Merge branch 'eb/hash-transition'

Work to support a repository that work with both SHA-1 and SHA-256
hash algorithms has started.

* eb/hash-transition: (30 commits)
  t1016-compatObjectFormat: add tests to verify the conversion between objects
  t1006: test oid compatibility with cat-file
  t1006: rename sha1 to oid
  test-lib: compute the compatibility hash so tests may use it
  builtin/ls-tree: let the oid determine the output algorithm
  object-file: handle compat objects in check_object_signature
  tree-walk: init_tree_desc take an oid to get the hash algorithm
  builtin/cat-file: let the oid determine the output algorithm
  rev-parse: add an --output-object-format parameter
  repository: implement extensions.compatObjectFormat
  object-file: update object_info_extended to reencode objects
  object-file-convert: convert commits that embed signed tags
  object-file-convert: convert commit objects when writing
  object-file-convert: don't leak when converting tag objects
  object-file-convert: convert tag objects when writing
  object-file-convert: add a function to convert trees between algorithms
  object: factor out parse_mode out of fast-import and tree-walk into in object.h
  cache: add a function to read an OID of a specific algorithm
  tag: sign both hashes
  commit: export add_header_signature to support handling signatures on tags
  ...

62 files changed, 1844 insertions, 349 deletions

diff --git a/Documentation/config/extensions.txt b/Documentation/config/extensions.txt
index 66db0e15da..38dce3df35 100644
--- a/Documentation/config/extensions.txt
+++ b/Documentation/config/extensions.txt
@@ -7,6 +7,18 @@ Note that this setting should only be set by linkgit:git-init[1] or
 linkgit:git-clone[1].  Trying to change it after initialization will not
 work and will produce hard-to-diagnose issues.
 
+extensions.compatObjectFormat::
+
+	Specify a compatitbility hash algorithm to use.  The acceptable values
+	are `sha1` and `sha256`.  The value specified must be different from the
+	value of extensions.objectFormat.  This allows client level
+	interoperability between git repositories whose objectFormat matches
+	this compatObjectFormat.  In particular when fully implemented the
+	pushes and pulls from a repository in whose objectFormat matches
+	compatObjectFormat.  As well as being able to use oids encoded in
+	compatObjectFormat in addition to oids encoded with objectFormat to
+	locally specify objects.
+
 extensions.refStorage::
 	Specify the ref storage format to use. The acceptable values are:
 +
diff --git a/Documentation/git-rev-parse.txt b/Documentation/git-rev-parse.txt
index 5d83dd36da..f9d5a35fa0 100644
--- a/Documentation/git-rev-parse.txt
+++ b/Documentation/git-rev-parse.txt
@@ -159,6 +159,18 @@ for another option.
 	unfortunately named tag "master"), and shows them as full
 	refnames (e.g. "refs/heads/master").
 
+--output-object-format=(sha1|sha256|storage)::
+
+	Allow oids to be input from any object format that the current
+	repository supports.
+
+	Specifying "sha1" translates if necessary and returns a sha1 oid.
+
+	Specifying "sha256" translates if necessary and returns a sha256 oid.
+
+	Specifying "storage" translates if necessary and returns an oid in
+	encoded in the storage hash algorithm.
+
 Options for Objects
 ~~~~~~~~~~~~~~~~~~~
 
diff --git a/Makefile b/Makefile
index 4e255c81f2..e955678908 100644
--- a/Makefile
+++ b/Makefile
@@ -797,6 +797,7 @@ TEST_BUILTINS_OBJS += test-config.o
 TEST_BUILTINS_OBJS += test-crontab.o
 TEST_BUILTINS_OBJS += test-csprng.o
 TEST_BUILTINS_OBJS += test-date.o
+TEST_BUILTINS_OBJS += test-delete-gpgsig.o
 TEST_BUILTINS_OBJS += test-delta.o
 TEST_BUILTINS_OBJS += test-dir-iterator.o
 TEST_BUILTINS_OBJS += test-drop-caches.o
@@ -1060,6 +1061,7 @@ LIB_OBJS += list-objects-filter.o
 LIB_OBJS += list-objects.o
 LIB_OBJS += lockfile.o
 LIB_OBJS += log-tree.o
+LIB_OBJS += loose.o
 LIB_OBJS += ls-refs.o
 LIB_OBJS += mailinfo.o
 LIB_OBJS += mailmap.o
@@ -1080,6 +1082,7 @@ LIB_OBJS += notes-cache.o
 LIB_OBJS += notes-merge.o
 LIB_OBJS += notes-utils.o
 LIB_OBJS += notes.o
+LIB_OBJS += object-file-convert.o
 LIB_OBJS += object-file.o
 LIB_OBJS += object-name.o
 LIB_OBJS += object.o
diff --git a/archive.c b/archive.c
index a6730bebfa..5287fcdd8e 100644
--- a/archive.c
+++ b/archive.c
@@ -339,7 +339,8 @@ int write_archive_entries(struct archiver_args *args,
 		opts.src_index = args->repo->index;
 		opts.dst_index = args->repo->index;
 		opts.fn = oneway_merge;
-		init_tree_desc(&t, args->tree->buffer, args->tree->size);
+		init_tree_desc(&t, &args->tree->object.oid,
+			       args->tree->buffer, args->tree->size);
 		if (unpack_trees(1, &t, &opts))
 			return -1;
 		git_attr_set_direction(GIT_ATTR_INDEX);
diff --git a/builtin/am.c b/builtin/am.c
index d1990d7edc..e8fb27a8ef 100644
--- a/builtin/am.c
+++ b/builtin/am.c
@@ -1994,8 +1994,8 @@ static int fast_forward_to(struct tree *head, struct tree *remote, int reset)
 	opts.reset = reset ? UNPACK_RESET_PROTECT_UNTRACKED : 0;
 	opts.preserve_ignored = 0; /* FIXME: !overwrite_ignore */
 	opts.fn = twoway_merge;
-	init_tree_desc(&t[0], head->buffer, head->size);
-	init_tree_desc(&t[1], remote->buffer, remote->size);
+	init_tree_desc(&t[0], &head->object.oid, head->buffer, head->size);
+	init_tree_desc(&t[1], &remote->object.oid, remote->buffer, remote->size);
 
 	if (unpack_trees(2, t, &opts)) {
 		rollback_lock_file(&lock_file);
@@ -2029,7 +2029,7 @@ static int merge_tree(struct tree *tree)
 	opts.dst_index = &the_index;
 	opts.merge = 1;
 	opts.fn = oneway_merge;
-	init_tree_desc(&t[0], tree->buffer, tree->size);
+	init_tree_desc(&t[0], &tree->object.oid, tree->buffer, tree->size);
 
 	if (unpack_trees(1, t, &opts)) {
 		rollback_lock_file(&lock_file);
diff --git a/builtin/cat-file.c b/builtin/cat-file.c
index bbf851138e..3f5ce7d34c 100644
--- a/builtin/cat-file.c
+++ b/builtin/cat-file.c
@@ -106,7 +106,10 @@ static int cat_one_file(int opt, const char *exp_type, const char *obj_name,
 	struct object_info oi = OBJECT_INFO_INIT;
 	struct strbuf sb = STRBUF_INIT;
 	unsigned flags = OBJECT_INFO_LOOKUP_REPLACE;
-	unsigned get_oid_flags = GET_OID_RECORD_PATH | GET_OID_ONLY_TO_DIE;
+	unsigned get_oid_flags =
+		GET_OID_RECORD_PATH |
+		GET_OID_ONLY_TO_DIE |
+		GET_OID_HASH_ANY;
 	const char *path = force_path;
 	const int opt_cw = (opt == 'c' || opt == 'w');
 	if (!path && opt_cw)
@@ -226,7 +229,8 @@ static int cat_one_file(int opt, const char *exp_type, const char *obj_name,
 					die(_("unable to read %s"), oid_to_hex(&oid));
 
 				if (!skip_prefix(buffer, "object ", &target) ||
-				    get_oid_hex(target, &blob_oid))
+				    get_oid_hex_algop(target, &blob_oid,
+						      &hash_algos[oid.algo]))
 					die("%s not a valid tag", oid_to_hex(&oid));
 				free(buffer);
 			} else
@@ -517,7 +521,9 @@ static void batch_one_object(const char *obj_name,
 			     struct expand_data *data)
 {
 	struct object_context ctx;
-	int flags = opt->follow_symlinks ? GET_OID_FOLLOW_SYMLINKS : 0;
+	int flags =
+		GET_OID_HASH_ANY |
+		(opt->follow_symlinks ? GET_OID_FOLLOW_SYMLINKS : 0);
 	enum get_oid_result result;
 
 	result = get_oid_with_context(the_repository, obj_name,
diff --git a/builtin/checkout.c b/builtin/checkout.c
index 902c97ab23..2e8b0d18f4 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -706,7 +706,7 @@ static int reset_tree(struct tree *tree, const struct checkout_opts *o,
 			       NULL);
 	if (parse_tree(tree) < 0)
 		return 128;
-	init_tree_desc(&tree_desc, tree->buffer, tree->size);
+	init_tree_desc(&tree_desc, &tree->object.oid, tree->buffer, tree->size);
 	switch (unpack_trees(1, &tree_desc, &opts)) {
 	case -2:
 		*writeout_error = 1;
@@ -826,11 +826,13 @@ static int merge_working_tree(const struct checkout_opts *opts,
 			die(_("unable to parse commit %s"),
 				oid_to_hex(old_commit_oid));
 
-		init_tree_desc(&trees[0], tree->buffer, tree->size);
+		init_tree_desc(&trees[0], &tree->object.oid,
+			       tree->buffer, tree->size);
 		if (parse_tree(new_tree) < 0)
 			exit(128);
 		tree = new_tree;
-		init_tree_desc(&trees[1], tree->buffer, tree->size);
+		init_tree_desc(&trees[1], &tree->object.oid,
+			       tree->buffer, tree->size);
 
 		ret = unpack_trees(2, trees, &topts);
 		clear_unpack_trees_porcelain(&topts);
diff --git a/builtin/clone.c b/builtin/clone.c
index f3bc6edef8..74ec14542e 100644
--- a/builtin/clone.c
+++ b/builtin/clone.c
@@ -740,7 +740,7 @@ static int checkout(int submodule_progress, int filter_submodules)
 		die(_("unable to parse commit %s"), oid_to_hex(&oid));
 	if (parse_tree(tree) < 0)
 		exit(128);
-	init_tree_desc(&t, tree->buffer, tree->size);
+	init_tree_desc(&t, &tree->object.oid, tree->buffer, tree->size);
 	if (unpack_trees(1, &t, &opts) < 0)
 		die(_("unable to checkout working tree"));
 
diff --git a/builtin/commit.c b/builtin/commit.c
index a91197245f..f5a5a0ec53 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -333,7 +333,7 @@ static void create_base_index(const struct commit *current_head)
 		die(_("failed to unpack HEAD tree object"));
 	if (parse_tree(tree) < 0)
 		exit(128);
-	init_tree_desc(&t, tree->buffer, tree->size);
+	init_tree_desc(&t, &tree->object.oid, tree->buffer, tree->size);
 	if (unpack_trees(1, &t, &opts))
 		exit(128); /* We've already reported the error, finish dying */
 }
diff --git a/builtin/fast-import.c b/builtin/fast-import.c
index 71a195ca22..782bda007c 100644
--- a/builtin/fast-import.c
+++ b/builtin/fast-import.c
@@ -1236,20 +1236,6 @@ static void *gfi_unpack_entry(
 	return unpack_entry(the_repository, p, oe->idx.offset, &type, sizep);
 }
 
-static const char *get_mode(const char *str, uint16_t *modep)
-{
-	unsigned char c;
-	uint16_t mode = 0;
-
-	while ((c = *str++) != ' ') {
-		if (c < '0' || c > '7')
-			return NULL;
-		mode = (mode << 3) + (c - '0');
-	}
-	*modep = mode;
-	return str;
-}
-
 static void load_tree(struct tree_entry *root)
 {
 	struct object_id *oid = &root->versions[1].oid;
@@ -1287,7 +1273,7 @@ static void load_tree(struct tree_entry *root)
 		t->entries[t->entry_count++] = e;
 
 		e->tree = NULL;
-		c = get_mode(c, &e->versions[1].mode);
+		c = parse_mode(c, &e->versions[1].mode);
 		if (!c)
 			die("Corrupt mode in %s", oid_to_hex(oid));
 		e->versions[0].mode = e->versions[1].mode;
@@ -2280,7 +2266,7 @@ static void file_change_m(const char *p, struct branch *b)
 	struct object_id oid;
 	uint16_t mode, inline_data = 0;
 
-	p = get_mode(p, &mode);
+	p = parse_mode(p, &mode);
 	if (!p)
 		die("Corrupt mode: %s", command_buf.buf);
 	switch (mode) {
diff --git a/builtin/grep.c b/builtin/grep.c
index 982bcfc4b1..5777ba82a9 100644
--- a/builtin/grep.c
+++ b/builtin/grep.c
@@ -527,7 +527,7 @@ static int grep_submodule(struct grep_opt *opt,
 		strbuf_addstr(&base, filename);
 		strbuf_addch(&base, '/');
 
-		init_tree_desc(&tree, data, size);
+		init_tree_desc(&tree, oid, data, size);
 		hit = grep_tree(&subopt, pathspec, &tree, &base, base.len,
 				object_type == OBJ_COMMIT);
 		strbuf_release(&base);
@@ -573,7 +573,7 @@ static int grep_cache(struct grep_opt *opt,
 						     &type, &size);
 			if (!data)
 				die(_("unable to read tree %s"), oid_to_hex(&ce->oid));
-			init_tree_desc(&tree, data, size);
+			init_tree_desc(&tree, &ce->oid, data, size);
 
 			hit |= grep_tree(opt, pathspec, &tree, &name, 0, 0);
 			strbuf_setlen(&name, name_base_len);
@@ -669,7 +669,7 @@ static int grep_tree(struct grep_opt *opt, const struct pathspec *pathspec,
 				    oid_to_hex(&entry.oid));
 
 			strbuf_addch(base, '/');
-			init_tree_desc(&sub, data, size);
+			init_tree_desc(&sub, &entry.oid, data, size);
 			hit |= grep_tree(opt, pathspec, &sub, base, tn_len,
 					 check_attr);
 			free(data);
@@ -713,7 +713,7 @@ static int grep_object(struct grep_opt *opt, const struct pathspec *pathspec,
 			strbuf_add(&base, name, len);
 			strbuf_addch(&base, ':');
 		}
-		init_tree_desc(&tree, data, size);
+		init_tree_desc(&tree, &obj->oid, data, size);
 		hit = grep_tree(opt, pathspec, &tree, &base, base.len,
 				obj->type == OBJ_COMMIT);
 		strbuf_release(&base);
diff --git a/builtin/ls-tree.c b/builtin/ls-tree.c
index e4a891337c..61a2965c8a 100644
--- a/builtin/ls-tree.c
+++ b/builtin/ls-tree.c
@@ -375,6 +375,7 @@ int cmd_ls_tree(int argc, const char **argv, const char *prefix)
 		OPT_END()
 	};
 	struct ls_tree_cmdmode_to_fmt *m2f = ls_tree_cmdmode_format;
+	struct object_context obj_context;
 	int ret;
 
 	git_config(git_default_config, NULL);
@@ -406,7 +407,9 @@ int cmd_ls_tree(int argc, const char **argv, const char *prefix)
 			ls_tree_usage, ls_tree_options);
 	if (argc < 1)
 		usage_with_options(ls_tree_usage, ls_tree_options);
-	if (repo_get_oid(the_repository, argv[0], &oid))
+	if (get_oid_with_context(the_repository, argv[0],
+				 GET_OID_HASH_ANY, &oid,
