[PATCH] Add commit & tag signing/verification via SSH keys using ssh-keygen

From: Fabian Stelzer <redacted>

set gpg.format = ssh and user.signingkey to a ssh public key string (like from an
authorized_keys file) and commits/tags can be signed using the private
key from your ssh-agent.

Verification uses a allowed_signers_file (see ssh-keygen(1)) which
defaults to .gitsigners but can be set via gpg.ssh.allowedsigners
A possible gpg.ssh.revocationfile is also passed to ssh-keygen on
verification.

needs openssh>8.2p1

Signed-off-by: Fabian Stelzer <redacted>
---
    RFC: Add commit & tag signing/verification via SSH keys using ssh-keygen
    
    Support for using private keyfiles directly is still missing and i'm
    unsure on how to configure it or if the pubkey in the signingkey field
    is such a good idea. A SSH Fingerprint as signingkey would be nicer, but
    key lookup would be quite cumbersome. Maybe storing the fingerprint in
    signingkey and then have a gpg.ssh.$FINGERPRINT.publickey/privatekeyfile
    setting? As a default we could get the first ssh key from ssh-add and
    store it in the config to avoid unintentional changes of the used
    signing key. I've started with some tests for SSH Signing but having
    static private keyfiles would make this a lot easier. So still on my
    TODO.
    
    This feature makes git signing much more accessible to the average user.
    Usually they have a SSH Key for pushing code already. Using it for
    signing commits allows us to verify not only the transport but the
    pushed code as well. The allowed_signers file could be kept in the
    repository if all receives are verified (allowing only useris with valid
    signatures to add/change them) or outside if generated/managed
    differently. Tools like gitolite could optionally generate and enforce
    them from the already existing user ssh keys for example.
    
    In our corporate environemnt we use PIV x509 Certs on Yubikeys for email
    signing/encryption and ssh keys which i think is quite common (at least
    for the email part). This way we can establish the correct trust for the
    SSH Keys without setting up a separate GPG Infrastructure (which is
    still quite painful for users) or implementing x509 signing support for
    git (which lacks good forwarding mechanisms). Using ssh agent forwarding
    makes this feature easily usable in todays development environments
    where code is often checked out in remote VMs / containers.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-1041%2FFStelzer%2Fsshsign-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-1041/FStelzer/sshsign-v1
Pull-Request: https://github.com/git/git/pull/1041

 Documentation/config/gpg.txt  |  13 ++-
 Documentation/config/user.txt |   4 +
 gpg-interface.c               | 212 ++++++++++++++++++++++++++++++----
 gpg-interface.h               |   3 +
 4 files changed, 205 insertions(+), 27 deletions(-)

diff --git a/Documentation/config/gpg.txt b/Documentation/config/gpg.txt
index d94025cb368..fd71bd782ec 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

@@ -27,6 +27,15 @@ gpg.minTrustLevel::
 	with at least `undefined` trust.  Setting this option overrides
 	the required trust-level for all operations.  Supported values,
 	in increasing order of significance:
+
+gpg.ssh.allowedSigners::
+	A file containing all valid SSH signing principals. 
+	Similar to an .ssh/authorized_keys file. See ssh-keygen(1) for details.
+	Defaults to .gitsigners
+
+gpg.ssh.revocationFile::
+	Either a SSH KRL or a list of revoked public keys.
+	See ssh-keygen(1) for details.
 +
 * `undefined`
 * `never`

diff --git a/Documentation/config/user.txt b/Documentation/config/user.txt
index 59aec7c3aed..1632e7b320f 100644
--- a/Documentation/config/user.txt
+++ b/Documentation/config/user.txt

@@ -36,3 +36,7 @@ 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 needs to contain the valid
+	ssh public key (e.g.: "ssh-rsa XXXXXX identifier") which corresponds
+	to the private key used for signing. The private key needs to be available
+	via ssh-agent. Direct private key files are not supported yet.

diff --git a/gpg-interface.c b/gpg-interface.c
index 127aecfc2b0..53504f64410 100644
--- a/gpg-interface.c
+++ b/gpg-interface.c

@@ -8,6 +8,7 @@
 #include "tempfile.h"
 
 static char *configured_signing_key;
+const char *ssh_allowed_signers, *ssh_revocation_file;
 static enum signature_trust_level configured_min_trust_level = TRUST_UNDEFINED;
 
 struct gpg_format {

@@ -35,6 +36,14 @@ static const char *x509_sigs[] = {
 	NULL
 };
 
+static const char *ssh_verify_args[] = {
+	NULL
+};
+static const char *ssh_sigs[] = {
+	"-----BEGIN SSH SIGNATURE-----",
+	NULL
+};
+
 static struct gpg_format gpg_format[] = {
 	{ .name = "openpgp", .program = "gpg",
 	  .verify_args = openpgp_verify_args,

@@ -44,6 +53,9 @@ static struct gpg_format gpg_format[] = {
 	  .verify_args = x509_verify_args,
 	  .sigs = x509_sigs
 	},
+	{ .name = "ssh", .program = "ssh-keygen",
+	  .verify_args = ssh_verify_args,
+	  .sigs = ssh_sigs },
 };
 
 static struct gpg_format *use_format = &gpg_format[0];

@@ -144,6 +156,38 @@ static int parse_gpg_trust_level(const char *level,
 	return 1;
 }
 
+static void parse_ssh_output(struct signature_check *sigc)
+{
+	const char *output = NULL;
+	char *next = NULL;
+
+	// ssh-keysign output should be:
+	// Good "git" signature for PRINCIPAL with RSA key SHA256:FINGERPRINT
+
+	output = xmemdupz(sigc->gpg_status, strcspn(sigc->gpg_status, " \n"));
+	if (skip_prefix(sigc->gpg_status, "Good \"git\" signature for ", &output)) {
+		sigc->result = 'G';
+
+		next = strchrnul(output, ' ');
+		replace_cstring(&sigc->signer, output, next);
+		output = next + 1;
+		next = strchrnul(output, ' '); // 'with'
+		output = next + 1;
+		next = strchrnul(output, ' '); // KEY Type
+		output = next + 1;
+		next = strchrnul(output, ' '); // 'key'
+		output = next + 1;
+		next = strchrnul(output, ' '); // key
+		replace_cstring(&sigc->fingerprint, output, next);
+	} else {
+		sigc->result = 'B';
+	}
+
+	// SSH-Keygen prints onto stdout instead of stderr like the output code expects - so we just copy it over
+	free(sigc->gpg_output);
+	sigc->gpg_output = xmemdupz(sigc->gpg_status, strlen(sigc->gpg_status));
+}
+
 static void parse_gpg_output(struct signature_check *sigc)
 {
 	const char *buf = sigc->gpg_status;

@@ -262,11 +306,17 @@ static int verify_signed_buffer(const char *payload, size_t payload_size,
 				struct strbuf *gpg_output,
 				struct strbuf *gpg_status)
 {
-	struct child_process gpg = CHILD_PROCESS_INIT;
+	struct child_process gpg = CHILD_PROCESS_INIT,
+			     ssh_keygen = CHILD_PROCESS_INIT;
 	struct gpg_format *fmt;
 	struct tempfile *temp;
 	int ret;
-	struct strbuf buf = STRBUF_INIT;
+	const char *line;
+	size_t trust_size;
+	char *principal;
+	struct strbuf buf = STRBUF_INIT,
+		      principal_out = STRBUF_INIT,
+		      principal_err = STRBUF_INIT;
 
 	temp = mks_tempfile_t(".git_vtag_tmpXXXXXX");
 	if (!temp)

@@ -283,24 +333,77 @@ static int verify_signed_buffer(const char *payload, size_t payload_size,
 	if (!fmt)
 		BUG("bad signature '%s'", signature);
 
-	strvec_push(&gpg.args, fmt->program);
-	strvec_pushv(&gpg.args, fmt->verify_args);
-	strvec_pushl(&gpg.args,
-		     "--status-fd=1",
-		     "--verify", temp->filename.buf, "-",
-		     NULL);
+	if (!strcmp(use_format->name, "ssh")) {
+		// Find the principal from the  signers
+		strvec_push(&ssh_keygen.args, fmt->program);
+		strvec_pushl(&ssh_keygen.args,  "-Y", "find-principals",
+						"-f", get_ssh_allowed_signers(),
+						"-s", temp->filename.buf,
+						NULL);
+		ret = pipe_command(&ssh_keygen, NULL, 0, &principal_out, 0, &principal_err, 0);
+		if (strstr(principal_err.buf, "unknown option")) {
+			error(_("openssh version > 8.2p1 is needed for ssh signature verification (ssh-keygen needs -Y find-principals/verify option)"));
+		}
+		if (ret || !principal_out.len)
+			goto out;
+
+		/* Iterate over all lines */
+		for (line = principal_out.buf; *line; line = strchrnul(line + 1, '\n')) {
+			while (*line == '\n')
+				line++;
+			if (!*line)
+				break;
 
-	if (!gpg_status)
-		gpg_status = &buf;
+			trust_size = strcspn(line, " \n");
+			principal = xmemdupz(line, trust_size);
 
-	sigchain_push(SIGPIPE, SIG_IGN);
-	ret = pipe_command(&gpg, payload, payload_size,
-			   gpg_status, 0, gpg_output, 0);
-	sigchain_pop(SIGPIPE);
+			strvec_push(&gpg.args,fmt->program);
+			// We found principals - Try with each until we find a match
+			strvec_pushl(&gpg.args, "-Y", "verify",
+						"-n", "git",
+						"-f", get_ssh_allowed_signers(),
+						"-I", principal,
+						"-s", temp->filename.buf,
+						 NULL);
 
-	delete_tempfile(&temp);
+			if (ssh_revocation_file) {
+				strvec_pushl(&gpg.args, "-r", ssh_revocation_file, 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);
+			sigchain_pop(SIGPIPE);
 
-	ret |= !strstr(gpg_status->buf, "\n[GNUPG:] GOODSIG ");
+			ret |= !strstr(gpg_status->buf, "Good");
+			if (ret == 0)
+				break;
+		}
+	} else {
+		strvec_push(&gpg.args, fmt->program);
+		strvec_pushv(&gpg.args, fmt->verify_args);
+		strvec_pushl(&gpg.args,
+				"--status-fd=1",
+				"--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);
+		sigchain_pop(SIGPIPE);
+		ret |= !strstr(gpg_status->buf, "\n[GNUPG:] GOODSIG ");
+	}
+
+out:
+	delete_tempfile(&temp);
+	strbuf_release(&principal_out);
+	strbuf_release(&principal_err);
 	strbuf_release(&buf); /* no matter it was used or not */
 
 	return ret;

@@ -323,7 +426,11 @@ int check_signature(const char *payload, size_t plen, const char *signature,
 	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);
+	if (!strcmp(use_format->name, "ssh")) {
+		parse_ssh_output(sigc);
+	} else {
+		parse_gpg_output(sigc);
+	}
 	status |= sigc->result != 'G';
 	status |= sigc->trust_level < configured_min_trust_level;
 

@@ -394,6 +501,14 @@ int git_gpg_config(const char *var, const char *value, void *cb)
 		return 0;
 	}
 
+	if (!strcmp(var, "gpg.ssh.allowedsigners")) {
+		return git_config_string(&ssh_allowed_signers, var, value);
+	}
+
+	if (!strcmp(var, "gpg.ssh.revocationfile")) {
+		return git_config_string(&ssh_revocation_file, var, value);
+	}
+
 	if (!strcmp(var, "gpg.format")) {
 		if (!value)
 			return config_error_nonbool(var);

@@ -425,6 +540,9 @@ 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);

@@ -437,7 +555,19 @@ const char *get_signing_key(void)
 {
 	if (configured_signing_key)
 		return configured_signing_key;
-	return git_committer_info(IDENT_STRICT|IDENT_NO_DATE);
+	if (!strcmp(use_format->name, "ssh")) {
+		// We could simply use the first key listed by ssh-add -L and risk signing with the wrong key
+		return "";
+	} else {
+		return git_committer_info(IDENT_STRICT | IDENT_NO_DATE);
+	}
+}
+
+const char *get_ssh_allowed_signers(void)
+{
+	if (ssh_allowed_signers)
+		return ssh_allowed_signers;
+	return GPG_SSH_ALLOWED_SIGNERS;
 }
 
 int sign_buffer(struct strbuf *buffer, struct strbuf *signature, const char *signing_key)

@@ -446,12 +576,35 @@ int sign_buffer(struct strbuf *buffer, struct strbuf *signature, const char *sig
 	int ret;
 	size_t i, j, bottom;
 	struct strbuf gpg_status = STRBUF_INIT;
-
-	strvec_pushl(&gpg.args,
-		     use_format->program,
-		     "--status-fd=2",
-		     "-bsau", signing_key,
-		     NULL);
+	struct tempfile *temp = NULL;
+
+	if (!strcmp(use_format->name, "ssh")) {
+		if (!signing_key)
+			return error(_("user.signingkey needs to be set to a ssh public key for ssh signing"));
+
+		// signing_key is a public ssh key
+		// FIXME: Allow specifying a key file so we can use private keyfiles instead of ssh-agent
+		temp = mks_tempfile_t(".git_signing_key_tmpXXXXXX");
+		if (!temp)
+			return error_errno(_("could not create temporary file"));
+		if (write_in_full(temp->fd, signing_key,
+					strlen(signing_key)) < 0 ||
+			close_tempfile_gently(temp) < 0) {
+			error_errno(_("failed writing ssh signing key to '%s'"), temp->filename.buf);
+			delete_tempfile(&temp);
+			return -1;
+		}
+		strvec_pushl(&gpg.args, use_format->program ,
+					"-Y", "sign",
+					"-n", "git",
+					"-f", temp->filename.buf,
+					NULL);
+	} else {
+		strvec_pushl(&gpg.args, use_format->program ,
+					"--status-fd=2",
+					"-bsau", signing_key,
+					NULL);
+	}
 
 	bottom = signature->len;
 

@@ -464,7 +617,16 @@ int sign_buffer(struct strbuf *buffer, struct strbuf *signature, const char *sig
 			   signature, 1024, &gpg_status, 0);
 	sigchain_pop(SIGPIPE);
 
-	ret |= !strstr(gpg_status.buf, "\n[GNUPG:] SIG_CREATED ");
+	if (temp)
+		delete_tempfile(&temp);
+
+	if (!strcmp(use_format->name, "ssh")) {
+		if (strstr(gpg_status.buf, "unknown option")) {
+			error(_("openssh version > 8.2p1 is needed for ssh signing (ssh-keygen needs -Y sign option)"));
+		}
+	} else {
+		ret |= !strstr(gpg_status.buf, "\n[GNUPG:] SIG_CREATED ");
+	}
 	strbuf_release(&gpg_status);
 	if (ret)
 		return error(_("gpg failed to sign the data"));

diff --git a/gpg-interface.h b/gpg-interface.h
index 80567e48948..286c1b4167a 100644
--- a/gpg-interface.h
+++ b/gpg-interface.h

@@ -7,6 +7,8 @@ struct strbuf;
 #define GPG_VERIFY_RAW			2
 #define GPG_VERIFY_OMIT_STATUS	4
 
+#define GPG_SSH_ALLOWED_SIGNERS ".gitsigners"
+
 enum signature_trust_level {
 	TRUST_UNDEFINED,
 	TRUST_NEVER,

@@ -64,6 +66,7 @@ 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);
+const char *get_ssh_allowed_signers(void);
 int check_signature(const char *payload, size_t plen,
 		    const char *signature, size_t slen,
 		    struct signature_check *sigc);

base-commit: 670b81a890388c60b7032a4f5b879f2ece8c4558
-- 
gitgitgadget

On Tue, Jul 6, 2021 at 10:20 AM Fabian Stelzer via GitGitGadget
[off-list ref] wrote:

From: Fabian Stelzer <redacted>

set gpg.format = ssh and user.signingkey to a ssh public key string (like from an
authorized_keys file) and commits/tags can be signed using the private
key from your ssh-agent.

