Merge branch 'jc/push-cert'

Allow "git push" request to be signed, so that it can be verified and
audited, using the GPG signature of the person who pushed, that the
tips of branches at a public repository really point the commits
the pusher wanted to, without having to "trust" the server.

* jc/push-cert: (24 commits)
  receive-pack::hmac_sha1(): copy the entire SHA-1 hash out
  signed push: allow stale nonce in stateless mode
  signed push: teach smart-HTTP to pass "git push --signed" around
  signed push: fortify against replay attacks
  signed push: add "pushee" header to push certificate
  signed push: remove duplicated protocol info
  send-pack: send feature request on push-cert packet
  receive-pack: GPG-validate push certificates
  push: the beginning of "git push --signed"
  pack-protocol doc: typofix for PKT-LINE
  gpg-interface: move parse_signature() to where it should be
  gpg-interface: move parse_gpg_output() to where it should be
  send-pack: clarify that cmds_sent is a boolean
  send-pack: refactor inspecting and resetting status and sending commands
  send-pack: rename "new_refs" to "need_pack_data"
  receive-pack: factor out capability string generation
  send-pack: factor out capability string generation
  send-pack: always send capabilities
  send-pack: refactor decision to send update per ref
  send-pack: move REF_STATUS_REJECT_NODELETE logic a bit higher
  ...

23 files changed, 932 insertions, 159 deletions

diff --git a/Documentation/config.txt b/Documentation/config.txt
index 3b5b24aeb..04a1e2f37 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -2044,6 +2044,25 @@ receive.autogc::
 	receiving data from git-push and updating refs.  You can stop
 	it by setting this variable to false.
 
+receive.certnonceseed::
+	By setting this variable to a string, `git receive-pack`
+	will accept a `git push --signed` and verifies it by using
+	a "nonce" protected by HMAC using this string as a secret
+	key.
+
+receive.certnonceslop::
+	When a `git push --signed` sent a push certificate with a
+	"nonce" that was issued by a receive-pack serving the same
+	repository within this many seconds, export the "nonce"
+	found in the certificate to `GIT_PUSH_CERT_NONCE` to the
+	hooks (instead of what the receive-pack asked the sending
+	side to include).  This may allow writing checks in
+	`pre-receive` and `post-receive` a bit easier.  Instead of
+	checking `GIT_PUSH_CERT_NONCE_SLOP` environment variable
+	that records by how many seconds the nonce is stale to
+	decide if they want to accept the certificate, they only
+	can check `GIT_PUSH_CERT_NONCE_STATUS` is `OK`.
+
 receive.fsckObjects::
 	If it is set to true, git-receive-pack will check all received
 	objects. It will abort in the case of a malformed object or a
diff --git a/Documentation/git-push.txt b/Documentation/git-push.txt
index c0d7403b9..b17283ab7 100644
--- a/Documentation/git-push.txt
+++ b/Documentation/git-push.txt
@@ -10,7 +10,8 @@ SYNOPSIS
 --------
 [verse]
 'git push' [--all | --mirror | --tags] [--follow-tags] [-n | --dry-run] [--receive-pack=<git-receive-pack>]
-	   [--repo=<repository>] [-f | --force] [--prune] [-v | --verbose] [-u | --set-upstream]
+	   [--repo=<repository>] [-f | --force] [--prune] [-v | --verbose]
+	   [-u | --set-upstream] [--signed]
 	   [--force-with-lease[=<refname>[:<expect>]]]
 	   [--no-verify] [<repository> [<refspec>...]]
 
@@ -129,6 +130,12 @@ already exists on the remote side.
 	from the remote but are pointing at commit-ish that are
 	reachable from the refs being pushed.
 
+--signed::
+	GPG-sign the push request to update refs on the receiving
+	side, to allow it to be checked by the hooks and/or be
+	logged.  See linkgit:git-receive-pack[1] for the details
+	on the receiving end.
+
 --receive-pack=<git-receive-pack>::
 --exec=<git-receive-pack>::
 	Path to the 'git-receive-pack' program on the remote
diff --git a/Documentation/git-receive-pack.txt b/Documentation/git-receive-pack.txt
index b1f7dc643..9016960e2 100644
--- a/Documentation/git-receive-pack.txt
+++ b/Documentation/git-receive-pack.txt
@@ -53,6 +53,56 @@ the update.  Refs to be created will have sha1-old equal to 0\{40},
 while refs to be deleted will have sha1-new equal to 0\{40}, otherwise
 sha1-old and sha1-new should be valid objects in the repository.
 
+When accepting a signed push (see linkgit:git-push[1]), the signed
+push certificate is stored in a blob and an environment variable
+`GIT_PUSH_CERT` can be consulted for its object name.  See the
+description of `post-receive` hook for an example.  In addition, the
+certificate is verified using GPG and the result is exported with
+the following environment variables:
+
+`GIT_PUSH_CERT_SIGNER`::
+	The name and the e-mail address of the owner of the key that
+	signed the push certificate.
+
+`GIT_PUSH_CERT_KEY`::
+	The GPG key ID of the key that signed the push certificate.
+
+`GIT_PUSH_CERT_STATUS`::
+	The status of GPG verification of the push certificate,
+	using the same mnemonic as used in `%G?` format of `git log`
+	family of commands (see linkgit:git-log[1]).
+
+`GIT_PUSH_CERT_NONCE`::
+	The nonce string the process asked the signer to include
+	in the push certificate.  If this does not match the value
+	recorded on the "nonce" header in the push certificate, it
+	may indicate that the certificate is a valid one that is
+	being replayed from a separate "git push" session.
+
+`GIT_PUSH_CERT_NONCE_STATUS`::
+`UNSOLICITED`;;
+	"git push --signed" sent a nonce when we did not ask it to
+	send one.
+`MISSING`;;
+	"git push --signed" did not send any nonce header.
+`BAD`;;
+	"git push --signed" sent a bogus nonce.
+`OK`;;
+	"git push --signed" sent the nonce we asked it to send.
+`SLOP`;;
+	"git push --signed" sent a nonce different from what we
+	asked it to send now, but in a previous session.  See
+	`GIT_PUSH_CERT_NONCE_SLOP` environment variable.
+
+`GIT_PUSH_CERT_NONCE_SLOP`::
+	"git push --signed" sent a nonce different from what we
+	asked it to send now, but in a different session whose
+	starting time is different by this many seconds from the
+	current session.  Only meaningful when
+	`GIT_PUSH_CERT_NONCE_STATUS` says `SLOP`.
+	Also read about `receive.certnonceslop` variable in
+	linkgit:git-config[1].
+
 This hook is called before any refname is updated and before any
 fast-forward checks are performed.
 
@@ -101,9 +151,14 @@ the update.  Refs that were created will have sha1-old equal to
 0\{40}, otherwise sha1-old and sha1-new should be valid objects in
 the repository.
 
+The `GIT_PUSH_CERT*` environment variables can be inspected, just as
+in `pre-receive` hook, after accepting a signed push.
+
 Using this hook, it is easy to generate mails describing the updates
 to the repository.  This example script sends one mail message per
-ref listing the commits pushed to the repository:
+ref listing the commits pushed to the repository, and logs the push
+certificates of signed pushes with good signatures to a logger
+service:
 
 	#!/bin/sh
 	# mail out commit update information.
@@ -119,6 +174,14 @@ ref listing the commits pushed to the repository:
 		fi |
 		mail -s "Changes to ref $ref" commit-list@mydomain
 	done
+	# log signed push certificate, if any
+	if test -n "${GIT_PUSH_CERT-}" && test ${GIT_PUSH_CERT_STATUS} = G
+	then
+		(
+			echo expected nonce is ${GIT_PUSH_NONCE}
+			git cat-file blob ${GIT_PUSH_CERT}
+		) | mail -s "push certificate from $GIT_PUSH_CERT_SIGNER" push-log@mydomain
+	fi
 	exit 0
 
 The exit code from this hook invocation is ignored, however a
diff --git a/Documentation/technical/pack-protocol.txt b/Documentation/technical/pack-protocol.txt
index 569c48a35..462e20645 100644
--- a/Documentation/technical/pack-protocol.txt
+++ b/Documentation/technical/pack-protocol.txt
@@ -212,9 +212,9 @@ out of what the server said it could do with the first 'want' line.
   want-list         =  first-want
 		       *additional-want
 
-  shallow-line      =  PKT_LINE("shallow" SP obj-id)
+  shallow-line      =  PKT-LINE("shallow" SP obj-id)
 
-  depth-request     =  PKT_LINE("deepen" SP depth)
+  depth-request     =  PKT-LINE("deepen" SP depth)
 
   first-want        =  PKT-LINE("want" SP obj-id SP capability-list LF)
   additional-want   =  PKT-LINE("want" SP obj-id LF)
@@ -465,7 +465,7 @@ contain all the objects that the server will need to complete the new
 references.
 
