Merge branch 'jc/signed-commit' and 'jc/pull-signed-tag'

They both use the extended headers in commit objects, and the former has
necessary infrastructure to show them that is useful to view the result of
the latter.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

15 files changed, 364 insertions, 19 deletions

diff --git a/Documentation/config.txt b/Documentation/config.txt
index b30c7e6278..094c1c9de3 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -1094,6 +1094,17 @@ grep.lineNumber::
 grep.extendedRegexp::
 	If set to true, enable '--extended-regexp' option by default.
 
+gpg.program::
+	Use this custom program instead of "gpg" found on $PATH when
+	making or verifying a PGP signature. The program must support the
+	same command line interface as GPG, namely, to verify a detached
+	signature, "gpg --verify $file - <$signature" is run, and the
+	program is expected to signal a good signature by exiting with
+	code 0, and to generate an ascii-armored detached signature, the
+	standard input of "gpg -bsau $key" is fed with the contents to be
+	signed, and the program is expected to send the result to its
+	standard output.
+
 gui.commitmsgwidth::
 	Defines how wide the commit message window is in the
 	linkgit:git-gui[1]. "75" is the default.
diff --git a/Documentation/git-tag.txt b/Documentation/git-tag.txt
index c83cb13de6..74fc7e006f 100644
--- a/Documentation/git-tag.txt
+++ b/Documentation/git-tag.txt
@@ -38,7 +38,9 @@ created (i.e. a lightweight tag).
 A GnuPG signed tag object will be created when `-s` or `-u
 <key-id>` is used.  When `-u <key-id>` is not used, the
 committer identity for the current user is used to find the
-GnuPG key for signing.
+GnuPG key for signing. 	The configuration variable `gpg.program`
+is used to specify custom GnuPG binary.
+
 
 OPTIONS
 -------
@@ -48,11 +50,11 @@ OPTIONS
 
 -s::
 --sign::
-	Make a GPG-signed tag, using the default e-mail address's key
+	Make a GPG-signed tag, using the default e-mail address's key.
 
 -u <key-id>::
 --local-user=<key-id>::
-	Make a GPG-signed tag, using the given key
+	Make a GPG-signed tag, using the given key.
 
 -f::
 --force::
diff --git a/builtin/commit-tree.c b/builtin/commit-tree.c
index b9c331225f..d5e19af547 100644
--- a/builtin/commit-tree.c
+++ b/builtin/commit-tree.c
@@ -8,8 +8,9 @@
 #include "tree.h"
 #include "builtin.h"
 #include "utf8.h"
+#include "gpg-interface.h"
 
-static const char commit_tree_usage[] = "git commit-tree [(-p <sha1>)...] [-m <message>] [-F <file>] <sha1> <changelog";
+static const char commit_tree_usage[] = "git commit-tree [(-p <sha1>)...] [-S<signer>] [-m <message>] [-F <file>] <sha1> <changelog";
 
 static void new_parent(struct commit *parent, struct commit_list **parents_p)
 {
@@ -25,6 +26,14 @@ static void new_parent(struct commit *parent, struct commit_list **parents_p)
 	commit_list_insert(parent, parents_p);
 }
 
+static int commit_tree_config(const char *var, const char *value, void *cb)
+{
+	int status = git_gpg_config(var, value, NULL);
+	if (status)
+		return status;
+	return git_default_config(var, value, cb);
+}
+
 int cmd_commit_tree(int argc, const char **argv, const char *prefix)
 {
 	int i, got_tree = 0;
@@ -32,12 +41,16 @@ int cmd_commit_tree(int argc, const char **argv, const char *prefix)
 	unsigned char tree_sha1[20];
 	unsigned char commit_sha1[20];
 	struct strbuf buffer = STRBUF_INIT;
+	const char *sign_commit = NULL;
 
-	git_config(git_default_config, NULL);
+	git_config(commit_tree_config, NULL);
 
 	if (argc < 2 || !strcmp(argv[1], "-h"))
 		usage(commit_tree_usage);
 