Verification uses a allowed_signers_file (see ssh-keygen(1)) which
defaults to .gitsigners but can be set via gpg.ssh.allowedsigners
A possible gpg.ssh.revocationfile is also passed to ssh-keygen on
verification.

...

    In our corporate environemnt we use PIV x509 Certs on Yubikeys for email
    signing/encryption and ssh keys which i think is quite common (at least
    for the email part). This way we can establish the correct trust for the
    SSH Keys without setting up a separate GPG Infrastructure (which is
    still quite painful for users) or implementing x509 signing support for
    git (which lacks good forwarding mechanisms). Using ssh agent forwarding
    makes this feature easily usable in todays development environments
    where code is often checked out in remote VMs / containers.

Thanks for working on this, and I support this initiative. I
coincidentally have started proselytizing something similar just weeks
ago.

My interest is in signing pushes rather than commits/tags, as that (in
combination with SSH U2F support) provides a simple mechanism to
require (forwardable!) 2-factor authentication on pushes over HTTP. I
haven't looked at the signing code in detail, but I had the impression
that adding SSH signatures would automatically also add support for
signed pushes? (aka. push-certs) Do you know?

-- 
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I work 80%. Don't expect answers from me on Fridays.
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Sitz der Gesellschaft: Hamburg

Geschäftsführer: Paul Manicle, Halimah DeLaine Prado

quoted

From: Fabian Stelzer <redacted>

set gpg.format = ssh and user.signingkey to a ssh public key string (like from an
authorized_keys file) and commits/tags can be signed using the private
key from your ssh-agent.

Verification uses a allowed_signers_file (see ssh-keygen(1)) which
defaults to .gitsigners but can be set via gpg.ssh.allowedsigners
A possible gpg.ssh.revocationfile is also passed to ssh-keygen on
verification.

...

quoted

     In our corporate environemnt we use PIV x509 Certs on Yubikeys for email
     signing/encryption and ssh keys which i think is quite common (at least
     for the email part). This way we can establish the correct trust for the
     SSH Keys without setting up a separate GPG Infrastructure (which is
     still quite painful for users) or implementing x509 signing support for
     git (which lacks good forwarding mechanisms). Using ssh agent forwarding
     makes this feature easily usable in todays development environments
     where code is often checked out in remote VMs / containers.

Thanks for working on this, and I support this initiative. I
coincidentally have started proselytizing something similar just weeks
ago.

My interest is in signing pushes rather than commits/tags, as that (in
combination with SSH U2F support) provides a simple mechanism to
require (forwardable!) 2-factor authentication on pushes over HTTP. I
haven't looked at the signing code in detail, but I had the impression
that adding SSH signatures would automatically also add support for
signed pushes? (aka. push-certs) Do you know?

Up until now i was not actually aware of the "push signing" 
functionality in git.
I can see that the send/receive-pack use the same api function calls as 
commit/tag signing.
So this should work just as well. Especially if using an ssh agent the whole
process is identical to git. I still need to try private key files 
directly to see
how user interaction (like entering a passphrase or touching the U2F 
Token) would work.

On 2021-07-06 at 08:19:53, Fabian Stelzer via GitGitGadget wrote:

From: Fabian Stelzer <redacted>

set gpg.format = ssh and user.signingkey to a ssh public key string (like from an
authorized_keys file) and commits/tags can be signed using the private
key from your ssh-agent.

Verification uses a allowed_signers_file (see ssh-keygen(1)) which
defaults to .gitsigners but can be set via gpg.ssh.allowedsigners
A possible gpg.ssh.revocationfile is also passed to ssh-keygen on
verification.

needs openssh>8.2p1

Usually we'll want to write the explanation here in full sentences with
typical capitalization.

Signed-off-by: Fabian Stelzer <redacted>
---
    RFC: Add commit & tag signing/verification via SSH keys using ssh-keygen
    
    Support for using private keyfiles directly is still missing and i'm
    unsure on how to configure it or if the pubkey in the signingkey field
    is such a good idea. A SSH Fingerprint as signingkey would be nicer, but
    key lookup would be quite cumbersome. Maybe storing the fingerprint in
    signingkey and then have a gpg.ssh.$FINGERPRINT.publickey/privatekeyfile
    setting? As a default we could get the first ssh key from ssh-add and
    store it in the config to avoid unintentional changes of the used
    signing key. I've started with some tests for SSH Signing but having
    static private keyfiles would make this a lot easier. So still on my
    TODO.

I think user.signingKey could be helpful for signing here.  That could
be a file name, not just a fingerprint, although we'd probably want to
have support for tilde expansion.  You could add an additional option,
gpg.ssh.keyring, that specifies the signatures to verify.  That would be
named the same thing as a potential option of gpg.openpgp.keyring,
which would be convenient.  Also, gpg.ssh.revokedKeyring could maybe be
the name for revoked keys?

    This feature makes git signing much more accessible to the average user.
    Usually they have a SSH Key for pushing code already. Using it for
    signing commits allows us to verify not only the transport but the
    pushed code as well. The allowed_signers file could be kept in the
    repository if all receives are verified (allowing only useris with valid
    signatures to add/change them) or outside if generated/managed
    differently. Tools like gitolite could optionally generate and enforce
    them from the already existing user ssh keys for example.
    
    In our corporate environemnt we use PIV x509 Certs on Yubikeys for email
    signing/encryption and ssh keys which i think is quite common (at least
    for the email part). This way we can establish the correct trust for the
    SSH Keys without setting up a separate GPG Infrastructure (which is
    still quite painful for users) or implementing x509 signing support for
    git (which lacks good forwarding mechanisms). Using ssh agent forwarding
    makes this feature easily usable in todays development environments
    where code is often checked out in remote VMs / containers.

I think some of this rationale would work well in the commit message,
especially the part about the fact that using an SSH key may be easier
for users and the fact that it can be well supported by smart cards.
Those are compelling arguments about why this is a desirable change, and
should be in the commit message.

I haven't looked too deeply at the intricacies of the change, but I'm in
favor of it.  I would, however, like to see some tests here, including
for commits, tags, and push certificates.  Note that you'll probably
need to run the testsuite both with and without
GIT_TEST_DEFAULT_HASH=sha256 to verify everything works.

We probably don't want to store this in the repository.  If OpenSSH has
a standard location for this, then we can default to that; otherwise, we
should pick something in .ssh or in $XDG_CONFIG_HOME/git.
-- 
brian m. carlson (he/him or they/them)
Toronto, Ontario, CA

From: Fabian Stelzer <redacted>

quoted

set gpg.format = ssh and user.signingkey to a ssh public key string (like from an
authorized_keys file) and commits/tags can be signed using the private
key from your ssh-agent.

Verification uses a allowed_signers_file (see ssh-keygen(1)) which
defaults to .gitsigners but can be set via gpg.ssh.allowedsigners
A possible gpg.ssh.revocationfile is also passed to ssh-keygen on
verification.

needs openssh>8.2p1

Usually we'll want to write the explanation here in full sentences with
typical capitalization.

Thanks, i was unsure about what to put in the commit and what into the 
cover letter.
I'll fix this with the next patch update and move some of it into the 
commit message.
In our env the commit messages are usually kept quite short.

quoted

Signed-off-by: Fabian Stelzer <redacted>
---
     RFC: Add commit & tag signing/verification via SSH keys using ssh-keygen
     
     Support for using private keyfiles directly is still missing and i'm
     unsure on how to configure it or if the pubkey in the signingkey field
     is such a good idea. A SSH Fingerprint as signingkey would be nicer, but
     key lookup would be quite cumbersome. Maybe storing the fingerprint in
     signingkey and then have a gpg.ssh.$FINGERPRINT.publickey/privatekeyfile
     setting? As a default we could get the first ssh key from ssh-add and
     store it in the config to avoid unintentional changes of the used
     signing key. I've started with some tests for SSH Signing but having
     static private keyfiles would make this a lot easier. So still on my
     TODO.

I think user.signingKey could be helpful for signing here.  That could
be a file name, not just a fingerprint, although we'd probably want to
have support for tilde expansion.  You could add an additional option,
gpg.ssh.keyring, that specifies the signatures to verify.  That would be
named the same thing as a potential option of gpg.openpgp.keyring,
which would be convenient.  Also, gpg.ssh.revokedKeyring could maybe be
the name for revoked keys?

The problem ist that looking up a key by fingerprint alone is not really 
possible with ssh :/
A referenced file (which could contain a public or private key) would be 
fine and i could return the fingerprint in the get_signing_key api which 
the pushcerts code uses as "pusher" info in the cert.
I'll change the keyring naming to what you suggested. Makes sense to 
have this option for gpg as well.

quoted

     This feature makes git signing much more accessible to the average user.
     Usually they have a SSH Key for pushing code already. Using it for
     signing commits allows us to verify not only the transport but the
     pushed code as well. The allowed_signers file could be kept in the
     repository if all receives are verified (allowing only useris with valid
     signatures to add/change them) or outside if generated/managed
     differently. Tools like gitolite could optionally generate and enforce
     them from the already existing user ssh keys for example.
     
     In our corporate environemnt we use PIV x509 Certs on Yubikeys for email
     signing/encryption and ssh keys which i think is quite common (at least
     for the email part). This way we can establish the correct trust for the
     SSH Keys without setting up a separate GPG Infrastructure (which is
     still quite painful for users) or implementing x509 signing support for
     git (which lacks good forwarding mechanisms). Using ssh agent forwarding
     makes this feature easily usable in todays development environments
     where code is often checked out in remote VMs / containers.

I think some of this rationale would work well in the commit message,
especially the part about the fact that using an SSH key may be easier
for users and the fact that it can be well supported by smart cards.
Those are compelling arguments about why this is a desirable change, and
should be in the commit message.

I haven't looked too deeply at the intricacies of the change, but I'm in
favor of it.  I would, however, like to see some tests here, including
for commits, tags, and push certificates.  Note that you'll probably
need to run the testsuite both with and without
GIT_TEST_DEFAULT_HASH=sha256 to verify everything works.

I'm working on some tests but there are lots of GPG / GPGSM tests in the 
suite and i'm unsure of how many i should duplicate.
Thanks for the info with the hash setting.

quoted

diff --git a/Documentation/config/gpg.txt b/Documentation/config/gpg.txt
index d94025cb368..fd71bd782ec 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

@@ -27,6 +27,15 @@ gpg.minTrustLevel::
  	with at least `undefined` trust.  Setting this option overrides
  	the required trust-level for all operations.  Supported values,
  	in increasing order of significance:
+
+gpg.ssh.allowedSigners::
+	A file containing all valid SSH signing principals.
+	Similar to an .ssh/authorized_keys file. See ssh-keygen(1) for details.
+	Defaults to .gitsigners

We probably don't want to store this in the repository.  If OpenSSH has
a standard location for this, then we can default to that; otherwise, we
should pick something in .ssh or in $XDG_CONFIG_HOME/git.

I'm not aware of a standard location. I think there are use cases to 
store this in the repo, but i'm of course fine not defaulting to it.

On 06/07/21 15.19, Fabian Stelzer via GitGitGadget wrote:

From: Fabian Stelzer <redacted>

set gpg.format = ssh and user.signingkey to a ssh public key string (like from an
authorized_keys file) and commits/tags can be signed using the private
key from your ssh-agent.

Verification uses a allowed_signers_file (see ssh-keygen(1)) which
defaults to .gitsigners but can be set via gpg.ssh.allowedsigners
A possible gpg.ssh.revocationfile is also passed to ssh-keygen on
verification.

needs openssh>8.2p1

Why did you choose to implement SSH-based signing as GPG interface? Why 
not create similar one?

If at later times we need to implement other signing methods (besides 
GPG and SSH), we can refactor gpg-interface into generic signing 
interface (say `signing.h`) and let each signing methods implement from it.

-- 
An old man doll... just what I always wanted! - Clara

On 07.07.21 08:26, Bagas Sanjaya wrote:

On 06/07/21 15.19, Fabian Stelzer via GitGitGadget wrote:

quoted

From: Fabian Stelzer <redacted>

set gpg.format = ssh and user.signingkey to a ssh public key string 
(like from an
authorized_keys file) and commits/tags can be signed using the private
key from your ssh-agent.

Verification uses a allowed_signers_file (see ssh-keygen(1)) which
defaults to .gitsigners but can be set via gpg.ssh.allowedsigners
A possible gpg.ssh.revocationfile is also passed to ssh-keygen on
verification.

needs openssh>8.2p1

Why did you choose to implement SSH-based signing as GPG interface? 
Why not create similar one?

If at later times we need to implement other signing methods (besides 
GPG and SSH), we can refactor gpg-interface into generic signing 
interface (say `signing.h`) and let each signing methods implement 
from it. 

I agree that a general purpose "signing" would be cleaner. The GPG 
kewords are scattered all over the codebase but all the paths i found 
just call the generic sign_buffer / verify_signed_buffer from 
gpg-interface.c in the end whose api works quite well for other signing 
mechanisms as well. I will rename some struct fields to be more generic 
and adjust a few messages printed to the user which currently say things 
like "gpg failed to sign the data" or "has a gpg signature" to be 
generic. Do we just want to call this "signature" and remove the gpg 
prefix or would that be too generic?

Refactoring the whole gpg part to a generic "signing" would be quite 
involved and should probably be a different patch even though its mostly 
renaming stuff.
If we want to go into that direction i could add the new config keys 
under signing.* (signing.format = ssh|gpg, ...) and keep the 
compatibility for the older gpg.* keys.

From: Fabian Stelzer <redacted>

Openssh v8.2p1 added some new options to ssh-keygen for signature
creation and verification. These allow us to use ssh keys for git
signatures easily.

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.
If no signingkey is set then git will call 'ssh-add -L' to check for
available agent keys and use the first one for signing.

Verification uses the gpg.ssh.keyring file (see ssh-keygen(1) "ALLOWED
SIGNERS") which contains valid public keys and an principal (usually
user@domain). Depending on the environment this file can be managed by
the individual developer or for example generated by the central
repository server from known ssh keys with push access. If the
repository only allows signed commits / pushes then the file can even be
stored inside it.

To revoke a key put the public key without the principal prefix into
gpg.ssh.revocationKeyring or generate a KRL (see ssh-keygen(1)
"KEY REVOCATION LISTS"). The same considerations about who to trust for
verification as with the keyring file apply.

This feature makes git signing much more accessible to the average user.
Usually they have a SSH Key for pushing code already. Using it
for signing commits allows us to verify not only the transport but the
pushed code as well.