 ----
-  update-request    =  *shallow command-list [pack-file]
+  update-request    =  *shallow ( command-list | push-cert ) [pack-file]
 
   shallow           =  PKT-LINE("shallow" SP obj-id LF)
 
@@ -481,12 +481,27 @@ references.
   old-id            =  obj-id
   new-id            =  obj-id
 
+  push-cert         = PKT-LINE("push-cert" NUL capability-list LF)
+		      PKT-LINE("certificate version 0.1" LF)
+		      PKT-LINE("pusher" SP ident LF)
+		      PKT-LINE("pushee" SP url LF)
+		      PKT-LINE("nonce" SP nonce LF)
+		      PKT-LINE(LF)
+		      *PKT-LINE(command LF)
+		      *PKT-LINE(gpg-signature-lines LF)
+		      PKT-LINE("push-cert-end" LF)
+
   pack-file         = "PACK" 28*(OCTET)
 ----
 
 If the receiving end does not support delete-refs, the sending end MUST
 NOT ask for delete command.
 
+If the receiving end does not support push-cert, the sending end
+MUST NOT send a push-cert command.  When a push-cert command is
+sent, command-list MUST NOT be sent; the commands recorded in the
+push certificate is used instead.
+
 The pack-file MUST NOT be sent if the only command used is 'delete'.
 
 A pack-file MUST be sent if either create or update command is used,
@@ -501,6 +516,34 @@ was being processed (the obj-id is still the same as the old-id), and
 it will run any update hooks to make sure that the update is acceptable.
 If all of that is fine, the server will then update the references.
 
+Push Certificate
+----------------
+
+A push certificate begins with a set of header lines.  After the
+header and an empty line, the protocol commands follow, one per
+line.
+
+Currently, the following header fields are defined:
+
+`pusher` ident::
+	Identify the GPG key in "Human Readable Name <email@address>"
+	format.
+
+`pushee` url::
+	The repository URL (anonymized, if the URL contains
+	authentication material) the user who ran `git push`
+	intended to push into.
+
+`nonce` nonce::
+	The 'nonce' string the receiving repository asked the
+	pushing user to include in the certificate, to prevent
+	replay attacks.
+
+The GPG signature lines are a detached signature for the contents
+recorded in the push certificate before the signature block begins.
+The detached signature is used to certify that the commands were
+given by the pusher, who must be the signer.
+
 Report Status
 -------------
 
diff --git a/Documentation/technical/protocol-capabilities.txt b/Documentation/technical/protocol-capabilities.txt
index e17434384..0c92deebc 100644
--- a/Documentation/technical/protocol-capabilities.txt
+++ b/Documentation/technical/protocol-capabilities.txt
@@ -18,8 +18,8 @@ was sent.  Server MUST NOT ignore capabilities that client requested
 and server advertised.  As a consequence of these rules, server MUST
 NOT advertise capabilities it does not understand.
 
-The 'report-status', 'delete-refs', and 'quiet' capabilities are sent and
-recognized by the receive-pack (push to server) process.
+The 'report-status', 'delete-refs', 'quiet', and 'push-cert' capabilities
+are sent and recognized by the receive-pack (push to server) process.
 
 The 'ofs-delta' and 'side-band-64k' capabilities are sent and recognized
 by both upload-pack and receive-pack protocols.  The 'agent' capability
@@ -250,3 +250,12 @@ allow-tip-sha1-in-want
 If the upload-pack server advertises this capability, fetch-pack may
 send "want" lines with SHA-1s that exist at the server but are not
 advertised by upload-pack.
+
+push-cert=<nonce>
+-----------------
+
+The receive-pack server that advertises this capability is willing
+to accept a signed push certificate, and asks the <nonce> to be
+included in the push certificate.  A send-pack client MUST NOT
+send a push-cert packet unless the receive-pack server advertises
+this capability.
diff --git a/builtin/push.c b/builtin/push.c
index f50e3d5e7..ae56f73a6 100644
--- a/builtin/push.c
+++ b/builtin/push.c
@@ -506,6 +506,7 @@ int cmd_push(int argc, const char **argv, const char *prefix)
 		OPT_BIT(0, "no-verify", &flags, N_("bypass pre-push hook"), TRANSPORT_PUSH_NO_HOOK),
 		OPT_BIT(0, "follow-tags", &flags, N_("push missing but relevant tags"),
 			TRANSPORT_PUSH_FOLLOW_TAGS),
+		OPT_BIT(0, "signed", &flags, N_("GPG sign the push"), TRANSPORT_PUSH_CERT),
 		OPT_END()
 	};
 
diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index daf0600ca..a01ac2096 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -15,6 +15,8 @@
 #include "connected.h"
 #include "argv-array.h"
 #include "version.h"
+#include "tag.h"
+#include "gpg-interface.h"
 #include "sigchain.h"
 
 static const char receive_pack_usage[] = "git receive-pack <git-dir>";
@@ -42,11 +44,27 @@ static int prefer_ofs_delta = 1;
 static int auto_update_server_info;
 static int auto_gc = 1;
 static int fix_thin = 1;
+static int stateless_rpc;
+static const char *service_dir;
 static const char *head_name;
 static void *head_name_to_free;
 static int sent_capabilities;
 static int shallow_update;
 static const char *alt_shallow_file;
+static struct strbuf push_cert = STRBUF_INIT;
+static unsigned char push_cert_sha1[20];
+static struct signature_check sigcheck;
+static const char *push_cert_nonce;
+static const char *cert_nonce_seed;
+
+static const char *NONCE_UNSOLICITED = "UNSOLICITED";
+static const char *NONCE_BAD = "BAD";
+static const char *NONCE_MISSING = "MISSING";
+static const char *NONCE_OK = "OK";
+static const char *NONCE_SLOP = "SLOP";
+static const char *nonce_status;
+static long nonce_stamp_slop;
+static unsigned long nonce_stamp_slop_limit;
 
 static enum deny_action parse_deny_action(const char *var, const char *value)
 {
@@ -130,6 +148,14 @@ static int receive_pack_config(const char *var, const char *value, void *cb)
 		return 0;
 	}
 
+	if (strcmp(var, "receive.certnonceseed") == 0)
+		return git_config_string(&cert_nonce_seed, var, value);
+
+	if (strcmp(var, "receive.certnonceslop") == 0) {
+		nonce_stamp_slop_limit = git_config_ulong(var, value);
+		return 0;
+	}
+
 	return git_default_config(var, value, cb);
 }
 
@@ -138,15 +164,23 @@ static void show_ref(const char *path, const unsigned char *sha1)
 	if (ref_is_hidden(path))
 		return;
 