+				 &obj_context))
 		die("Not a valid object name %s", argv[0]);
 
 	/*
diff --git a/builtin/merge.c b/builtin/merge.c
index a0ba1f9815..1cbd6a899c 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -677,7 +677,8 @@ static int read_tree_trivial(struct object_id *common, struct object_id *head,
 	cache_tree_free(&the_index.cache_tree);
 	for (i = 0; i < nr_trees; i++) {
 		parse_tree(trees[i]);
-		init_tree_desc(t+i, trees[i]->buffer, trees[i]->size);
+		init_tree_desc(t+i, &trees[i]->object.oid,
+			       trees[i]->buffer, trees[i]->size);
 	}
 	if (unpack_trees(nr_trees, t, &opts))
 		return -1;
diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index 329aeac804..baf0090fc8 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -1826,7 +1826,8 @@ static void add_pbase_object(struct tree_desc *tree,
 			tree = pbase_tree_get(&entry.oid);
 			if (!tree)
 				return;
-			init_tree_desc(&sub, tree->tree_data, tree->tree_size);
+			init_tree_desc(&sub, &tree->oid,
+				       tree->tree_data, tree->tree_size);
 
 			add_pbase_object(&sub, down, downlen, fullname);
 			pbase_tree_put(tree);
@@ -1886,7 +1887,8 @@ static void add_preferred_base_object(const char *name)
 		}
 		else {
 			struct tree_desc tree;
-			init_tree_desc(&tree, it->pcache.tree_data, it->pcache.tree_size);
+			init_tree_desc(&tree, &it->pcache.oid,
+				       it->pcache.tree_data, it->pcache.tree_size);
 			add_pbase_object(&tree, name, cmplen, name);
 		}
 	}
diff --git a/builtin/read-tree.c b/builtin/read-tree.c
index 1ffd863cff..6f89cec0fb 100644
--- a/builtin/read-tree.c
+++ b/builtin/read-tree.c
@@ -263,7 +263,7 @@ int cmd_read_tree(int argc, const char **argv, const char *cmd_prefix)
 		struct tree *tree = trees[i];
 		if (parse_tree(tree) < 0)
 			return 128;
-		init_tree_desc(t+i, tree->buffer, tree->size);
+		init_tree_desc(t+i, &tree->object.oid, tree->buffer, tree->size);
 	}
 	if (unpack_trees(nr_trees, t, &opts))
 		return 128;
diff --git a/builtin/rev-parse.c b/builtin/rev-parse.c
index 181c703d4c..624182e507 100644
--- a/builtin/rev-parse.c
+++ b/builtin/rev-parse.c
@@ -25,6 +25,7 @@
 #include "submodule.h"
 #include "commit-reach.h"
 #include "shallow.h"
+#include "object-file-convert.h"
 
 #define DO_REVS		1
 #define DO_NOREV	2
@@ -676,6 +677,8 @@ static void print_path(const char *path, const char *prefix, enum format_type fo
 int cmd_rev_parse(int argc, const char **argv, const char *prefix)
 {
 	int i, as_is = 0, verify = 0, quiet = 0, revs_count = 0, type = 0;
+	const struct git_hash_algo *output_algo = NULL;
+	const struct git_hash_algo *compat = NULL;
 	int did_repo_setup = 0;
 	int has_dashdash = 0;
 	int output_prefix = 0;
@@ -747,6 +750,7 @@ int cmd_rev_parse(int argc, const char **argv, const char *prefix)
 
 			prepare_repo_settings(the_repository);
 			the_repository->settings.command_requires_full_index = 0;
+			compat = the_repository->compat_hash_algo;
 		}
 
 		if (!strcmp(arg, "--")) {
@@ -834,6 +838,22 @@ int cmd_rev_parse(int argc, const char **argv, const char *prefix)
 				flags |= GET_OID_QUIETLY;
 				continue;
 			}
+			if (opt_with_value(arg, "--output-object-format", &arg)) {
+				if (!arg)
+					die(_("no object format specified"));
+				if (!strcmp(arg, the_hash_algo->name) ||
+				    !strcmp(arg, "storage")) {
+					flags |= GET_OID_HASH_ANY;
+					output_algo = the_hash_algo;
+					continue;
+				}
+				else if (compat && !strcmp(arg, compat->name)) {
+					flags |= GET_OID_HASH_ANY;
+					output_algo = compat;
+					continue;
+				}
+				else die(_("unsupported object format: %s"), arg);
+			}
 			if (opt_with_value(arg, "--short", &arg)) {
 				filter &= ~(DO_FLAGS|DO_NOREV);
 				verify = 1;
@@ -883,7 +903,7 @@ int cmd_rev_parse(int argc, const char **argv, const char *prefix)
 				continue;
 			}
 			if (skip_prefix(arg, "--disambiguate=", &arg)) {
-				repo_for_each_abbrev(the_repository, arg,
+				repo_for_each_abbrev(the_repository, arg, the_hash_algo,
 						     show_abbrev, NULL);
 				continue;
 			}
@@ -1091,6 +1111,9 @@ int cmd_rev_parse(int argc, const char **argv, const char *prefix)
 		}
 		if (!get_oid_with_context(the_repository, name,
 					  flags, &oid, &unused)) {
+			if (output_algo)
+				repo_oid_to_algop(the_repository, &oid,
+						  output_algo, &oid);
 			if (verify)
 				revs_count++;
 			else
diff --git a/builtin/stash.c b/builtin/stash.c
index 7fb355bff0..062be1fbc0 100644
--- a/builtin/stash.c
+++ b/builtin/stash.c
@@ -284,7 +284,7 @@ static int reset_tree(struct object_id *i_tree, int update, int reset)
 	if (parse_tree(tree))
 		return -1;
 
-	init_tree_desc(t, tree->buffer, tree->size);
+	init_tree_desc(t, &tree->object.oid, tree->buffer, tree->size);
 
 	opts.head_idx = 1;
 	opts.src_index = &the_index;
@@ -870,7 +870,8 @@ static void diff_include_untracked(const struct stash_info *info, struct diff_op
 		tree[i] = parse_tree_indirect(oid[i]);
 		if (parse_tree(tree[i]) < 0)
 			die(_("failed to parse tree"));
-		init_tree_desc(&tree_desc[i], tree[i]->buffer, tree[i]->size);
+		init_tree_desc(&tree_desc[i], &tree[i]->object.oid,
+			       tree[i]->buffer, tree[i]->size);
 	}
 
 	unpack_tree_opt.head_idx = -1;
diff --git a/builtin/tag.c b/builtin/tag.c
index 19a7e06bf4..e2ff749832 100644
--- a/builtin/tag.c
+++ b/builtin/tag.c
@@ -27,6 +27,7 @@
 #include "ref-filter.h"
 #include "date.h"
 #include "write-or-die.h"
+#include "object-file-convert.h"
 
 static const char * const git_tag_usage[] = {
 	N_("git tag [-a | -s | -u <key-id>] [-f] [-m <msg> | -F <file>] [-e]\n"
@@ -151,9 +152,43 @@ static int verify_tag(const char *name, const char *ref UNUSED,
 	return 0;
 }
 
-static int do_sign(struct strbuf *buffer)
+static int do_sign(struct strbuf *buffer, struct object_id **compat_oid,
+		   struct object_id *compat_oid_buf)
 {
-	return sign_buffer(buffer, buffer, get_signing_key()) ? -1 : 0;
+	const struct git_hash_algo *compat = the_repository->compat_hash_algo;
+	struct strbuf sig = STRBUF_INIT, compat_sig = STRBUF_INIT;
+	struct strbuf compat_buf = STRBUF_INIT;
+	const char *keyid = get_signing_key();
+	int ret = -1;
+
+	if (sign_buffer(buffer, &sig, keyid))
+		return -1;
+
+	if (compat) {
+		const struct git_hash_algo *algo = the_repository->hash_algo;
+
+		if (convert_object_file(&compat_buf, algo, compat,
+					buffer->buf, buffer->len, OBJ_TAG, 1))
+			goto out;
+		if (sign_buffer(&compat_buf, &compat_sig, keyid))
+			goto out;
+		add_header_signature(&compat_buf, &sig, algo);
+		strbuf_addbuf(&compat_buf, &compat_sig);
+		hash_object_file(compat, compat_buf.buf, compat_buf.len,
+				 OBJ_TAG, compat_oid_buf);
+		*compat_oid = compat_oid_buf;
+	}
+
+	if (compat_sig.len)
+		add_header_signature(buffer, &compat_sig, compat);
+
+	strbuf_addbuf(buffer, &sig);
+	ret = 0;
+out:
+	strbuf_release(&sig);
+	strbuf_release(&compat_sig);
+	strbuf_release(&compat_buf);
+	return ret;
 }
 
 static const char tag_template[] =
@@ -226,9 +261,11 @@ static void write_tag_body(int fd, const struct object_id *oid)
 
 static int build_tag_object(struct strbuf *buf, int sign, struct object_id *result)
 {
-	if (sign && do_sign(buf) < 0)
+	struct object_id *compat_oid = NULL, compat_oid_buf;
+	if (sign && do_sign(buf, &compat_oid, &compat_oid_buf) < 0)
 		return error(_("unable to sign the tag"));
-	if (write_object_file(buf->buf, buf->len, OBJ_TAG, result) < 0)
+	if (write_object_file_flags(buf->buf, buf->len, OBJ_TAG, result,
+				    compat_oid, 0) < 0)
 		return error(_("unable to write tag file"));
 	return 0;
 }
diff --git a/cache-tree.c b/cache-tree.c
index 80ca832477..387c0a3e5b 100644
--- a/cache-tree.c
+++ b/cache-tree.c
@@ -447,7 +447,7 @@ static int update_one(struct cache_tree *it,
 		hash_object_file(the_hash_algo, buffer.buf, buffer.len,
 				 OBJ_TREE, &it->oid);
 	} else if (write_object_file_flags(buffer.buf, buffer.len, OBJ_TREE,
-					   &it->oid, flags & WRITE_TREE_SILENT
+					   &it->oid, NULL, flags & WRITE_TREE_SILENT
 					   ? HASH_SILENT : 0)) {
 		strbuf_release(&buffer);
 		return -1;
@@ -769,7 +769,7 @@ static void prime_cache_tree_rec(struct repository *r,
 
 	oidcpy(&it->oid, &tree->object.oid);
 
-	init_tree_desc(&desc, tree->buffer, tree->size);
+	init_tree_desc(&desc, &tree->object.oid, tree->buffer, tree->size);
 	cnt = 0;
 	while (tree_entry(&desc, &entry)) {
 		if (!S_ISDIR(entry.mode))
diff --git a/commit.c b/commit.c
index 467be9f7f9..1da10ec916 100644
--- a/commit.c
+++ b/commit.c
@@ -27,6 +27,7 @@
 #include "tree.h"
 #include "hook.h"
 #include "parse.h"
+#include "object-file-convert.h"
 
 static struct commit_extra_header *read_commit_extra_header_lines(const char *buf, size_t len, const char **);
 
@@ -1113,12 +1114,11 @@ static const char *gpg_sig_headers[] = {
 	"gpgsig-sha256",
 };
 
-int sign_with_header(struct strbuf *buf, const char *keyid)
+int add_header_signature(struct strbuf *buf, struct strbuf *sig, const struct git_hash_algo *algo)
 {
-	struct strbuf sig = STRBUF_INIT;
 	int inspos, copypos;
 	const char *eoh;
-	const char *gpg_sig_header = gpg_sig_headers[hash_algo_by_ptr(the_hash_algo)];
+	const char *gpg_sig_header = gpg_sig_headers[hash_algo_by_ptr(algo)];
 	int gpg_sig_header_len = strlen(gpg_sig_header);
 
 	/* find the end of the header */
@@ -1128,15 +1128,8 @@ int sign_with_header(struct strbuf *buf, const char *keyid)
 	else
 		inspos = eoh - 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;
+	for (copypos = 0; sig->buf[copypos]; ) {
+		const char *bol = sig->buf + copypos;
 		const char *eol = strchrnul(bol, '\n');
 		int len = (eol - bol) + !!*eol;
 
@@ -1149,11 +1142,17 @@ int sign_with_header(struct strbuf *buf, const char *keyid)
 		inspos += len;
 		copypos += len;
 	}
-	strbuf_release(&sig);
 	return 0;
 }
 
-
+static int sign_commit_to_strbuf(struct strbuf *sig, struct strbuf *buf, const char *keyid)
+{
+	if (!keyid || !*keyid)
+		keyid = get_signing_key();
+	if (sign_buffer(buf, sig, keyid))
+		return -1;
+	return 0;
+}
 
 int parse_signed_commit(const struct commit *commit,
 			struct strbuf *payload, struct strbuf *signature,
@@ -1369,6 +1368,39 @@ void append_merge_tag_headers(struct commit_list *parents,
 	}
 }
 
+static int convert_commit_extra_headers(struct commit_extra_header *orig,
+					struct commit_extra_header **result)
+{
+	const struct git_hash_algo *compat = the_repository->compat_hash_algo;
+	const struct git_hash_algo *algo = the_repository->hash_algo;
+	struct commit_extra_header *extra = NULL, **tail = &extra;
+	struct strbuf out = STRBUF_INIT;
+	while (orig) {
+		struct commit_extra_header *new;
+		CALLOC_ARRAY(new, 1);
+		if (!strcmp(orig->key, "mergetag")) {
+			if (convert_object_file(&out, algo, compat,
+						orig->value, orig->len,
+						OBJ_TAG, 1)) {
+				free(new);
+				free_commit_extra_headers(extra);
+				return -1;
+			}
+			new->key = xstrdup("mergetag");
+			new->value = strbuf_detach(&out, &new->len);
+		} else {
+			new->key = xstrdup(orig->key);
+			new->len = orig->len;
+			new->value = xmemdupz(orig->value, orig->len);
+		}
+		*tail = new;
+		tail = &new->next;
+		orig = orig->next;
+	}
+	*result = extra;
+	return 0;
+}
+
 static void add_extra_header(struct strbuf *buffer,
 			     struct commit_extra_header *extra)
 {
@@ -1612,70 +1644,169 @@ N_("Warning: commit message did not conform to UTF-8.\n"
    "You may want to amend it after fixing the message, or set the config\n"
    "variable i18n.commitEncoding to the encoding your project uses.\n");
 
-int commit_tree_extended(const char *msg, size_t msg_len,
-			 const struct object_id *tree,
-			 struct commit_list *parents, struct object_id *ret,
-			 const char *author, const char *committer,
-			 const char *sign_commit,
-			 struct commit_extra_header *extra)
+static void write_commit_tree(struct strbuf *buffer, const char *msg, size_t msg_len,
+			      const struct object_id *tree,
+			      const struct object_id *parents, size_t parents_len,
+			      const char *author, const char *committer,
+			      struct commit_extra_header *extra)
 {
-	int result;
 	int encoding_is_utf8;
-	struct strbuf buffer;
-
-	assert_oid_type(tree, OBJ_TREE);
-
-	if (memchr(msg, '\0', msg_len))
-		return error("a NUL byte in commit log message not allowed.");
+	size_t i;
 
 	/* Not having i18n.commitencoding is the same as having utf-8 */
 	encoding_is_utf8 = is_encoding_utf8(git_commit_encoding);
 
-	strbuf_init(&buffer, 8192); /* should avoid reallocs for the headers */
-	strbuf_addf(&buffer, "tree %s\n", oid_to_hex(tree));
+	strbuf_grow(buffer, 8192); /* should avoid reallocs for the headers */
+	strbuf_addf(buffer, "tree %s\n", oid_to_hex(tree));
 
 	/*
 	 * NOTE! This ordering means that the same exact tree merged with a
 	 * different order of parents will be a _different_ changeset even
 	 * if everything else stays the same.