In our corporate environemnt we use PIV x509 Certs on Yubikeys for email
signing/encryption and ssh keys which i think is quite common
(at least for the email part). This way we can establish the correct
trust for the SSH Keys without setting up a separate GPG Infrastructure
(which is still quite painful for users) or implementing x509 signing
support for git (which lacks good forwarding mechanisms).
Using ssh agent forwarding makes this feature easily usable in todays
development environments where code is often checked out in remote VMs / containers.
In such a setup the keyring & revocationKeyring can be centrally
generated from the x509 CA information and distributed to the users.

Signed-off-by: Fabian Stelzer <redacted>
---
    RFC: Add commit & tag signing/verification via SSH keys using ssh-keygen
    
    I have added support for using keyfiles directly, lots of tests and
    generally cleaned up the signing & verification code a lot.
    
    I can still rename things from being gpg specific to a more general
    "signing" but thats rather cosmetic. Also i'm not sure if i named the
    new test files correctly.
    
    There is a patch in the pipeline for openssh by Damien Miller that will
    add valid-after, valid-before options to the allowed keys keyring. This
    allows us to pass the commit timestamp to the verification call and make
    key rollover possible and still be able to verify older commits. Set
    valid-after=NOW when adding your key to the keyring and set valid-before
    to make it fail if used after a certain date. Software like
    gitolite/github or corporate automation can do this automatically when
    ssh push keys are addded / removed

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-1041%2FFStelzer%2Fsshsign-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-1041/FStelzer/sshsign-v2
Pull-Request: https://github.com/git/git/pull/1041

Range-diff vs v1:

 1:  f238392bfa8 ! 1:  b8b16f8e6ec Add commit & tag signing/verification via SSH keys using ssh-keygen
     @@ Metadata
      Author: Fabian Stelzer [off-list ref]
      
       ## Commit message ##
     -    Add commit & tag signing/verification via SSH keys using ssh-keygen
     +    Add commit, tag & push signing/verification via SSH keys using ssh-keygen
      
     -    set gpg.format = ssh and user.signingkey to a ssh public key string (like from an
     -    authorized_keys file) and commits/tags can be signed using the private
     -    key from your ssh-agent.
     +    Openssh v8.2p1 added some new options to ssh-keygen for signature
     +    creation and verification. These allow us to use ssh keys for git
     +    signatures easily.
      
     -    Verification uses a allowed_signers_file (see ssh-keygen(1)) which
     -    defaults to .gitsigners but can be set via gpg.ssh.allowedsigners
     -    A possible gpg.ssh.revocationfile is also passed to ssh-keygen on
     -    verification.
     +    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.
     +    If no signingkey is set then git will call 'ssh-add -L' to check for
     +    available agent keys and use the first one for signing.
      
     -    needs openssh>8.2p1
     +    Verification uses the gpg.ssh.keyring file (see ssh-keygen(1) "ALLOWED
     +    SIGNERS") which contains valid public keys and an principal (usually
     +    user@domain). Depending on the environment this file can be managed by
     +    the individual developer or for example generated by the central
     +    repository server from known ssh keys with push access. If the
     +    repository only allows signed commits / pushes then the file can even be
     +    stored inside it.
     +
     +    To revoke a key put the public key without the principal prefix into
     +    gpg.ssh.revocationKeyring or generate a KRL (see ssh-keygen(1)
     +    "KEY REVOCATION LISTS"). The same considerations about who to trust for
     +    verification as with the keyring file apply.
     +
     +    This feature makes git signing much more accessible to the average user.
     +    Usually they have a SSH Key for pushing code already. Using it
     +    for signing commits allows us to verify not only the transport but the
     +    pushed code as well.
     +
     +    In our corporate environemnt we use PIV x509 Certs on Yubikeys for email
     +    signing/encryption and ssh keys which i think is quite common
     +    (at least for the email part). This way we can establish the correct
     +    trust for the SSH Keys without setting up a separate GPG Infrastructure
     +    (which is still quite painful for users) or implementing x509 signing
     +    support for git (which lacks good forwarding mechanisms).
     +    Using ssh agent forwarding makes this feature easily usable in todays
     +    development environments where code is often checked out in remote VMs / containers.
     +    In such a setup the keyring & revocationKeyring can be centrally
     +    generated from the x509 CA information and distributed to the users.
      
          Signed-off-by: Fabian Stelzer [off-list ref]
      
     @@ Documentation/config/gpg.txt: gpg.program::
       gpg.minTrustLevel::
       	Specifies a minimum trust level for signature verification.  If
      @@ Documentation/config/gpg.txt: gpg.minTrustLevel::
     - 	with at least `undefined` trust.  Setting this option overrides
     - 	the required trust-level for all operations.  Supported values,
     - 	in increasing order of significance:
     + * `marginal`
     + * `fully`
     + * `ultimate`
     ++
     ++gpg.ssh.keyring::
     ++	A file containing all valid SSH public signing keys. 
     ++	Similar to an .ssh/authorized_keys file.
     ++	See ssh-keygen(1) "ALLOWED SIGNERS" for details.
     ++	If a signing key is found in this file then the trust level will
     ++	be set to "fully". Otherwise if the key is not present
     ++	but the signature is still valid then the trust level will be "undefined".
     ++
     ++	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 some automation that already handles developer ssh keys. 
     ++	
     ++	A repository that is only allowing signed commits can store the file 
     ++	in the repository itself using a relative path. This way only committers
     ++	with an already valid key can add or change keys in the keyring.
      +
     -+gpg.ssh.allowedSigners::
     -+	A file containing all valid SSH signing principals. 
     -+	Similar to an .ssh/authorized_keys file. See ssh-keygen(1) for details.
     -+	Defaults to .gitsigners
     ++	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.
     ++	To revoke a key place the public key without the principal into the 
     ++	revocationKeyring.
     ++
     ++gpg.ssh.revocationKeyring::
     ++	Either a SSH KRL or a list of revoked public keys (without the principal prefix).
      +	See ssh-keygen(1) for details.
     - +
     - * `undefined`
     - * `never`
     ++	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.
      
       ## Documentation/config/user.txt ##
      @@ Documentation/config/user.txt: 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 needs to contain the valid
     -+	ssh public key (e.g.: "ssh-rsa XXXXXX identifier") which corresponds
     -+	to the private key used for signing. The private key needs to be available
     -+	via ssh-agent. Direct private key files are not supported yet.
     ++	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 
     ++	"ssh-add -L" and try to use the first key available.
     +
     + ## builtin/receive-pack.c ##
     +@@ builtin/receive-pack.c: static int receive_pack_config(const char *var, const char *value, void *cb)
     + {
     + 	int status = parse_hide_refs_config(var, value, "receive");
     + 
     ++	git_gpg_config(var, value, NULL);
     ++
     + 	if (status)
     + 		return status;
     + 
     +@@ builtin/receive-pack.c: static void prepare_push_cert_sha1(struct child_process *proc)
     + 		bogs = parse_signed_buffer(push_cert.buf, push_cert.len);
     + 		check_signature(push_cert.buf, bogs, push_cert.buf + bogs,
     + 				push_cert.len - bogs, &sigcheck);
     +-
     ++		
     + 		nonce_status = check_nonce(push_cert.buf, bogs);
     + 	}
     + 	if (!is_null_oid(&push_cert_oid)) {
     +
     + ## fmt-merge-msg.c ##
     +@@ fmt-merge-msg.c: static void fmt_merge_msg_sigs(struct strbuf *out)
     + 			len = payload.len;
     + 			if (check_signature(payload.buf, payload.len, sig.buf,
     + 					 sig.len, &sigc) &&
     +-				!sigc.gpg_output)
     ++				!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);
     + 
      
       ## gpg-interface.c ##
      @@
     + #include "config.h"
     + #include "run-command.h"
     + #include "strbuf.h"
     ++#include "dir.h"
     + #include "gpg-interface.h"
     + #include "sigchain.h"
       #include "tempfile.h"
       
       static char *configured_signing_key;
     @@ gpg-interface.c: static struct gpg_format gpg_format[] = {
       };
       
       static struct gpg_format *use_format = &gpg_format[0];
     +@@ gpg-interface.c: 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->output);
     + 	FREE_AND_NULL(sigc->gpg_output);
     + 	FREE_AND_NULL(sigc->gpg_status);
     + 	FREE_AND_NULL(sigc->signer);
      @@ gpg-interface.c: static int parse_gpg_trust_level(const char *level,
       	return 1;
       }
     @@ gpg-interface.c: static int parse_gpg_trust_level(const char *level,
      +	const char *output = NULL;
      +	char *next = NULL;
      +
     -+	// ssh-keysign output should be:
     -+	// Good "git" signature for PRINCIPAL with RSA key SHA256:FINGERPRINT
     ++	/* ssh-keysign 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
     ++	 */
      +
     -+	output = xmemdupz(sigc->gpg_status, strcspn(sigc->gpg_status, " \n"));
     -+	if (skip_prefix(sigc->gpg_status, "Good \"git\" signature for ", &output)) {
     ++	output = xmemdupz(sigc->output, strcspn(sigc->output, " \n"));
     ++	if (skip_prefix(sigc->output, "Good \"git\" signature for ", &output)) {
     ++		// Valid signature for a trusted signer
      +		sigc->result = 'G';
     ++		sigc->trust_level = TRUST_FULLY;
      +
     -+		next = strchrnul(output, ' ');
     ++		next = strchrnul(output, ' '); // 'principal'
      +		replace_cstring(&sigc->signer, output, next);
      +		output = next + 1;
      +		next = strchrnul(output, ' '); // 'with'
     @@ gpg-interface.c: static int parse_gpg_trust_level(const char *level,
      +		output = next + 1;
      +		next = strchrnul(output, ' '); // 'key'
      +		output = next + 1;
     -+		next = strchrnul(output, ' '); // key
     ++		next = strchrnul(output, '\n'); // key
      +		replace_cstring(&sigc->fingerprint, output, next);
     ++		replace_cstring(&sigc->key, output, next);
     ++	} else if (skip_prefix(sigc->output, "Good \"git\" signature with ", &output)) {
     ++		// Valid signature, but key unknown
     ++		sigc->result = 'G';
     ++		sigc->trust_level = TRUST_UNDEFINED;
     ++
     ++		next = strchrnul(output, ' '); // KEY Type
     ++		output = next + 1;
     ++		next = strchrnul(output, ' '); // 'key'
     ++		output = next + 1;
     ++		next = strchrnul(output, '\n'); // key
     ++		replace_cstring(&sigc->fingerprint, output, next);
     ++		replace_cstring(&sigc->key, output, next);
      +	} else {
      +		sigc->result = 'B';
     ++		sigc->trust_level = TRUST_NEVER;
      +	}
     -+
     -+	// SSH-Keygen prints onto stdout instead of stderr like the output code expects - so we just copy it over
     -+	free(sigc->gpg_output);
     -+	sigc->gpg_output = xmemdupz(sigc->gpg_status, strlen(sigc->gpg_status));
      +}
      +
       static void parse_gpg_output(struct signature_check *sigc)
       {
       	const char *buf = sigc->gpg_status;
     -@@ gpg-interface.c: static int verify_signed_buffer(const char *payload, size_t payload_size,
     - 				struct strbuf *gpg_output,
     - 				struct strbuf *gpg_status)
     +@@ gpg-interface.c: 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_ssh_signature(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 child_process gpg = CHILD_PROCESS_INIT,
     -+			     ssh_keygen = CHILD_PROCESS_INIT;
     - 	struct gpg_format *fmt;
     +-	struct gpg_format *fmt;
     ++	struct child_process ssh_keygen = CHILD_PROCESS_INIT;
       	struct tempfile *temp;
       	int ret;
      -	struct strbuf buf = STRBUF_INIT;
      +	const char *line;
      +	size_t trust_size;
      +	char *principal;
     -+	struct strbuf buf = STRBUF_INIT,
     -+		      principal_out = STRBUF_INIT,
     -+		      principal_err = STRBUF_INIT;
     ++	struct strbuf ssh_keygen_out = STRBUF_INIT;
     ++	struct strbuf ssh_keygen_err = STRBUF_INIT;
       
       	temp = mks_tempfile_t(".git_vtag_tmpXXXXXX");
       	if (!temp)
      @@ gpg-interface.c: static int verify_signed_buffer(const char *payload, size_t payload_size,
     - 	if (!fmt)
     - 		BUG("bad signature '%s'", signature);
     + 		return -1;
     + 	}
       