+	if (get_sha1(argv[1], tree_sha1))
+		die("Not a valid object name %s", argv[1]);
+
 	for (i = 1; i < argc; i++) {
 		const char *arg = argv[i];
 		if (!strcmp(arg, "-p")) {
@@ -51,6 +64,11 @@ int cmd_commit_tree(int argc, const char **argv, const char *prefix)
 			continue;
 		}
 
+		if (!memcmp(arg, "-S", 2)) {
+			sign_commit = arg + 2;
+			continue;
+		}
+
 		if (!strcmp(arg, "-m")) {
 			if (argc <= ++i)
 				usage(commit_tree_usage);
@@ -98,7 +116,8 @@ int cmd_commit_tree(int argc, const char **argv, const char *prefix)
 			die_errno("git commit-tree: failed to read");
 	}
 
-	if (commit_tree(buffer.buf, tree_sha1, parents, commit_sha1, NULL)) {
+	if (commit_tree(buffer.buf, tree_sha1, parents, commit_sha1,
+			NULL, sign_commit)) {
 		strbuf_release(&buffer);
 		return 1;
 	}
diff --git a/builtin/commit.c b/builtin/commit.c
index 0c64c880d5..fa41ec8c87 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -26,6 +26,7 @@
 #include "unpack-trees.h"
 #include "quote.h"
 #include "submodule.h"
+#include "gpg-interface.h"
 
 static const char * const builtin_commit_usage[] = {
 	"git commit [options] [--] <filepattern>...",
@@ -85,6 +86,8 @@ static int all, edit_flag, also, interactive, patch_interactive, only, amend, si
 static int quiet, verbose, no_verify, allow_empty, dry_run, renew_authorship;
 static int no_post_rewrite, allow_empty_message;
 static char *untracked_files_arg, *force_date, *ignore_submodule_arg;
+static char *sign_commit;
+
 /*
  * The default commit message cleanup mode will remove the lines
  * beginning with # (shell comments) and leading and trailing
@@ -144,6 +147,8 @@ static struct option builtin_commit_options[] = {
 	OPT_BOOLEAN('e', "edit", &edit_flag, "force edit of commit"),
 	OPT_STRING(0, "cleanup", &cleanup_arg, "default", "how to strip spaces and #comments from message"),
 	OPT_BOOLEAN(0, "status", &include_status, "include status in commit message template"),
+	{ OPTION_STRING, 'S', "gpg-sign", &sign_commit, "key id",
+	  "GPG sign commit", PARSE_OPT_OPTARG, NULL, (intptr_t) "" },
 	/* end commit message options */
 
 	OPT_GROUP("Commit contents options"),
@@ -1324,6 +1329,7 @@ static void print_summary(const char *prefix, const unsigned char *sha1,
 static int git_commit_config(const char *k, const char *v, void *cb)
 {
 	struct wt_status *s = cb;
+	int status;
 
 	if (!strcmp(k, "commit.template"))
 		return git_config_pathname(&template_file, k, v);
@@ -1332,6 +1338,9 @@ static int git_commit_config(const char *k, const char *v, void *cb)
 		return 0;
 	}
 
+	status = git_gpg_config(k, v, NULL);
+	if (status)
+		return status;
 	return git_status_config(k, v, s);
 }
 
@@ -1492,7 +1501,7 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 	}
 
 	if (commit_tree_extended(sb.buf, active_cache_tree->sha1, parents, sha1,
-				 author_ident.buf, extra)) {
+				 author_ident.buf, sign_commit, extra)) {
 		rollback_index_files();
 		die(_("failed to write commit object"));
 	}
diff --git a/builtin/merge.c b/builtin/merge.c
index 99f1429b35..e5afe64cef 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -26,6 +26,7 @@
 #include "merge-recursive.h"
 #include "resolve-undo.h"
 #include "remote.h"
+#include "gpg-interface.h"
 