 	 */
-	while (parents) {
-		struct commit *parent = pop_commit(&parents);
-		strbuf_addf(&buffer, "parent %s\n",
-			    oid_to_hex(&parent->object.oid));
-	}
+	for (i = 0; i < parents_len; i++)
+		strbuf_addf(buffer, "parent %s\n", oid_to_hex(&parents[i]));
 
 	/* Person/date information */
 	if (!author)
 		author = git_author_info(IDENT_STRICT);
-	strbuf_addf(&buffer, "author %s\n", author);
+	strbuf_addf(buffer, "author %s\n", author);
 	if (!committer)
 		committer = git_committer_info(IDENT_STRICT);
-	strbuf_addf(&buffer, "committer %s\n", committer);
+	strbuf_addf(buffer, "committer %s\n", committer);
 	if (!encoding_is_utf8)
-		strbuf_addf(&buffer, "encoding %s\n", git_commit_encoding);
+		strbuf_addf(buffer, "encoding %s\n", git_commit_encoding);
 
 	while (extra) {
-		add_extra_header(&buffer, extra);
+		add_extra_header(buffer, extra);
 		extra = extra->next;
 	}
-	strbuf_addch(&buffer, '\n');
+	strbuf_addch(buffer, '\n');
 
 	/* And add the comment */
-	strbuf_add(&buffer, msg, msg_len);
+	strbuf_add(buffer, msg, msg_len);
+}
 
-	/* And check the encoding */
-	if (encoding_is_utf8 && !verify_utf8(&buffer))
-		fprintf(stderr, _(commit_utf8_warn));
+int commit_tree_extended(const char *msg, size_t msg_len,
+			 const struct object_id *tree,
+			 struct commit_list *parents, struct object_id *ret,
+			 const char *author, const char *committer,
+			 const char *sign_commit,
+			 struct commit_extra_header *extra)
+{
+	struct repository *r = the_repository;
+	int result = 0;
+	int encoding_is_utf8;
+	struct strbuf buffer = STRBUF_INIT, compat_buffer = STRBUF_INIT;
+	struct strbuf sig = STRBUF_INIT, compat_sig = STRBUF_INIT;
+	struct object_id *parent_buf = NULL, *compat_oid = NULL;
+	struct object_id compat_oid_buf;
+	size_t i, nparents;
+
+	/* Not having i18n.commitencoding is the same as having utf-8 */
+	encoding_is_utf8 = is_encoding_utf8(git_commit_encoding);
+
+	assert_oid_type(tree, OBJ_TREE);
 
-	if (sign_commit && sign_with_header(&buffer, sign_commit)) {
+	if (memchr(msg, '\0', msg_len))
+		return error("a NUL byte in commit log message not allowed.");
+
+	nparents = commit_list_count(parents);
+	CALLOC_ARRAY(parent_buf, nparents);
+	i = 0;
+	while (parents) {
+		struct commit *parent = pop_commit(&parents);
+		oidcpy(&parent_buf[i++], &parent->object.oid);
+	}
+
+	write_commit_tree(&buffer, msg, msg_len, tree, parent_buf, nparents, author, committer, extra);
+	if (sign_commit && sign_commit_to_strbuf(&sig, &buffer, sign_commit)) {
 		result = -1;
 		goto out;
 	}
+	if (r->compat_hash_algo) {
+		struct commit_extra_header *compat_extra = NULL;
+		struct object_id mapped_tree;
+		struct object_id *mapped_parents;
+
+		CALLOC_ARRAY(mapped_parents, nparents);
 
-	result = write_object_file(buffer.buf, buffer.len, OBJ_COMMIT, ret);
+		if (repo_oid_to_algop(r, tree, r->compat_hash_algo, &mapped_tree)) {
+			result = -1;
+			free(mapped_parents);
+			goto out;
+		}
+		for (i = 0; i < nparents; i++)
+			if (repo_oid_to_algop(r, &parent_buf[i], r->compat_hash_algo, &mapped_parents[i])) {
+				result = -1;
+				free(mapped_parents);
+				goto out;
+			}
+		if (convert_commit_extra_headers(extra, &compat_extra)) {
+			result = -1;
+			free(mapped_parents);
+			goto out;
+		}
+		write_commit_tree(&compat_buffer, msg, msg_len, &mapped_tree,
+				  mapped_parents, nparents, author, committer, compat_extra);
+		free_commit_extra_headers(compat_extra);
+		free(mapped_parents);
+
+		if (sign_commit && sign_commit_to_strbuf(&compat_sig, &compat_buffer, sign_commit)) {
+			result = -1;
+			goto out;
+		}
+	}
+
+	if (sign_commit) {
+		struct sig_pairs {
+			struct strbuf *sig;
+			const struct git_hash_algo *algo;
+		} bufs [2] = {
+			{ &compat_sig, r->compat_hash_algo },
+			{ &sig, r->hash_algo },
+		};
+		int i;
+
+		/*
+		 * We write algorithms in the order they were implemented in
+		 * Git to produce a stable hash when multiple algorithms are
+		 * used.
+		 */
+		if (r->compat_hash_algo && hash_algo_by_ptr(bufs[0].algo) > hash_algo_by_ptr(bufs[1].algo))
+			SWAP(bufs[0], bufs[1]);
+
+		/*
+		 * We traverse each algorithm in order, and apply the signature
+		 * to each buffer.
+		 */
+		for (i = 0; i < ARRAY_SIZE(bufs); i++) {
+			if (!bufs[i].algo)
+				continue;
+			add_header_signature(&buffer, bufs[i].sig, bufs[i].algo);
+			if (r->compat_hash_algo)
+				add_header_signature(&compat_buffer, bufs[i].sig, bufs[i].algo);
+		}
+	}
+
+	/* And check the encoding. */
+	if (encoding_is_utf8 && (!verify_utf8(&buffer) || !verify_utf8(&compat_buffer)))
+		fprintf(stderr, _(commit_utf8_warn));
+
+	if (r->compat_hash_algo) {
+		hash_object_file(r->compat_hash_algo, compat_buffer.buf, compat_buffer.len,
+			OBJ_COMMIT, &compat_oid_buf);
+		compat_oid = &compat_oid_buf;
+	}
+
+	result = write_object_file_flags(buffer.buf, buffer.len, OBJ_COMMIT,
+					 ret, compat_oid, 0);
 out:
+	free(parent_buf);
 	strbuf_release(&buffer);
+	strbuf_release(&compat_buffer);
+	strbuf_release(&sig);
+	strbuf_release(&compat_sig);
 	return result;
 }
 
diff --git a/commit.h b/commit.h
index 1cc872f225..62fe0d77a7 100644
--- a/commit.h
+++ b/commit.h
@@ -370,5 +370,6 @@ int parse_buffer_signed_by_header(const char *buffer,
 				  struct strbuf *payload,
 				  struct strbuf *signature,
 				  const struct git_hash_algo *algop);
+int add_header_signature(struct strbuf *buf, struct strbuf *sig, const struct git_hash_algo *algo);
 
 #endif /* COMMIT_H */
diff --git a/delta-islands.c b/delta-islands.c
index ee2318d45a..f7e079425f 100644
--- a/delta-islands.c
+++ b/delta-islands.c
@@ -284,7 +284,7 @@ void resolve_tree_islands(struct repository *r,
 		if (!tree || parse_tree(tree) < 0)
 			die(_("bad tree object %s"), oid_to_hex(&ent->idx.oid));
 
-		init_tree_desc(&desc, tree->buffer, tree->size);
+		init_tree_desc(&desc, &tree->object.oid, tree->buffer, tree->size);
 		while (tree_entry(&desc, &entry)) {
 			struct object *obj;
 
diff --git a/diff-lib.c b/diff-lib.c
index 5e8717c774..1cd790a4d2 100644
--- a/diff-lib.c
+++ b/diff-lib.c
@@ -562,7 +562,7 @@ static int diff_cache(struct rev_info *revs,
 	opts.pathspec = &revs->diffopt.pathspec;
 	opts.pathspec->recursive = 1;
 
-	init_tree_desc(&t, tree->buffer, tree->size);
+	init_tree_desc(&t, &tree->object.oid, tree->buffer, tree->size);
 	return unpack_trees(1, &t, &opts);
 }
 
diff --git a/fsck.c b/fsck.c
index 8ded0a473a..78af29d264 100644
--- a/fsck.c
+++ b/fsck.c
@@ -327,7 +327,8 @@ static int fsck_walk_tree(struct tree *tree, void *data, struct fsck_options *op
 		return -1;
 
 	name = fsck_get_object_name(options, &tree->object.oid);
-	if (init_tree_desc_gently(&desc, tree->buffer, tree->size, 0))
+	if (init_tree_desc_gently(&desc, &tree->object.oid,
+				  tree->buffer, tree->size, 0))
 		return -1;
 	while (tree_entry_gently(&desc, &entry)) {
 		struct object *obj;
@@ -598,7 +599,8 @@ static int fsck_tree(const struct object_id *tree_oid,
 	const char *o_name;
 	struct name_stack df_dup_candidates = { NULL };
 
-	if (init_tree_desc_gently(&desc, buffer, size, TREE_DESC_RAW_MODES)) {
+	if (init_tree_desc_gently(&desc, tree_oid, buffer, size,
+				  TREE_DESC_RAW_MODES)) {
 		retval += report(options, tree_oid, OBJ_TREE,
 				 FSCK_MSG_BAD_TREE,
 				 "cannot be parsed as a tree");
diff --git a/hash-ll.h b/hash-ll.h
index 10d84cc208..2cfde63ae1 100644
--- a/hash-ll.h
+++ b/hash-ll.h
@@ -145,6 +145,7 @@ struct object_id {
 #define GET_OID_RECORD_PATH     0200
 #define GET_OID_ONLY_TO_DIE    04000
 #define GET_OID_REQUIRE_PATH  010000
+#define GET_OID_HASH_ANY      020000
 
 #define GET_OID_DISAMBIGUATORS \
 	(GET_OID_COMMIT | GET_OID_COMMITTISH | \
diff --git a/hash.h b/hash.h
index 615ae0691d..e064807c17 100644
--- a/hash.h
+++ b/hash.h
@@ -73,10 +73,15 @@ static inline void oidclr(struct object_id *oid)
 	oid->algo = hash_algo_by_ptr(the_hash_algo);
 }
 
+static inline void oidread_algop(struct object_id *oid, const unsigned char *hash, const struct git_hash_algo *algop)
+{
+	memcpy(oid->hash, hash, algop->rawsz);
+	oid->algo = hash_algo_by_ptr(algop);
+}
+
 static inline void oidread(struct object_id *oid, const unsigned char *hash)
 {
-	memcpy(oid->hash, hash, the_hash_algo->rawsz);
-	oid->algo = hash_algo_by_ptr(the_hash_algo);
+	oidread_algop(oid, hash, the_hash_algo);
 }
 
 static inline int is_empty_blob_sha1(const unsigned char *sha1)
diff --git a/http-push.c b/http-push.c
index 33db41bfac..1fe51226fd 100644
--- a/http-push.c
+++ b/http-push.c
@@ -1307,7 +1307,7 @@ static struct object_list **process_tree(struct tree *tree,
 	obj->flags |= SEEN;
 	p = add_one_object(obj, p);
 
-	init_tree_desc(&desc, tree->buffer, tree->size);
+	init_tree_desc(&desc, &tree->object.oid, tree->buffer, tree->size);
 
 	while (tree_entry(&desc, &entry))
 		switch (object_type(entry.mode)) {
diff --git a/list-objects.c b/list-objects.c
index f39b68faf5..11ad8be411 100644
--- a/list-objects.c
+++ b/list-objects.c
@@ -102,7 +102,7 @@ static void process_tree_contents(struct traversal_context *ctx,
 	enum interesting match = ctx->revs->diffopt.pathspec.nr == 0 ?
