ssh_keygen.args, fmt->program,
+			     "-Y", "check-novalidate",
+			     "-n", "git",
+			     "-s", buffer_file->filename.buf,
+			     NULL);
+		pipe_command(&ssh_keygen, payload, payload_size,
+				   &ssh_keygen_out, 0, &ssh_keygen_err, 0);
+
+		/*
+		 * Fail on unknown keys
+		 * we still call check-novalidate to display the signature info
+		 */
+		ret = -1;
+	} else {
+		/* Check every principal we found (one per line) */
+		for (line = ssh_principals_out.buf; *line;
+		     line = strchrnul(line + 1, '\n')) {
+			while (*line == '\n')
+				line++;
+			if (!*line)
+				break;
+
+			trust_size = strcspn(line, "\n");
+			principal = xmemdupz(line, trust_size);
+
+			child_process_init(&ssh_keygen);
+			strbuf_release(&ssh_keygen_out);
+			strbuf_release(&ssh_keygen_err);
+			strvec_push(&ssh_keygen.args, fmt->program);
+			/*
+			 * We found principals
+			 * Try with each until we find a match
+			 */
+			strvec_pushl(&ssh_keygen.args, "-Y", "verify",
+				     "-n", "git",
+				     "-f", ssh_allowed_signers,
+				     "-I", principal,
+				     "-s", buffer_file->filename.buf,
+				     NULL);
+
+			if (ssh_revocation_file) {
+				if (file_exists(ssh_revocation_file)) {
+					strvec_pushl(&ssh_keygen.args, "-r",
+						     ssh_revocation_file, NULL);
+				} else {
+					warning(_("ssh signing revocation file configured but not found: %s"),
+						ssh_revocation_file);
+				}
+			}
+
+			sigchain_push(SIGPIPE, SIG_IGN);
+			ret = pipe_command(&ssh_keygen, payload, payload_size,
+					   &ssh_keygen_out, 0, &ssh_keygen_err, 0);
+			sigchain_pop(SIGPIPE);
+
+			FREE_AND_NULL(principal);
+
+			if (!ret)
+				ret = !starts_with(ssh_keygen_out.buf, "Good");
+
+			if (!ret)
+				break;
+		}
+	}
+
+	sigc->payload = xmemdupz(payload, payload_size);
+	strbuf_stripspace(&ssh_keygen_out, 0);
+	strbuf_stripspace(&ssh_keygen_err, 0);
+	/* Add stderr outputs to show the user actual ssh-keygen errors */
+	strbuf_add(&ssh_keygen_out, ssh_principals_err.buf, ssh_principals_err.len);
+	strbuf_add(&ssh_keygen_out, ssh_keygen_err.buf, ssh_keygen_err.len);
+	sigc->output = strbuf_detach(&ssh_keygen_out, NULL);
+	sigc->gpg_status = xstrdup(sigc->output);
+
+	parse_ssh_output(sigc);
+
+out:
+	if (buffer_file)
+		delete_tempfile(&buffer_file);
+	strbuf_release(&ssh_principals_out);
+	strbuf_release(&ssh_principals_err);
+	strbuf_release(&ssh_keygen_out);
+	strbuf_release(&ssh_keygen_err);
 
 	return ret;
 }
@@ -309,35 +559,32 @@ static int verify_signed_buffer(const char *payload, size_t payload_size,
 int check_signature(const char *payload, size_t plen, const char *signature,
 	size_t slen, struct signature_check *sigc)
 {
-	struct strbuf gpg_output = STRBUF_INIT;
-	struct strbuf gpg_status = STRBUF_INIT;
+	struct gpg_format *fmt;
 	int status;
 
 	sigc->result = 'N';
 	sigc->trust_level = -1;
 
-	status = verify_signed_buffer(payload, plen, signature, slen,
-				      &gpg_output, &gpg_status);
-	if (status && !gpg_output.len)
-		goto out;
-	sigc->payload = xmemdupz(payload, plen);
-	sigc->gpg_output = strbuf_detach(&gpg_output, NULL);
-	sigc->gpg_status = strbuf_detach(&gpg_status, NULL);
-	parse_gpg_output(sigc);
+	fmt = get_format_by_sig(signature);
+	if (!fmt)
+		die(_("bad/incompatible signature '%s'"), signature);
+
+	status = fmt->verify_signed_buffer(sigc, fmt, payload, plen, signature,
+					   slen);
+
+	if (status && !sigc->output)
+		return !!status;
+
 	status |= sigc->result != 'G';
 	status |= sigc->trust_level < configured_min_trust_level;
 