 #define DEFAULT_TWOHEAD (1<<0)
 #define DEFAULT_OCTOPUS (1<<1)
@@ -62,6 +63,7 @@ static int allow_rerere_auto;
 static int abort_current_merge;
 static int show_progress = -1;
 static int default_to_upstream;
+static const char *sign_commit;
 
 static struct strategy all_strategy[] = {
 	{ "recursive",  DEFAULT_TWOHEAD | NO_TRIVIAL },
@@ -207,6 +209,8 @@ static struct option builtin_merge_options[] = {
 	OPT_BOOLEAN(0, "abort", &abort_current_merge,
 		"abort the current in-progress merge"),
 	OPT_SET_INT(0, "progress", &show_progress, "force progress reporting", 1),
+	{ OPTION_STRING, 'S', "gpg-sign", &sign_commit, "key id",
+	  "GPG sign commit", PARSE_OPT_OPTARG, NULL, (intptr_t) "" },
 	OPT_END()
 };
 
@@ -533,6 +537,8 @@ static void parse_branch_merge_options(char *bmo)
 
 static int git_merge_config(const char *k, const char *v, void *cb)
 {
+	int status;
+
 	if (branch && !prefixcmp(k, "branch.") &&
 		!prefixcmp(k + 7, branch) &&
 		!strcmp(k + 7 + strlen(branch), ".mergeoptions")) {
@@ -570,6 +576,10 @@ static int git_merge_config(const char *k, const char *v, void *cb)
 		default_to_upstream = git_config_bool(k, v);
 		return 0;
 	}
+
+	status = git_gpg_config(k, v, NULL);
+	if (status)
+		return status;
 	return git_diff_ui_config(k, v, cb);
 }
 
@@ -902,7 +912,8 @@ static int merge_trivial(struct commit *head)
 	parent->next->item = remoteheads->item;
 	parent->next->next = NULL;
 	prepare_to_commit();
-	commit_tree(merge_msg.buf, result_tree, parent, result_commit, NULL);
+	commit_tree(merge_msg.buf, result_tree, parent, result_commit, NULL,
+		    sign_commit);
 	finish(head, result_commit, "In-index merge");
 	drop_save();
 	return 0;
@@ -933,7 +944,8 @@ static int finish_automerge(struct commit *head,
 	strbuf_addch(&merge_msg, '\n');
 	prepare_to_commit();
 	free_commit_list(remoteheads);
-	commit_tree(merge_msg.buf, result_tree, parents, result_commit, NULL);
+	commit_tree(merge_msg.buf, result_tree, parents, result_commit,
+		    NULL, sign_commit);
 	strbuf_addf(&buf, "Merge made by the '%s' strategy.", wt_strategy);
 	finish(head, result_commit, buf.buf);
 	strbuf_release(&buf);
diff --git a/commit.c b/commit.c
index b78127403b..27c7226abb 100644
--- a/commit.c
+++ b/commit.c
@@ -6,6 +6,7 @@
 #include "diff.h"
 #include "revision.h"
 #include "notes.h"
+#include "gpg-interface.h"
 
 int save_commit_buffer = 1;
 
@@ -840,6 +841,86 @@ struct commit_list *reduce_heads(struct commit_list *heads)
 	return result;
 }
 