 		all_entries_interesting : entry_not_interesting;
 
-	init_tree_desc(&desc, tree->buffer, tree->size);
+	init_tree_desc(&desc, &tree->object.oid, tree->buffer, tree->size);
 
 	while (tree_entry(&desc, &entry)) {
 		if (match != all_entries_interesting) {
diff --git a/loose.c b/loose.c
new file mode 100644
index 0000000000..f6faa6216a
--- /dev/null
+++ b/loose.c
@@ -0,0 +1,259 @@
+#include "git-compat-util.h"
+#include "hash.h"
+#include "path.h"
+#include "object-store.h"
+#include "hex.h"
+#include "wrapper.h"
+#include "gettext.h"
+#include "loose.h"
+#include "lockfile.h"
+#include "oidtree.h"
+
+static const char *loose_object_header = "# loose-object-idx\n";
+
+static inline int should_use_loose_object_map(struct repository *repo)
+{
+	return repo->compat_hash_algo && repo->gitdir;
+}
+
+void loose_object_map_init(struct loose_object_map **map)
+{
+	struct loose_object_map *m;
+	m = xmalloc(sizeof(**map));
+	m->to_compat = kh_init_oid_map();
+	m->to_storage = kh_init_oid_map();
+	*map = m;
+}
+
+static int insert_oid_pair(kh_oid_map_t *map, const struct object_id *key, const struct object_id *value)
+{
+	khiter_t pos;
+	int ret;
+	struct object_id *stored;
+
+	pos = kh_put_oid_map(map, *key, &ret);
+
+	/* This item already exists in the map. */
+	if (ret == 0)
+		return 0;
+
+	stored = xmalloc(sizeof(*stored));
+	oidcpy(stored, value);
+	kh_value(map, pos) = stored;
+	return 1;
+}
+
+static int insert_loose_map(struct object_directory *odb,
+			    const struct object_id *oid,
+			    const struct object_id *compat_oid)
+{
+	struct loose_object_map *map = odb->loose_map;
+	int inserted = 0;
+
+	inserted |= insert_oid_pair(map->to_compat, oid, compat_oid);
+	inserted |= insert_oid_pair(map->to_storage, compat_oid, oid);
+	if (inserted)
+		oidtree_insert(odb->loose_objects_cache, compat_oid);
+
+	return inserted;
+}
+
+static int load_one_loose_object_map(struct repository *repo, struct object_directory *dir)
+{
+	struct strbuf buf = STRBUF_INIT, path = STRBUF_INIT;
+	FILE *fp;
+
+	if (!dir->loose_map)
+		loose_object_map_init(&dir->loose_map);
+	if (!dir->loose_objects_cache) {
+		ALLOC_ARRAY(dir->loose_objects_cache, 1);
+		oidtree_init(dir->loose_objects_cache);
+	}
+
+	insert_loose_map(dir, repo->hash_algo->empty_tree, repo->compat_hash_algo->empty_tree);
+	insert_loose_map(dir, repo->hash_algo->empty_blob, repo->compat_hash_algo->empty_blob);
+	insert_loose_map(dir, repo->hash_algo->null_oid, repo->compat_hash_algo->null_oid);
+
+	strbuf_git_common_path(&path, repo, "objects/loose-object-idx");
+	fp = fopen(path.buf, "rb");
+	if (!fp) {
+		strbuf_release(&path);
+		return 0;
+	}
+
+	errno = 0;
+	if (strbuf_getwholeline(&buf, fp, '\n') || strcmp(buf.buf, loose_object_header))
+		goto err;
+	while (!strbuf_getline_lf(&buf, fp)) {
+		const char *p;
+		struct object_id oid, compat_oid;
+		if (parse_oid_hex_algop(buf.buf, &oid, &p, repo->hash_algo) ||
+		    *p++ != ' ' ||
+		    parse_oid_hex_algop(p, &compat_oid, &p, repo->compat_hash_algo) ||
+		    p != buf.buf + buf.len)
+			goto err;
+		insert_loose_map(dir, &oid, &compat_oid);
+	}
+
+	strbuf_release(&buf);
+	strbuf_release(&path);
+	return errno ? -1 : 0;
+err:
+	strbuf_release(&buf);
+	strbuf_release(&path);
+	return -1;
+}
+
+int repo_read_loose_object_map(struct repository *repo)
+{
+	struct object_directory *dir;
+
+	if (!should_use_loose_object_map(repo))
+		return 0;
+
+	prepare_alt_odb(repo);
+
+	for (dir = repo->objects->odb; dir; dir = dir->next) {
+		if (load_one_loose_object_map(repo, dir) < 0) {
+			return -1;
+		}
+	}
+	return 0;
+}
+
+int repo_write_loose_object_map(struct repository *repo)
+{
+	kh_oid_map_t *map = repo->objects->odb->loose_map->to_compat;
+	struct lock_file lock;
+	int fd;
+	khiter_t iter;
+	struct strbuf buf = STRBUF_INIT, path = STRBUF_INIT;
+
+	if (!should_use_loose_object_map(repo))
+		return 0;
+
+	strbuf_git_common_path(&path, repo, "objects/loose-object-idx");
+	fd = hold_lock_file_for_update_timeout(&lock, path.buf, LOCK_DIE_ON_ERROR, -1);
+	iter = kh_begin(map);
+	if (write_in_full(fd, loose_object_header, strlen(loose_object_header)) < 0)
+		goto errout;
+
+	for (; iter != kh_end(map); iter++) {
+		if (kh_exist(map, iter)) {
+			if (oideq(&kh_key(map, iter), the_hash_algo->empty_tree) ||
+			    oideq(&kh_key(map, iter), the_hash_algo->empty_blob))
+				continue;
+			strbuf_addf(&buf, "%s %s\n", oid_to_hex(&kh_key(map, iter)), oid_to_hex(kh_value(map, iter)));
+			if (write_in_full(fd, buf.buf, buf.len) < 0)
+				goto errout;
+			strbuf_reset(&buf);
+		}
+	}
+	strbuf_release(&buf);
+	if (commit_lock_file(&lock) < 0) {
+		error_errno(_("could not write loose object index %s"), path.buf);
+		strbuf_release(&path);
+		return -1;
+	}
+	strbuf_release(&path);
+	return 0;
+errout:
+	rollback_lock_file(&lock);
+	strbuf_release(&buf);
+	error_errno(_("failed to write loose object index %s\n"), path.buf);
+	strbuf_release(&path);
+	return -1;
+}
+
+static int write_one_object(struct repository *repo, const struct object_id *oid,
+			    const struct object_id *compat_oid)
+{
+	struct lock_file lock;
+	int fd;
+	struct stat st;
+	struct strbuf buf = STRBUF_INIT, path = STRBUF_INIT;
+
+	strbuf_git_common_path(&path, repo, "objects/loose-object-idx");
+	hold_lock_file_for_update_timeout(&lock, path.buf, LOCK_DIE_ON_ERROR, -1);
+
+	fd = open(path.buf, O_WRONLY | O_CREAT | O_APPEND, 0666);
+	if (fd < 0)
+		goto errout;
+	if (fstat(fd, &st) < 0)
+		goto errout;
+	if (!st.st_size && write_in_full(fd, loose_object_header, strlen(loose_object_header)) < 0)
+		goto errout;
+
+	strbuf_addf(&buf, "%s %s\n", oid_to_hex(oid), oid_to_hex(compat_oid));
+	if (write_in_full(fd, buf.buf, buf.len) < 0)
+		goto errout;
+	if (close(fd))
+		goto errout;
+	adjust_shared_perm(path.buf);
+	rollback_lock_file(&lock);
+	strbuf_release(&buf);
+	strbuf_release(&path);
+	return 0;
+errout:
+	error_errno(_("failed to write loose object index %s\n"), path.buf);
+	close(fd);
+	rollback_lock_file(&lock);
+	strbuf_release(&buf);
+	strbuf_release(&path);
+	return -1;
+}
+
+int repo_add_loose_object_map(struct repository *repo, const struct object_id *oid,
+			      const struct object_id *compat_oid)
+{
+	int inserted = 0;
+
+	if (!should_use_loose_object_map(repo))
+		return 0;
+
+	inserted = insert_loose_map(repo->objects->odb, oid, compat_oid);
+	if (inserted)
+		return write_one_object(repo, oid, compat_oid);
+	return 0;
+}
+
+int repo_loose_object_map_oid(struct repository *repo,
+			      const struct object_id *src,
+			      const struct git_hash_algo *to,
+			      struct object_id *dest)
+{
+	struct object_directory *dir;
+	kh_oid_map_t *map;
+	khiter_t pos;
+
+	for (dir = repo->objects->odb; dir; dir = dir->next) {
+		struct loose_object_map *loose_map = dir->loose_map;
+		if (!loose_map)
+			continue;
+		map = (to == repo->compat_hash_algo) ?
+			loose_map->to_compat :
+			loose_map->to_storage;
+		pos = kh_get_oid_map(map, *src);
+		if (pos < kh_end(map)) {
+			oidcpy(dest, kh_value(map, pos));
+			return 0;
+		}
+	}
+	return -1;
+}
+
+void loose_object_map_clear(struct loose_object_map **map)
+{
+	struct loose_object_map *m = *map;
+	struct object_id *oid;
+
+	if (!m)
+		return;
+
+	kh_foreach_value(m->to_compat, oid, free(oid));
+	kh_foreach_value(m->to_storage, oid, free(oid));
+	kh_destroy_oid_map(m->to_compat);
+	kh_destroy_oid_map(m->to_storage);
+	free(m);
+	*map = NULL;
+}
diff --git a/loose.h b/loose.h
new file mode 100644
index 0000000000..2c2957072c
--- /dev/null
+++ b/loose.h
@@ -0,0 +1,22 @@
+#ifndef LOOSE_H
+#define LOOSE_H
+
+#include "khash.h"
+
+struct loose_object_map {
+	kh_oid_map_t *to_compat;
+	kh_oid_map_t *to_storage;
+};
+
+void loose_object_map_init(struct loose_object_map **map);
+void loose_object_map_clear(struct loose_object_map **map);
+int repo_loose_object_map_oid(struct repository *repo,
+			      const struct object_id *src,
+			      const struct git_hash_algo *dest_algo,
+			      struct object_id *dest);
+int repo_add_loose_object_map(struct repository *repo, const struct object_id *oid,
+			      const struct object_id *compat_oid);
+int repo_read_loose_object_map(struct repository *repo);
+int repo_write_loose_object_map(struct repository *repo);
+
+#endif
diff --git a/match-trees.c b/match-trees.c
index 0885ac681c..3412b6a140 100644
--- a/match-trees.c
+++ b/match-trees.c
@@ -63,7 +63,7 @@ static void *fill_tree_desc_strict(struct tree_desc *desc,
 		die("unable to read tree (%s)", oid_to_hex(hash));
 	if (type != OBJ_TREE)
 		die("%s is not a tree", oid_to_hex(hash));
-	init_tree_desc(desc, buffer, size);
+	init_tree_desc(desc, hash, buffer, size);
 	return buffer;
 }
 
@@ -194,7 +194,7 @@ static int splice_tree(const struct object_id *oid1, const char *prefix,
 	buf = repo_read_object_file(the_repository, oid1, &type, &sz);
 	if (!buf)
 		die("cannot read tree %s", oid_to_hex(oid1));
-	init_tree_desc(&desc, buf, sz);
+	init_tree_desc(&desc, oid1, buf, sz);
 
 	rewrite_here = NULL;
 	while (desc.size) {
diff --git a/merge-ort.c b/merge-ort.c
index 201f8f7775..ac225cc33c 100644
--- a/merge-ort.c
+++ b/merge-ort.c
@@ -1665,9 +1665,10 @@ static int collect_merge_info(struct merge_options *opt,
 	    parse_tree(side1) < 0 ||
 	    parse_tree(side2) < 0)
 		return -1;
-	init_tree_desc(t + 0, merge_base->buffer, merge_base->size);
-	init_tree_desc(t + 1, side1->buffer, side1->size);
-	init_tree_desc(t + 2, side2->buffer, side2->size);
+	init_tree_desc(t + 0, &merge_base->object.oid,
+		       merge_base->buffer, merge_base->size);
+	init_tree_desc(t + 1, &side1->object.oid, side1->buffer, side1->size);
+	init_tree_desc(t + 2, &side2->object.oid, side2->buffer, side2->size);
 
 	trace2_region_enter("merge", "traverse_trees", opt->repo);
 	ret = traverse_trees(NULL, 3, t, &info);
@@ -4446,10 +4447,10 @@ static int checkout(struct merge_options *opt,
 	unpack_opts.preserve_ignored = 0; /* FIXME: !opts->overwrite_ignore */
 	if (parse_tree(prev) < 0)
 		return -1;
-	init_tree_desc(&trees[0], prev->buffer, prev->size);
+	init_tree_desc(&trees[0], &prev->object.oid, prev->buffer, prev->size);
 	if (parse_tree(next) < 0)
 		return -1;
-	init_tree_desc(&trees[1], next->buffer, next->size);
+	init_tree_desc(&trees[1], &next->object.oid, next->buffer, next->size);
 
 	ret = unpack_trees(2, trees, &unpack_opts);
 	clear_unpack_trees_porcelain(&unpack_opts);
diff --git a/merge-recursive.c b/merge-recursive.c
index 103ee321ae..69d67bef5a 100644
--- a/merge-recursive.c
+++ b/merge-recursive.c
@@ -407,7 +407,7 @@ static void init_tree_desc_from_tree(struct tree_desc *desc, struct tree *tree)
 {
 	if (parse_tree(tree) < 0)
 		exit(128);
-	init_tree_desc(desc, tree->buffer, tree->size);
+	init_tree_desc(desc, &tree->object.oid, tree->buffer, tree->size);
 }
 
 static int unpack_trees_start(struct merge_options *opt,
diff --git a/merge.c b/merge.c
index 563281b10f..752a937fa9 100644
--- a/merge.c
+++ b/merge.c
@@ -81,7 +81,8 @@ int checkout_fast_forward(struct repository *r,
 			rollback_lock_file(&lock_file);
 			return -1;
 		}
-		init_tree_desc(t+i, trees[i]->buffer, trees[i]->size);
+		init_tree_desc(t+i, &trees[i]->object.oid,
+			       trees[i]->buffer, trees[i]->size);
 	}
 
 	memset(&opts, 0, sizeof(opts));
diff --git a/object-file-convert.c b/object-file-convert.c
new file mode 100644
index 0000000000..4f6189095b
--- /dev/null
+++ b/object-file-convert.c
@@ -0,0 +1,277 @@
+#include "git-compat-util.h"
+#include "gettext.h"
+#include "strbuf.h"
+#include "hex.h"
+#include "repository.h"
+#include "hash-ll.h"
+#include "hash.h"
+#include "object.h"
+#include "loose.h"
+#include "commit.h"
+#include "gpg-interface.h"
+#include "object-file-convert.h"
+
+int repo_oid_to_algop(struct repository *repo, const struct object_id *src,
+		      const struct git_hash_algo *to, struct object_id *dest)
+{
+	/*
+	 * If the source algorithm is not set, then we're using the
+	 * default hash algorithm for that object.
+	 */
+	const struct git_hash_algo *from =
+		src->algo ? &hash_algos[src->algo] : repo->hash_algo;
+
+	if (from == to) {
+		if (src != dest)
+			oidcpy(dest, src);
+		return 0;
+	}
+	if (repo_loose_object_map_oid(repo, src, to, dest)) {
+		/*
+		 * We may have loaded the object map at repo initialization but
+		 * another process (perhaps upstream of a pipe from us) may have
+		 * written a new object into the map.  If the object is missing,
+		 * let's reload the map to see if the object has appeared.
+		 */
+		repo_read_loose_object_map(repo);
+		if (repo_loose_object_map_oid(repo, src, to, dest))
+			return -1;
+	}
+	return 0;
+}
+
+static int decode_tree_entry_raw(struct object_id *oid, const char **path,
+				 size_t *len, const struct git_hash_algo *algo,
+				 const char *buf, unsigned long size)
+{
+	uint16_t mode;
+	const unsigned hashsz = algo->rawsz;
+
+	if (size < hashsz + 3 || buf[size - (hashsz + 1)]) {
+		return -1;
+	}
+
+	*path = parse_mode(buf, &mode);
+	if (!*path || !**path)
+		return -1;
+	*len = strlen(*path) + 1;
+
+	oidread_algop(oid, (const unsigned char *)*path + *len, algo);
+	return 0;
+}
+
+static int convert_tree_object(struct strbuf *out,
+			       const struct git_hash_algo *from,
+			       const struct git_hash_algo *to,
+			       const char *buffer, size_t size)
+{
+	const char *p = buffer, *end = buffer + size;
+
+	while (p < end) {
+		struct object_id entry_oid, mapped_oid;
+		const char *path = NULL;
+		size_t pathlen;
+
+		if (decode_tree_entry_raw(&entry_oid, &path, &pathlen, from, p,
+					  end - p))
+			return error(_("failed to decode tree entry"));
+		if (repo_oid_to_algop(the_repository, &entry_oid, to, &mapped_oid))
+			return error(_("failed to map tree entry for %s"), oid_to_hex(&entry_oid));
+		strbuf_add(out, p, path - p);
+		strbuf_add(out, path, pathlen);
+		strbuf_add(out, mapped_oid.hash, to->rawsz);
+		p = path + pathlen + from->rawsz;
+	}
+	return 0;
+}
+
+static int convert_tag_object(struct strbuf *out,
+			      const struct git_hash_algo *from,
+			      const struct git_hash_algo *to,
+			      const char *buffer, size_t size)
+{
+	struct strbuf payload = STRBUF_INIT, oursig = STRBUF_INIT, othersig = STRBUF_INIT;
+	const int entry_len = from->hexsz + 7;
+	size_t payload_size;
+	struct object_id oid, mapped_oid;
+	const char *p;
+
+	/* Consume the object line */
+	if ((entry_len >= size) ||
+	    memcmp(buffer, "object ", 7) || buffer[entry_len] != '\n')
+		return error("bogus tag object");
+	if (parse_oid_hex_algop(buffer + 7, &oid, &p, from) < 0)
+		return error("bad tag object ID");
+	if (repo_oid_to_algop(the_repository, &oid, to, &mapped_oid))
+		return error("unable to map tree %s in tag object",
+			     oid_to_hex(&oid));
+	size -= ((p + 1) - buffer);
+	buffer = p + 1;
+
+	/* Is there a signature for our algorithm? */
+	payload_size = parse_signed_buffer(buffer, size);
+	if (payload_size != size) {
+		/* Yes, there is. */
+		strbuf_add(&oursig, buffer + payload_size, size - payload_size);
+	}
+
+	/* Now, is there a signature for the other algorithm? */
+	parse_buffer_signed_by_header(buffer, payload_size, &payload, &othersig, to);
+	/*
+	 * Our payload is now in payload and we may have up to two signatrures
+	 * in oursig and othersig.