-	if (sent_capabilities)
+	if (sent_capabilities) {
 		packet_write(1, "%s %s\n", sha1_to_hex(sha1), path);
-	else
-		packet_write(1, "%s %s%c%s%s agent=%s\n",
-			     sha1_to_hex(sha1), path, 0,
-			     " report-status delete-refs side-band-64k quiet",
-			     prefer_ofs_delta ? " ofs-delta" : "",
-			     git_user_agent_sanitized());
-	sent_capabilities = 1;
+	} else {
+		struct strbuf cap = STRBUF_INIT;
+
+		strbuf_addstr(&cap,
+			      "report-status delete-refs side-band-64k quiet");
+		if (prefer_ofs_delta)
+			strbuf_addstr(&cap, " ofs-delta");
+		if (push_cert_nonce)
+			strbuf_addf(&cap, " push-cert=%s", push_cert_nonce);
+		strbuf_addf(&cap, " agent=%s", git_user_agent_sanitized());
+		packet_write(1, "%s %s%c%s\n",
+			     sha1_to_hex(sha1), path, 0, cap.buf);
+		strbuf_release(&cap);
+		sent_capabilities = 1;
+	}
 }
 
 static int show_ref_cb(const char *path, const unsigned char *sha1, int flag, void *unused)
@@ -253,6 +287,222 @@ static int copy_to_sideband(int in, int out, void *arg)
 	return 0;
 }
 
+#define HMAC_BLOCK_SIZE 64
+
+static void hmac_sha1(unsigned char *out,
+		      const char *key_in, size_t key_len,
+		      const char *text, size_t text_len)
+{
+	unsigned char key[HMAC_BLOCK_SIZE];
+	unsigned char k_ipad[HMAC_BLOCK_SIZE];
+	unsigned char k_opad[HMAC_BLOCK_SIZE];
+	int i;
+	git_SHA_CTX ctx;
+
+	/* RFC 2104 2. (1) */
+	memset(key, '\0', HMAC_BLOCK_SIZE);
+	if (HMAC_BLOCK_SIZE < key_len) {
+		git_SHA1_Init(&ctx);
+		git_SHA1_Update(&ctx, key_in, key_len);
+		git_SHA1_Final(key, &ctx);
+	} else {
+		memcpy(key, key_in, key_len);
+	}
+
+	/* RFC 2104 2. (2) & (5) */
+	for (i = 0; i < sizeof(key); i++) {
+		k_ipad[i] = key[i] ^ 0x36;
+		k_opad[i] = key[i] ^ 0x5c;
+	}
+
+	/* RFC 2104 2. (3) & (4) */
+	git_SHA1_Init(&ctx);
+	git_SHA1_Update(&ctx, k_ipad, sizeof(k_ipad));
+	git_SHA1_Update(&ctx, text, text_len);
+	git_SHA1_Final(out, &ctx);
+
+	/* RFC 2104 2. (6) & (7) */
+	git_SHA1_Init(&ctx);
+	git_SHA1_Update(&ctx, k_opad, sizeof(k_opad));
+	git_SHA1_Update(&ctx, out, 20);
+	git_SHA1_Final(out, &ctx);
+}
+
+static char *prepare_push_cert_nonce(const char *path, unsigned long stamp)
+{
+	struct strbuf buf = STRBUF_INIT;
+	unsigned char sha1[20];
+
+	strbuf_addf(&buf, "%s:%lu", path, stamp);
+	hmac_sha1(sha1, buf.buf, buf.len, cert_nonce_seed, strlen(cert_nonce_seed));;
+	strbuf_release(&buf);
+
+	/* RFC 2104 5. HMAC-SHA1-80 */
+	strbuf_addf(&buf, "%lu-%.*s", stamp, 20, sha1_to_hex(sha1));
+	return strbuf_detach(&buf, NULL);
+}
+
+/*
+ * NEEDSWORK: reuse find_commit_header() from jk/commit-author-parsing
+ * after dropping "_commit" from its name and possibly moving it out
+ * of commit.c
+ */
+static char *find_header(const char *msg, size_t len, const char *key)
+{
+	int key_len = strlen(key);
+	const char *line = msg;
+
+	while (line && line < msg + len) {
+		const char *eol = strchrnul(line, '\n');
+
+		if ((msg + len <= eol) || line == eol)
+			return NULL;
+		if (line + key_len < eol &&
+		    !memcmp(line, key, key_len) && line[key_len] == ' ') {
+			int offset = key_len + 1;
+			return xmemdupz(line + offset, (eol - line) - offset);
+		}
+		line = *eol ? eol + 1 : NULL;
+	}
+	return NULL;
+}
+
+static const char *check_nonce(const char *buf, size_t len)
+{
+	char *nonce = find_header(buf, len, "nonce");
+	unsigned long stamp, ostamp;
+	char *bohmac, *expect = NULL;
+	const char *retval = NONCE_BAD;
+
+	if (!nonce) {
+		retval = NONCE_MISSING;
+		goto leave;
+	} else if (!push_cert_nonce) {
+		retval = NONCE_UNSOLICITED;
+		goto leave;
+	} else if (!strcmp(push_cert_nonce, nonce)) {
+		retval = NONCE_OK;
+		goto leave;
+	}
+
+	if (!stateless_rpc) {
+		/* returned nonce MUST match what we gave out earlier */
+		retval = NONCE_BAD;
+		goto leave;
+	}
+
+	/*
+	 * In stateless mode, we may be receiving a nonce issued by
+	 * another instance of the server that serving the same
+	 * repository, and the timestamps may not match, but the
+	 * nonce-seed and dir should match, so we can recompute and
+	 * report the time slop.
+	 *
+	 * In addition, when a nonce issued by another instance has
+	 * timestamp within receive.certnonceslop seconds, we pretend
+	 * as if we issued that nonce when reporting to the hook.
+	 */
+
+	/* nonce is concat(<seconds-since-epoch>, "-", <hmac>) */
+	if (*nonce <= '0' || '9' < *nonce) {
+		retval = NONCE_BAD;
+		goto leave;
+	}
+	stamp = strtoul(nonce, &bohmac, 10);
+	if (bohmac == nonce || bohmac[0] != '-') {
+		retval = NONCE_BAD;
+		goto leave;
+	}
+
+	expect = prepare_push_cert_nonce(service_dir, stamp);
+	if (strcmp(expect, nonce)) {
+		/* Not what we would have signed earlier */
+		retval = NONCE_BAD;
+		goto leave;
+	}
+
+	/*
+	 * By how many seconds is this nonce stale?  Negative value
+	 * would mean it was issued by another server with its clock
+	 * skewed in the future.
+	 */
+	ostamp = strtoul(push_cert_nonce, NULL, 10);
+	nonce_stamp_slop = (long)ostamp - (long)stamp;
+
+	if (nonce_stamp_slop_limit &&
+	    abs(nonce_stamp_slop) <= nonce_stamp_slop_limit) {
+		/*
+		 * Pretend as if the received nonce (which passes the
+		 * HMAC check, so it is not a forged by third-party)
+		 * is what we issued.
+		 */
+		free((void *)push_cert_nonce);
+		push_cert_nonce = xstrdup(nonce);
+		retval = NONCE_OK;
+	} else {
+		retval = NONCE_SLOP;
+	}
+
+leave:
+	free(nonce);
+	free(expect);
+	return retval;
+}
+
+static void prepare_push_cert_sha1(struct child_process *proc)
+{
+	static int already_done;
+	struct argv_array env = ARGV_ARRAY_INIT;
+
+	if (!push_cert.len)
+		return;
+
+	if (!already_done) {
+		struct strbuf gpg_output = STRBUF_INIT;
+		struct strbuf gpg_status = STRBUF_INIT;
+		int bogs /* beginning_of_gpg_sig */;
+
+		already_done = 1;
+		if (write_sha1_file(push_cert.buf, push_cert.len, "blob", push_cert_sha1))
+			hashclr(push_cert_sha1);
+
+		memset(&sigcheck, '\0', sizeof(sigcheck));
+		sigcheck.result = 'N';
+
+		bogs = parse_signature(push_cert.buf, push_cert.len);
+		if (verify_signed_buffer(push_cert.buf, bogs,
+					 push_cert.buf + bogs, push_cert.len - bogs,
+					 &gpg_output, &gpg_status) < 0) {
+			; /* error running gpg */
+		} else {
+			sigcheck.payload = push_cert.buf;
+			sigcheck.gpg_output = gpg_output.buf;
+			sigcheck.gpg_status = gpg_status.buf;
+			parse_gpg_output(&sigcheck);
+		}
+
+		strbuf_release(&gpg_output);
+		strbuf_release(&gpg_status);
+		nonce_status = check_nonce(push_cert.buf, bogs);
+	}
+	if (!is_null_sha1(push_cert_sha1)) {
+		argv_array_pushf(&env, "GIT_PUSH_CERT=%s", sha1_to_hex(push_cert_sha1));
+		argv_array_pushf(&env, "GIT_PUSH_CERT_SIGNER=%s",
+				 sigcheck.signer ? sigcheck.signer : "");
+		argv_array_pushf(&env, "GIT_PUSH_CERT_KEY=%s",
+				 sigcheck.key ? sigcheck.key : "");
+		argv_array_pushf(&env, "GIT_PUSH_CERT_STATUS=%c", sigcheck.result);
+		if (push_cert_nonce) {
+			argv_array_pushf(&env, "GIT_PUSH_CERT_NONCE=%s", push_cert_nonce);
+			argv_array_pushf(&env, "GIT_PUSH_CERT_NONCE_STATUS=%s", nonce_status);
+			if (nonce_status == NONCE_SLOP)
+				argv_array_pushf(&env, "GIT_PUSH_CERT_NONCE_SLOP=%ld",
+						 nonce_stamp_slop);
+		}
+		proc->env = env.argv;
+	}
+}
+
 typedef int (*feed_fn)(void *, const char **, size_t *);
 static int run_and_feed_hook(const char *hook_name, feed_fn feed, void *feed_state)
 {
@@ -271,6 +521,8 @@ static int run_and_feed_hook(const char *hook_name, feed_fn feed, void *feed_sta
 	proc.in = -1;
 	proc.stdout_to_stderr = 1;
 