+static const char gpg_sig_header[] = "gpgsig";
+static const int gpg_sig_header_len = sizeof(gpg_sig_header) - 1;
+
+static int do_sign_commit(struct strbuf *buf, const char *keyid)
+{
+	struct strbuf sig = STRBUF_INIT;
+	int inspos, copypos;
+
+	/* find the end of the header */
+	inspos = strstr(buf->buf, "\n\n") - buf->buf + 1;
+
+	if (!keyid || !*keyid)
+		keyid = get_signing_key();
+	if (sign_buffer(buf, &sig, keyid)) {
+		strbuf_release(&sig);
+		return -1;
+	}
+
+	for (copypos = 0; sig.buf[copypos]; ) {
+		const char *bol = sig.buf + copypos;
+		const char *eol = strchrnul(bol, '\n');
+		int len = (eol - bol) + !!*eol;
+
+		if (!copypos) {
+			strbuf_insert(buf, inspos, gpg_sig_header, gpg_sig_header_len);
+			inspos += gpg_sig_header_len;
+		}
+		strbuf_insert(buf, inspos++, " ", 1);
+		strbuf_insert(buf, inspos, bol, len);
+		inspos += len;
+		copypos += len;
+	}
+	strbuf_release(&sig);
+	return 0;
+}
+
+int parse_signed_commit(const unsigned char *sha1,
+			struct strbuf *payload, struct strbuf *signature)
+{
+	unsigned long size;
+	enum object_type type;
+	char *buffer = read_sha1_file(sha1, &type, &size);
+	int in_signature, saw_signature = -1;
+	char *line, *tail;
+
+	if (!buffer || type != OBJ_COMMIT)
+		goto cleanup;
+
+	line = buffer;
+	tail = buffer + size;
+	in_signature = 0;
+	saw_signature = 0;
+	while (line < tail) {
+		const char *sig = NULL;
+		char *next = memchr(line, '\n', tail - line);
+
+		next = next ? next + 1 : tail;
+		if (in_signature && line[0] == ' ')
+			sig = line + 1;
+		else if (!prefixcmp(line, gpg_sig_header) &&
+			 line[gpg_sig_header_len] == ' ')
+			sig = line + gpg_sig_header_len + 1;
+		if (sig) {
+			strbuf_add(signature, sig, next - sig);
+			saw_signature = 1;
+			in_signature = 1;
+		} else {
+			if (*line == '\n')
+				/* dump the whole remainder of the buffer */
+				next = tail;
+			strbuf_add(payload, line, next - line);
+			in_signature = 0;
+		}
+		line = next;
+	}
+ cleanup:
+	free(buffer);
+	return saw_signature;
+}
+
 static void handle_signed_tag(struct commit *parent, struct commit_extra_header ***tail)
 {
 	struct merge_remote_desc *desc;
@@ -975,13 +1056,14 @@ void free_commit_extra_headers(struct commit_extra_header *extra)
 
 int commit_tree(const char *msg, unsigned char *tree,
 		struct commit_list *parents, unsigned char *ret,
-		const char *author)
+		const char *author, const char *sign_commit)
 {
 	struct commit_extra_header *extra = NULL, **tail = &extra;
 	int result;
 
 	append_merge_tag_headers(parents, &tail);
-	result = commit_tree_extended(msg, tree, parents, ret, author, extra);
+	result = commit_tree_extended(msg, tree, parents, ret,
+				      author, sign_commit, extra);
 	free_commit_extra_headers(extra);
 	return result;
 }
@@ -993,7 +1075,8 @@ static const char commit_utf8_warn[] =
 
 int commit_tree_extended(const char *msg, unsigned char *tree,
 			 struct commit_list *parents, unsigned char *ret,
-			 const char *author, struct commit_extra_header *extra)
+			 const char *author, const char *sign_commit,
+			 struct commit_extra_header *extra)
 {
 	int result;
 	int encoding_is_utf8;
@@ -1043,6 +1126,9 @@ int commit_tree_extended(const char *msg, unsigned char *tree,
 	if (encoding_is_utf8 && !is_utf8(buffer.buf))
 		fprintf(stderr, commit_utf8_warn);
 
+	if (sign_commit && do_sign_commit(&buffer, sign_commit))
+		return -1;
+
 	result = write_sha1_file(buffer.buf, buffer.len, commit_type, ret);
 	strbuf_release(&buffer);
 	return result;
diff --git a/commit.h b/commit.h
index 3745f12099..61076486df 100644
--- a/commit.h
+++ b/commit.h
@@ -193,11 +193,11 @@ extern void append_merge_tag_headers(struct commit_list *parents,
 
 extern int commit_tree(const char *msg, unsigned char *tree,
 		       struct commit_list *parents, unsigned char *ret,
-		       const char *author);
+		       const char *author, const char *sign_commit);
 
 extern int commit_tree_extended(const char *msg, unsigned char *tree,
 				struct commit_list *parents, unsigned char *ret,
-				const char *author,
+				const char *author, const char *sign_commit,
 				struct commit_extra_header *);
 
 extern struct commit_extra_header *read_commit_extra_headers(struct commit *);
@@ -218,4 +218,6 @@ struct merge_remote_desc {
  */
 struct commit *get_merge_parent(const char *name);
 