+	 */
+
+	/* Add some slop for longer signature header in the new algorithm. */
+	strbuf_grow(out, (7 + to->hexsz + 1) + size + 7);
+	strbuf_addf(out, "object %s\n", oid_to_hex(&mapped_oid));
+	strbuf_addbuf(out, &payload);
+	if (oursig.len)
+		add_header_signature(out, &oursig, from);
+	strbuf_addbuf(out, &othersig);
+
+	strbuf_release(&payload);
+	strbuf_release(&othersig);
+	strbuf_release(&oursig);
+	return 0;
+}
+
+static int convert_commit_object(struct strbuf *out,
+				 const struct git_hash_algo *from,
+				 const struct git_hash_algo *to,
+				 const char *buffer, size_t size)
+{
+	const char *tail = buffer;
+	const char *bufptr = buffer;
+	const int tree_entry_len = from->hexsz + 5;
+	const int parent_entry_len = from->hexsz + 7;
+	struct object_id oid, mapped_oid;
+	const char *p, *eol;
+
+	tail += size;
+
+	while ((bufptr < tail) && (*bufptr != '\n')) {
+		eol = memchr(bufptr, '\n', tail - bufptr);
+		if (!eol)
+			return error(_("bad %s in commit"), "line");
+
+		if (((bufptr + 5) < eol) && !memcmp(bufptr, "tree ", 5))
+		{
+			if (((bufptr + tree_entry_len) != eol) ||
+			    parse_oid_hex_algop(bufptr + 5, &oid, &p, from) ||
+			    (p != eol))
+				return error(_("bad %s in commit"), "tree");
+
+			if (repo_oid_to_algop(the_repository, &oid, to, &mapped_oid))
+				return error(_("unable to map %s %s in commit object"),
+					     "tree", oid_to_hex(&oid));
+			strbuf_addf(out, "tree %s\n", oid_to_hex(&mapped_oid));
+		}
+		else if (((bufptr + 7) < eol) && !memcmp(bufptr, "parent ", 7))
+		{
+			if (((bufptr + parent_entry_len) != eol) ||
+			    parse_oid_hex_algop(bufptr + 7, &oid, &p, from) ||
+			    (p != eol))
+				return error(_("bad %s in commit"), "parent");
+
+			if (repo_oid_to_algop(the_repository, &oid, to, &mapped_oid))
+				return error(_("unable to map %s %s in commit object"),
+					     "parent", oid_to_hex(&oid));
+
+			strbuf_addf(out, "parent %s\n", oid_to_hex(&mapped_oid));
+		}
+		else if (((bufptr + 9) < eol) && !memcmp(bufptr, "mergetag ", 9))
+		{
+			struct strbuf tag = STRBUF_INIT, new_tag = STRBUF_INIT;
+
+			/* Recover the tag object from the mergetag */
+			strbuf_add(&tag, bufptr + 9, (eol - (bufptr + 9)) + 1);
+
+			bufptr = eol + 1;
+			while ((bufptr < tail) && (*bufptr == ' ')) {
+				eol = memchr(bufptr, '\n', tail - bufptr);
+				if (!eol) {
+					strbuf_release(&tag);
+					return error(_("bad %s in commit"), "mergetag continuation");
+				}
+				strbuf_add(&tag, bufptr + 1, (eol - (bufptr + 1)) + 1);
+				bufptr = eol + 1;
+			}
+
+			/* Compute the new tag object */
+			if (convert_tag_object(&new_tag, from, to, tag.buf, tag.len)) {
+				strbuf_release(&tag);
+				strbuf_release(&new_tag);
+				return -1;
+			}
+
+			/* Write the new mergetag */
+			strbuf_addstr(out, "mergetag");
+			strbuf_add_lines(out, " ", new_tag.buf, new_tag.len);
+			strbuf_release(&tag);
+			strbuf_release(&new_tag);
+		}
+		else if (((bufptr + 7) < tail) && !memcmp(bufptr, "author ", 7))
+			strbuf_add(out, bufptr, (eol - bufptr) + 1);
+		else if (((bufptr + 10) < tail) && !memcmp(bufptr, "committer ", 10))
+			strbuf_add(out, bufptr, (eol - bufptr) + 1);
+		else if (((bufptr + 9) < tail) && !memcmp(bufptr, "encoding ", 9))
+			strbuf_add(out, bufptr, (eol - bufptr) + 1);
+		else if (((bufptr + 6) < tail) && !memcmp(bufptr, "gpgsig", 6))
+			strbuf_add(out, bufptr, (eol - bufptr) + 1);
+		else {
+			/* Unknown line fail it might embed an oid */
+			return -1;
+		}
+		/* Consume any trailing continuation lines */
+		bufptr = eol + 1;
+		while ((bufptr < tail) && (*bufptr == ' ')) {
+			eol = memchr(bufptr, '\n', tail - bufptr);
+			if (!eol)
+				return error(_("bad %s in commit"), "continuation");
+			strbuf_add(out, bufptr, (eol - bufptr) + 1);
+			bufptr = eol + 1;
+		}
+	}
+	if (bufptr < tail)
+		strbuf_add(out, bufptr, tail - bufptr);
+	return 0;
+}
+
+int convert_object_file(struct strbuf *outbuf,
+			const struct git_hash_algo *from,
+			const struct git_hash_algo *to,
+			const void *buf, size_t len,
+			enum object_type type,
+			int gentle)
+{
+	int ret;
+
+	/* Don't call this function when no conversion is necessary */
+	if ((from == to) || (type == OBJ_BLOB))
+		BUG("Refusing noop object file conversion");
+
+	switch (type) {
+	case OBJ_COMMIT:
+		ret = convert_commit_object(outbuf, from, to, buf, len);
+		break;
+	case OBJ_TREE:
+		ret = convert_tree_object(outbuf, from, to, buf, len);
+		break;
+	case OBJ_TAG:
+		ret = convert_tag_object(outbuf, from, to, buf, len);
+		break;
+	default:
+		/* Not implemented yet, so fail. */
+		ret = -1;
+		break;
+	}
+	if (!ret)
+		return 0;
+	if (gentle) {
+		strbuf_release(outbuf);
+		return ret;
+	}
+	die(_("Failed to convert object from %s to %s"),
+		from->name, to->name);
+}
diff --git a/object-file-convert.h b/object-file-convert.h
new file mode 100644
index 0000000000..a4f802aa8e
--- /dev/null
+++ b/object-file-convert.h
@@ -0,0 +1,24 @@
+#ifndef OBJECT_CONVERT_H
+#define OBJECT_CONVERT_H
+
+struct repository;
+struct object_id;
+struct git_hash_algo;
+struct strbuf;
+#include "object.h"
+
+int repo_oid_to_algop(struct repository *repo, const struct object_id *src,
+		      const struct git_hash_algo *to, struct object_id *dest);
+
+/*
+ * Convert an object file from one hash algorithm to another algorithm.
+ * Return -1 on failure, 0 on success.
+ */
+int convert_object_file(struct strbuf *outbuf,
+			const struct git_hash_algo *from,
+			const struct git_hash_algo *to,
+			const void *buf, size_t len,
+			enum object_type type,
+			int gentle);
+
+#endif /* OBJECT_CONVERT_H */
diff --git a/object-file.c b/object-file.c
index 619f039ebc..610b1f465c 100644
--- a/object-file.c
+++ b/object-file.c
@@ -35,6 +35,8 @@
 #include "setup.h"
 #include "submodule.h"
 #include "fsck.h"
+#include "loose.h"
+#include "object-file-convert.h"
 
 /* The maximum size for an object header. */
 #define MAX_HEADER_LEN 32
@@ -1084,9 +1086,11 @@ int check_object_signature(struct repository *r, const struct object_id *oid,
 			   void *buf, unsigned long size,
 			   enum object_type type)
 {
+	const struct git_hash_algo *algo =
+		oid->algo ? &hash_algos[oid->algo] : r->hash_algo;
 	struct object_id real_oid;
 
-	hash_object_file(r->hash_algo, buf, size, type, &real_oid);
+	hash_object_file(algo, buf, size, type, &real_oid);
 
 	return !oideq(oid, &real_oid) ? -1 : 0;
 }
@@ -1652,10 +1656,101 @@ static int do_oid_object_info_extended(struct repository *r,
 	return 0;
 }
 
+static int oid_object_info_convert(struct repository *r,
+				   const struct object_id *input_oid,
+				   struct object_info *input_oi, unsigned flags)
+{
+	const struct git_hash_algo *input_algo = &hash_algos[input_oid->algo];
+	int do_die = flags & OBJECT_INFO_DIE_IF_CORRUPT;
+	struct strbuf type_name = STRBUF_INIT;
+	struct object_id oid, delta_base_oid;
+	struct object_info new_oi, *oi;
+	unsigned long size;
+	void *content;
+	int ret;
+
+	if (repo_oid_to_algop(r, input_oid, the_hash_algo, &oid)) {
+		if (do_die)
+			die(_("missing mapping of %s to %s"),
+			    oid_to_hex(input_oid), the_hash_algo->name);
+		return -1;
+	}
+
+	/* Is new_oi needed? */
+	oi = input_oi;
+	if (input_oi && (input_oi->delta_base_oid || input_oi->sizep ||
+			 input_oi->contentp)) {
+		new_oi = *input_oi;
+		/* Does delta_base_oid need to be converted? */
+		if (input_oi->delta_base_oid)
+			new_oi.delta_base_oid = &delta_base_oid;
+		/* Will the attributes differ when converted? */
+		if (input_oi->sizep || input_oi->contentp) {
+			new_oi.contentp = &content;
+			new_oi.sizep = &size;
+			new_oi.type_name = &type_name;
+		}
+		oi = &new_oi;
+	}
+
+	ret = oid_object_info_extended(r, &oid, oi, flags);
+	if (ret)
+		return -1;
+	if (oi == input_oi)
+		return ret;
+
+	if (new_oi.contentp) {
+		struct strbuf outbuf = STRBUF_INIT;
+		enum object_type type;
+
+		type = type_from_string_gently(type_name.buf, type_name.len,
+					       !do_die);
+		if (type == -1)
+			return -1;
+		if (type != OBJ_BLOB) {
+			ret = convert_object_file(&outbuf,
+						  the_hash_algo, input_algo,
+						  content, size, type, !do_die);
+			if (ret == -1)
+				return -1;
+			free(content);
+			size = outbuf.len;
+			content = strbuf_detach(&outbuf, NULL);
+		}
+		if (input_oi->sizep)
+			*input_oi->sizep = size;
+		if (input_oi->contentp)
+			*input_oi->contentp = content;
+		else
+			free(content);
+		if (input_oi->type_name)
+			*input_oi->type_name = type_name;
+		else
+			strbuf_release(&type_name);
+	}
+	if (new_oi.delta_base_oid == &delta_base_oid) {
+		if (repo_oid_to_algop(r, &delta_base_oid, input_algo,
+				 input_oi->delta_base_oid)) {
+			if (do_die)
+				die(_("missing mapping of %s to %s"),
+				    oid_to_hex(&delta_base_oid),
+				    input_algo->name);
+			return -1;
+		}
+	}
+	input_oi->whence = new_oi.whence;
+	input_oi->u = new_oi.u;
+	return ret;
+}
+
 int oid_object_info_extended(struct repository *r, const struct object_id *oid,
 			     struct object_info *oi, unsigned flags)
 {
 	int ret;
+
+	if (oid->algo && (hash_algo_by_ptr(r->hash_algo) != oid->algo))
+		return oid_object_info_convert(r, oid, oi, flags);
+
 	obj_read_lock();
 	ret = do_oid_object_info_extended(r, oid, oi, flags);
 	obj_read_unlock();
@@ -1944,9 +2039,12 @@ static int start_loose_object_common(struct strbuf *tmp_file,
 				     const char *filename, unsigned flags,
 				     git_zstream *stream,
 				     unsigned char *buf, size_t buflen,
-				     git_hash_ctx *c,
+				     git_hash_ctx *c, git_hash_ctx *compat_c,
 				     char *hdr, int hdrlen)
 {
+	struct repository *repo = the_repository;
+	const struct git_hash_algo *algo = repo->hash_algo;
+	const struct git_hash_algo *compat = repo->compat_hash_algo;
 	int fd;
 
 	fd = create_tmpfile(tmp_file, filename);
@@ -1966,14 +2064,18 @@ static int start_loose_object_common(struct strbuf *tmp_file,
 	git_deflate_init(stream, zlib_compression_level);
 	stream->next_out = buf;
 	stream->avail_out = buflen;
-	the_hash_algo->init_fn(c);
+	algo->init_fn(c);
+	if (compat && compat_c)
+		compat->init_fn(compat_c);
 
 	/*  Start to feed header to zlib stream */
 	stream->next_in = (unsigned char *)hdr;
 	stream->avail_in = hdrlen;
 	while (git_deflate(stream, 0) == Z_OK)
 		; /* nothing */
-	the_hash_algo->update_fn(c, hdr, hdrlen);
+	algo->update_fn(c, hdr, hdrlen);
+	if (compat && compat_c)
+		compat->update_fn(compat_c, hdr, hdrlen);
 
 	return fd;
 }
@@ -1982,16 +2084,21 @@ static int start_loose_object_common(struct strbuf *tmp_file,
  * Common steps for the inner git_deflate() loop for writing loose
  * objects. Returns what git_deflate() returns.
  */
-static int write_loose_object_common(git_hash_ctx *c,
+static int write_loose_object_common(git_hash_ctx *c, git_hash_ctx *compat_c,
 				     git_zstream *stream, const int flush,
 				     unsigned char *in0, const int fd,
 				     unsigned char *compressed,
 				     const size_t compressed_len)
 {
+	struct repository *repo = the_repository;
+	const struct git_hash_algo *algo = repo->hash_algo;
+	const struct git_hash_algo *compat = repo->compat_hash_algo;
 	int ret;
 
 	ret = git_deflate(stream, flush ? Z_FINISH : 0);
-	the_hash_algo->update_fn(c, in0, stream->next_in - in0);
+	algo->update_fn(c, in0, stream->next_in - in0);
+	if (compat && compat_c)
+		compat->update_fn(compat_c, in0, stream->next_in - in0);
 	if (write_in_full(fd, compressed, stream->next_out - compressed) < 0)
 		die_errno(_("unable to write loose object file"));
 	stream->next_out = compressed;
@@ -2006,15 +2113,21 @@ static int write_loose_object_common(git_hash_ctx *c,
  * - End the compression of zlib stream.
  * - Get the calculated oid to "oid".