+	prepare_push_cert_sha1(&proc);
+
 	if (use_sideband) {
 		memset(&muxer, 0, sizeof(muxer));
 		muxer.proc = copy_to_sideband;
@@ -841,40 +1093,79 @@ static void execute_commands(struct command *commands,
 		      "the reported refs above");
 }
 
+static struct command **queue_command(struct command **tail,
+				      const char *line,
+				      int linelen)
+{
+	unsigned char old_sha1[20], new_sha1[20];
+	struct command *cmd;
+	const char *refname;
+	int reflen;
+
+	if (linelen < 83 ||
+	    line[40] != ' ' ||
+	    line[81] != ' ' ||
+	    get_sha1_hex(line, old_sha1) ||
+	    get_sha1_hex(line + 41, new_sha1))
+		die("protocol error: expected old/new/ref, got '%s'", line);
+
+	refname = line + 82;
+	reflen = linelen - 82;
+	cmd = xcalloc(1, sizeof(struct command) + reflen + 1);
+	hashcpy(cmd->old_sha1, old_sha1);
+	hashcpy(cmd->new_sha1, new_sha1);
+	memcpy(cmd->ref_name, refname, reflen);
+	cmd->ref_name[reflen] = '\0';
+	*tail = cmd;
+	return &cmd->next;
+}
+
+static void queue_commands_from_cert(struct command **tail,
+				     struct strbuf *push_cert)
+{
+	const char *boc, *eoc;
+
+	if (*tail)
+		die("protocol error: got both push certificate and unsigned commands");
+
+	boc = strstr(push_cert->buf, "\n\n");
+	if (!boc)
+		die("malformed push certificate %.*s", 100, push_cert->buf);
+	else
+		boc += 2;
+	eoc = push_cert->buf + parse_signature(push_cert->buf, push_cert->len);
+
+	while (boc < eoc) {
+		const char *eol = memchr(boc, '\n', eoc - boc);
+		tail = queue_command(tail, boc, eol ? eol - boc : eoc - eol);
+		boc = eol ? eol + 1 : eoc;
+	}
+}
+
 static struct command *read_head_info(struct sha1_array *shallow)
 {
 	struct command *commands = NULL;
 	struct command **p = &commands;
 	for (;;) {
 		char *line;
-		unsigned char old_sha1[20], new_sha1[20];
-		struct command *cmd;
-		char *refname;
-		int len, reflen;
+		int len, linelen;
 
 		line = packet_read_line(0, &len);
 		if (!line)
 			break;
 
 		if (len == 48 && starts_with(line, "shallow ")) {
-			if (get_sha1_hex(line + 8, old_sha1))
-				die("protocol error: expected shallow sha, got '%s'", line + 8);
-			sha1_array_append(shallow, old_sha1);
+			unsigned char sha1[20];
+			if (get_sha1_hex(line + 8, sha1))
+				die("protocol error: expected shallow sha, got '%s'",
+				    line + 8);
+			sha1_array_append(shallow, sha1);
 			continue;
 		}
 
-		if (len < 83 ||
-		    line[40] != ' ' ||
-		    line[81] != ' ' ||
-		    get_sha1_hex(line, old_sha1) ||
-		    get_sha1_hex(line + 41, new_sha1))
-			die("protocol error: expected old/new/ref, got '%s'",
-			    line);
-
-		refname = line + 82;
-		reflen = strlen(refname);
-		if (reflen + 82 < len) {
-			const char *feature_list = refname + reflen + 1;
+		linelen = strlen(line);
+		if (linelen < len) {
+			const char *feature_list = line + linelen + 1;
 			if (parse_feature_request(feature_list, "report-status"))
 				report_status = 1;
 			if (parse_feature_request(feature_list, "side-band-64k"))
@@ -882,13 +1173,34 @@ static struct command *read_head_info(struct sha1_array *shallow)
 			if (parse_feature_request(feature_list, "quiet"))
 				quiet = 1;
 		}
-		cmd = xcalloc(1, sizeof(struct command) + len - 80);
-		hashcpy(cmd->old_sha1, old_sha1);
-		hashcpy(cmd->new_sha1, new_sha1);
-		memcpy(cmd->ref_name, line + 82, len - 81);
-		*p = cmd;
-		p = &cmd->next;
+
+		if (!strcmp(line, "push-cert")) {
+			int true_flush = 0;
+			char certbuf[1024];
+
+			for (;;) {
+				len = packet_read(0, NULL, NULL,
+						  certbuf, sizeof(certbuf), 0);
+				if (!len) {
+					true_flush = 1;
+					break;
+				}
+				if (!strcmp(certbuf, "push-cert-end\n"))
+					break; /* end of cert */
+				strbuf_addstr(&push_cert, certbuf);
+			}
+
+			if (true_flush)
+				break;
+			continue;
+		}
+
+		p = queue_command(p, line, linelen);
 	}
+
+	if (push_cert.len)
+		queue_commands_from_cert(p, &push_cert);
+
 	return commands;
 }
 
@@ -1129,9 +1441,7 @@ static int delete_only(struct command *commands)
 int cmd_receive_pack(int argc, const char **argv, const char *prefix)
 {
 	int advertise_refs = 0;
-	int stateless_rpc = 0;
 	int i;
-	const char *dir = NULL;
 	struct command *commands;
 	struct sha1_array shallow = SHA1_ARRAY_INIT;
 	struct sha1_array ref = SHA1_ARRAY_INIT;
@@ -1164,19 +1474,21 @@ int cmd_receive_pack(int argc, const char **argv, const char *prefix)
 
 			usage(receive_pack_usage);
 		}
-		if (dir)
+		if (service_dir)
 			usage(receive_pack_usage);
-		dir = arg;
+		service_dir = arg;
 	}
-	if (!dir)
+	if (!service_dir)
 		usage(receive_pack_usage);
 
 	setup_path();
 