     --	strvec_push(&gpg.args, fmt->program);
     --	strvec_pushv(&gpg.args, fmt->verify_args);
     --	strvec_pushl(&gpg.args,
     --		     "--status-fd=1",
     --		     "--verify", temp->filename.buf, "-",
     --		     NULL);
     -+	if (!strcmp(use_format->name, "ssh")) {
     -+		// Find the principal from the  signers
     -+		strvec_push(&ssh_keygen.args, fmt->program);
     -+		strvec_pushl(&ssh_keygen.args,  "-Y", "find-principals",
     -+						"-f", get_ssh_allowed_signers(),
     +-	fmt = get_format_by_sig(signature);
     +-	if (!fmt)
     +-		BUG("bad signature '%s'", signature);
     ++	// Find the principal from the  signers
     ++	strvec_pushl(&ssh_keygen.args,  fmt->program,
     ++					"-Y", "find-principals",
     ++					"-f", get_ssh_allowed_signers(),
     ++					"-s", temp->filename.buf,
     ++					NULL);
     ++	ret = pipe_command(&ssh_keygen, NULL, 0, &ssh_keygen_out, 0, &ssh_keygen_err, 0);
     ++	if (strstr(ssh_keygen_err.buf, "unknown option")) {
     ++		error(_("openssh version > 8.2p1 is needed for ssh signature verification (ssh-keygen needs -Y find-principals/verify option)"));
     ++	}
     ++	if (ret || !ssh_keygen_out.len) {
     ++		// We did not find a matching principal in the keyring - Check without validation
     ++		child_process_init(&ssh_keygen);
     ++		strvec_pushl(&ssh_keygen.args,  fmt->program,
     ++						"-Y", "check-novalidate",
     ++						"-n", "git",
      +						"-s", temp->filename.buf,
      +						NULL);
     -+		ret = pipe_command(&ssh_keygen, NULL, 0, &principal_out, 0, &principal_err, 0);
     -+		if (strstr(principal_err.buf, "unknown option")) {
     -+			error(_("openssh version > 8.2p1 is needed for ssh signature verification (ssh-keygen needs -Y find-principals/verify option)"));
     -+		}
     -+		if (ret || !principal_out.len)
     -+			goto out;
     -+
     -+		/* Iterate over all lines */
     -+		for (line = principal_out.buf; *line; line = strchrnul(line + 1, '\n')) {
     ++		ret = pipe_command(&ssh_keygen, payload, payload_size, &ssh_keygen_out, 0, &ssh_keygen_err, 0);
     ++	} else {
     ++		// Check every principal we found (one per line)
     ++		for (line = ssh_keygen_out.buf; *line; line = strchrnul(line + 1, '\n')) {
      +			while (*line == '\n')
      +				line++;
      +			if (!*line)
      +				break;
     - 
     --	if (!gpg_status)
     --		gpg_status = &buf;
     ++
      +			trust_size = strcspn(line, " \n");
      +			principal = xmemdupz(line, trust_size);
     - 
     --	sigchain_push(SIGPIPE, SIG_IGN);
     --	ret = pipe_command(&gpg, payload, payload_size,
     --			   gpg_status, 0, gpg_output, 0);
     --	sigchain_pop(SIGPIPE);
     -+			strvec_push(&gpg.args,fmt->program);
     ++
     ++			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(&gpg.args, "-Y", "verify",
     -+						"-n", "git",
     -+						"-f", get_ssh_allowed_signers(),
     -+						"-I", principal,
     -+						"-s", temp->filename.buf,
     -+						 NULL);
     - 
     --	delete_tempfile(&temp);
     -+			if (ssh_revocation_file) {
     -+				strvec_pushl(&gpg.args, "-r", ssh_revocation_file, NULL);
     -+			}
     ++			strvec_pushl(&ssh_keygen.args,  "-Y", "verify",
     ++							//TODO: sprintf("-Overify-time=%s", commit->date...),
     ++							"-n", "git",
     ++							"-f", get_ssh_allowed_signers(),
     ++							"-I", principal,
     ++							"-s", temp->filename.buf,
     ++							NULL);
      +
     -+			if (!gpg_status)
     -+				gpg_status = &buf;
     ++			if (ssh_revocation_file && file_exists(ssh_revocation_file)) {
     ++				strvec_pushl(&ssh_keygen.args, "-r", ssh_revocation_file, NULL);
     ++			}
      +
      +			sigchain_push(SIGPIPE, SIG_IGN);
     -+			ret = pipe_command(&gpg, payload, payload_size,
     -+					   gpg_status, 0, gpg_output, 0);
     ++			ret = pipe_command(&ssh_keygen, payload, payload_size,
     ++					&ssh_keygen_out, 0, &ssh_keygen_err, 0);
      +			sigchain_pop(SIGPIPE);
     - 
     --	ret |= !strstr(gpg_status->buf, "\n[GNUPG:] GOODSIG ");
     -+			ret |= !strstr(gpg_status->buf, "Good");
     ++
     ++			ret &= starts_with(ssh_keygen_out.buf, "Good");
      +			if (ret == 0)
      +				break;
      +		}
     -+	} else {
     -+		strvec_push(&gpg.args, fmt->program);
     -+		strvec_pushv(&gpg.args, fmt->verify_args);
     -+		strvec_pushl(&gpg.args,
     -+				"--status-fd=1",
     -+				"--verify", temp->filename.buf, "-",
     -+				NULL);
     ++	}
      +
     -+		if (!gpg_status)
     -+			gpg_status = &buf;
     ++	sigc->payload = xmemdupz(payload, payload_size);
     ++	strbuf_stripspace(&ssh_keygen_out, 0);
     ++	strbuf_stripspace(&ssh_keygen_err, 0);
     ++	strbuf_add(&ssh_keygen_out, ssh_keygen_err.buf, ssh_keygen_err.len);
     ++	sigc->output = strbuf_detach(&ssh_keygen_out, NULL);
      +
     -+		sigchain_push(SIGPIPE, SIG_IGN);
     -+		ret = pipe_command(&gpg, payload, payload_size, gpg_status, 0,
     -+				   gpg_output, 0);
     -+		sigchain_pop(SIGPIPE);
     -+		ret |= !strstr(gpg_status->buf, "\n[GNUPG:] GOODSIG ");
     ++	//sigc->gpg_output = strbuf_detach(&ssh_keygen_err, NULL); // This flip around is broken...
     ++	sigc->gpg_status = strbuf_detach(&ssh_keygen_out, NULL);
     ++
     ++	parse_ssh_output(sigc);
     ++
     ++	delete_tempfile(&temp);
     ++	strbuf_release(&ssh_keygen_out);
     ++	strbuf_release(&ssh_keygen_err);
     ++
     ++	return ret;
     ++}
     ++
     ++static int verify_gpg_signature(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 tempfile *temp;
     ++	int ret;
     ++	struct strbuf gpg_out = STRBUF_INIT;
     ++	struct strbuf gpg_err = STRBUF_INIT;
     ++
     ++	temp = mks_tempfile_t(".git_vtag_tmpXXXXXX");
     ++	if (!temp)
     ++		return error_errno(_("could not create temporary file"));
     ++	if (write_in_full(temp->fd, signature, signature_size) < 0 ||
     ++	    close_tempfile_gently(temp) < 0) {
     ++		error_errno(_("failed writing detached signature to '%s'"),
     ++			    temp->filename.buf);
     ++		delete_tempfile(&temp);
     ++		return -1;
      +	}
     + 
     + 	strvec_push(&gpg.args, fmt->program);
     + 	strvec_pushv(&gpg.args, fmt->verify_args);
     + 	strvec_pushl(&gpg.args,
     +-		     "--status-fd=1",
     +-		     "--verify", temp->filename.buf, "-",
     +-		     NULL);
     +-
     +-	if (!gpg_status)
     +-		gpg_status = &buf;
     ++			"--status-fd=1",
     ++			"--verify", temp->filename.buf, "-",
     ++			NULL);
     + 
     + 	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_out, 0,
     ++				&gpg_err, 0);
     + 	sigchain_pop(SIGPIPE);
     ++	ret |= !strstr(gpg_out.buf, "\n[GNUPG:] GOODSIG ");
     + 
     +-	delete_tempfile(&temp);
     ++	sigc->payload = xmemdupz(payload, payload_size);
     ++	sigc->output = strbuf_detach(&gpg_err, NULL);
     ++	sigc->gpg_status = strbuf_detach(&gpg_out, NULL);
     + 
     +-	ret |= !strstr(gpg_status->buf, "\n[GNUPG:] GOODSIG ");
     +-	strbuf_release(&buf); /* no matter it was used or not */
     ++	parse_gpg_output(sigc);
      +
     -+out:
      +	delete_tempfile(&temp);
     -+	strbuf_release(&principal_out);
     -+	strbuf_release(&principal_err);
     - 	strbuf_release(&buf); /* no matter it was used or not */
     ++	strbuf_release(&gpg_out);
     ++	strbuf_release(&gpg_err);
       
       	return ret;
     -@@ gpg-interface.c: int check_signature(const char *payload, size_t plen, const char *signature,
     - 	sigc->payload = xmemdupz(payload, plen);
     - 	sigc->gpg_output = strbuf_detach(&gpg_output, NULL);
     - 	sigc->gpg_status = strbuf_detach(&gpg_status, NULL);
     + }
     +@@ gpg-interface.c: 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);
     -+	if (!strcmp(use_format->name, "ssh")) {
     -+		parse_ssh_output(sigc);
     ++	fmt = get_format_by_sig(signature);
     ++	if (!fmt)
     ++		BUG("bad signature '%s'", signature);
     ++
     ++	if (!strcmp(fmt->name, "ssh")) {
     ++		status = verify_ssh_signature(sigc, fmt, payload, plen, signature, slen);
      +	} else {
     -+		parse_gpg_output(sigc);
     ++		status = verify_gpg_signature(sigc, fmt, payload, plen, signature, slen);
      +	}
     ++	if (status && !sigc->gpg_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;
     ++		sigc->gpg_status : sigc->output;
     + 
     + 	if (flags & GPG_VERIFY_VERBOSE && sigc->payload)
     + 		fputs(sigc->payload, stdout);
      @@ gpg-interface.c: int git_gpg_config(const char *var, const char *value, void *cb)
     + 	int ret;
     + 
     + 	if (!strcmp(var, "user.signingkey")) {
     ++		/* user.signingkey can contain one of the following
     ++		 * when format = openpgp/x509
     ++		 *   - GPG KeyID
     ++		 * when format = ssh
     ++		 *   - literal ssh public key (e.g. ssh-rsa XXXKEYXXX comment)
     ++		 *   - path to a file containing a public or a private ssh key
     ++		 */
     + 		if (!value)
     + 			return config_error_nonbool(var);
     + 		set_signing_key(value);
       		return 0;
       	}
       
     -+	if (!strcmp(var, "gpg.ssh.allowedsigners")) {
     ++	if (!strcmp(var, "gpg.ssh.keyring")) {
     ++		if (!value)
     ++			return config_error_nonbool(var);
      +		return git_config_string(&ssh_allowed_signers, var, value);
      +	}
      +
     -+	if (!strcmp(var, "gpg.ssh.revocationfile")) {
     ++	if (!strcmp(var, "gpg.ssh.revocationkeyring")) {
     ++		if (!value)
     ++			return config_error_nonbool(var);
      +		return git_config_string(&ssh_revocation_file, var, value);
      +	}
      +
     @@ gpg-interface.c: int git_gpg_config(const char *var, const char *value, void *cb
       	if (fmtname) {
       		fmt = get_format_by_name(fmtname);
       		return git_config_string(&fmt->program, var, value);
     -@@ gpg-interface.c: const char *get_signing_key(void)
     +@@ gpg-interface.c: 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;
     ++
     ++	/* For 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]) {
     ++			return strbuf_detach(fingerprint[1], NULL);
     ++		}
     ++	}
     ++	die_errno(_("failed to get the ssh fingerprint for key '%s'"), signing_key);
     ++}
     ++
     ++// Returns the first public key from an ssh-agent to use for signing
     ++static char *get_default_ssh_signing_key(void) {
     ++	struct child_process ssh_add = CHILD_PROCESS_INIT;
     ++	int ret = -1;
     ++	struct strbuf key_stdout = STRBUF_INIT;
     ++	struct strbuf **keys;
     ++
     ++	strvec_pushl(&ssh_add.args, "ssh-add", "-L", NULL);
     ++	ret = pipe_command(&ssh_add, NULL, 0, &key_stdout, 0, NULL, 0);
     ++	if (!ret) { 
     ++		keys = strbuf_split_max(&key_stdout, '\n', 2);
     ++		if (keys[0])
     ++			return strbuf_detach(keys[0], NULL);
     ++	}
     ++
     ++	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)
       		return configured_signing_key;
      -	return git_committer_info(IDENT_STRICT|IDENT_NO_DATE);
      +	if (!strcmp(use_format->name, "ssh")) {
     -+		// We could simply use the first key listed by ssh-add -L and risk signing with the wrong key
     -+		return "";
     ++		return get_default_ssh_signing_key();
      +	} else {
      +		return git_committer_info(IDENT_STRICT | IDENT_NO_DATE);
      +	}
     @@ gpg-interface.c: const char *get_signing_key(void)
      +{
      +	if (ssh_allowed_signers)
      +		return ssh_allowed_signers;
     -+	return GPG_SSH_ALLOWED_SIGNERS;
     ++
     ++	die("A Path to an allowed signers ssh keyring is needed for validation");
       }
       
       int sign_buffer(struct strbuf *buffer, struct strbuf *signature, const char *signing_key)
     @@ gpg-interface.c: int sign_buffer(struct strbuf *buffer, struct strbuf *signature
       	int ret;
       	size_t i, j, bottom;
       	struct strbuf gpg_status = STRBUF_INIT;
     --
     ++	struct tempfile *temp = NULL, *buffer_file = NULL;
     ++	char *ssh_signing_key_file = NULL;
     ++	struct strbuf ssh_signature_filename = STRBUF_INIT;
     ++
     ++	if (!strcmp(use_format->name, "ssh")) {
     ++		if (!signing_key || signing_key[0] == '\0')
     ++			return error(_("user.signingkey needs to be set for ssh signing"));
     ++
     ++
     ++		if (istarts_with(signing_key, "ssh-")) {
     ++			// A literal ssh key
     ++			temp = mks_tempfile_t(".git_signing_key_tmpXXXXXX");
     ++			if (!temp)
     ++				return error_errno(_("could not create temporary file"));
     ++			if (write_in_full(temp->fd, signing_key, strlen(signing_key)) < 0 ||
     ++				close_tempfile_gently(temp) < 0) {
     ++				error_errno(_("failed writing ssh signing key to '%s'"), temp->filename.buf);
     ++				delete_tempfile(&temp);
     ++				return -1;
     ++			}
     ++			ssh_signing_key_file= temp->filename.buf;
     ++		} else {
     ++			// We assume a file
     ++			ssh_signing_key_file = expand_user_path(signing_key, 1);
     ++		}
     + 
      -	strvec_pushl(&gpg.args,
      -		     use_format->program,
      -		     "--status-fd=2",
      -		     "-bsau", signing_key,
      -		     NULL);
     -+	struct tempfile *temp = NULL;
     -+
     -+	if (!strcmp(use_format->name, "ssh")) {
     -+		if (!signing_key)
     -+			return error(_("user.signingkey needs to be set to a ssh public key for ssh signing"));
     -+
     -+		// signing_key is a public ssh key
     -+		// FIXME: Allow specifying a key file so we can use private keyfiles instead of ssh-agent
     -+		temp = mks_tempfile_t(".git_signing_key_tmpXXXXXX");
     -+		if (!temp)
     ++		buffer_file = mks_tempfile_t(".git_signing_buffer_tmpXXXXXX");
     ++		if (!buffer_file)
      +			return error_errno(_("could not create temporary file"));
     -+		if (write_in_full(temp->fd, signing_key,
     -+					strlen(signing_key)) < 0 ||
     -+			close_tempfile_gently(temp) < 0) {
     -+			error_errno(_("failed writing ssh signing key to '%s'"), temp->filename.buf);
     -+			delete_tempfile(&temp);
     ++		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);
     ++			delete_tempfile(&buffer_file);
      +			return -1;
      +		}
     ++
      +		strvec_pushl(&gpg.args, use_format->program ,
      +					"-Y", "sign",
      +					"-n", "git",
     -+					"-f", temp->filename.buf,
     ++					"-f", ssh_signing_key_file,
     ++					buffer_file->filename.buf,
      +					NULL);
     ++
     ++		sigchain_push(SIGPIPE, SIG_IGN);
     ++		ret = pipe_command(&gpg, NULL, 0, NULL, 0, &gpg_status, 0);
     ++		sigchain_pop(SIGPIPE);
     ++
     ++		strbuf_addbuf(&ssh_signature_filename, &buffer_file->filename);
     ++		strbuf_addstr(&ssh_signature_filename, ".sig");
     ++		if (strbuf_read_file(signature, ssh_signature_filename.buf, 2048) < 0) {
     ++			error_errno(_("failed reading ssh signing data buffer from '%s'"), ssh_signature_filename.buf);
     ++		}
     ++		unlink_or_warn(ssh_signature_filename.buf);
     ++		strbuf_release(&ssh_signature_filename);
     ++		delete_tempfile(&buffer_file);
      +	} else {
      +		strvec_pushl(&gpg.args, use_format->program ,
      +					"--status-fd=2",
      +					"-bsau", signing_key,
      +					NULL);
     ++
     ++		/*
     ++		* When the username signingkey is bad, program could be terminated
     ++		* because gpg exits without reading and then write gets SIGPIPE.
     ++		*/
     ++		sigchain_push(SIGPIPE, SIG_IGN);
     ++		ret = pipe_command(&gpg, buffer->buf, buffer->len, signature, 1024, &gpg_status, 0);
     ++		sigchain_pop(SIGPIPE);
      +	}
       
       	bottom = signature->len;
       