  */
-static int end_loose_object_common(git_hash_ctx *c, git_zstream *stream,
-				   struct object_id *oid)
+static int end_loose_object_common(git_hash_ctx *c, git_hash_ctx *compat_c,
+				   git_zstream *stream, struct object_id *oid,
+				   struct object_id *compat_oid)
 {
+	struct repository *repo = the_repository;
+	const struct git_hash_algo *algo = repo->hash_algo;
+	const struct git_hash_algo *compat = repo->compat_hash_algo;
 	int ret;
 
 	ret = git_deflate_end_gently(stream);
 	if (ret != Z_OK)
 		return ret;
-	the_hash_algo->final_oid_fn(oid, c);
+	algo->final_oid_fn(oid, c);
+	if (compat && compat_c)
+		compat->final_oid_fn(compat_oid, compat_c);
 
 	return Z_OK;
 }
@@ -2038,7 +2151,7 @@ static int write_loose_object(const struct object_id *oid, char *hdr,
 
 	fd = start_loose_object_common(&tmp_file, filename.buf, flags,
 				       &stream, compressed, sizeof(compressed),
-				       &c, hdr, hdrlen);
+				       &c, NULL, hdr, hdrlen);
 	if (fd < 0)
 		return -1;
 
@@ -2048,14 +2161,14 @@ static int write_loose_object(const struct object_id *oid, char *hdr,
 	do {
 		unsigned char *in0 = stream.next_in;
 
-		ret = write_loose_object_common(&c, &stream, 1, in0, fd,
+		ret = write_loose_object_common(&c, NULL, &stream, 1, in0, fd,
 						compressed, sizeof(compressed));
 	} while (ret == Z_OK);
 
 	if (ret != Z_STREAM_END)
 		die(_("unable to deflate new object %s (%d)"), oid_to_hex(oid),
 		    ret);
-	ret = end_loose_object_common(&c, &stream, &parano_oid);
+	ret = end_loose_object_common(&c, NULL, &stream, &parano_oid, NULL);
 	if (ret != Z_OK)
 		die(_("deflateEnd on object %s failed (%d)"), oid_to_hex(oid),
 		    ret);
@@ -2100,10 +2213,12 @@ static int freshen_packed_object(const struct object_id *oid)
 int stream_loose_object(struct input_stream *in_stream, size_t len,
 			struct object_id *oid)
 {
+	const struct git_hash_algo *compat = the_repository->compat_hash_algo;
+	struct object_id compat_oid;
 	int fd, ret, err = 0, flush = 0;
 	unsigned char compressed[4096];
 	git_zstream stream;
-	git_hash_ctx c;
+	git_hash_ctx c, compat_c;
 	struct strbuf tmp_file = STRBUF_INIT;
 	struct strbuf filename = STRBUF_INIT;
 	int dirlen;
@@ -2127,7 +2242,7 @@ int stream_loose_object(struct input_stream *in_stream, size_t len,
 	 */
 	fd = start_loose_object_common(&tmp_file, filename.buf, 0,
 				       &stream, compressed, sizeof(compressed),
-				       &c, hdr, hdrlen);
+				       &c, &compat_c, hdr, hdrlen);
 	if (fd < 0) {
 		err = -1;
 		goto cleanup;
@@ -2145,7 +2260,7 @@ int stream_loose_object(struct input_stream *in_stream, size_t len,
 			if (in_stream->is_finished)
 				flush = 1;
 		}
-		ret = write_loose_object_common(&c, &stream, flush, in0, fd,
+		ret = write_loose_object_common(&c, &compat_c, &stream, flush, in0, fd,
 						compressed, sizeof(compressed));
 		/*
 		 * Unlike write_loose_object(), we do not have the entire
@@ -2168,7 +2283,7 @@ int stream_loose_object(struct input_stream *in_stream, size_t len,
 	 */
 	if (ret != Z_STREAM_END)
 		die(_("unable to stream deflate new object (%d)"), ret);
-	ret = end_loose_object_common(&c, &stream, oid);
+	ret = end_loose_object_common(&c, &compat_c, &stream, oid, &compat_oid);
 	if (ret != Z_OK)
 		die(_("deflateEnd on stream object failed (%d)"), ret);
 	close_loose_object(fd, tmp_file.buf);
@@ -2195,6 +2310,8 @@ int stream_loose_object(struct input_stream *in_stream, size_t len,
 	}
 
 	err = finalize_object_file(tmp_file.buf, filename.buf);
+	if (!err && compat)
+		err = repo_add_loose_object_map(the_repository, oid, &compat_oid);
 cleanup:
 	strbuf_release(&tmp_file);
 	strbuf_release(&filename);
@@ -2203,19 +2320,42 @@ cleanup:
 
 int write_object_file_flags(const void *buf, unsigned long len,
 			    enum object_type type, struct object_id *oid,
-			    unsigned flags)
+			    struct object_id *compat_oid_in, unsigned flags)
 {
+	struct repository *repo = the_repository;
+	const struct git_hash_algo *algo = repo->hash_algo;
+	const struct git_hash_algo *compat = repo->compat_hash_algo;
+	struct object_id compat_oid;
 	char hdr[MAX_HEADER_LEN];
 	int hdrlen = sizeof(hdr);
 
+	/* Generate compat_oid */
+	if (compat) {
+		if (compat_oid_in)
+			oidcpy(&compat_oid, compat_oid_in);
+		else if (type == OBJ_BLOB)
+			hash_object_file(compat, buf, len, type, &compat_oid);
+		else {
+			struct strbuf converted = STRBUF_INIT;
+			convert_object_file(&converted, algo, compat,
+					    buf, len, type, 0);
+			hash_object_file(compat, converted.buf, converted.len,
+					 type, &compat_oid);
+			strbuf_release(&converted);
+		}
+	}
+
 	/* Normally if we have it in the pack then we do not bother writing
 	 * it out into .git/objects/??/?{38} file.
 	 */
-	write_object_file_prepare(the_hash_algo, buf, len, type, oid, hdr,
-				  &hdrlen);
+	write_object_file_prepare(algo, buf, len, type, oid, hdr, &hdrlen);
 	if (freshen_packed_object(oid) || freshen_loose_object(oid))
 		return 0;
-	return write_loose_object(oid, hdr, hdrlen, buf, len, 0, flags);
+	if (write_loose_object(oid, hdr, hdrlen, buf, len, 0, flags))
+		return -1;
+	if (compat)
+		return repo_add_loose_object_map(repo, oid, &compat_oid);
+	return 0;
 }
 
 int write_object_file_literally(const void *buf, unsigned long len,
@@ -2223,7 +2363,27 @@ int write_object_file_literally(const void *buf, unsigned long len,
 				unsigned flags)
 {
 	char *header;
+	struct repository *repo = the_repository;
+	const struct git_hash_algo *algo = repo->hash_algo;
+	const struct git_hash_algo *compat = repo->compat_hash_algo;
+	struct object_id compat_oid;
 	int hdrlen, status = 0;
+	int compat_type = -1;
+
+	if (compat) {
+		compat_type = type_from_string_gently(type, -1, 1);
+		if (compat_type == OBJ_BLOB)
+			hash_object_file(compat, buf, len, compat_type,
+					 &compat_oid);
+		else if (compat_type != -1) {
+			struct strbuf converted = STRBUF_INIT;
+			convert_object_file(&converted, algo, compat,
+					    buf, len, compat_type, 0);
+			hash_object_file(compat, converted.buf, converted.len,
+					 compat_type, &compat_oid);
+			strbuf_release(&converted);
+		}
+	}
 
 	/* type string, SP, %lu of the length plus NUL must fit this */
 	hdrlen = strlen(type) + MAX_HEADER_LEN;
@@ -2236,6 +2396,8 @@ int write_object_file_literally(const void *buf, unsigned long len,
 	if (freshen_packed_object(oid) || freshen_loose_object(oid))
 		goto cleanup;
 	status = write_loose_object(oid, header, hdrlen, buf, len, 0, 0);
+	if (compat_type != -1)
+		return repo_add_loose_object_map(repo, oid, &compat_oid);
 
 cleanup:
 	free(header);
@@ -2244,9 +2406,12 @@ cleanup:
 
 int force_object_loose(const struct object_id *oid, time_t mtime)
 {
+	struct repository *repo = the_repository;
+	const struct git_hash_algo *compat = repo->compat_hash_algo;
 	void *buf;
 	unsigned long len;
 	struct object_info oi = OBJECT_INFO_INIT;
+	struct object_id compat_oid;
 	enum object_type type;
 	char hdr[MAX_HEADER_LEN];
 	int hdrlen;
@@ -2259,8 +2424,15 @@ int force_object_loose(const struct object_id *oid, time_t mtime)
 	oi.contentp = &buf;
 	if (oid_object_info_extended(the_repository, oid, &oi, 0))
 		return error(_("cannot read object for %s"), oid_to_hex(oid));
+	if (compat) {
+		if (repo_oid_to_algop(repo, oid, compat, &compat_oid))
+			return error(_("cannot map object %s to %s"),
+				     oid_to_hex(oid), compat->name);
+	}
 	hdrlen = format_object_header(hdr, sizeof(hdr), type, len);
 	ret = write_loose_object(oid, hdr, hdrlen, buf, len, mtime, 0);
+	if (!ret && compat)
+		ret = repo_add_loose_object_map(the_repository, oid, &compat_oid);
 	free(buf);
 
 	return ret;
diff --git a/object-name.c b/object-name.c
index bd77695d7e..523af6f64f 100644
--- a/object-name.c
+++ b/object-name.c
@@ -23,6 +23,7 @@
 #include "midx.h"
 #include "commit-reach.h"
 #include "date.h"
+#include "object-file-convert.h"
 
 static int get_oid_oneline(struct repository *r, const char *, struct object_id *, struct commit_list *);
 
@@ -47,6 +48,7 @@ struct disambiguate_state {
 
 static void update_candidates(struct disambiguate_state *ds, const struct object_id *current)
 {
+	/* The hash algorithm of current has already been filtered */
 	if (ds->always_call_fn) {
 		ds->ambiguous = ds->fn(ds->repo, current, ds->cb_data) ? 1 : 0;
 		return;
@@ -132,6 +134,8 @@ static void unique_in_midx(struct multi_pack_index *m,
 {
 	uint32_t num, i, first = 0;
 	const struct object_id *current = NULL;
+	int len = ds->len > ds->repo->hash_algo->hexsz ?
+		ds->repo->hash_algo->hexsz : ds->len;
 	num = m->num_objects;
 
 	if (!num)
@@ -147,7 +151,7 @@ static void unique_in_midx(struct multi_pack_index *m,
 	for (i = first; i < num && !ds->ambiguous; i++) {
 		struct object_id oid;
 		current = nth_midxed_object_oid(&oid, m, i);
-		if (!match_hash(ds->len, ds->bin_pfx.hash, current->hash))
+		if (!match_hash(len, ds->bin_pfx.hash, current->hash))
 			break;
 		update_candidates(ds, current);
 	}
@@ -157,6 +161,8 @@ static void unique_in_pack(struct packed_git *p,
 			   struct disambiguate_state *ds)
 {
 	uint32_t num, i, first = 0;
+	int len = ds->len > ds->repo->hash_algo->hexsz ?
+		ds->repo->hash_algo->hexsz : ds->len;
 
 	if (p->multi_pack_index)
 		return;
@@ -175,7 +181,7 @@ static void unique_in_pack(struct packed_git *p,
 	for (i = first; i < num && !ds->ambiguous; i++) {
 		struct object_id oid;
 		nth_packed_object_id(&oid, p, i);
-		if (!match_hash(ds->len, ds->bin_pfx.hash, oid.hash))
+		if (!match_hash(len, ds->bin_pfx.hash, oid.hash))
 			break;
 		update_candidates(ds, &oid);
 	}
@@ -186,6 +192,10 @@ static void find_short_packed_object(struct disambiguate_state *ds)
 	struct multi_pack_index *m;
 	struct packed_git *p;
 
+	/* Skip, unless oids from the storage hash algorithm are wanted */
+	if (ds->bin_pfx.algo && (&hash_algos[ds->bin_pfx.algo] != ds->repo->hash_algo))
+		return;
+
 	for (m = get_multi_pack_index(ds->repo); m && !ds->ambiguous;
 	     m = m->next)
 		unique_in_midx(m, ds);
@@ -324,11 +334,12 @@ int set_disambiguate_hint_config(const char *var, const char *value)
 
 static int init_object_disambiguation(struct repository *r,
 				      const char *name, int len,
+				      const struct git_hash_algo *algo,
 				      struct disambiguate_state *ds)
 {
 	int i;
 
-	if (len < MINIMUM_ABBREV || len > the_hash_algo->hexsz)
+	if (len < MINIMUM_ABBREV || len > GIT_MAX_HEXSZ)
 		return -1;
 
 	memset(ds, 0, sizeof(*ds));
@@ -355,6 +366,7 @@ static int init_object_disambiguation(struct repository *r,
 	ds->len = len;
 	ds->hex_pfx[len] = '\0';
 	ds->repo = r;
+	ds->bin_pfx.algo = algo ? hash_algo_by_ptr(algo) : GIT_HASH_UNKNOWN;
 	prepare_alt_odb(r);
 	return 0;
 }
@@ -489,9 +501,10 @@ static int repo_collect_ambiguous(struct repository *r UNUSED,
 	return collect_ambiguous(oid, data);
 }
 
-static int sort_ambiguous(const void *a, const void *b, void *ctx)
+static int sort_ambiguous(const void *va, const void *vb, void *ctx)
 {
 	struct repository *sort_ambiguous_repo = ctx;
+	const struct object_id *a = va, *b = vb;
 	int a_type = oid_object_info(sort_ambiguous_repo, a, NULL);
 	int b_type = oid_object_info(sort_ambiguous_repo, b, NULL);
 	int a_type_sort;
@@ -501,8 +514,12 @@ static int sort_ambiguous(const void *a, const void *b, void *ctx)
 	 * Sorts by hash within the same object type, just as
 	 * oid_array_for_each_unique() would do.
 	 */
-	if (a_type == b_type)
-		return oidcmp(a, b);
+	if (a_type == b_type) {
+		if (a->algo == b->algo)
+			return oidcmp(a, b);
+		else
+			return a->algo > b->algo ? 1 : -1;
+	}
 
 	/*
 	 * Between object types show tags, then commits, and finally
@@ -531,8 +548,12 @@ static enum get_oid_result get_short_oid(struct repository *r,
 	int status;
 	struct disambiguate_state ds;
 	int quietly = !!(flags & GET_OID_QUIETLY);
+	const struct git_hash_algo *algo = r->hash_algo;
+
+	if (flags & GET_OID_HASH_ANY)
+		algo = NULL;
 
-	if (init_object_disambiguation(r, name, len, &ds) < 0)
+	if (init_object_disambiguation(r, name, len, algo, &ds) < 0)
 		return -1;
 
 	if (HAS_MULTI_BITS(flags & GET_OID_DISAMBIGUATORS))
@@ -586,7 +607,7 @@ static enum get_oid_result get_short_oid(struct repository *r,
 		if (!ds.ambiguous)
 			ds.fn = NULL;
 
-		repo_for_each_abbrev(r, ds.hex_pfx, collect_ambiguous, &collect);
+		repo_for_each_abbrev(r, ds.hex_pfx, algo, collect_ambiguous, &collect);
 		sort_ambiguous_oid_array(r, &collect);
 
 		if (oid_array_for_each(&collect, show_ambiguous_object, &out))
@@ -608,13 +629,14 @@ static enum get_oid_result get_short_oid(struct repository *r,
 }
 
 int repo_for_each_abbrev(struct repository *r, const char *prefix,
+			 const struct git_hash_algo *algo,
 			 each_abbrev_fn fn, void *cb_data)
 {
 	struct oid_array collect = OID_ARRAY_INIT;
 	struct disambiguate_state ds;
 	int ret;
 
-	if (init_object_disambiguation(r, prefix, strlen(prefix), &ds) < 0)
+	if (init_object_disambiguation(r, prefix, strlen(prefix), algo, &ds) < 0)
 		return -1;
 
 	ds.always_call_fn = 1;
@@ -785,10 +807,12 @@ void strbuf_add_unique_abbrev(struct strbuf *sb, const struct object_id *oid,
 int repo_find_unique_abbrev_r(struct repository *r, char *hex,
 			      const struct object_id *oid, int len)
 {
+	const struct git_hash_algo *algo =
+		oid->algo ? &hash_algos[oid->algo] : r->hash_algo;
 	struct disambiguate_state ds;
 	struct min_abbrev_data mad;
 	struct object_id oid_ret;
-	const unsigned hexsz = r->hash_algo->hexsz;
+	const unsigned hexsz = algo->hexsz;
 
 	if (len < 0) {
 		unsigned long count = repo_approximate_object_count(r);
@@ -824,7 +848,7 @@ int repo_find_unique_abbrev_r(struct repository *r, char *hex,
 
 	find_abbrev_len_packed(&mad);
 
-	if (init_object_disambiguation(r, hex, mad.cur_len, &ds) < 0)
+	if (init_object_disambiguation(r, hex, mad.cur_len, algo, &ds) < 0)
 		return -1;
 
 	ds.fn = repo_extend_abbrev_len;
diff --git a/object-name.h b/object-name.h
index 9ae5223071..064ddc97d1 100644
--- a/object-name.h
+++ b/object-name.h
@@ -67,7 +67,8 @@ enum get_oid_result get_oid_with_context(struct repository *repo, const char *st
 
 
 typedef int each_abbrev_fn(const struct object_id *oid, void *);
-int repo_for_each_abbrev(struct repository *r, const char *prefix, each_abbrev_fn, void *);
+int repo_for_each_abbrev(struct repository *r, const char *prefix,
+			 const struct git_hash_algo *algo, each_abbrev_fn, void *);
 
 int set_disambiguate_hint_config(const char *var, const char *value);
 
diff --git a/object-store-ll.h b/object-store-ll.h
index 26a3895c82..c5f2bb2fc2 100644
--- a/object-store-ll.h
+++ b/object-store-ll.h
@@ -26,6 +26,9 @@ struct object_directory {
 	uint32_t loose_objects_subdir_seen[8]; /* 256 bits */
 	struct oidtree *loose_objects_cache;
 
+	/* Map between object IDs for loose objects. */
+	struct loose_object_map *loose_map;
+
 	/*
 	 * This is a temporary object store created by the tmp_objdir
 	 * facility. Disable ref updates since the objects in the store
@@ -252,11 +255,11 @@ void hash_object_file(const struct git_hash_algo *algo, const void *buf,
 
 int write_object_file_flags(const void *buf, unsigned long len,
 			    enum object_type type, struct object_id *oid,
-			    unsigned flags);
+			    struct object_id *comapt_oid_in, unsigned flags);
 static inline int write_object_file(const void *buf, unsigned long len,
 				    enum object_type type, struct object_id *oid)
 {
-	return write_object_file_flags(buf, len, type, oid, 0);
+	return write_object_file_flags(buf, len, type, oid, NULL, 0);
 }
 
 int write_object_file_literally(const void *buf, unsigned long len,
diff --git a/object.c b/object.c
index f11c59ac0c..51e384828e 100644
--- a/object.c
+++ b/object.c
@@ -13,6 +13,7 @@
 #include "alloc.h"
 #include "packfile.h"
 #include "commit-graph.h"
+#include "loose.h"
 
 unsigned int get_max_object_index(void)
 {
@@ -553,6 +554,7 @@ void free_object_directory(struct object_directory *odb)
 {
 	free(odb->path);
 	odb_clear_loose_cache(odb);
+	loose_object_map_clear(&odb->loose_map);
 	free(odb);
 }
 
diff --git a/object.h b/object.h
index c7123cade6..9293e703cc 100644
--- a/object.h
+++ b/object.h
@@ -190,6 +190,24 @@ void *create_object(struct repository *r, const struct object_id *oid, void *obj
 
 void *object_as_type(struct object *obj, enum object_type type, int quiet);
 
+
+static inline const char *parse_mode(const char *str, uint16_t *modep)
+{
+	unsigned char c;
+	unsigned int mode = 0;
+
+	if (*str == ' ')
+		return NULL;
+
+	while ((c = *str++) != ' ') {
+		if (c < '0' || c > '7')
+			return NULL;
+		mode = (mode << 3) + (c - '0');
+	}
+	*modep = mode;
+	return str;
+}
+
 /*
  * Returns the object, having parsed it to find out what it is.