-	if (!enter_repo(dir, 0))
-		die("'%s' does not appear to be a git repository", dir);
+	if (!enter_repo(service_dir, 0))
+		die("'%s' does not appear to be a git repository", service_dir);
 
 	git_config(receive_pack_config, NULL);
+	if (cert_nonce_seed)
+		push_cert_nonce = prepare_push_cert_nonce(service_dir, time(NULL));
 
 	if (0 <= transfer_unpack_limit)
 		unpack_limit = transfer_unpack_limit;
@@ -1221,5 +1533,6 @@ int cmd_receive_pack(int argc, const char **argv, const char *prefix)
 		packet_flush(1);
 	sha1_array_clear(&shallow);
 	sha1_array_clear(&ref);
+	free((void *)push_cert_nonce);
 	return 0;
 }
diff --git a/builtin/send-pack.c b/builtin/send-pack.c
index 4b1bc0fef..b564a7784 100644
--- a/builtin/send-pack.c
+++ b/builtin/send-pack.c
@@ -154,6 +154,10 @@ int cmd_send_pack(int argc, const char **argv, const char *prefix)
 				args.verbose = 1;
 				continue;
 			}
+			if (!strcmp(arg, "--signed")) {
+				args.push_cert = 1;
+				continue;
+			}
 			if (!strcmp(arg, "--progress")) {
 				progress = 1;
 				continue;
diff --git a/commit.c b/commit.c
index 9c4439fed..19cf8f9c6 100644
--- a/commit.c
+++ b/commit.c
@@ -1214,42 +1214,6 @@ free_return:
 	free(buf);
 }
 
-static struct {
-	char result;
-	const char *check;
-} sigcheck_gpg_status[] = {
-	{ 'G', "\n[GNUPG:] GOODSIG " },
-	{ 'B', "\n[GNUPG:] BADSIG " },
-	{ 'U', "\n[GNUPG:] TRUST_NEVER" },
-	{ 'U', "\n[GNUPG:] TRUST_UNDEFINED" },
-};
-
-static void parse_gpg_output(struct signature_check *sigc)
-{
-	const char *buf = sigc->gpg_status;
-	int i;
-
-	/* Iterate over all search strings */
-	for (i = 0; i < ARRAY_SIZE(sigcheck_gpg_status); i++) {
-		const char *found, *next;
-
-		if (!skip_prefix(buf, sigcheck_gpg_status[i].check + 1, &found)) {
-			found = strstr(buf, sigcheck_gpg_status[i].check);
-			if (!found)
-				continue;
-			found += strlen(sigcheck_gpg_status[i].check);
-		}
-		sigc->result = sigcheck_gpg_status[i].result;
-		/* The trust messages are not followed by key/signer information */
-		if (sigc->result != 'U') {
-			sigc->key = xmemdupz(found, 16);
-			found += 17;
-			next = strchrnul(found, '\n');
-			sigc->signer = xmemdupz(found, next - found);
-		}
-	}
-}
-
 void check_commit_signature(const struct commit *commit, struct signature_check *sigc)
 {
 	struct strbuf payload = STRBUF_INIT;
diff --git a/gpg-interface.c b/gpg-interface.c
index 1ef73fb7d..68b0c814f 100644
--- a/gpg-interface.c
+++ b/gpg-interface.c
@@ -7,6 +7,9 @@
 static char *configured_signing_key;
 static const char *gpg_program = "gpg";
 
+#define PGP_SIGNATURE "-----BEGIN PGP SIGNATURE-----"
+#define PGP_MESSAGE "-----BEGIN PGP MESSAGE-----"
+
 void signature_check_clear(struct signature_check *sigc)
 {
 	free(sigc->payload);
@@ -21,6 +24,60 @@ void signature_check_clear(struct signature_check *sigc)
 	sigc->key = NULL;
 }
 
+static struct {
+	char result;
+	const char *check;
+} sigcheck_gpg_status[] = {
+	{ 'G', "\n[GNUPG:] GOODSIG " },
+	{ 'B', "\n[GNUPG:] BADSIG " },
+	{ 'U', "\n[GNUPG:] TRUST_NEVER" },
+	{ 'U', "\n[GNUPG:] TRUST_UNDEFINED" },
+};
+
+void parse_gpg_output(struct signature_check *sigc)
+{
+	const char *buf = sigc->gpg_status;
+	int i;
+
+	/* Iterate over all search strings */
+	for (i = 0; i < ARRAY_SIZE(sigcheck_gpg_status); i++) {
+		const char *found, *next;
+
+		if (!skip_prefix(buf, sigcheck_gpg_status[i].check + 1, &found)) {
+			found = strstr(buf, sigcheck_gpg_status[i].check);
+			if (!found)
+				continue;
+			found += strlen(sigcheck_gpg_status[i].check);
+		}
+		sigc->result = sigcheck_gpg_status[i].result;
+		/* The trust messages are not followed by key/signer information */
+		if (sigc->result != 'U') {
+			sigc->key = xmemdupz(found, 16);
+			found += 17;
+			next = strchrnul(found, '\n');
+			sigc->signer = xmemdupz(found, next - found);
+		}
+	}
+}
+
+/*
+ * Look at GPG signed content (e.g. a signed tag object), whose
+ * payload is followed by a detached signature on it.  Return the
+ * offset where the embedded detached signature begins, or the end of
+ * the data when there is no such signature.
+ */
+size_t parse_signature(const char *buf, unsigned long size)
+{
+	char *eol;
+	size_t len = 0;
+	while (len < size && !starts_with(buf + len, PGP_SIGNATURE) &&
+			!starts_with(buf + len, PGP_MESSAGE)) {
+		eol = memchr(buf + len, '\n', size - len);
+		len += eol ? eol - (buf + len) + 1 : size - len;
+	}
+	return len;
+}
+
 void set_signing_key(const char *key)
 {
 	free(configured_signing_key);
diff --git a/gpg-interface.h b/gpg-interface.h
index 37c23daff..87a4f2e3f 100644
--- a/gpg-interface.h
+++ b/gpg-interface.h
@@ -5,16 +5,23 @@ struct signature_check {
 	char *payload;
 	char *gpg_output;
 	char *gpg_status;
-	char result; /* 0 (not checked),
-		      * N (checked but no further result),
-		      * U (untrusted good),
-		      * G (good)
-		      * B (bad) */
+
+	/*
+	 * possible "result":
+	 * 0 (not checked)
+	 * N (checked but no further result)
+	 * U (untrusted good)
+	 * G (good)
+	 * B (bad)
+	 */
+	char result;
 	char *signer;
 	char *key;
 };
 
 extern void signature_check_clear(struct signature_check *sigc);
+extern size_t parse_signature(const char *buf, unsigned long size);
+extern void parse_gpg_output(struct signature_check *);
 extern int sign_buffer(struct strbuf *buffer, struct strbuf *signature, const char *signing_key);
 extern 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);
 extern int git_gpg_config(const char *, const char *, void *);
diff --git a/remote-curl.c b/remote-curl.c
index cd626d15e..dd63bc27a 100644
--- a/remote-curl.c
+++ b/remote-curl.c
@@ -25,7 +25,8 @@ struct options {
 		update_shallow : 1,
 		followtags : 1,
 		dry_run : 1,
-		thin : 1;
+		thin : 1,
+		push_cert : 1;
 };
 static struct options options;
 static struct string_list cas_options = STRING_LIST_INIT_DUP;
@@ -106,6 +107,14 @@ static int set_option(const char *name, const char *value)
 		else
 			return -1;
 		return 0;
+	} else if (!strcmp(name, "pushcert")) {
+		if (!strcmp(value, "true"))
+			options.push_cert = 1;
+		else if (!strcmp(value, "false"))
+			options.push_cert = 0;
+		else
+			return -1;
+		return 0;
 	} else {
 		return 1 /* unsupported */;
 	}
@@ -872,6 +881,8 @@ static int push_git(struct discovery *heads, int nr_spec, char **specs)
 		argv_array_push(&args, "--thin");
 	if (options.dry_run)
 		argv_array_push(&args, "--dry-run");
+	if (options.push_cert)
+		argv_array_push(&args, "--signed");
 	if (options.verbosity == 0)
 		argv_array_push(&args, "--quiet");
 	else if (options.verbosity > 1)
diff --git a/send-pack.c b/send-pack.c
index 8b4cbf049..949cb61aa 100644
--- a/send-pack.c
+++ b/send-pack.c
@@ -11,6 +11,7 @@
 #include "transport.h"
 #include "version.h"
 #include "sha1-array.h"
+#include "gpg-interface.h"
 
 static int feed_object(const unsigned char *sha1, int fd, int negative)
 {
@@ -189,6 +190,94 @@ static void advertise_shallow_grafts_buf(struct strbuf *sb)
 	for_each_commit_graft(advertise_shallow_grafts_cb, sb);
 }
 