+extern int parse_signed_commit(const unsigned char *sha1,
+			       struct strbuf *message, struct strbuf *signature);
 #endif /* COMMIT_H */
diff --git a/gpg-interface.c b/gpg-interface.c
index ff232c8c5d..18630ff8da 100644
--- a/gpg-interface.c
+++ b/gpg-interface.c
@@ -5,6 +5,7 @@
 #include "sigchain.h"
 
 static char *configured_signing_key;
+static const char *gpg_program = "gpg";
 
 void set_signing_key(const char *key)
 {
@@ -15,9 +16,12 @@ void set_signing_key(const char *key)
 int git_gpg_config(const char *var, const char *value, void *cb)
 {
 	if (!strcmp(var, "user.signingkey")) {
+		set_signing_key(value);
+	}
+	if (!strcmp(var, "gpg.program")) {
 		if (!value)
 			return config_error_nonbool(var);
-		set_signing_key(value);
+		gpg_program = xstrdup(value);
 	}
 	return 0;
 }
@@ -46,7 +50,7 @@ int sign_buffer(struct strbuf *buffer, struct strbuf *signature, const char *sig
 	gpg.argv = args;
 	gpg.in = -1;
 	gpg.out = -1;
-	args[0] = "gpg";
+	args[0] = gpg_program;
 	args[1] = "-bsau";
 	args[2] = signing_key;
 	args[3] = NULL;
@@ -101,10 +105,11 @@ int verify_signed_buffer(const char *payload, size_t payload_size,
 			 struct strbuf *gpg_output)
 {
 	struct child_process gpg;
-	const char *args_gpg[] = {"gpg", "--verify", "FILE", "-", NULL};
+	const char *args_gpg[] = {NULL, "--verify", "FILE", "-", NULL};
 	char path[PATH_MAX];
 	int fd, ret;
 
+	args_gpg[0] = gpg_program;
 	fd = git_mkstemp(path, PATH_MAX, ".git_vtag_tmpXXXXXX");
 	if (fd < 0)
 		return error("could not create temporary file '%s': %s",
diff --git a/log-tree.c b/log-tree.c
index e7694a3a4c..142ba51427 100644
--- a/log-tree.c
+++ b/log-tree.c
@@ -8,6 +8,7 @@
 #include "refs.h"
 #include "string-list.h"
 #include "color.h"
+#include "gpg-interface.h"
 
 struct decoration name_decoration = { "object names" };
 
@@ -403,6 +404,41 @@ void log_write_email_headers(struct rev_info *opt, struct commit *commit,
 	*extra_headers_p = extra_headers;
 }
 
+static void show_signature(struct rev_info *opt, struct commit *commit)
+{
+	struct strbuf payload = STRBUF_INIT;
+	struct strbuf signature = STRBUF_INIT;
+	struct strbuf gpg_output = STRBUF_INIT;
+	int status;
+	const char *color, *reset, *bol, *eol;
+
+	if (parse_signed_commit(commit->object.sha1, &payload, &signature) <= 0)
+		goto out;
+
+	status = verify_signed_buffer(payload.buf, payload.len,
+				      signature.buf, signature.len,
+				      &gpg_output);
+	if (status && !gpg_output.len)
+		strbuf_addstr(&gpg_output, "No signature\n");
+
+	color = diff_get_color_opt(&opt->diffopt,
+				   status ? DIFF_WHITESPACE : DIFF_FRAGINFO);
+	reset = diff_get_color_opt(&opt->diffopt, DIFF_RESET);
+
+	bol = gpg_output.buf;
+	while (*bol) {
+		eol = strchrnul(bol, '\n');
+		printf("%s%.*s%s%s", color, (int)(eol - bol), bol, reset,
+		       *eol ? "\n" : "");
+		bol = (*eol) ? (eol + 1) : eol;
+	}
+
+ out:
+	strbuf_release(&gpg_output);
+	strbuf_release(&payload);
+	strbuf_release(&signature);
+}
+
 void show_log(struct rev_info *opt)
 {
 	struct strbuf msgbuf = STRBUF_INIT;
@@ -514,6 +550,9 @@ void show_log(struct rev_info *opt)
 		}
 	}
 