  *
diff --git a/oid-array.c b/oid-array.c
index 8e4717746c..1f36651754 100644
--- a/oid-array.c
+++ b/oid-array.c
@@ -6,12 +6,20 @@ void oid_array_append(struct oid_array *array, const struct object_id *oid)
 {
 	ALLOC_GROW(array->oid, array->nr + 1, array->alloc);
 	oidcpy(&array->oid[array->nr++], oid);
+	if (!oid->algo)
+		oid_set_algo(&array->oid[array->nr - 1], the_hash_algo);
 	array->sorted = 0;
 }
 
-static int void_hashcmp(const void *a, const void *b)
+static int void_hashcmp(const void *va, const void *vb)
 {
-	return oidcmp(a, b);
+	const struct object_id *a = va, *b = vb;
+	int ret;
+	if (a->algo == b->algo)
+		ret = oidcmp(a, b);
+	else
+		ret = a->algo > b->algo ? 1 : -1;
+	return ret;
 }
 
 void oid_array_sort(struct oid_array *array)
diff --git a/pack-bitmap-write.c b/pack-bitmap-write.c
index 990a9498d7..c6c8f94cc5 100644
--- a/pack-bitmap-write.c
+++ b/pack-bitmap-write.c
@@ -370,7 +370,7 @@ static int fill_bitmap_tree(struct bitmap *bitmap,
 	if (parse_tree(tree) < 0)
 		die("unable to load tree object %s",
 		    oid_to_hex(&tree->object.oid));
-	init_tree_desc(&desc, tree->buffer, tree->size);
+	init_tree_desc(&desc, &tree->object.oid, tree->buffer, tree->size);
 
 	while (tree_entry(&desc, &entry)) {
 		switch (object_type(entry.mode)) {
diff --git a/packfile.c b/packfile.c
index 84a005674d..d4df7fdeea 100644
--- a/packfile.c
+++ b/packfile.c
@@ -2249,7 +2249,8 @@ static int add_promisor_object(const struct object_id *oid,
 		struct tree *tree = (struct tree *)obj;
 		struct tree_desc desc;
 		struct name_entry entry;
-		if (init_tree_desc_gently(&desc, tree->buffer, tree->size, 0))
+		if (init_tree_desc_gently(&desc, &tree->object.oid,
+					  tree->buffer, tree->size, 0))
 			/*
 			 * Error messages are given when packs are
 			 * verified, so do not print any here.
diff --git a/reflog.c b/reflog.c
index 0a1bc35e8c..647f3ca398 100644
--- a/reflog.c
+++ b/reflog.c
@@ -39,7 +39,7 @@ static int tree_is_complete(const struct object_id *oid)
 		tree->buffer = data;
 		tree->size = size;
 	}
-	init_tree_desc(&desc, tree->buffer, tree->size);
+	init_tree_desc(&desc, &tree->object.oid, tree->buffer, tree->size);
 	complete = 1;
 	while (tree_entry(&desc, &entry)) {
 		if (!repo_has_object_file(the_repository, &entry.oid) ||
diff --git a/repository.c b/repository.c
index 7aacb51b65..e15b416944 100644
--- a/repository.c
+++ b/repository.c
@@ -14,6 +14,7 @@
 #include "read-cache-ll.h"
 #include "remote.h"
 #include "setup.h"
+#include "loose.h"
 #include "submodule-config.h"
 #include "sparse-index.h"
 #include "trace2.h"
@@ -104,6 +105,15 @@ void repo_set_hash_algo(struct repository *repo, int hash_algo)
 	repo->hash_algo = &hash_algos[hash_algo];
 }
 
+void repo_set_compat_hash_algo(struct repository *repo, int algo)
+{
+	if (hash_algo_by_ptr(repo->hash_algo) == algo)
+		BUG("hash_algo and compat_hash_algo match");
+	repo->compat_hash_algo = algo ? &hash_algos[algo] : NULL;
+	if (repo->compat_hash_algo)
+		repo_read_loose_object_map(repo);
+}
+
 void repo_set_ref_storage_format(struct repository *repo, unsigned int format)
 {
 	repo->ref_storage_format = format;
@@ -189,6 +199,7 @@ int repo_init(struct repository *repo,
 		goto error;
 
 	repo_set_hash_algo(repo, format.hash_algo);
+	repo_set_compat_hash_algo(repo, format.compat_hash_algo);
 	repo_set_ref_storage_format(repo, format.ref_storage_format);
 	repo->repository_format_worktree_config = format.worktree_config;
 
@@ -199,6 +210,9 @@ int repo_init(struct repository *repo,
 	if (worktree)
 		repo_set_worktree(repo, worktree);
 
+	if (repo->compat_hash_algo)
+		repo_read_loose_object_map(repo);
+
 	clear_repository_format(&format);
 	return 0;
 
diff --git a/repository.h b/repository.h
index 9bf1e33d25..268436779c 100644
--- a/repository.h
+++ b/repository.h
@@ -163,6 +163,9 @@ struct repository {
 	/* Repository's current hash algorithm, as serialized on disk. */
 	const struct git_hash_algo *hash_algo;
 
+	/* Repository's compatibility hash algorithm. */
+	const struct git_hash_algo *compat_hash_algo;
+
 	/* Repository's reference storage format, as serialized on disk. */
 	unsigned int ref_storage_format;
 
@@ -205,6 +208,7 @@ void repo_set_gitdir(struct repository *repo, const char *root,
 		     const struct set_gitdir_args *extra_args);
 void repo_set_worktree(struct repository *repo, const char *path);
 void repo_set_hash_algo(struct repository *repo, int algo);
+void repo_set_compat_hash_algo(struct repository *repo, int compat_algo);
 void repo_set_ref_storage_format(struct repository *repo, unsigned int format);
 void initialize_the_repository(void);
 RESULT_MUST_BE_USED
diff --git a/revision.c b/revision.c
index d6436ee66b..7e45f765d9 100644
--- a/revision.c
+++ b/revision.c
@@ -81,7 +81,7 @@ static void mark_tree_contents_uninteresting(struct repository *r,
 	if (parse_tree_gently(tree, 1) < 0)
 		return;
 
-	init_tree_desc(&desc, tree->buffer, tree->size);
+	init_tree_desc(&desc, &tree->object.oid, tree->buffer, tree->size);
 	while (tree_entry(&desc, &entry)) {
 		switch (object_type(entry.mode)) {
 		case OBJ_TREE:
@@ -188,7 +188,7 @@ static void add_children_by_path(struct repository *r,
 	if (parse_tree_gently(tree, 1) < 0)
 		return;
 
-	init_tree_desc(&desc, tree->buffer, tree->size);
+	init_tree_desc(&desc, &tree->object.oid, tree->buffer, tree->size);
 	while (tree_entry(&desc, &entry)) {
 		switch (object_type(entry.mode)) {
 		case OBJ_TREE:
diff --git a/setup.c b/setup.c
index 0b798591c0..f4b32f76e3 100644
--- a/setup.c
+++ b/setup.c
@@ -591,6 +591,25 @@ static enum extension_result handle_extension(const char *var,
 				     "extensions.objectformat", value);
 		data->hash_algo = format;
 		return EXTENSION_OK;
+	} else if (!strcmp(ext, "compatobjectformat")) {
+		struct string_list_item *item;
+		int format;
+
+		if (!value)
+			return config_error_nonbool(var);
+		format = hash_algo_by_name(value);
+		if (format == GIT_HASH_UNKNOWN)
+			return error(_("invalid value for '%s': '%s'"),
+				     "extensions.compatobjectformat", value);
+		/* For now only support compatObjectFormat being specified once. */
+		for_each_string_list_item(item, &data->v1_only_extensions) {
+			if (!strcmp(item->string, "compatobjectformat"))
+				return error(_("'%s' already specified as '%s'"),
+					"extensions.compatobjectformat",
+					hash_algos[data->compat_hash_algo].name);
+		}
+		data->compat_hash_algo = format;
+		return EXTENSION_OK;
 	} else if (!strcmp(ext, "refstorage")) {
 		unsigned int format;
 
@@ -1603,6 +1622,8 @@ const char *setup_git_directory_gently(int *nongit_ok)
 		}
 		if (startup_info->have_repository) {
 			repo_set_hash_algo(the_repository, repo_fmt.hash_algo);
+			repo_set_compat_hash_algo(the_repository,
+						  repo_fmt.compat_hash_algo);
 			repo_set_ref_storage_format(the_repository,
 						    repo_fmt.ref_storage_format);
 			the_repository->repository_format_worktree_config =
@@ -1698,6 +1719,7 @@ void check_repository_format(struct repository_format *fmt)
 	check_repository_format_gently(get_git_dir(), fmt, NULL);
 	startup_info->have_repository = 1;
 	repo_set_hash_algo(the_repository, fmt->hash_algo);
+	repo_set_compat_hash_algo(the_repository, fmt->compat_hash_algo);
 	repo_set_ref_storage_format(the_repository,
 				    fmt->ref_storage_format);
 	the_repository->repository_format_worktree_config =
diff --git a/setup.h b/setup.h
index 3599aec93c..d88bb37aaf 100644
--- a/setup.h
+++ b/setup.h
@@ -115,6 +115,7 @@ struct repository_format {
 	int worktree_config;
 	int is_bare;
 	int hash_algo;
+	int compat_hash_algo;
 	unsigned int ref_storage_format;
 	int sparse_index;
 	char *work_tree;
diff --git a/t/helper/test-delete-gpgsig.c b/t/helper/test-delete-gpgsig.c
new file mode 100644
index 0000000000..e36831af03
--- /dev/null
+++ b/t/helper/test-delete-gpgsig.c
@@ -0,0 +1,62 @@
+#include "test-tool.h"
+#include "gpg-interface.h"
+#include "strbuf.h"
+
+
+int cmd__delete_gpgsig(int argc, const char **argv)
+{
+	struct strbuf buf = STRBUF_INIT;
+	const char *pattern = "gpgsig";
+	const char *bufptr, *tail, *eol;
+	int deleting = 0;
+	size_t plen;
+
+	if (argc >= 2) {
+		pattern = argv[1];
+		argv++;
+		argc--;
+	}
+
+	plen = strlen(pattern);
+	strbuf_read(&buf, 0, 0);
+
+	if (!strcmp(pattern, "trailer")) {
+		size_t payload_size = parse_signed_buffer(buf.buf, buf.len);
+		fwrite(buf.buf, 1, payload_size, stdout);
+		fflush(stdout);
+		return 0;
+	}
+
+	bufptr = buf.buf;
+	tail = bufptr + buf.len;
+
+	while (bufptr < tail) {
+		/* Find the end of the line */
+		eol = memchr(bufptr, '\n', tail - bufptr);
+		if (!eol)
+			eol = tail;
+
+		/* Drop continuation lines */
+		if (deleting && (bufptr < eol) && (bufptr[0] == ' ')) {
+			bufptr = eol + 1;
+			continue;
+		}
+		deleting = 0;
+
+		/* Does the line match the prefix? */
+		if (((bufptr + plen) < eol) &&
+		    !memcmp(bufptr, pattern, plen) &&
+		    (bufptr[plen] == ' ')) {
+			deleting = 1;
+			bufptr = eol + 1;
+			continue;
+		}
+
+		/* Print all other lines */
+		fwrite(bufptr, 1, (eol - bufptr) + 1, stdout);
+		bufptr = eol + 1;
+	}
+	fflush(stdout);
+
+	return 0;
+}
diff --git a/t/helper/test-tool.c b/t/helper/test-tool.c
index 482a1e58a4..80a946b847 100644
--- a/t/helper/test-tool.c
+++ b/t/helper/test-tool.c
@@ -20,6 +20,7 @@ static struct test_cmd cmds[] = {
 	{ "crontab", cmd__crontab },
 	{ "csprng", cmd__csprng },
 	{ "date", cmd__date },
+	{ "delete-gpgsig", cmd__delete_gpgsig },
 	{ "delta", cmd__delta },
 	{ "dir-iterator", cmd__dir_iterator },
 	{ "drop-caches", cmd__drop_caches },
diff --git a/t/helper/test-tool.h b/t/helper/test-tool.h
index b1be7cfcf5..2808b92419 100644
--- a/t/helper/test-tool.h
+++ b/t/helper/test-tool.h
@@ -14,6 +14,7 @@ int cmd__crontab(int argc, const char **argv);
 int cmd__csprng(int argc, const char **argv);
 int cmd__date(int argc, const char **argv);
 int cmd__delta(int argc, const char **argv);
+int cmd__delete_gpgsig(int argc, const char **argv);
 int cmd__dir_iterator(int argc, const char **argv);
 int cmd__drop_caches(int argc, const char **argv);
 int cmd__dump_cache_tree(int argc, const char **argv);
diff --git a/t/t1006-cat-file.sh b/t/t1006-cat-file.sh
index e0c6482797..e12b221972 100755
--- a/t/t1006-cat-file.sh
+++ b/t/t1006-cat-file.sh
@@ -112,65 +112,65 @@ strlen () {
 
 run_tests () {
     type=$1
-    sha1=$2
+    oid=$2
     size=$3
     content=$4
     pretty_content=$5
 
-    batch_output="$sha1 $type $size
+    batch_output="$oid $type $size
 $content"
 
     test_expect_success "$type exists" '
-	git cat-file -e $sha1
+	git cat-file -e $oid
     '
 
     test_expect_success "Type of $type is correct" '
 	echo $type >expect &&
-	git cat-file -t $sha1 >actual &&
+	git cat-file -t $oid >actual &&
 	test_cmp expect actual
     '
 
     test_expect_success "Size of $type is correct" '
 	echo $size >expect &&
-	git cat-file -s $sha1 >actual &&
+	git cat-file -s $oid >actual &&
 	test_cmp expect actual
     '
 
     test_expect_success "Type of $type is correct using --allow-unknown-type" '
 	echo $type >expect &&
-	git cat-file -t --allow-unknown-type $sha1 >actual &&
+	git cat-file -t --allow-unknown-type $oid >actual &&
 	test_cmp expect actual
     '
 
     test_expect_success "Size of $type is correct using --allow-unknown-type" '
 	echo $size >expect &&
-	git cat-file -s --allow-unknown-type $sha1 >actual &&
+	git cat-file -s --allow-unknown-type $oid >actual &&
 	test_cmp expect actual
     '
 
     test -z "$content" ||
     test_expect_success "Content of $type is correct" '
 	echo_without_newline "$content" >expect &&
-	git cat-file $type $sha1 >actual &&
+	git cat-file $type $oid >actual &&
 	test_cmp expect actual
     '
 
     test_expect_success "Pretty content of $type is correct" '
 	echo_without_newline "$pretty_content" >expect &&
-	git cat-file -p $sha1 >actual &&
+	git cat-file -p $oid >actual &&
 	test_cmp expect actual
     '
 
     test -z "$content" ||
     test_expect_success "--batch output of $type is correct" '
 	echo "$batch_output" >expect &&
-	echo $sha1 | git cat-file --batch >actual &&
+	echo $oid | git cat-file --batch >actual &&
 	test_cmp expect actual
     '
 
     test_expect_success "--batch-check output of $type is correct" '
-	echo "$sha1 $type $size" >expect &&