+static int ref_update_to_be_sent(const struct ref *ref, const struct send_pack_args *args)
+{
+	if (!ref->peer_ref && !args->send_mirror)
+		return 0;
+
+	/* Check for statuses set by set_ref_status_for_push() */
+	switch (ref->status) {
+	case REF_STATUS_REJECT_NONFASTFORWARD:
+	case REF_STATUS_REJECT_ALREADY_EXISTS:
+	case REF_STATUS_REJECT_FETCH_FIRST:
+	case REF_STATUS_REJECT_NEEDS_FORCE:
+	case REF_STATUS_REJECT_STALE:
+	case REF_STATUS_REJECT_NODELETE:
+	case REF_STATUS_UPTODATE:
+		return 0;
+	default:
+		return 1;
+	}
+}
+
+/*
+ * the beginning of the next line, or the end of buffer.
+ *
+ * NEEDSWORK: perhaps move this to git-compat-util.h or somewhere and
+ * convert many similar uses found by "git grep -A4 memchr".
+ */
+static const char *next_line(const char *line, size_t len)
+{
+	const char *nl = memchr(line, '\n', len);
+	if (!nl)
+		return line + len; /* incomplete line */
+	return nl + 1;
+}
+
+static int generate_push_cert(struct strbuf *req_buf,
+			      const struct ref *remote_refs,
+			      struct send_pack_args *args,
+			      const char *cap_string,
+			      const char *push_cert_nonce)
+{
+	const struct ref *ref;
+	char *signing_key = xstrdup(get_signing_key());
+	const char *cp, *np;
+	struct strbuf cert = STRBUF_INIT;
+	int update_seen = 0;
+
+	strbuf_addf(&cert, "certificate version 0.1\n");
+	strbuf_addf(&cert, "pusher %s ", signing_key);
+	datestamp(&cert);
+	strbuf_addch(&cert, '\n');
+	if (args->url && *args->url) {
+		char *anon_url = transport_anonymize_url(args->url);
+		strbuf_addf(&cert, "pushee %s\n", anon_url);
+		free(anon_url);
+	}
+	if (push_cert_nonce[0])
+		strbuf_addf(&cert, "nonce %s\n", push_cert_nonce);
+	strbuf_addstr(&cert, "\n");
+
+	for (ref = remote_refs; ref; ref = ref->next) {
+		if (!ref_update_to_be_sent(ref, args))
+			continue;
+		update_seen = 1;
+		strbuf_addf(&cert, "%s %s %s\n",
+			    sha1_to_hex(ref->old_sha1),
+			    sha1_to_hex(ref->new_sha1),
+			    ref->name);
+	}
+	if (!update_seen)
+		goto free_return;
+
+	if (sign_buffer(&cert, &cert, signing_key))
+		die(_("failed to sign the push certificate"));
+
+	packet_buf_write(req_buf, "push-cert%c%s", 0, cap_string);
+	for (cp = cert.buf; cp < cert.buf + cert.len; cp = np) {
+		np = next_line(cp, cert.buf + cert.len - cp);
+		packet_buf_write(req_buf,
+				 "%.*s", (int)(np - cp), cp);
+	}
+	packet_buf_write(req_buf, "push-cert-end\n");
+
+free_return:
+	free(signing_key);
+	strbuf_release(&cert);
+	return update_seen;
+}
+
 int send_pack(struct send_pack_args *args,
 	      int fd[], struct child_process *conn,
 	      struct ref *remote_refs,
@@ -197,8 +286,9 @@ int send_pack(struct send_pack_args *args,
 	int in = fd[0];
 	int out = fd[1];
 	struct strbuf req_buf = STRBUF_INIT;
+	struct strbuf cap_buf = STRBUF_INIT;
 	struct ref *ref;
-	int new_refs;
+	int need_pack_data = 0;
 	int allow_deleting_refs = 0;
 	int status_report = 0;
 	int use_sideband = 0;
@@ -207,6 +297,7 @@ int send_pack(struct send_pack_args *args,
 	unsigned cmds_sent = 0;
 	int ret;
 	struct async demux;
+	const char *push_cert_nonce = NULL;
 
 	/* Does the other end support the reporting? */
 	if (server_supports("report-status"))
@@ -223,6 +314,14 @@ int send_pack(struct send_pack_args *args,
 		agent_supported = 1;
 	if (server_supports("no-thin"))
 		args->use_thin_pack = 0;
+	if (args->push_cert) {
+		int len;
+
+		push_cert_nonce = server_feature_value("push-cert", &len);
+		if (!push_cert_nonce)
+			die(_("the receiving end does not support --signed push"));
+		push_cert_nonce = xmemdupz(push_cert_nonce, len);
+	}
 
 	if (!remote_refs) {
 		fprintf(stderr, "No refs in common and none specified; doing nothing.\n"
@@ -230,64 +329,71 @@ int send_pack(struct send_pack_args *args,
 		return 0;
 	}
 
+	if (status_report)
+		strbuf_addstr(&cap_buf, " report-status");
+	if (use_sideband)
+		strbuf_addstr(&cap_buf, " side-band-64k");
+	if (quiet_supported && (args->quiet || !args->progress))
+		strbuf_addstr(&cap_buf, " quiet");
+	if (agent_supported)
+		strbuf_addf(&cap_buf, " agent=%s", git_user_agent_sanitized());
+
+	/*
+	 * NEEDSWORK: why does delete-refs have to be so specific to
+	 * send-pack machinery that set_ref_status_for_push() cannot
+	 * set this bit for us???
+	 */
+	for (ref = remote_refs; ref; ref = ref->next)
+		if (ref->deletion && !allow_deleting_refs)
+			ref->status = REF_STATUS_REJECT_NODELETE;
+
 	if (!args->dry_run)
 		advertise_shallow_grafts_buf(&req_buf);
 
+	if (!args->dry_run && args->push_cert)
+		cmds_sent = generate_push_cert(&req_buf, remote_refs, args,
+					       cap_buf.buf, push_cert_nonce);
+
 	/*
-	 * Finally, tell the other end!
+	 * Clear the status for each ref and see if we need to send
+	 * the pack data.
 	 */
-	new_refs = 0;
 	for (ref = remote_refs; ref; ref = ref->next) {
-		if (!ref->peer_ref && !args->send_mirror)
+		if (!ref_update_to_be_sent(ref, args))
 			continue;
 
-		/* Check for statuses set by set_ref_status_for_push() */
-		switch (ref->status) {
-		case REF_STATUS_REJECT_NONFASTFORWARD:
-		case REF_STATUS_REJECT_ALREADY_EXISTS:
-		case REF_STATUS_REJECT_FETCH_FIRST:
-		case REF_STATUS_REJECT_NEEDS_FORCE:
-		case REF_STATUS_REJECT_STALE:
-		case REF_STATUS_UPTODATE:
-			continue;
-		default:
-			; /* do nothing */
-		}
+		if (!ref->deletion)
+			need_pack_data = 1;
 
-		if (ref->deletion && !allow_deleting_refs) {
-			ref->status = REF_STATUS_REJECT_NODELETE;
+		if (args->dry_run || !status_report)
+			ref->status = REF_STATUS_OK;
+		else
+			ref->status = REF_STATUS_EXPECTING_REPORT;
+	}
+
+	/*
+	 * Finally, tell the other end!
+	 */
+	for (ref = remote_refs; ref; ref = ref->next) {
+		char *old_hex, *new_hex;
+
+		if (args->dry_run || args->push_cert)
 			continue;
-		}
 