     -@@ gpg-interface.c: int sign_buffer(struct strbuf *buffer, struct strbuf *signature, const char *sig
     - 			   signature, 1024, &gpg_status, 0);
     - 	sigchain_pop(SIGPIPE);
     - 
     --	ret |= !strstr(gpg_status.buf, "\n[GNUPG:] SIG_CREATED ");
     +-	/*
     +-	 * When the username signingkey is bad, program could be terminated
     +-	 * because gpg exits without reading and then write gets SIGPIPE.
     +-	 */
     +-	sigchain_push(SIGPIPE, SIG_IGN);
     +-	ret = pipe_command(&gpg, buffer->buf, buffer->len,
     +-			   signature, 1024, &gpg_status, 0);
     +-	sigchain_pop(SIGPIPE);
      +	if (temp)
      +		delete_tempfile(&temp);
     -+
     + 
     +-	ret |= !strstr(gpg_status.buf, "\n[GNUPG:] SIG_CREATED ");
      +	if (!strcmp(use_format->name, "ssh")) {
      +		if (strstr(gpg_status.buf, "unknown option")) {
      +			error(_("openssh version > 8.2p1 is needed for ssh signing (ssh-keygen needs -Y sign option)"));
     @@ gpg-interface.c: int sign_buffer(struct strbuf *buffer, struct strbuf *signature
       		return error(_("gpg failed to sign the data"));
      
       ## gpg-interface.h ##
     -@@ gpg-interface.h: struct strbuf;
     - #define GPG_VERIFY_RAW			2
     - #define GPG_VERIFY_OMIT_STATUS	4
     +@@ gpg-interface.h: enum signature_trust_level {
       