+	if (opt->show_signature)
+		show_signature(opt, commit);
+
 	if (!commit->buffer)
 		return;
 
diff --git a/notes-cache.c b/notes-cache.c
index 4c8984ede1..c36a960bc3 100644
--- a/notes-cache.c
+++ b/notes-cache.c
@@ -56,7 +56,7 @@ int notes_cache_write(struct notes_cache *c)
 
 	if (write_notes_tree(&c->tree, tree_sha1))
 		return -1;
-	if (commit_tree(c->validity, tree_sha1, NULL, commit_sha1, NULL) < 0)
+	if (commit_tree(c->validity, tree_sha1, NULL, commit_sha1, NULL, NULL) < 0)
 		return -1;
 	if (update_ref("update notes cache", c->tree.ref, commit_sha1, NULL,
 		       0, QUIET_ON_ERR) < 0)
diff --git a/notes-merge.c b/notes-merge.c
index e9e4199311..61cf18eeab 100644
--- a/notes-merge.c
+++ b/notes-merge.c
@@ -546,7 +546,7 @@ void create_notes_commit(struct notes_tree *t, struct commit_list *parents,
 		/* else: t->ref points to nothing, assume root/orphan commit */
 	}
 
-	if (commit_tree(msg, tree_sha1, parents, result_sha1, NULL))
+	if (commit_tree(msg, tree_sha1, parents, result_sha1, NULL, NULL))
 		die("Failed to commit notes tree to database");
 }
 
diff --git a/pretty.c b/pretty.c
index f45eb54e4c..392d656595 100644
--- a/pretty.c
+++ b/pretty.c
@@ -9,6 +9,7 @@
 #include "notes.h"
 #include "color.h"
 #include "reflog-walk.h"
+#include "gpg-interface.h"
 
 static char *user_format;
 static struct cmt_fmt_map {
@@ -640,6 +641,12 @@ struct format_commit_context {
 	const struct pretty_print_context *pretty_ctx;
 	unsigned commit_header_parsed:1;
 	unsigned commit_message_parsed:1;
+	unsigned commit_signature_parsed:1;
+	struct {
+		char *gpg_output;
+		char good_bad;
+		char *signer;
+	} signature;
 	char *message;
 	size_t width, indent1, indent2;
 
@@ -822,6 +829,59 @@ static void rewrap_message_tail(struct strbuf *sb,
 	c->indent2 = new_indent2;
 }
 
+static struct {
+	char result;
+	const char *check;
+} signature_check[] = {
+	{ 'G', ": Good signature from " },
+	{ 'B', ": BAD signature from " },
+};
+
+static void parse_signature_lines(struct format_commit_context *ctx)
+{
+	const char *buf = ctx->signature.gpg_output;
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(signature_check); i++) {
+		const char *found = strstr(buf, signature_check[i].check);
+		const char *next;
+		if (!found)
+			continue;
+		ctx->signature.good_bad = signature_check[i].result;
+		found += strlen(signature_check[i].check);
+		next = strchrnul(found, '\n');
+		ctx->signature.signer = xmemdupz(found, next - found);
+		break;
+	}
+}
+
+static void parse_commit_signature(struct format_commit_context *ctx)
+{
+	struct strbuf payload = STRBUF_INIT;
+	struct strbuf signature = STRBUF_INIT;
+	struct strbuf gpg_output = STRBUF_INIT;
+	int status;
+
+	ctx->commit_signature_parsed = 1;
+
+	if (parse_signed_commit(ctx->commit->object.sha1,
+				&payload, &signature) <= 0)
+		goto out;
+	status = verify_signed_buffer(payload.buf, payload.len,
+				      signature.buf, signature.len,
+				      &gpg_output);
+	if (status && !gpg_output.len)
+		goto out;
+	ctx->signature.gpg_output = strbuf_detach(&gpg_output, NULL);
+	parse_signature_lines(ctx);
+
+ out:
+	strbuf_release(&gpg_output);
+	strbuf_release(&payload);
+	strbuf_release(&signature);
+}
+
+
 static size_t format_commit_one(struct strbuf *sb, const char *placeholder,
 				void *context)
 {
@@ -974,6 +1034,30 @@ static size_t format_commit_one(struct strbuf *sb, const char *placeholder,
 		return 0;
 	}
 