-		if (!ref->deletion)
-			new_refs++;
+		if (!ref_update_to_be_sent(ref, args))
+			continue;
 
-		if (args->dry_run) {
-			ref->status = REF_STATUS_OK;
+		old_hex = sha1_to_hex(ref->old_sha1);
+		new_hex = sha1_to_hex(ref->new_sha1);
+		if (!cmds_sent) {
+			packet_buf_write(&req_buf,
+					 "%s %s %s%c%s",
+					 old_hex, new_hex, ref->name, 0,
+					 cap_buf.buf);
+			cmds_sent = 1;
 		} else {
-			char *old_hex = sha1_to_hex(ref->old_sha1);
-			char *new_hex = sha1_to_hex(ref->new_sha1);
-			int quiet = quiet_supported && (args->quiet || !args->progress);
-
-			if (!cmds_sent && (status_report || use_sideband ||
-					   quiet || agent_supported)) {
-				packet_buf_write(&req_buf,
-						 "%s %s %s%c%s%s%s%s%s",
-						 old_hex, new_hex, ref->name, 0,
-						 status_report ? " report-status" : "",
-						 use_sideband ? " side-band-64k" : "",
-						 quiet ? " quiet" : "",
-						 agent_supported ? " agent=" : "",
-						 agent_supported ? git_user_agent_sanitized() : ""
-						);
-			}
-			else
-				packet_buf_write(&req_buf, "%s %s %s",
-						 old_hex, new_hex, ref->name);
-			ref->status = status_report ?
-				REF_STATUS_EXPECTING_REPORT :
-				REF_STATUS_OK;
-			cmds_sent++;
+			packet_buf_write(&req_buf, "%s %s %s",
+					 old_hex, new_hex, ref->name);
 		}
 	}
 
@@ -301,6 +407,7 @@ int send_pack(struct send_pack_args *args,
 		packet_flush(out);
 	}
 	strbuf_release(&req_buf);
+	strbuf_release(&cap_buf);
 
 	if (use_sideband && cmds_sent) {
 		memset(&demux, 0, sizeof(demux));
@@ -312,7 +419,7 @@ int send_pack(struct send_pack_args *args,
 		in = demux.out;
 	}
 
-	if (new_refs && cmds_sent) {
+	if (need_pack_data && cmds_sent) {
 		if (pack_objects(out, remote_refs, extra_have, args) < 0) {
 			for (ref = remote_refs; ref; ref = ref->next)
 				ref->status = REF_STATUS_NONE;
diff --git a/send-pack.h b/send-pack.h
index 8e843924c..563545774 100644
--- a/send-pack.h
+++ b/send-pack.h
@@ -2,6 +2,7 @@
 #define SEND_PACK_H
 
 struct send_pack_args {
+	const char *url;
 	unsigned verbose:1,
 		quiet:1,
 		porcelain:1,
@@ -11,6 +12,7 @@ struct send_pack_args {
 		use_thin_pack:1,
 		use_ofs_delta:1,
 		dry_run:1,
+		push_cert:1,
 		stateless_rpc:1;
 };
 
diff --git a/t/lib-httpd/apache.conf b/t/lib-httpd/apache.conf
index b384d7993..7713dd260 100644
--- a/t/lib-httpd/apache.conf
+++ b/t/lib-httpd/apache.conf
@@ -68,6 +68,7 @@ LockFile accept.lock
 
 PassEnv GIT_VALGRIND
 PassEnv GIT_VALGRIND_OPTIONS
+PassEnv GNUPGHOME
 
 Alias /dumb/ www/
 Alias /auth/dumb/ www/auth/dumb/
diff --git a/t/t5534-push-signed.sh b/t/t5534-push-signed.sh
new file mode 100755
index 000000000..2786346f9
--- /dev/null
+++ b/t/t5534-push-signed.sh
@@ -0,0 +1,127 @@
+#!/bin/sh
+
+test_description='signed push'
+
+. ./test-lib.sh
+. "$TEST_DIRECTORY"/lib-gpg.sh
+
+prepare_dst () {
+	rm -fr dst &&
+	test_create_repo dst &&
+
+	git push dst master:noop master:ff master:noff
+}
+
+test_expect_success setup '
+	# master, ff and noff branches pointing at the same commit
+	test_tick &&
+	git commit --allow-empty -m initial &&
+
+	git checkout -b noop &&
+	git checkout -b ff &&
+	git checkout -b noff &&
+
+	# noop stays the same, ff advances, noff rewrites
+	test_tick &&
+	git commit --allow-empty --amend -m rewritten &&
+	git checkout ff &&
+
+	test_tick &&
+	git commit --allow-empty -m second
+'
+
+test_expect_success 'unsigned push does not send push certificate' '
+	prepare_dst &&
+	mkdir -p dst/.git/hooks &&
+	write_script dst/.git/hooks/post-receive <<-\EOF &&
+	# discard the update list
+	cat >/dev/null
+	# record the push certificate
+	if test -n "${GIT_PUSH_CERT-}"
+	then
+		git cat-file blob $GIT_PUSH_CERT >../push-cert
+	fi
+	EOF
+
+	git push dst noop ff +noff &&
+	! test -f dst/push-cert
+'
+
+test_expect_success 'talking with a receiver without push certificate support' '
+	prepare_dst &&
+	mkdir -p dst/.git/hooks &&
+	write_script dst/.git/hooks/post-receive <<-\EOF &&
+	# discard the update list
+	cat >/dev/null
+	# record the push certificate
+	if test -n "${GIT_PUSH_CERT-}"
+	then
+		git cat-file blob $GIT_PUSH_CERT >../push-cert
+	fi
+	EOF
+
+	git push dst noop ff +noff &&
+	! test -f dst/push-cert
+'
+
+test_expect_success 'push --signed fails with a receiver without push certificate support' '
+	prepare_dst &&
+	mkdir -p dst/.git/hooks &&
+	test_must_fail git push --signed dst noop ff +noff 2>err &&
+	test_i18ngrep "the receiving end does not support" err
+'
+
+test_expect_success GPG 'no certificate for a signed push with no update' '
+	prepare_dst &&
+	mkdir -p dst/.git/hooks &&
+	write_script dst/.git/hooks/post-receive <<-\EOF &&
+	if test -n "${GIT_PUSH_CERT-}"
+	then
+		git cat-file blob $GIT_PUSH_CERT >../push-cert
+	fi
+	EOF
+	git push dst noop &&
+	! test -f dst/push-cert
+'
+
+test_expect_success GPG 'signed push sends push certificate' '
+	prepare_dst &&
+	mkdir -p dst/.git/hooks &&
+	git -C dst config receive.certnonceseed sekrit &&
+	write_script dst/.git/hooks/post-receive <<-\EOF &&
+	# discard the update list
+	cat >/dev/null
+	# record the push certificate
+	if test -n "${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
+
+	git push --signed dst noop ff +noff &&
+
+	(
+		cat <<-\EOF &&
+		SIGNER=C O Mitter <committer@example.com>
+		KEY=13B6F51ECDDE430D
+		STATUS=G
+		NONCE_STATUS=OK
+		EOF
+		sed -n -e "s/^nonce /NONCE=/p" -e "/^$/q" dst/push-cert
+	) >expect &&
+
+	grep "$(git rev-parse noop ff) refs/heads/ff" dst/push-cert &&
+	grep "$(git rev-parse noop noff) refs/heads/noff" dst/push-cert &&
+	test_cmp expect dst/push-cert-status
+'
+
+test_done
diff --git a/t/t5541-http-push-smart.sh b/t/t5541-http-push-smart.sh
index db1998873..d2c681ebf 100755
--- a/t/t5541-http-push-smart.sh
+++ b/t/t5541-http-push-smart.sh
@@ -12,6 +12,7 @@ if test -n "$NO_CURL"; then
 fi
 