     -+#define GPG_SSH_ALLOWED_SIGNERS ".gitsigners"
     -+
     - enum signature_trust_level {
     - 	TRUST_UNDEFINED,
     - 	TRUST_NEVER,
     + struct signature_check {
     + 	char *payload;
     +-	char *gpg_output;
     +-	char *gpg_status;
     ++	char *output;
     ++	char *gpg_output; // This will be printed in commit logs
     ++	char *gpg_status; // Only used internally -> remove
     + 
     + 	/*
     + 	 * possible "result":
      @@ gpg-interface.h: 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);
     ++
      +const char *get_ssh_allowed_signers(void);
       int check_signature(const char *payload, size_t plen,
       		    const char *signature, size_t slen,
       		    struct signature_check *sigc);
     +
     + ## log-tree.c ##
     +@@ log-tree.c: 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:
     +@@ log-tree.c: 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);
     +
     + ## pretty.c ##
     +@@ pretty.c: 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) {
     +
     + ## send-pack.c ##
     +@@ send-pack.c: 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) {
     +@@ send-pack.c: static int generate_push_cert(struct strbuf *req_buf,
     + 
     + free_return:
     + 	free(signing_key);
     ++	free(signing_key_id);
     + 	strbuf_release(&cert);
     + 	return update_seen;
     + }
     +
     + ## t/lib-gpg.sh ##
     +@@ t/lib-gpg.sh: test_lazy_prereq RFC1991 '
     + 	echo | gpg --homedir "${GNUPGHOME}" -b --rfc1991 >/dev/null
     + '
     + 
     ++test_lazy_prereq GPGSSH '
     ++	ssh_version=$(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 "${GNUPGHOME}" &&
     ++	ssh-keygen -t ed25519 -N "" -f "${GNUPGHOME}/ed25519_ssh_signing_key" >/dev/null &&
     ++	ssh-keygen -t rsa -b 2048 -N "" -f "${GNUPGHOME}/rsa_2048_ssh_signing_key" >/dev/null &&
     ++	ssh-keygen -t ed25519 -N "super_secret" -f "${GNUPGHOME}/protected_ssh_signing_key" >/dev/null &&
     ++	find "${GNUPGHOME}" -name *ssh_signing_key.pub -exec cat {} \; | awk "{print \"principal_\" NR \" \" \$0}" > "${GNUPGHOME}/ssh.all_valid.keyring" &&
     ++	cat "${GNUPGHOME}/ssh.all_valid.keyring" &&
     ++	ssh-keygen -t ed25519 -N "" -f "${GNUPGHOME}/untrusted_ssh_signing_key" >/dev/null
     ++'
     ++
     ++SIGNING_KEY_PRIMARY="${GNUPGHOME}/ed25519_ssh_signing_key"
     ++SIGNING_KEY_SECONDARY="${GNUPGHOME}/rsa_2048_ssh_signing_key"
     ++SIGNING_KEY_UNTRUSTED="${GNUPGHOME}/untrusted_ssh_signing_key"
     ++SIGNING_KEY_WITH_PASSPHRASE="${GNUPGHOME}/protected_ssh_signing_key"
     ++SIGNING_KEY_PASSPHRASE="super_secret"
     ++SIGNING_KEYRING="${GNUPGHOME}/ssh.all_valid.keyring"
     ++
     ++GOOD_SIGNATURE_TRUSTED='Good "git" signature for'
     ++GOOD_SIGNATURE_UNTRUSTED='Good "git" signature with'
     ++KEY_NOT_TRUSTED="No principal matched"
     ++BAD_SIGNATURE="Signature verification failed"
     ++
     + sanitize_pgp() {
     + 	perl -ne '
     + 		/^-----END PGP/ and $in_pgp = 0;
     +
     + ## t/t4202-log.sh ##
     +@@ t/t4202-log.sh: test_expect_success GPGSM 'setup signed branch x509' '
     + 	git commit -S -m signed_commit
     + '
     + 
     ++test_expect_success GPGSSH 'setup sshkey signed branch' '
     ++	test_config gpg.format ssh &&
     ++	test_config user.signingkey "${SIGNING_KEY_PRIMARY}" &&
     ++	test_when_finished "git reset --hard && git checkout main" &&
     ++	git checkout -b signed-ssh main &&
     ++	echo foo >foo &&
     ++	git add foo &&
     ++	git commit -S -m signed_commit
     ++'
     ++
     + test_expect_success GPGSM 'log x509 fingerprint' '
     + 	echo "F8BF62E0693D0694816377099909C779FA23FD65 | " >expect &&
     + 	git log -n1 --format="%GF | %GP" signed-x509 >actual &&
     +@@ t/t4202-log.sh: test_expect_success GPGSM 'log --graph --show-signature x509' '
     + 	grep "^| gpgsm: Good signature" actual
     + '
     + 
     ++test_expect_success GPGSSH 'log ssh key fingerprint' '
     ++	test_config gpg.ssh.keyring "${SIGNING_KEYRING}" &&
     ++	ssh-keygen -lf  "${SIGNING_KEY_PRIMARY}" | awk "{print \$2\" | \"}" >expect &&
     ++	git log -n1 --format="%GF | %GP" signed-ssh >actual &&
     ++	test_cmp expect actual
     ++'
     ++
     + test_expect_success GPG 'log --graph --show-signature for merged tag' '
     + 	test_when_finished "git reset --hard && git checkout main" &&
     + 	git checkout -b plain main &&
     +
     + ## t/t5534-push-signed.sh ##
     +@@ t/t5534-push-signed.sh: test_expect_success GPG 'signed push sends push certificate' '
     + 	test_cmp expect dst/push-cert-status
     + '
     + 
     ++test_expect_success GPGSSH 'ssh signed push sends push certificate' '
     ++	prepare_dst &&
     ++	mkdir -p dst/.git/hooks &&
     ++	git -C dst config gpg.ssh.keyring "${SIGNING_KEYRING}" &&
     ++	git -C dst config receive.certnonceseed sekrit &&
     ++	write_script dst/.git/hooks/post-receive <<-\EOF &&
     ++	# discard the update list
     ++	cat >/dev/null
     ++	# record the push certificate
     ++	if test -n "${GIT_PUSH_CERT-}"
     ++	then
     ++		git cat-file blob $GIT_PUSH_CERT >../push-cert
     ++	fi &&
     ++
     ++	cat >../push-cert-status <<E_O_F
     ++	SIGNER=${GIT_PUSH_CERT_SIGNER-nobody}
     ++	KEY=${GIT_PUSH_CERT_KEY-nokey}
     ++	STATUS=${GIT_PUSH_CERT_STATUS-nostatus}
     ++	NONCE_STATUS=${GIT_PUSH_CERT_NONCE_STATUS-nononcestatus}
     ++	NONCE=${GIT_PUSH_CERT_NONCE-nononce}
     ++	E_O_F
     ++
     ++	EOF
     ++
     ++	test_config gpg.format ssh &&
     ++	test_config user.signingkey "${SIGNING_KEY_PRIMARY}" &&
     ++	FINGERPRINT=$(ssh-keygen -lf "${SIGNING_KEY_PRIMARY}" | awk "{print \$2;}") &&
     ++	git push --signed dst noop ff +noff &&
     ++
     ++	(
     ++		cat <<-\EOF &&
     ++		SIGNER=principal_1
     ++		KEY=FINGERPRINT
     ++		STATUS=G
     ++		NONCE_STATUS=OK
     ++		EOF
     ++		sed -n -e "s/^nonce /NONCE=/p" -e "/^$/q" dst/push-cert
     ++	) | sed -e "s|FINGERPRINT|$FINGERPRINT|" >expect &&
     ++
     ++	noop=$(git rev-parse noop) &&
     ++	ff=$(git rev-parse ff) &&
     ++	noff=$(git rev-parse noff) &&
     ++	grep "$noop $ff refs/heads/ff" dst/push-cert &&
     ++	grep "$noop $noff refs/heads/noff" dst/push-cert &&
     ++	test_cmp expect dst/push-cert-status
     ++'
     ++
     + test_expect_success GPG 'inconsistent push options in signed push not allowed' '
     + 	# First, invoke receive-pack with dummy input to obtain its preamble.
     + 	prepare_dst &&
     +@@ t/t5534-push-signed.sh: test_expect_success GPGSM 'fail without key and heed user.signingkey x509' '
     + 	test_cmp expect dst/push-cert-status
     + '
     + 
     ++test_expect_success GPGSSH 'fail without key and heed user.signingkey ssh' '
     ++	test_config gpg.format ssh &&
     ++	prepare_dst &&
     ++	mkdir -p dst/.git/hooks &&
     ++	git -C dst config gpg.ssh.keyring "${SIGNING_KEYRING}" &&
     ++	git -C dst config receive.certnonceseed sekrit &&
     ++	write_script dst/.git/hooks/post-receive <<-\EOF &&
     ++	# discard the update list
     ++	cat >/dev/null
     ++	# record the push certificate
     ++	if test -n "${GIT_PUSH_CERT-}"
     ++	then
     ++		git cat-file blob $GIT_PUSH_CERT >../push-cert
     ++	fi &&
     ++
     ++	cat >../push-cert-status <<E_O_F
     ++	SIGNER=${GIT_PUSH_CERT_SIGNER-nobody}
     ++	KEY=${GIT_PUSH_CERT_KEY-nokey}
     ++	STATUS=${GIT_PUSH_CERT_STATUS-nostatus}
     ++	NONCE_STATUS=${GIT_PUSH_CERT_NONCE_STATUS-nononcestatus}
     ++	NONCE=${GIT_PUSH_CERT_NONCE-nononce}
     ++	E_O_F
     ++
     ++	EOF
     ++
     ++	test_config user.email hasnokey@nowhere.com &&
     ++	test_config gpg.format ssh &&
     ++	
     ++	test_config user.signingkey "" &&
     ++	(
     ++		sane_unset GIT_COMMITTER_EMAIL &&
     ++		test_must_fail git push --signed dst noop ff +noff
     ++	) &&
     ++	test_config user.signingkey "${SIGNING_KEY_PRIMARY}" &&
     ++	FINGERPRINT=$(ssh-keygen -lf "${SIGNING_KEY_PRIMARY}" | awk "{print \$2;}") &&
     ++	git push --signed dst noop ff +noff &&
     ++
     ++	(
     ++		cat <<-\EOF &&
     ++		SIGNER=principal_1
     ++		KEY=FINGERPRINT
     ++		STATUS=G
     ++		NONCE_STATUS=OK
     ++		EOF
     ++		sed -n -e "s/^nonce /NONCE=/p" -e "/^$/q" dst/push-cert
     ++	) | sed -e "s|FINGERPRINT|$FINGERPRINT|" >expect &&
     ++
     ++	noop=$(git rev-parse noop) &&
     ++	ff=$(git rev-parse ff) &&
     ++	noff=$(git rev-parse noff) &&
     ++	grep "$noop $ff refs/heads/ff" dst/push-cert &&
     ++	grep "$noop $noff refs/heads/noff" dst/push-cert &&
     ++	test_cmp expect dst/push-cert-status
     ++'
     ++
     + test_expect_success GPG 'failed atomic push does not execute GPG' '
     + 	prepare_dst &&
     + 	git -C dst config receive.certnonceseed sekrit &&
     +
     + ## t/t7031-verify-tag-signed-ssh.sh (new) ##
     +@@
     ++#!/bin/sh
     ++
     ++test_description='signed tag tests'
     ++GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main
     ++export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
     ++
     ++. ./test-lib.sh
     ++. "$TEST_DIRECTORY/lib-gpg.sh"
     ++
     ++test_expect_success GPGSSH 'create signed tags ssh' '
     ++	test_when_finished "test_unconfig commit.gpgsign" &&
     ++	test_config gpg.format ssh &&
     ++	test_config user.signingkey "${SIGNING_KEY_PRIMARY}" &&
     ++
     ++	echo 1 >file && git add file &&
     ++	test_tick && git commit -m initial &&
     ++	git tag -s -m initial initial &&
     ++	git branch side &&
     ++
     ++	echo 2 >file && test_tick && git commit -a -m second &&
     ++	git tag -s -m second second &&
     ++
     ++	git checkout side &&
     ++	echo 3 >elif && git add elif &&
     ++	test_tick && git commit -m "third on side" &&
     ++
     ++	git checkout main &&
     ++	test_tick && git merge -S side &&
     ++	git tag -s -m merge merge &&
     ++
     ++	echo 4 >file && test_tick && git commit -a -S -m "fourth unsigned" &&
     ++	git tag -a -m fourth-unsigned fourth-unsigned &&
     ++
     ++	test_tick && git commit --amend -S -m "fourth signed" &&
     ++	git tag -s -m fourth fourth-signed &&
     ++
     ++	echo 5 >file && test_tick && git commit -a -m "fifth" &&
     ++	git tag fifth-unsigned &&
     ++
     ++	git config commit.gpgsign true &&
     ++	echo 6 >file && test_tick && git commit -a -m "sixth" &&
     ++	git tag -a -m sixth sixth-unsigned &&
     ++
     ++	test_tick && git rebase -f HEAD^^ && git tag -s -m 6th sixth-signed HEAD^ &&
     ++	git tag -m seventh -s seventh-signed &&
     ++
     ++	echo 8 >file && test_tick && git commit -a -m eighth &&
     ++	git tag -u"${SIGNING_KEY_UNTRUSTED}" -m eighth eighth-signed-alt
     ++'
     ++
     ++test_expect_success GPGSSH 'verify and show ssh signatures' '
     ++	test_config gpg.ssh.keyring "${SIGNING_KEYRING}" &&
     ++	test_config gpg.mintrustlevel UNDEFINED &&
     ++	(
     ++		for tag in initial second merge fourth-signed sixth-signed seventh-signed
     ++		do
     ++			git verify-tag $tag 2>actual &&
     ++			grep "${GOOD_SIGNATURE_TRUSTED}" actual &&
     ++			! grep "${BAD_SIGNATURE}" actual &&
     ++			echo $tag OK || exit 1
     ++		done
     ++	) &&
     ++	(
     ++		for tag in fourth-unsigned fifth-unsigned sixth-unsigned
     ++		do
     ++			test_must_fail git verify-tag $tag 2>actual &&
     ++			! grep "${GOOD_SIGNATURE_TRUSTED}" actual &&
     ++			! grep "${BAD_SIGNATURE}" actual &&
     ++			echo $tag OK || exit 1
     ++		done
     ++	) &&
     ++	(
     ++		for tag in eighth-signed-alt
     ++		do
     ++			git verify-tag $tag 2>actual &&
     ++			grep "${GOOD_SIGNATURE_UNTRUSTED}" actual &&
     ++			! grep "${BAD_SIGNATURE}" actual &&
     ++			grep "${KEY_NOT_TRUSTED}" actual &&
     ++			echo $tag OK || exit 1
     ++		done
     ++	)
     ++'
     ++
     ++test_expect_success GPGSSH 'detect fudged ssh signature' '
     ++	test_config gpg.ssh.keyring "${SIGNING_KEYRING}" &&
     ++	git cat-file tag seventh-signed >raw &&
     ++	sed -e "/^tag / s/seventh/7th forged/" raw >forged1 &&
     ++	git hash-object -w -t tag forged1 >forged1.tag &&
     ++	test_must_fail git verify-tag $(cat forged1.tag) 2>actual1 &&
     ++	grep "${BAD_SIGNATURE}" actual1 &&
     ++	! grep "${GOOD_SIGNATURE_TRUSTED}" actual1 &&
     ++	! grep "${GOOD_SIGNATURE_UNTRUSTED}" actual1
     ++'
     ++
     ++# test_expect_success GPGSSH 'verify ssh signatures with --raw' '
     ++# 	test_config gpg.ssh.keyring "${SIGNING_KEYRING}" &&
     ++# 	(
     ++# 		for tag in initial second merge fourth-signed sixth-signed seventh-signed
     ++# 		do
     ++# 			git verify-tag --raw $tag 2>actual &&
     ++# 			grep "GOODSIG" actual &&
     ++# 			! grep "BADSIG" actual &&
     ++# 			echo $tag OK || exit 1
     ++# 		done
     ++# 	) &&
     ++# 	(
     ++# 		for tag in fourth-unsigned fifth-unsigned sixth-unsigned
     ++# 		do
     ++# 			test_must_fail git verify-tag --raw $tag 2>actual &&
     ++# 			! grep "GOODSIG" actual &&
     ++# 			! grep "BADSIG" actual &&
     ++# 			echo $tag OK || exit 1
     ++# 		done
     ++# 	) &&
     ++# 	(
     ++# 		for tag in eighth-signed-alt
     ++# 		do
     ++# 			git verify-tag --raw $tag 2>actual &&
     ++# 			grep "GOODSIG" actual &&
     ++# 			! grep "BADSIG" actual &&
     ++# 			grep "TRUST_UNDEFINED" actual &&
     ++# 			echo $tag OK || exit 1
     ++# 		done
     ++# 	)
     ++# '
     ++
     ++# test_expect_success GPGSM 'verify signatures with --raw x509' '
     ++# 	git verify-tag --raw ninth-signed-x509 2>actual &&
     ++# 	grep "GOODSIG" actual &&
     ++# 	! grep "BADSIG" actual &&
     ++# 	echo ninth-signed-x509 OK
     ++# '
     ++
     ++# test_expect_success GPGSSH 'verify multiple tags' '
     ++# 	test_config gpg.ssh.keyring "${SIGNING_KEYRING}" &&
     ++# 	tags="fourth-signed sixth-signed seventh-signed" &&
     ++# 	for i in $tags
     ++# 	do
     ++# 		git verify-tag -v --raw $i || return 1
     ++# 	done >expect.stdout 2>expect.stderr.1 &&
     ++# 	grep "^.GNUPG:." <expect.stderr.1 >expect.stderr &&
     ++# 	git verify-tag -v --raw $tags >actual.stdout 2>actual.stderr.1 &&
     ++# 	grep "^.GNUPG:." <actual.stderr.1 >actual.stderr &&
     ++# 	test_cmp expect.stdout actual.stdout &&
     ++# 	test_cmp expect.stderr actual.stderr
     ++# '
     ++
     ++# test_expect_success GPGSM 'verify multiple tags x509' '
     ++#	test_config gpg.ssh.keyring "${SIGNING_KEYRING}" &&
     ++# 	tags="seventh-signed ninth-signed-x509" &&
     ++# 	for i in $tags
     ++# 	do
     ++# 		git verify-tag -v --raw $i || return 1
     ++# 	done >expect.stdout 2>expect.stderr.1 &&
     ++# 	grep "^.GNUPG:." <expect.stderr.1 >expect.stderr &&
     ++# 	git verify-tag -v --raw $tags >actual.stdout 2>actual.stderr.1 &&
     ++# 	grep "^.GNUPG:." <actual.stderr.1 >actual.stderr &&
     ++# 	test_cmp expect.stdout actual.stdout &&
     ++# 	test_cmp expect.stderr actual.stderr
     ++# '
     ++
     ++test_expect_success GPGSSH 'verifying tag with --format' '
     ++	test_config gpg.ssh.keyring "${SIGNING_KEYRING}" &&
     ++	cat >expect <<-\EOF &&
     ++	tagname : fourth-signed
     ++	EOF
     ++	git verify-tag --format="tagname : %(tag)" "fourth-signed" >actual &&
     ++	test_cmp expect actual
     ++'
     ++
     ++test_expect_success GPGSSH 'verifying a forged tag with --format should fail silently' '
     ++	test_must_fail git verify-tag --format="tagname : %(tag)" $(cat forged1.tag) >actual-forged &&
     ++	test_must_be_empty actual-forged
     ++'
     ++
     ++test_done
     +
     + ## t/t7527-signed-commit-ssh.sh (new) ##
     +@@
     ++#!/bin/sh
     ++
     ++test_description='ssh signed commit tests'
     ++GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main
     ++export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
     ++
     ++. ./test-lib.sh
     ++GNUPGHOME_NOT_USED=$GNUPGHOME
     ++. "$TEST_DIRECTORY/lib-gpg.sh"
     ++
     ++test_expect_success GPGSSH 'create signed commits' '
     ++	test_oid_cache <<-\EOF &&
     ++	header sha1:gpgsig
     ++	header sha256:gpgsig-sha256
     ++	EOF
     ++
     ++	test_when_finished "test_unconfig commit.gpgsign" &&
     ++	test_config gpg.format ssh &&
     ++	test_config user.signingkey "${SIGNING_KEY_PRIMARY}" &&
     ++
     ++	echo 1 >file && git add file &&
     ++	test_tick && git commit -S -m initial &&
     ++	git tag initial &&
     ++	git branch side &&
     ++
     ++	echo 2 >file && test_tick && git commit -a -S -m second &&
     ++	git tag second &&
     ++
     ++	git checkout side &&
     ++	echo 3 >elif && git add elif &&
     ++	test_tick && git commit -m "third on side" &&
     ++
     ++	git checkout main &&
     ++	test_tick && git merge -S side &&
     ++	git tag merge &&
     ++
     ++	echo 4 >file && test_tick && git commit -a -m "fourth unsigned" &&
     ++	git tag fourth-unsigned &&
     ++
     ++	test_tick && git commit --amend -S -m "fourth signed" &&
     ++	git tag fourth-signed &&
     ++
     ++	git config commit.gpgsign true &&
     ++	echo 5 >file && test_tick && git commit -a -m "fifth signed" &&
     ++	git tag fifth-signed &&
     ++
     ++	git config commit.gpgsign false &&
     ++	echo 6 >file && test_tick && git commit -a -m "sixth" &&
     ++	git tag sixth-unsigned &&
     ++
     ++	git config commit.gpgsign true &&
     ++	echo 7 >file && test_tick && git commit -a -m "seventh" --no-gpg-sign &&
     ++	git tag seventh-unsigned &&
     ++
     ++	test_tick && git rebase -f HEAD^^ && git tag sixth-signed HEAD^ &&
     ++	git tag seventh-signed &&
     ++
     ++	echo 8 >file && test_tick && git commit -a -m eighth -S"${SIGNING_KEY_UNTRUSTED}" &&
     ++	git tag eighth-signed-alt &&
     ++
     ++	# commit.gpgsign is still on but this must not be signed
     ++	echo 9 | git commit-tree HEAD^{tree} >oid &&
     ++	test_line_count = 1 oid &&
     ++	git tag ninth-unsigned $(cat oid) &&
     ++	# explicit -S of course must sign.
     ++	echo 10 | git commit-tree -S HEAD^{tree} >oid &&
     ++	test_line_count = 1 oid &&
     ++	git tag tenth-signed $(cat oid) &&
     ++
     ++	# --gpg-sign[=<key-id>] must sign.
     ++	echo 11 | git commit-tree --gpg-sign HEAD^{tree} >oid &&
     ++	test_line_count = 1 oid &&
     ++	git tag eleventh-signed $(cat oid) &&
     ++	echo 12 | git commit-tree --gpg-sign="${SIGNING_KEY_UNTRUSTED}" HEAD^{tree} >oid &&
     ++	test_line_count = 1 oid &&
     ++	git tag twelfth-signed-alt $(cat oid)
     ++'
     ++
     ++test_expect_success GPGSSH 'verify and show signatures' '
     ++	test_config gpg.ssh.keyring "${SIGNING_KEYRING}" &&
     ++	test_config gpg.mintrustlevel UNDEFINED &&
     ++	(
     ++		for commit in initial second merge fourth-signed \
     ++			fifth-signed sixth-signed seventh-signed tenth-signed \
     ++			eleventh-signed
     ++		do
     ++			git verify-commit $commit &&
     ++			git show --pretty=short --show-signature $commit >actual &&
     ++			grep "${GOOD_SIGNATURE_TRUSTED}" actual &&
     ++			! grep "${BAD_SIGNATURE}" actual &&
     ++			echo $commit OK || exit 1
     ++		done
     ++	) &&
     ++	(
     ++		for commit in merge^2 fourth-unsigned sixth-unsigned \
     ++			seventh-unsigned ninth-unsigned
     ++		do
     ++			test_must_fail git verify-commit $commit &&
     ++			git show --pretty=short --show-signature $commit >actual &&
     ++			! grep "${GOOD_SIGNATURE_TRUSTED}" actual &&
     ++			! grep "${BAD_SIGNATURE}" actual &&
     ++			echo $commit OK || exit 1
     ++		done
     ++	) &&
     ++	(
     ++		for commit in eighth-signed-alt twelfth-signed-alt
     ++		do
     ++			git show --pretty=short --show-signature $commit >actual &&
     ++			grep "${GOOD_SIGNATURE_UNTRUSTED}" actual &&
     ++			! grep "${BAD_SIGNATURE}" actual &&
     ++			grep "${KEY_NOT_TRUSTED}" actual &&
     ++			echo $commit OK || exit 1
     ++		done
     ++	)
     ++'
     ++
     ++test_expect_success GPGSSH 'verify-commit exits success on untrusted signature' '
     ++	test_config gpg.ssh.keyring "${SIGNING_KEYRING}" &&
     ++	git verify-commit eighth-signed-alt 2>actual &&
     ++	grep "${GOOD_SIGNATURE_UNTRUSTED}" actual &&
     ++	! grep "${BAD_SIGNATURE}" actual &&
     ++	grep "${KEY_NOT_TRUSTED}" actual
     ++'
     ++
     ++test_expect_success GPGSSH 'verify-commit exits success with matching minTrustLevel' '
     ++	test_config gpg.ssh.keyring "${SIGNING_KEYRING}" &&
     ++	test_config gpg.minTrustLevel fully &&
     ++	git verify-commit sixth-signed
     ++'
     ++
     ++test_expect_success GPGSSH 'verify-commit exits success with low minTrustLevel' '
     ++	test_config gpg.ssh.keyring "${SIGNING_KEYRING}" &&
     ++	test_config gpg.minTrustLevel marginal &&
     ++	git verify-commit sixth-signed
     ++'
     ++
     ++test_expect_success GPGSSH 'verify-commit exits failure with high minTrustLevel' '
     ++	test_config gpg.minTrustLevel ultimate &&
     ++	test_must_fail git verify-commit eighth-signed-alt
     ++'
     ++
     ++# test_expect_success GPGSSH 'verify signatures with --raw' '
     ++# 	(
     ++# 		for commit in initial second merge fourth-signed fifth-signed sixth-signed seventh-signed
     ++# 		do
     ++# 			git verify-commit --raw $commit 2>actual &&
     ++# 			grep "GOODSIG" actual &&
     ++# 			! grep "BADSIG" actual &&
     ++# 			echo $commit OK || exit 1
     ++# 		done
     ++# 	) &&
     ++# 	(
     ++# 		for commit in merge^2 fourth-unsigned sixth-unsigned seventh-unsigned
     ++# 		do
     ++# 			test_must_fail git verify-commit --raw $commit 2>actual &&
     ++# 			! grep "GOODSIG" actual &&
     ++# 			! grep "BADSIG" actual &&
     ++# 			echo $commit OK || exit 1
     ++# 		done
     ++# 	) &&
     ++# 	(
     ++# 		for commit in eighth-signed-alt
     ++# 		do
     ++# 			git verify-commit --raw $commit 2>actual &&
     ++# 			grep "GOODSIG" actual &&
     ++# 			! grep "BADSIG" actual &&
     ++# 			grep "TRUST_UNDEFINED" actual &&
     ++# 			echo $commit OK || exit 1
     ++# 		done
     ++# 	)
     ++# '
     ++
     ++test_expect_success GPGSSH 'proper header is used for hash algorithm' '
     ++	git cat-file commit fourth-signed >output &&
     ++	grep "^$(test_oid header) -----BEGIN SSH SIGNATURE-----" output
     ++'
     ++
     ++test_expect_success GPGSSH 'show signed commit with signature' '
     ++	test_config gpg.ssh.keyring "${SIGNING_KEYRING}" &&
     ++	git show -s initial >commit &&
     ++	git show -s --show-signature initial >show &&
     ++	git verify-commit -v initial >verify.1 2>verify.2 &&
     ++	git cat-file commit initial >cat &&
     ++	grep -v -e "${GOOD_SIGNATURE_TRUSTED}" -e "Warning: " show >show.commit &&
     ++	grep -e "${GOOD_SIGNATURE_TRUSTED}" -e "Warning: " show >show.gpg &&
     ++	grep -v "^ " cat | grep -v "^gpgsig.* " >cat.commit &&
     ++	test_cmp show.commit commit &&
     ++	test_cmp show.gpg verify.2 &&
     ++	test_cmp cat.commit verify.1
     ++'
     ++
     ++test_expect_success GPGSSH 'detect fudged signature' '
     ++	test_config gpg.ssh.keyring "${SIGNING_KEYRING}" &&
     ++	git cat-file commit seventh-signed >raw &&
     ++	sed -e "s/^seventh/7th forged/" raw >forged1 &&
     ++	git hash-object -w -t commit forged1 >forged1.commit &&
     ++	test_must_fail git verify-commit $(cat forged1.commit) &&
     ++	git show --pretty=short --show-signature $(cat forged1.commit) >actual1 &&
     ++	grep "${BAD_SIGNATURE}" actual1 &&
     ++	! grep "${GOOD_SIGNATURE_TRUSTED}" actual1 &&
     ++	! grep "${GOOD_SIGNATURE_UNTRUSTED}" actual1
     ++'
     ++
     ++test_expect_success GPGSSH 'detect fudged signature with NUL' '
     ++	test_config gpg.ssh.keyring "${SIGNING_KEYRING}" &&
     ++	git cat-file commit seventh-signed >raw &&
     ++	cat raw >forged2 &&
     ++	echo Qwik | tr "Q" "\000" >>forged2 &&
     ++	git hash-object -w -t commit forged2 >forged2.commit &&
     ++	test_must_fail git verify-commit $(cat forged2.commit) &&
     ++	git show --pretty=short --show-signature $(cat forged2.commit) >actual2 &&
     ++	grep "${BAD_SIGNATURE}" actual2 &&
     ++	! grep "${GOOD_SIGNATURE_TRUSTED}" actual2
     ++'
     ++
     ++test_expect_success GPGSSH 'amending already signed commit' '
     ++	test_config gpg.format ssh &&
     ++	test_config user.signingkey "${SIGNING_KEY_PRIMARY}" &&
     ++	test_config gpg.ssh.keyring "${SIGNING_KEYRING}" &&
     ++	git checkout fourth-signed^0 &&
     ++	git commit --amend -S --no-edit &&
     ++	git verify-commit HEAD &&
     ++	git show -s --show-signature HEAD >actual &&
     ++	grep "${GOOD_SIGNATURE_TRUSTED}" actual &&
     ++	! grep "${BAD_SIGNATURE}" actual
     ++'
     ++
     ++test_expect_success GPGSSH 'show good signature with custom format' '
     ++	test_config gpg.ssh.keyring "${SIGNING_KEYRING}" &&
     ++	FINGERPRINT=$(ssh-keygen -lf "${SIGNING_KEY_PRIMARY}" | awk "{print \$2;}") &&
     ++	cat >expect.tmpl <<-\EOF &&
     ++	G
     ++	FINGERPRINT
     ++	principal_1
     ++	FINGERPRINT
     ++	
     ++	EOF
     ++	sed "s|FINGERPRINT|$FINGERPRINT|g" expect.tmpl >expect &&
     ++	git log -1 --format="%G?%n%GK%n%GS%n%GF%n%GP" sixth-signed >actual &&
     ++	test_cmp expect actual
     ++'
     ++
     ++test_expect_success GPGSSH 'show bad signature with custom format' '
     ++	test_config gpg.ssh.keyring "${SIGNING_KEYRING}" &&
     ++	cat >expect <<-\EOF &&
     ++	B
     ++
     ++
     ++
     ++
     ++	EOF
     ++	git log -1 --format="%G?%n%GK%n%GS%n%GF%n%GP" $(cat forged1.commit) >actual &&
     ++	test_cmp expect actual
     ++'
     ++
     ++test_expect_success GPGSSH 'show untrusted signature with custom format' '
     ++	test_config gpg.ssh.keyring "${SIGNING_KEYRING}" &&
     ++	cat >expect.tmpl <<-\EOF &&
     ++	U
     ++	FINGERPRINT
     ++
     ++	FINGERPRINT
     ++	
     ++	EOF
     ++	git log -1 --format="%G?%n%GK%n%GS%n%GF%n%GP" eighth-signed-alt >actual &&
     ++	FINGERPRINT=$(ssh-keygen -lf "${SIGNING_KEY_UNTRUSTED}" | awk "{print \$2;}") &&
     ++	sed "s|FINGERPRINT|$FINGERPRINT|g" expect.tmpl >expect &&
     ++	test_cmp expect actual
     ++'
     ++
     ++test_expect_success GPGSSH 'show untrusted signature with undefined trust level' '
     ++	test_config gpg.ssh.keyring "${SIGNING_KEYRING}" &&
     ++	cat >expect.tmpl <<-\EOF &&
     ++	undefined
     ++	FINGERPRINT
     ++
     ++	FINGERPRINT
     ++
     ++	EOF
     ++	git log -1 --format="%GT%n%GK%n%GS%n%GF%n%GP" eighth-signed-alt >actual &&
     ++	FINGERPRINT=$(ssh-keygen -lf "${SIGNING_KEY_UNTRUSTED}" | awk "{print \$2;}") &&
     ++	sed "s|FINGERPRINT|$FINGERPRINT|g" expect.tmpl >expect &&
     ++	test_cmp expect actual
     ++'
     ++
     ++test_expect_success GPGSSH 'show untrusted signature with ultimate trust level' '
     ++	test_config gpg.ssh.keyring "${SIGNING_KEYRING}" &&
     ++	cat >expect.tmpl <<-\EOF &&
     ++	fully
     ++	FINGERPRINT
     ++	principal_1
     ++	FINGERPRINT
     ++
     ++	EOF
     ++	git log -1 --format="%GT%n%GK%n%GS%n%GF%n%GP" sixth-signed >actual &&
     ++	FINGERPRINT=$(ssh-keygen -lf "${SIGNING_KEY_PRIMARY}" | awk "{print \$2;}") &&
     ++	sed "s|FINGERPRINT|$FINGERPRINT|g" expect.tmpl >expect &&
     ++	test_cmp expect actual
     ++'
     ++
     ++test_expect_success GPGSSH 'show lack of signature with custom format' '
     ++	cat >expect <<-\EOF &&
     ++	N
     ++
     ++
     ++
     ++
     ++	EOF
     ++	git log -1 --format="%G?%n%GK%n%GS%n%GF%n%GP" seventh-unsigned >actual &&
     ++	test_cmp expect actual
     ++'
     ++
     ++test_expect_success GPGSSH 'log.showsignature behaves like --show-signature' '
     ++	test_config gpg.ssh.keyring "${SIGNING_KEYRING}" &&
     ++	test_config log.showsignature true &&
     ++	git show initial >actual &&
     ++	grep "${GOOD_SIGNATURE_TRUSTED}" actual
     ++'
     ++
     ++test_expect_success GPGSSH 'check config gpg.format values' '
     ++	test_config gpg.format ssh &&
     ++	test_config user.signingkey "${SIGNING_KEY_PRIMARY}" &&
     ++	test_config gpg.format ssh &&
     ++	git commit -S --amend -m "success" &&
     ++	test_config gpg.format OpEnPgP &&
     ++	test_must_fail git commit -S --amend -m "fail"
     ++'
     ++
     ++# test_expect_success GPGSSH 'detect fudged commit with double signature' '
     ++# 	sed -e "/gpgsig/,/END PGP/d" forged1 >double-base &&
     ++# 	sed -n -e "/gpgsig/,/END PGP/p" forged1 | \
     ++# 		sed -e "s/^$(test_oid header)//;s/^ //" | gpg --dearmor >double-sig1.sig &&
     ++# 	gpg -o double-sig2.sig -u 29472784 --detach-sign double-base &&
     ++# 	cat double-sig1.sig double-sig2.sig | gpg --enarmor >double-combined.asc &&
     ++# 	sed -e "s/^\(-.*\)ARMORED FILE/\1SIGNATURE/;1s/^/$(test_oid header) /;2,\$s/^/ /" \
     ++# 		double-combined.asc > double-gpgsig &&
     ++# 	sed -e "/committer/r double-gpgsig" double-base >double-commit &&
     ++# 	git hash-object -w -t commit double-commit >double-commit.commit &&
     ++# 	test_must_fail git verify-commit $(cat double-commit.commit) &&
     ++# 	git show --pretty=short --show-signature $(cat double-commit.commit) >double-actual &&
     ++# 	grep "BAD signature from" double-actual &&
     ++# 	grep "Good signature from" double-actual
     ++# '
     ++
     ++# test_expect_success GPGSSH 'show double signature with custom format' '
     ++# 	cat >expect <<-\EOF &&
     ++# 	E
     ++
     ++
     ++
     ++
     ++# 	EOF
     ++# 	git log -1 --format="%G?%n%GK%n%GS%n%GF%n%GP" $(cat double-commit.commit) >actual &&
     ++# 	test_cmp expect actual
     ++# '
     ++
     ++
     ++# test_expect_success GPGSSH 'verify-commit verifies multiply signed commits' '
     ++# 	git init multiply-signed &&
     ++# 	cd multiply-signed &&
     ++# 	test_commit first &&
     ++# 	echo 1 >second &&
     ++# 	git add second &&
     ++# 	tree=$(git write-tree) &&
     ++# 	parent=$(git rev-parse HEAD^{commit}) &&
     ++# 	git commit --gpg-sign -m second &&
     ++# 	git cat-file commit HEAD &&
     ++# 	# Avoid trailing whitespace.
     ++# 	sed -e "s/^Q//" -e "s/^Z/ /" >commit <<-EOF &&
     ++# 	Qtree $tree
     ++# 	Qparent $parent
     ++# 	Qauthor A U Thor [off-list ref] 1112912653 -0700
     ++# 	Qcommitter C O Mitter [off-list ref] 1112912653 -0700
     ++# 	Qgpgsig -----BEGIN PGP SIGNATURE-----
     ++# 	QZ
     ++# 	Q iHQEABECADQWIQRz11h0S+chaY7FTocTtvUezd5DDQUCX/uBDRYcY29tbWl0dGVy
     ++# 	Q QGV4YW1wbGUuY29tAAoJEBO29R7N3kMNd+8AoK1I8mhLHviPH+q2I5fIVgPsEtYC
     ++# 	Q AKCTqBh+VabJceXcGIZuF0Ry+udbBQ==
     ++# 	Q =tQ0N
     ++# 	Q -----END PGP SIGNATURE-----
     ++# 	Qgpgsig-sha256 -----BEGIN PGP SIGNATURE-----
     ++# 	QZ
     ++# 	Q iHQEABECADQWIQRz11h0S+chaY7FTocTtvUezd5DDQUCX/uBIBYcY29tbWl0dGVy
     ++# 	Q QGV4YW1wbGUuY29tAAoJEBO29R7N3kMN/NEAn0XO9RYSBj2dFyozi0JKSbssYMtO
     ++# 	Q AJwKCQ1BQOtuwz//IjU8TiS+6S4iUw==
     ++# 	Q =pIwP
     ++# 	Q -----END PGP SIGNATURE-----
     ++# 	Q
     ++# 	Qsecond
     ++# 	EOF
     ++# 	head=$(git hash-object -t commit -w commit) &&
     ++# 	git reset --hard $head &&
     ++# 	git verify-commit $head 2>actual &&
     ++# 	grep "Good signature from" actual &&
     ++# 	! grep "BAD signature from" actual
     ++# '
     ++
     ++test_done