+	if (placeholder[0] == 'G') {
+		if (!c->commit_signature_parsed)
+			parse_commit_signature(c);
+		switch (placeholder[1]) {
+		case 'G':
+			if (c->signature.gpg_output)
+				strbuf_addstr(sb, c->signature.gpg_output);
+			break;
+		case '?':
+			switch (c->signature.good_bad) {
+			case 'G':
+			case 'B':
+				strbuf_addch(sb, c->signature.good_bad);
+			}
+			break;
+		case 'S':
+			if (c->signature.signer)
+				strbuf_addstr(sb, c->signature.signer);
+			break;
+		}
+		return 2;
+	}
+
+
 	/* For the rest we have to parse the commit header. */
 	if (!c->commit_header_parsed)
 		parse_commit_header(c);
@@ -1114,6 +1198,8 @@ void format_commit_message(const struct commit *commit,
 
 	if (context.message != commit->buffer)
 		free(context.message);
+	free(context.signature.gpg_output);
+	free(context.signature.signer);
 }
 
 static void pp_header(const struct pretty_print_context *pp,
diff --git a/revision.c b/revision.c
index 8764dde381..064e351084 100644
--- a/revision.c
+++ b/revision.c
@@ -1469,6 +1469,8 @@ static int handle_revision_opt(struct rev_info *revs, int argc, const char **arg
 		revs->show_notes = 1;
 		revs->show_notes_given = 1;
 		revs->notes_opt.use_default_notes = 1;
+	} else if (!strcmp(arg, "--show-signature")) {
+		revs->show_signature = 1;
 	} else if (!prefixcmp(arg, "--show-notes=") ||
 		   !prefixcmp(arg, "--notes=")) {
 		struct strbuf buf = STRBUF_INIT;
diff --git a/revision.h b/revision.h
index 6aa53d1aa7..b8e9223954 100644
--- a/revision.h
+++ b/revision.h
@@ -110,6 +110,7 @@ struct rev_info {
 			show_merge:1,
 			show_notes:1,
 			show_notes_given:1,
+			show_signature:1,
 			pretty_given:1,
 			abbrev_commit:1,
 			abbrev_commit_given:1,
diff --git a/t/t7510-signed-commit.sh b/t/t7510-signed-commit.sh
new file mode 100755
index 0000000000..30401ced07
--- /dev/null
+++ b/t/t7510-signed-commit.sh
@@ -0,0 +1,71 @@
+#!/bin/sh
+
+test_description='signed commit tests'
+. ./test-lib.sh
+. "$TEST_DIRECTORY/lib-gpg.sh"
+
+test_expect_success GPG 'create signed commits' '
+	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 master &&
+	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"
+'
+
+test_expect_success GPG 'show signatures' '
+	(
+		for commit in initial second merge master
+		do
+			git show --pretty=short --show-signature $commit >actual &&
+			grep "Good signature from" actual || exit 1
+			! grep "BAD signature from" actual || exit 1
+			echo $commit OK
+		done
+	) &&
+	(
+		for commit in merge^2 fourth-unsigned
+		do
+			git show --pretty=short --show-signature $commit >actual &&
+			grep "Good signature from" actual && exit 1
+			! grep "BAD signature from" actual || exit 1
+			echo $commit OK
+		done
+	)
+'
+
+test_expect_success GPG 'detect fudged signature' '
+	git cat-file commit master >raw &&
+
+	sed -e "s/fourth signed/4th forged/" raw >forged1 &&
+	git hash-object -w -t commit forged1 >forged1.commit &&
+	git show --pretty=short --show-signature $(cat forged1.commit) >actual1 &&
+	grep "BAD signature from" actual1 &&
+	! grep "Good signature from" actual1
+'
+
+test_expect_success GPG 'detect fudged signature with NUL' '
+	git cat-file commit master >raw &&
+	cat raw >forged2 &&
+	echo Qwik | tr "Q" "\000" >>forged2 &&
+	git hash-object -w -t commit forged2 >forged2.commit &&
+	git show --pretty=short --show-signature $(cat forged2.commit) >actual2 &&
+	grep "BAD signature from" actual2 &&
+	! grep "Good signature from" actual2
+'
+
+test_done