 ROOT_PATH="$PWD"
+. "$TEST_DIRECTORY"/lib-gpg.sh
 . "$TEST_DIRECTORY"/lib-httpd.sh
 . "$TEST_DIRECTORY"/lib-terminal.sh
 start_httpd
@@ -338,5 +339,45 @@ test_expect_success CMDLINE_LIMIT 'push 2000 tags over http' '
 	run_with_limited_cmdline git push --mirror
 '
 
+test_expect_success GPG 'push with post-receive to inspect certificate' '
+	(
+		cd "$HTTPD_DOCUMENT_ROOT_PATH"/test_repo.git &&
+		mkdir -p hooks &&
+		write_script hooks/post-receive <<-\EOF &&
+		# discard the update list
+		cat >/dev/null
+		# record the push certificate
+		if test -n "${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
+
+		git config receive.certnonceseed sekrit &&
+		git config receive.certnonceslop 30
+	) &&
+	cd "$ROOT_PATH/test_repo_clone" &&
+	test_commit cert-test &&
+	git push --signed "$HTTPD_URL/smart/test_repo.git" &&
+	(
+		cd "$HTTPD_DOCUMENT_ROOT_PATH" &&
+		cat <<-\EOF &&
+		SIGNER=C O Mitter <committer@example.com>
+		KEY=13B6F51ECDDE430D
+		STATUS=G
+		NONCE_STATUS=OK
+		EOF
+		sed -n -e "s/^nonce /NONCE=/p" -e "/^$/q" push-cert
+	) >expect &&
+	test_cmp expect "$HTTPD_DOCUMENT_ROOT_PATH/push-cert-status"
+'
+
 stop_httpd
 test_done
diff --git a/t/test-lib.sh b/t/test-lib.sh
index 82095e34e..0f4a67bfc 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -813,7 +813,8 @@ rm -fr "$TRASH_DIRECTORY" || {
 }
 
 HOME="$TRASH_DIRECTORY"
-export HOME
+GNUPGHOME="$HOME/gnupg-home-not-used"
+export HOME GNUPGHOME
 
 if test -z "$TEST_NO_CREATE_REPO"
 then
diff --git a/tag.c b/tag.c
index 82d841bf2..5b0ac62ed 100644
--- a/tag.c
+++ b/tag.c
@@ -4,9 +4,6 @@
 #include "tree.h"
 #include "blob.h"
 
-#define PGP_SIGNATURE "-----BEGIN PGP SIGNATURE-----"
-#define PGP_MESSAGE "-----BEGIN PGP MESSAGE-----"
-
 const char *tag_type = "tag";
 
 struct object *deref_tag(struct object *o, const char *warn, int warnlen)
@@ -143,20 +140,3 @@ int parse_tag(struct tag *item)
 	free(data);
 	return ret;
 }
-
-/*
- * Look at a signed tag object, and return the offset where
- * the embedded detached signature begins, or the end of the
- * data when there is no such signature.
- */
-size_t parse_signature(const char *buf, unsigned long size)
-{
-	char *eol;
-	size_t len = 0;
-	while (len < size && !starts_with(buf + len, PGP_SIGNATURE) &&
-			!starts_with(buf + len, PGP_MESSAGE)) {
-		eol = memchr(buf + len, '\n', size - len);
-		len += eol ? eol - (buf + len) + 1 : size - len;
-	}
-	return len;
-}
diff --git a/tag.h b/tag.h
index bc8a1e40f..f4580aea4 100644
--- a/tag.h
+++ b/tag.h
@@ -17,6 +17,5 @@ extern int parse_tag_buffer(struct tag *item, const void *data, unsigned long si
 extern int parse_tag(struct tag *item);
 extern struct object *deref_tag(struct object *, const char *, int);
 extern struct object *deref_tag_noverify(struct object *);
-extern size_t parse_signature(const char *buf, unsigned long size);
 
 #endif /* TAG_H */
diff --git a/transport-helper.c b/transport-helper.c
index 080a7a6ae..2b24d51a2 100644
--- a/transport-helper.c
+++ b/transport-helper.c
@@ -260,7 +260,8 @@ static const char *unsupported_options[] = {
 static const char *boolean_options[] = {
 	TRANS_OPT_THIN,
 	TRANS_OPT_KEEP,
-	TRANS_OPT_FOLLOWTAGS
+	TRANS_OPT_FOLLOWTAGS,
+	TRANS_OPT_PUSH_CERT
 	};
 
 static int set_helper_option(struct transport *transport,
@@ -836,6 +837,9 @@ static int push_refs_with_push(struct transport *transport,
 	if (flags & TRANSPORT_PUSH_DRY_RUN) {
 		if (set_helper_option(transport, "dry-run", "true") != 0)
 			die("helper %s does not support dry-run", data->name);
+	} else if (flags & TRANSPORT_PUSH_CERT) {
+		if (set_helper_option(transport, TRANS_OPT_PUSH_CERT, "true") != 0)
+			die("helper %s does not support --signed", data->name);
 	}
 
 	strbuf_addch(&buf, '\n');
@@ -860,6 +864,9 @@ static int push_refs_with_export(struct transport *transport,
 	if (flags & TRANSPORT_PUSH_DRY_RUN) {
 		if (set_helper_option(transport, "dry-run", "true") != 0)
 			die("helper %s does not support dry-run", data->name);
+	} else if (flags & TRANSPORT_PUSH_CERT) {
+		if (set_helper_option(transport, TRANS_OPT_PUSH_CERT, "true") != 0)
+			die("helper %s does not support dry-run", data->name);
 	}
 
 	if (flags & TRANSPORT_PUSH_FORCE) {
diff --git a/transport.c b/transport.c
index 7388bb87d..055d2a27d 100644
--- a/transport.c
+++ b/transport.c
@@ -477,6 +477,9 @@ static int set_git_option(struct git_transport_options *opts,
 				die("transport: invalid depth option '%s'", value);
 		}
 		return 0;
+	} else if (!strcmp(name, TRANS_OPT_PUSH_CERT)) {
+		opts->push_cert = !!value;
+		return 0;
 	}
 	return 1;
 }
@@ -820,6 +823,8 @@ static int git_transport_push(struct transport *transport, struct ref *remote_re
 	args.progress = transport->progress;
 	args.dry_run = !!(flags & TRANSPORT_PUSH_DRY_RUN);
 	args.porcelain = !!(flags & TRANSPORT_PUSH_PORCELAIN);
+	args.push_cert = !!(flags & TRANSPORT_PUSH_CERT);
+	args.url = transport->url;
 
 	ret = send_pack(&args, data->fd, data->conn, remote_refs,
 			&data->extra_have);
diff --git a/transport.h b/transport.h
index 02ea248db..3e0091eaa 100644
--- a/transport.h
+++ b/transport.h
@@ -12,6 +12,7 @@ struct git_transport_options {
 	unsigned check_self_contained_and_connected : 1;
 	unsigned self_contained_and_connected : 1;
 	unsigned update_shallow : 1;
+	unsigned push_cert : 1;
 	int depth;
 	const char *uploadpack;
 	const char *receivepack;
@@ -123,6 +124,7 @@ struct transport {
 #define TRANSPORT_RECURSE_SUBMODULES_ON_DEMAND 256
 #define TRANSPORT_PUSH_NO_HOOK 512
 #define TRANSPORT_PUSH_FOLLOW_TAGS 1024
+#define TRANSPORT_PUSH_CERT 2048
 
 #define TRANSPORT_SUMMARY_WIDTH (2 * DEFAULT_ABBREV + 3)
 #define TRANSPORT_SUMMARY(x) (int)(TRANSPORT_SUMMARY_WIDTH + strlen(x) - gettext_width(x)), (x)
@@ -156,6 +158,9 @@ struct transport *transport_get(struct remote *, const char *);
 /* Accept refs that may update .git/shallow without --depth */
 #define TRANS_OPT_UPDATE_SHALLOW "updateshallow"
 
+/* Send push certificates */
+#define TRANS_OPT_PUSH_CERT "pushcert"
+
 /**
  * Returns 0 if the option was used, non-zero otherwise. Prints a
  * message to stderr if the option is not used.