 Documentation/config/gpg.txt     |  35 ++-
 Documentation/config/user.txt    |   6 +
 builtin/receive-pack.c           |   4 +-
 fmt-merge-msg.c                  |   4 +-
 gpg-interface.c                  | 414 +++++++++++++++++++++++++++----
 gpg-interface.h                  |  12 +-
 log-tree.c                       |   8 +-
 pretty.c                         |   4 +-
 send-pack.c                      |   6 +-
 t/lib-gpg.sh                     |  27 ++
 t/t4202-log.sh                   |  17 ++
 t/t5534-push-signed.sh           | 102 ++++++++
 t/t7031-verify-tag-signed-ssh.sh | 176 +++++++++++++
 t/t7527-signed-commit-ssh.sh     | 398 +++++++++++++++++++++++++++++
 14 files changed, 1147 insertions(+), 66 deletions(-)
 create mode 100755 t/t7031-verify-tag-signed-ssh.sh
 create mode 100755 t/t7527-signed-commit-ssh.sh

diff --git a/Documentation/config/gpg.txt b/Documentation/config/gpg.txt
index d94025cb368..c2bc4d06a66 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 +33,34 @@ gpg.minTrustLevel::
 * `marginal`
 * `fully`
 * `ultimate`
+
+gpg.ssh.keyring::
+	A file containing all valid SSH public signing keys. 
+	Similar to an .ssh/authorized_keys file.
+	See ssh-keygen(1) "ALLOWED SIGNERS" for details.
+	If a signing key is found in this file then the trust level will
+	be set to "fully". Otherwise if the key is not present
+	but the signature is still valid then the trust level will be "undefined".
+
+	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 some automation that already handles developer ssh keys. 
+	
+	A repository that is only allowing signed commits can store the file 
+	in the repository itself using a relative path. 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.
+
+	To revoke a key place the public key without the principal into the 
+	revocationKeyring.
+
+gpg.ssh.revocationKeyring::
+	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 59aec7c3aed..e71a099b8b8 100644
--- a/Documentation/config/user.txt
+++ b/Documentation/config/user.txt

@@ -36,3 +36,9 @@ 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 
+	"ssh-add -L" and try to use the first key available.

diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index a34742513ac..fd790f7fd72 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c

@@ -131,6 +131,8 @@ static int receive_pack_config(const char *var, const char *value, void *cb)
 {
 	int status = parse_hide_refs_config(var, value, "receive");
 
+	git_gpg_config(var, value, NULL);
+
 	if (status)
 		return status;
 

@@ -767,7 +769,7 @@ static void prepare_push_cert_sha1(struct child_process *proc)
 		bogs = parse_signed_buffer(push_cert.buf, push_cert.len);
 		check_signature(push_cert.buf, bogs, push_cert.buf + bogs,
 				push_cert.len - bogs, &sigcheck);
-
+		
 		nonce_status = check_nonce(push_cert.buf, bogs);
 	}
 	if (!is_null_oid(&push_cert_oid)) {

diff --git a/fmt-merge-msg.c b/fmt-merge-msg.c
index 0f66818e0f8..1d7b64fa021 100644
--- a/fmt-merge-msg.c
+++ b/fmt-merge-msg.c

@@ -527,10 +527,10 @@ static void fmt_merge_msg_sigs(struct strbuf *out)
 			len = payload.len;
 			if (check_signature(payload.buf, payload.len, sig.buf,
 					 sig.len, &sigc) &&
-				!sigc.gpg_output)
+				!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 127aecfc2b0..3b1a350bcfd 100644
--- a/gpg-interface.c
+++ b/gpg-interface.c

@@ -3,11 +3,13 @@
 #include "config.h"
 #include "run-command.h"
 #include "strbuf.h"
+#include "dir.h"
 #include "gpg-interface.h"
 #include "sigchain.h"
 #include "tempfile.h"
 
 static char *configured_signing_key;
+const char *ssh_allowed_signers, *ssh_revocation_file;
 static enum signature_trust_level configured_min_trust_level = TRUST_UNDEFINED;
 
 struct gpg_format {

@@ -35,6 +37,14 @@ static const char *x509_sigs[] = {
 	NULL
 };
 
+static const char *ssh_verify_args[] = {
+	NULL
+};
+static const char *ssh_sigs[] = {
+	"-----BEGIN SSH SIGNATURE-----",
+	NULL
+};
+
 static struct gpg_format gpg_format[] = {
 	{ .name = "openpgp", .program = "gpg",
 	  .verify_args = openpgp_verify_args,