- out:
-	strbuf_release(&gpg_status);
-	strbuf_release(&gpg_output);
-
 	return !!status;
 }
 
 void print_signature_buffer(const struct signature_check *sigc, unsigned flags)
 {
-	const char *output = flags & GPG_VERIFY_RAW ?
-		sigc->gpg_status : sigc->gpg_output;
+	const char *output = flags & GPG_VERIFY_RAW ? sigc->gpg_status :
+							    sigc->output;
 
 	if (flags & GPG_VERIFY_VERBOSE && sigc->payload)
 		fputs(sigc->payload, stdout);
@@ -419,12 +666,33 @@ int git_gpg_config(const char *var, const char *value, void *cb)
 		return 0;
 	}
 
+	if (!strcmp(var, "gpg.ssh.defaultkeycommand")) {
+		if (!value)
+			return config_error_nonbool(var);
+		return git_config_string(&ssh_default_key_command, var, value);
+	}
+
+	if (!strcmp(var, "gpg.ssh.allowedsignersfile")) {
+		if (!value)
+			return config_error_nonbool(var);
+		return git_config_pathname(&ssh_allowed_signers, var, value);
+	}
+
+	if (!strcmp(var, "gpg.ssh.revocationfile")) {
+		if (!value)
+			return config_error_nonbool(var);
+		return git_config_pathname(&ssh_revocation_file, var, value);
+	}
+
 	if (!strcmp(var, "gpg.program") || !strcmp(var, "gpg.openpgp.program"))
 		fmtname = "openpgp";
 
 	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);
@@ -433,18 +701,144 @@ 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)
+{
+	struct child_process ssh_default_key = CHILD_PROCESS_INIT;
+	int ret = -1;
+	struct strbuf key_stdout = STRBUF_INIT, key_stderr = STRBUF_INIT;
+	struct strbuf **keys;
+	char *key_command = NULL;
+	const char **argv;
+	int n;
+	char *default_key = NULL;
+
+	if (!ssh_default_key_command)
+		die(_("either user.signingkey or gpg.ssh.defaultKeyCommand needs to be configured"));
+
+	key_command = xstrdup(ssh_default_key_command);
+	n = split_cmdline(key_command, &argv);
+
+	if (n < 0)
+		die("malformed build-time gpg.ssh.defaultKeyCommand: %s",
+		    split_cmdline_strerror(n));
+
+	strvec_pushv(&ssh_default_key.args, argv);
+	ret = pipe_command(&ssh_default_key, NULL, 0, &key_stdout, 0,
+			   &key_stderr, 0);
+
+	if (!ret) {
+		keys = strbuf_split_max(&key_stdout, '\n', 2);
+		if (keys[0] && starts_with(keys[0]->buf, "ssh-")) {
+			default_key = strbuf_detach(keys[0], NULL);
+		} else {
+			warning(_("gpg.ssh.defaultKeycommand succeeded but returned no keys: %s %s"),
+				key_stderr.buf, key_stdout.buf);
+		}
+
+		strbuf_list_free(keys);
+	} else {
+		warning(_("gpg.ssh.defaultKeyCommand failed: %s %s"),
+			key_stderr.buf, key_stdout.buf);
+	}
+
+	free(key_command);
+	free(argv);
+	strbuf_release(&key_stdout);
+
+	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)
 		return configured_signing_key;
-	return git_committer_info(IDENT_STRICT|IDENT_NO_DATE);
+	if (use_format->get_default_key) {
+		return use_format->get_default_key();
+	}
+
+	return git_committer_info(IDENT_STRICT | IDENT_NO_DATE);
 }
 
 int sign_buffer(struct strbuf *buffer, struct strbuf *signature, const char *signing_key)
 {
+	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,
@@ -470,13 +864,98 @@ int sign_buffer(struct strbuf *buffer, struct strbuf *signature, const char *sig
 		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;
+}
diff --git a/gpg-interface.h b/gpg-interface.h
index 80567e4894..beefacbb1e 100644
--- a/gpg-interface.h
+++ b/gpg-interface.h
@@ -17,7 +17,7 @@ enum signature_trust_level {
 
 struct signature_check {
 	char *payload;
-	char *gpg_output;
+	char *output;
 	char *gpg_status;
 