@@ -44,6 +54,9 @@ static struct gpg_format gpg_format[] = {
 	  .verify_args = x509_verify_args,
 	  .sigs = x509_sigs
 	},
+	{ .name = "ssh", .program = "ssh-keygen",
+	  .verify_args = ssh_verify_args,
+	  .sigs = ssh_sigs },
 };
 
 static struct gpg_format *use_format = &gpg_format[0];

@@ -72,6 +85,7 @@ 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->output);
 	FREE_AND_NULL(sigc->gpg_output);
 	FREE_AND_NULL(sigc->gpg_status);
 	FREE_AND_NULL(sigc->signer);

@@ -144,6 +158,53 @@ static int parse_gpg_trust_level(const char *level,
 	return 1;
 }
 
+static void parse_ssh_output(struct signature_check *sigc)
+{
+	const char *output = NULL;
+	char *next = NULL;
+
+	/* ssh-keysign 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
+	 */
+
+	output = xmemdupz(sigc->output, strcspn(sigc->output, " \n"));
+	if (skip_prefix(sigc->output, "Good \"git\" signature for ", &output)) {
+		// Valid signature for a trusted signer
+		sigc->result = 'G';
+		sigc->trust_level = TRUST_FULLY;
+
+		next = strchrnul(output, ' '); // 'principal'
+		replace_cstring(&sigc->signer, output, next);
+		output = next + 1;
+		next = strchrnul(output, ' '); // 'with'
+		output = next + 1;
+		next = strchrnul(output, ' '); // KEY Type
+		output = next + 1;
+		next = strchrnul(output, ' '); // 'key'
+		output = next + 1;
+		next = strchrnul(output, '\n'); // key
+		replace_cstring(&sigc->fingerprint, output, next);
+		replace_cstring(&sigc->key, output, next);
+	} else if (skip_prefix(sigc->output, "Good \"git\" signature with ", &output)) {
+		// Valid signature, but key unknown
+		sigc->result = 'G';
+		sigc->trust_level = TRUST_UNDEFINED;
+
+		next = strchrnul(output, ' '); // KEY Type
+		output = next + 1;
+		next = strchrnul(output, ' '); // 'key'
+		output = next + 1;
+		next = strchrnul(output, '\n'); // key
+		replace_cstring(&sigc->fingerprint, output, next);
+		replace_cstring(&sigc->key, output, next);
+	} else {
+		sigc->result = 'B';
+		sigc->trust_level = TRUST_NEVER;
+	}
+}
+
 static void parse_gpg_output(struct signature_check *sigc)
 {
 	const char *buf = sigc->gpg_status;

@@ -257,16 +318,18 @@ 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_ssh_signature(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 child_process ssh_keygen = CHILD_PROCESS_INIT;
 	struct tempfile *temp;
 	int ret;
-	struct strbuf buf = STRBUF_INIT;
+	const char *line;
+	size_t trust_size;
+	char *principal;
+	struct strbuf ssh_keygen_out = STRBUF_INIT;
+	struct strbuf ssh_keygen_err = STRBUF_INIT;
 
 	temp = mks_tempfile_t(".git_vtag_tmpXXXXXX");
 	if (!temp)

@@ -279,29 +342,125 @@ 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);
+	// Find the principal from the  signers
+	strvec_pushl(&ssh_keygen.args,  fmt->program,
+					"-Y", "find-principals",
+					"-f", get_ssh_allowed_signers(),
+					"-s", temp->filename.buf,
+					NULL);
+	ret = pipe_command(&ssh_keygen, NULL, 0, &ssh_keygen_out, 0, &ssh_keygen_err, 0);
+	if (strstr(ssh_keygen_err.buf, "unknown option")) {
+		error(_("openssh version > 8.2p1 is needed for ssh signature verification (ssh-keygen needs -Y find-principals/verify option)"));
+	}
+	if (ret || !ssh_keygen_out.len) {
+		// We did not find a matching principal in the keyring - Check without validation
+		child_process_init(&ssh_keygen);
+		strvec_pushl(&ssh_keygen.args,  fmt->program,
+						"-Y", "check-novalidate",
+						"-n", "git",
+						"-s", temp->filename.buf,
+						NULL);
+		ret = pipe_command(&ssh_keygen, payload, payload_size, &ssh_keygen_out, 0, &ssh_keygen_err, 0);
+	} else {
+		// Check every principal we found (one per line)
+		for (line = ssh_keygen_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",
+							//TODO: sprintf("-Overify-time=%s", commit->date...),
+							"-n", "git",
+							"-f", get_ssh_allowed_signers(),
+							"-I", principal,
+							"-s", temp->filename.buf,
+							NULL);
+
+			if (ssh_revocation_file && file_exists(ssh_revocation_file)) {
+				strvec_pushl(&ssh_keygen.args, "-r", ssh_revocation_file, NULL);
+			}
+
+			sigchain_push(SIGPIPE, SIG_IGN);
+			ret = pipe_command(&ssh_keygen, payload, payload_size,
+					&ssh_keygen_out, 0, &ssh_keygen_err, 0);
+			sigchain_pop(SIGPIPE);
+
+			ret &= starts_with(ssh_keygen_out.buf, "Good");
+			if (ret == 0)
+				break;
+		}
+	}
+
+	sigc->payload = xmemdupz(payload, payload_size);
+	strbuf_stripspace(&ssh_keygen_out, 0);
+	strbuf_stripspace(&ssh_keygen_err, 0);
+	strbuf_add(&ssh_keygen_out, ssh_keygen_err.buf, ssh_keygen_err.len);
+	sigc->output = strbuf_detach(&ssh_keygen_out, NULL);
+
+	//sigc->gpg_output = strbuf_detach(&ssh_keygen_err, NULL); // This flip around is broken...
+	sigc->gpg_status = strbuf_detach(&ssh_keygen_out, NULL);
+
+	parse_ssh_output(sigc);
+
+	delete_tempfile(&temp);
+	strbuf_release(&ssh_keygen_out);
+	strbuf_release(&ssh_keygen_err);
+
+	return ret;
+}
+
+static int verify_gpg_signature(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 tempfile *temp;
+	int ret;
+	struct strbuf gpg_out = STRBUF_INIT;
+	struct strbuf gpg_err = STRBUF_INIT;
+
+	temp = mks_tempfile_t(".git_vtag_tmpXXXXXX");
+	if (!temp)
+		return error_errno(_("could not create temporary file"));
+	if (write_in_full(temp->fd, signature, signature_size) < 0 ||
+	    close_tempfile_gently(temp) < 0) {
+		error_errno(_("failed writing detached signature to '%s'"),
+			    temp->filename.buf);
+		delete_tempfile(&temp);
+		return -1;
+	}
 
 	strvec_push(&gpg.args, fmt->program);
 	strvec_pushv(&gpg.args, fmt->verify_args);
 	strvec_pushl(&gpg.args,
-		     "--status-fd=1",
-		     "--verify", temp->filename.buf, "-",
-		     NULL);
-
-	if (!gpg_status)
-		gpg_status = &buf;
+			"--status-fd=1",
+			"--verify", temp->filename.buf, "-",
+			NULL);
 
 	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_out, 0,
+				&gpg_err, 0);
 	sigchain_pop(SIGPIPE);
+	ret |= !strstr(gpg_out.buf, "\n[GNUPG:] GOODSIG ");
 
-	delete_tempfile(&temp);
+	sigc->payload = xmemdupz(payload, payload_size);
+	sigc->output = strbuf_detach(&gpg_err, NULL);
+	sigc->gpg_status = strbuf_detach(&gpg_out, NULL);
 
-	ret |= !strstr(gpg_status->buf, "\n[GNUPG:] GOODSIG ");
-	strbuf_release(&buf); /* no matter it was used or not */
+	parse_gpg_output(sigc);
+
+	delete_tempfile(&temp);
+	strbuf_release(&gpg_out);
+	strbuf_release(&gpg_err);
 
 	return ret;
 }

@@ -309,35 +468,34 @@ 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)
+		BUG("bad signature '%s'", signature);
+
+	if (!strcmp(fmt->name, "ssh")) {
+		status = verify_ssh_signature(sigc, fmt, payload, plen, signature, slen);
+	} else {
+		status = verify_gpg_signature(sigc, fmt, payload, plen, signature, slen);
+	}
+	if (status && !sigc->gpg_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;
+		sigc->gpg_status : sigc->output;
 
 	if (flags & GPG_VERIFY_VERBOSE && sigc->payload)
 		fputs(sigc->payload, stdout);

@@ -388,12 +546,31 @@ int git_gpg_config(const char *var, const char *value, void *cb)
 	int ret;
 
 	if (!strcmp(var, "user.signingkey")) {
+		/* user.signingkey can contain one of the following
+		 * when format = openpgp/x509
+		 *   - GPG KeyID
+		 * when format = ssh
+		 *   - literal ssh public key (e.g. ssh-rsa XXXKEYXXX comment)
+		 *   - path to a file containing a public or a private ssh key
+		 */
 		if (!value)
 			return config_error_nonbool(var);
 		set_signing_key(value);
 		return 0;
 	}
 
+	if (!strcmp(var, "gpg.ssh.keyring")) {
+		if (!value)
+			return config_error_nonbool(var);
+		return git_config_string(&ssh_allowed_signers, var, value);
+	}
+
+	if (!strcmp(var, "gpg.ssh.revocationkeyring")) {
+		if (!value)
+			return config_error_nonbool(var);
+		return git_config_string(&ssh_revocation_file, var, value);
+	}
+
 	if (!strcmp(var, "gpg.format")) {
 		if (!value)
 			return config_error_nonbool(var);

@@ -425,6 +602,9 @@ 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);

@@ -433,11 +613,80 @@ 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;
+
+	/* For 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]) {
+			return strbuf_detach(fingerprint[1], NULL);
+		}
+	}
+	die_errno(_("failed to get the ssh fingerprint for key '%s'"), signing_key);
+}
+
+// Returns the first public key from an ssh-agent to use for signing
+static char *get_default_ssh_signing_key(void) {
+	struct child_process ssh_add = CHILD_PROCESS_INIT;
+	int ret = -1;
+	struct strbuf key_stdout = STRBUF_INIT;
+	struct strbuf **keys;
+
+	strvec_pushl(&ssh_add.args, "ssh-add", "-L", NULL);
+	ret = pipe_command(&ssh_add, NULL, 0, &key_stdout, 0, NULL, 0);
+	if (!ret) { 
+		keys = strbuf_split_max(&key_stdout, '\n', 2);
+		if (keys[0])
+			return strbuf_detach(keys[0], NULL);
+	}
+
+	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)
 		return configured_signing_key;
-	return git_committer_info(IDENT_STRICT|IDENT_NO_DATE);
+	if (!strcmp(use_format->name, "ssh")) {
+		return get_default_ssh_signing_key();
+	} else {
+		return git_committer_info(IDENT_STRICT | IDENT_NO_DATE);
+	}
+}
+
+const char *get_ssh_allowed_signers(void)
+{
+	if (ssh_allowed_signers)
+		return ssh_allowed_signers;
+
+	die("A Path to an allowed signers ssh keyring is needed for validation");
 }
 
 int sign_buffer(struct strbuf *buffer, struct strbuf *signature, const char *signing_key)

@@ -446,25 +695,88 @@ int sign_buffer(struct strbuf *buffer, struct strbuf *signature, const char *sig
 	int ret;
 	size_t i, j, bottom;
 	struct strbuf gpg_status = STRBUF_INIT;
+	struct tempfile *temp = NULL, *buffer_file = NULL;
+	char *ssh_signing_key_file = NULL;
+	struct strbuf ssh_signature_filename = STRBUF_INIT;
+
+	if (!strcmp(use_format->name, "ssh")) {
+		if (!signing_key || signing_key[0] == '\0')
+			return error(_("user.signingkey needs to be set for ssh signing"));
+
+
+		if (istarts_with(signing_key, "ssh-")) {
+			// A literal ssh key
+			temp = mks_tempfile_t(".git_signing_key_tmpXXXXXX");
+			if (!temp)
+				return error_errno(_("could not create temporary file"));
+			if (write_in_full(temp->fd, signing_key, strlen(signing_key)) < 0 ||
+				close_tempfile_gently(temp) < 0) {
+				error_errno(_("failed writing ssh signing key to '%s'"), temp->filename.buf);
+				delete_tempfile(&temp);
+				return -1;
+			}
+			ssh_signing_key_file= temp->filename.buf;
+		} else {
+			// We assume a file
+			ssh_signing_key_file = expand_user_path(signing_key, 1);
+		}
 
-	strvec_pushl(&gpg.args,
-		     use_format->program,
-		     "--status-fd=2",
-		     "-bsau", signing_key,
-		     NULL);
+		buffer_file = mks_tempfile_t(".git_signing_buffer_tmpXXXXXX");
+		if (!buffer_file)
+			return error_errno(_("could not create temporary file"));
+		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);
+			delete_tempfile(&buffer_file);
+			return -1;
+		}
+
+		strvec_pushl(&gpg.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(&gpg, NULL, 0, NULL, 0, &gpg_status, 0);
+		sigchain_pop(SIGPIPE);
+
+		strbuf_addbuf(&ssh_signature_filename, &buffer_file->filename);
+		strbuf_addstr(&ssh_signature_filename, ".sig");
+		if (strbuf_read_file(signature, ssh_signature_filename.buf, 2048) < 0) {
+			error_errno(_("failed reading ssh signing data buffer from '%s'"), ssh_signature_filename.buf);
+		}
+		unlink_or_warn(ssh_signature_filename.buf);
+		strbuf_release(&ssh_signature_filename);
+		delete_tempfile(&buffer_file);
+	} else {
+		strvec_pushl(&gpg.args, use_format->program ,
+					"--status-fd=2",
+					"-bsau", signing_key,
+					NULL);
+
+		/*
+		* When the username signingkey is bad, program could be terminated
+		* because gpg exits without reading and then write gets SIGPIPE.
+		*/
+		sigchain_push(SIGPIPE, SIG_IGN);
+		ret = pipe_command(&gpg, buffer->buf, buffer->len, signature, 1024, &gpg_status, 0);
+		sigchain_pop(SIGPIPE);
+	}
 
 	bottom = signature->len;
 
-	/*
-	 * When the username signingkey is bad, program could be terminated
-	 * because gpg exits without reading and then write gets SIGPIPE.
-	 */
-	sigchain_push(SIGPIPE, SIG_IGN);
-	ret = pipe_command(&gpg, buffer->buf, buffer->len,
-			   signature, 1024, &gpg_status, 0);
-	sigchain_pop(SIGPIPE);
+	if (temp)
+		delete_tempfile(&temp);
 
-	ret |= !strstr(gpg_status.buf, "\n[GNUPG:] SIG_CREATED ");
+	if (!strcmp(use_format->name, "ssh")) {
+		if (strstr(gpg_status.buf, "unknown option")) {
+			error(_("openssh version > 8.2p1 is needed for ssh signing (ssh-keygen needs -Y sign option)"));
+		}
+	} else {
+		ret |= !strstr(gpg_status.buf, "\n[GNUPG:] SIG_CREATED ");
+	}
 	strbuf_release(&gpg_status);
 	if (ret)
 		return error(_("gpg failed to sign the data"));