 	/*
@@ -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/log-tree.c b/log-tree.c
index 6dc4412268..644893fd8c 100644
--- a/log-tree.c
+++ b/log-tree.c
@@ -515,10 +515,10 @@ static void show_signature(struct rev_info *opt, struct commit *commit)
 
 	status = check_signature(payload.buf, payload.len, signature.buf,
 				 signature.len, &sigc);
-	if (status && !sigc.gpg_output)
+	if (status && !sigc.output)
 		show_sig_lines(opt, status, "No signature\n");
 	else
-		show_sig_lines(opt, status, sigc.gpg_output);
+		show_sig_lines(opt, status, sigc.output);
 	signature_check_clear(&sigc);
 
  out:
@@ -585,8 +585,8 @@ static int show_one_mergetag(struct commit *commit,
 		/* could have a good signature */
 		status = check_signature(payload.buf, payload.len,
 					 signature.buf, signature.len, &sigc);
-		if (sigc.gpg_output)
-			strbuf_addstr(&verify_message, sigc.gpg_output);
+		if (sigc.output)
+			strbuf_addstr(&verify_message, sigc.output);
 		else
 			strbuf_addstr(&verify_message, "No signature\n");
 		signature_check_clear(&sigc);
diff --git a/pretty.c b/pretty.c
index 73b5ead509..fe95107ae5 100644
--- a/pretty.c
+++ b/pretty.c
@@ -1436,8 +1436,8 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 			check_commit_signature(c->commit, &(c->signature_check));
 		switch (placeholder[1]) {
 		case 'G':
-			if (c->signature_check.gpg_output)
-				strbuf_addstr(sb, c->signature_check.gpg_output);
+			if (c->signature_check.output)
+				strbuf_addstr(sb, c->signature_check.output);
 			break;
 		case '?':
 			switch (c->signature_check.result) {
diff --git a/send-pack.c b/send-pack.c
index b3a495b7b1..bc0fcdbb00 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;
 }
diff --git a/t/lib-gpg.sh b/t/lib-gpg.sh
index 9fc5241228..f99ef3e859 100644
--- a/t/lib-gpg.sh
+++ b/t/lib-gpg.sh
@@ -87,6 +87,34 @@ test_lazy_prereq RFC1991 '
 	echo | gpg --homedir "${GNUPGHOME}" -b --rfc1991 >/dev/null
 '
 
+GPGSSH_KEY_PRIMARY="${GNUPGHOME}/ed25519_ssh_signing_key"
+GPGSSH_KEY_SECONDARY="${GNUPGHOME}/rsa_2048_ssh_signing_key"
+GPGSSH_KEY_UNTRUSTED="${GNUPGHOME}/untrusted_ssh_signing_key"
+GPGSSH_KEY_WITH_PASSPHRASE="${GNUPGHOME}/protected_ssh_signing_key"
+GPGSSH_KEY_PASSPHRASE="super_secret"
+GPGSSH_ALLOWED_SIGNERS="${GNUPGHOME}/ssh.all_valid.allowedSignersFile"
+
+GPGSSH_GOOD_SIGNATURE_TRUSTED='Good "git" signature for'
+GPGSSH_GOOD_SIGNATURE_UNTRUSTED='Good "git" signature with'
+GPGSSH_KEY_NOT_TRUSTED="No principal matched"
+GPGSSH_BAD_SIGNATURE="Signature verification failed"
+
+test_lazy_prereq GPGSSH '
+	ssh_version=$(ssh-keygen -Y find-principals -n "git" 2>&1)
+	test $? != 127 || exit 1
+	echo $ssh_version | grep -q "find-principals:missing signature file"
+	test $? = 0 || exit 1;
+	mkdir -p "${GNUPGHOME}" &&
+	chmod 0700 "${GNUPGHOME}" &&
+	ssh-keygen -t ed25519 -N "" -C "git ed25519 key" -f "${GPGSSH_KEY_PRIMARY}" >/dev/null &&
+	echo "\"principal with number 1\" $(cat "${GPGSSH_KEY_PRIMARY}.pub")" >> "${GPGSSH_ALLOWED_SIGNERS}" &&
+	ssh-keygen -t rsa -b 2048 -N "" -C "git rsa2048 key" -f "${GPGSSH_KEY_SECONDARY}" >/dev/null &&
+	echo "\"principal with number 2\" $(cat "${GPGSSH_KEY_SECONDARY}.pub")" >> "${GPGSSH_ALLOWED_SIGNERS}" &&
+	ssh-keygen -t ed25519 -N "${GPGSSH_KEY_PASSPHRASE}" -C "git ed25519 encrypted key" -f "${GPGSSH_KEY_WITH_PASSPHRASE}" >/dev/null &&
+	echo "\"principal with number 3\" $(cat "${GPGSSH_KEY_WITH_PASSPHRASE}.pub")" >> "${GPGSSH_ALLOWED_SIGNERS}" &&
+	ssh-keygen -t ed25519 -N "" -f "${GPGSSH_KEY_UNTRUSTED}" >/dev/null