-	echo_without_newline $sha1 | git cat-file --batch-check >actual &&
+	echo "$oid $type $size" >expect &&
+	echo_without_newline $oid | git cat-file --batch-check >actual &&
 	test_cmp expect actual
     '
 
@@ -179,33 +179,33 @@ $content"
 	test -z "$content" ||
 		test_expect_success "--batch-command $opt output of $type content is correct" '
 		echo "$batch_output" >expect &&
-		test_write_lines "contents $sha1" | git cat-file --batch-command $opt >actual &&
+		test_write_lines "contents $oid" | git cat-file --batch-command $opt >actual &&
 		test_cmp expect actual
 	'
 
 	test_expect_success "--batch-command $opt output of $type info is correct" '
-		echo "$sha1 $type $size" >expect &&
-		test_write_lines "info $sha1" |
+		echo "$oid $type $size" >expect &&
+		test_write_lines "info $oid" |
 		git cat-file --batch-command $opt >actual &&
 		test_cmp expect actual
 	'
     done
 
     test_expect_success "custom --batch-check format" '
-	echo "$type $sha1" >expect &&
-	echo $sha1 | git cat-file --batch-check="%(objecttype) %(objectname)" >actual &&
+	echo "$type $oid" >expect &&
+	echo $oid | git cat-file --batch-check="%(objecttype) %(objectname)" >actual &&
 	test_cmp expect actual
     '
 
     test_expect_success "custom --batch-command format" '
-	echo "$type $sha1" >expect &&
-	echo "info $sha1" | git cat-file --batch-command="%(objecttype) %(objectname)" >actual &&
+	echo "$type $oid" >expect &&
+	echo "info $oid" | git cat-file --batch-command="%(objecttype) %(objectname)" >actual &&
 	test_cmp expect actual
     '
 
     test_expect_success '--batch-check with %(rest)' '
 	echo "$type this is some extra content" >expect &&
-	echo "$sha1    this is some extra content" |
+	echo "$oid    this is some extra content" |
 		git cat-file --batch-check="%(objecttype) %(rest)" >actual &&
 	test_cmp expect actual
     '
@@ -216,7 +216,7 @@ $content"
 		echo "$size" &&
 		echo "$content"
 	} >expect &&
-	echo $sha1 | git cat-file --batch="%(objectsize)" >actual &&
+	echo $oid | git cat-file --batch="%(objectsize)" >actual &&
 	test_cmp expect actual
     '
 
@@ -226,114 +226,154 @@ $content"
 		echo "$type" &&
 		echo "$content"
 	} >expect &&
-	echo $sha1 | git cat-file --batch="%(objecttype)" >actual &&
+	echo $oid | git cat-file --batch="%(objecttype)" >actual &&
 	test_cmp expect actual
     '
 }
 
 hello_content="Hello World"
 hello_size=$(strlen "$hello_content")
-hello_sha1=$(echo_without_newline "$hello_content" | git hash-object --stdin)
+hello_oid=$(echo_without_newline "$hello_content" | git hash-object --stdin)
 
 test_expect_success "setup" '
+	git config core.repositoryformatversion 1 &&
+	git config extensions.objectformat $test_hash_algo &&
+	git config extensions.compatobjectformat $test_compat_hash_algo &&
 	echo_without_newline "$hello_content" > hello &&
 	git update-index --add hello
 '
 
-run_tests 'blob' $hello_sha1 $hello_size "$hello_content" "$hello_content"
+run_blob_tests () {
+    oid=$1
 
-test_expect_success '--batch-command --buffer with flush for blob info' '
-	echo "$hello_sha1 blob $hello_size" >expect &&
-	test_write_lines "info $hello_sha1" "flush" |
+    run_tests 'blob' $oid $hello_size "$hello_content" "$hello_content"
+
+    test_expect_success '--batch-command --buffer with flush for blob info' '
+	echo "$oid blob $hello_size" >expect &&
+	test_write_lines "info $oid" "flush" |
 	GIT_TEST_CAT_FILE_NO_FLUSH_ON_EXIT=1 \
 	git cat-file --batch-command --buffer >actual &&
 	test_cmp expect actual
-'
+    '
 
-test_expect_success '--batch-command --buffer without flush for blob info' '
+    test_expect_success '--batch-command --buffer without flush for blob info' '
 	touch output &&
-	test_write_lines "info $hello_sha1" |
+	test_write_lines "info $oid" |
 	GIT_TEST_CAT_FILE_NO_FLUSH_ON_EXIT=1 \
 	git cat-file --batch-command --buffer >>output &&
 	test_must_be_empty output
-'
+    '
+}
+
+hello_compat_oid=$(git rev-parse --output-object-format=$test_compat_hash_algo $hello_oid)
+run_blob_tests $hello_oid
+run_blob_tests $hello_compat_oid
 
 test_expect_success '--batch-check without %(rest) considers whole line' '
-	echo "$hello_sha1 blob $hello_size" >expect &&
-	git update-index --add --cacheinfo 100644 $hello_sha1 "white space" &&
+	echo "$hello_oid blob $hello_size" >expect &&
+	git update-index --add --cacheinfo 100644 $hello_oid "white space" &&
 	test_when_finished "git update-index --remove \"white space\"" &&
 	echo ":white space" | git cat-file --batch-check >actual &&
 	test_cmp expect actual
 '
 
-tree_sha1=$(git write-tree)
+tree_oid=$(git write-tree)
+tree_compat_oid=$(git rev-parse --output-object-format=$test_compat_hash_algo $tree_oid)
 tree_size=$(($(test_oid rawsz) + 13))
-tree_pretty_content="100644 blob $hello_sha1	hello${LF}"
+tree_compat_size=$(($(test_oid --hash=compat rawsz) + 13))
+tree_pretty_content="100644 blob $hello_oid	hello${LF}"
+tree_compat_pretty_content="100644 blob $hello_compat_oid	hello${LF}"
 
-run_tests 'tree' $tree_sha1 $tree_size "" "$tree_pretty_content"
+run_tests 'tree' $tree_oid $tree_size "" "$tree_pretty_content"
+run_tests 'tree' $tree_compat_oid $tree_compat_size "" "$tree_compat_pretty_content"
 
 commit_message="Initial commit"
-commit_sha1=$(echo_without_newline "$commit_message" | git commit-tree $tree_sha1)
+commit_oid=$(echo_without_newline "$commit_message" | git commit-tree $tree_oid)
+commit_compat_oid=$(git rev-parse --output-object-format=$test_compat_hash_algo $commit_oid)
 commit_size=$(($(test_oid hexsz) + 137))
-commit_content="tree $tree_sha1
+commit_compat_size=$(($(test_oid --hash=compat hexsz) + 137))
+commit_content="tree $tree_oid
+author $GIT_AUTHOR_NAME <$GIT_AUTHOR_EMAIL> $GIT_AUTHOR_DATE
+committer $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> $GIT_COMMITTER_DATE
+
+$commit_message"
+
+commit_compat_content="tree $tree_compat_oid
 author $GIT_AUTHOR_NAME <$GIT_AUTHOR_EMAIL> $GIT_AUTHOR_DATE
 committer $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> $GIT_COMMITTER_DATE
 
 $commit_message"
 
-run_tests 'commit' $commit_sha1 $commit_size "$commit_content" "$commit_content"
+run_tests 'commit' $commit_oid $commit_size "$commit_content" "$commit_content"
+run_tests 'commit' $commit_compat_oid $commit_compat_size "$commit_compat_content" "$commit_compat_content"
 
-tag_header_without_timestamp="object $hello_sha1
-type blob
+tag_header_without_oid="type blob
 tag hellotag
 tagger $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL>"
+tag_header_without_timestamp="object $hello_oid
+$tag_header_without_oid"
+tag_compat_header_without_timestamp="object $hello_compat_oid
+$tag_header_without_oid"
 tag_description="This is a tag"
 tag_content="$tag_header_without_timestamp 0 +0000
 
 $tag_description"
+tag_compat_content="$tag_compat_header_without_timestamp 0 +0000
 
-tag_sha1=$(echo_without_newline "$tag_content" | git hash-object -t tag --stdin -w)
+$tag_description"
+
+tag_oid=$(echo_without_newline "$tag_content" | git hash-object -t tag --stdin -w)
 tag_size=$(strlen "$tag_content")
 
-run_tests 'tag' $tag_sha1 $tag_size "$tag_content" "$tag_content"
+tag_compat_oid=$(git rev-parse --output-object-format=$test_compat_hash_algo $tag_oid)
+tag_compat_size=$(strlen "$tag_compat_content")
+
+run_tests 'tag' $tag_oid $tag_size "$tag_content" "$tag_content"
+run_tests 'tag' $tag_compat_oid $tag_compat_size "$tag_compat_content" "$tag_compat_content"
 
 test_expect_success "Reach a blob from a tag pointing to it" '
 	echo_without_newline "$hello_content" >expect &&
-	git cat-file blob $tag_sha1 >actual &&
+	git cat-file blob $tag_oid >actual &&
 	test_cmp expect actual
 '
 
-for batch in batch batch-check batch-command
+for oid in $hello_oid $hello_compat_oid
 do
-    for opt in t s e p
+    for batch in batch batch-check batch-command
     do
+	for opt in t s e p
+	do
 	test_expect_success "Passing -$opt with --$batch fails" '
-	    test_must_fail git cat-file --$batch -$opt $hello_sha1
+	    test_must_fail git cat-file --$batch -$opt $oid
 	'
 
 	test_expect_success "Passing --$batch with -$opt fails" '
-	    test_must_fail git cat-file -$opt --$batch $hello_sha1
+	    test_must_fail git cat-file -$opt --$batch $oid
 	'
-    done
+	done
 
-    test_expect_success "Passing <type> with --$batch fails" '
-	test_must_fail git cat-file --$batch blob $hello_sha1
-    '
+	test_expect_success "Passing <type> with --$batch fails" '
+	test_must_fail git cat-file --$batch blob $oid
+	'
 
-    test_expect_success "Passing --$batch with <type> fails" '
-	test_must_fail git cat-file blob --$batch $hello_sha1
-    '
+	test_expect_success "Passing --$batch with <type> fails" '
+	test_must_fail git cat-file blob --$batch $oid
+	'
 
-    test_expect_success "Passing sha1 with --$batch fails" '
-	test_must_fail git cat-file --$batch $hello_sha1
-    '
+	test_expect_success "Passing oid with --$batch fails" '
+	test_must_fail git cat-file --$batch $oid
+	'
+    done
 done
 
-for opt in t s e p
+for oid in $hello_oid $hello_compat_oid
 do
-    test_expect_success "Passing -$opt with --follow-symlinks fails" '
-	    test_must_fail git cat-file --follow-symlinks -$opt $hello_sha1
+    for opt in t s e p
+    do
+	test_expect_success "Passing -$opt with --follow-symlinks fails" '
+	    test_must_fail git cat-file --follow-symlinks -$opt $oid
 	'
+    done
 done
 
 test_expect_success "--batch-check for a non-existent named object" '
@@ -360,12 +400,12 @@ test_expect_success "--batch-check for a non-existent hash" '
 
 test_expect_success "--batch for an existent and a non-existent hash" '
 	cat >expect <<-EOF &&
-	$tag_sha1 tag $tag_size
+	$tag_oid tag $tag_size
 	$tag_content
 	0000000000000000000000000000000000000000 missing
 	EOF
 
-	printf "$tag_sha1\n0000000000000000000000000000000000000000" >in &&
+	printf "$tag_oid\n0000000000000000000000000000000000000000" >in &&
 	git cat-file --batch <in >actual &&
 	test_cmp expect actual
 '
@@ -386,112 +426,102 @@ test_expect_success 'empty --batch-check notices missing object' '
 	test_cmp expect actual
 '
 
-batch_input="$hello_sha1
-$commit_sha1
-$tag_sha1
+batch_tests () {
+    boid=$1
+    loid=$2
+    lsize=$3
+    coid=$4
+    csize=$5
+    ccontent=$6
+    toid=$7
+    tsize=$8
+    tcontent=$9
+
+    batch_input="$boid
+$coid
+$toid
 deadbeef
 
 "
 
-printf "%s\0" \
-	"$hello_sha1 blob $hello_size" \
+    printf "%s\0" \
+	"$boid blob $hello_size" \
 	"$hello_content" \
-	"$commit_sha1 commit $commit_size" \
-	"$commit_content" \
-	"$tag_sha1 tag $tag_size" \
-	"$tag_content" \
+	"$coid commit $csize" \
+	"$ccontent" \
+	"$toid tag $tsize" \
+	"$tcontent" \
 	"deadbeef missing" \
 	" missing" >batch_output
 
-test_expect_success '--batch with multiple sha1s gives correct format' '
+    test_expect_success '--batch with multiple oids gives correct format' '
 	tr "\0" "\n" <batch_output >expect &&
 	echo_without_newline "$batch_input" >in &&
 	git cat-file --batch <in >actual &&
 	test_cmp expect actual
-'
+    '
 
-test_expect_success '--batch, -z with multiple sha1s gives correct format' '
+    test_expect_success '--batch, -z with multiple oids gives correct format' '
 	echo_without_newline_nul "$batch_input" >in &&
 	tr "\0" "\n" <batch_output >expect &&
 	git cat-file --batch -z <in >actual &&
 	test_cmp expect actual
-'
+    '
 
-test_expect_success '--batch, -Z with multiple sha1s gives correct format' '
+    test_expect_success '--batch, -Z with multiple oids gives correct format' '
 	echo_without_newline_nul "$batch_input" >in &&
 	git cat-file --batch -Z <in >actual &&
 	test_cmp batch_output actual
-'
+    '
 
-batch_check_input="$hello_sha1
-$tree_sha1
-$commit_sha1
-$tag_sha1
+batch_check_input="$boid
+$loid
+$coid
+$toid
 deadbeef
 
 "
 
-printf "%s\0" \
-	"$hello_sha1 blob $hello_size" \
-	"$tree_sha1 tree $tree_size" \
-	"$commit_sha1 commit $commit_size" \
-	"$tag_sha1 tag $tag_size" \
+    printf "%s\0" \
+	"$boid blob $hello_size" \
+	"$loid tree $lsize" \
+	"$coid commit $csize" \
+	"$toid tag $tsize" \
 	"deadbeef missing" \
 	" missing" >batch_check_output
 
-test_expect_success "--batch-check with multiple sha1s gives correct format" '
+    test_expect_success "--batch-check with multiple oids gives correct format" '
 	tr "\0" "\n" <batch_check_output >expect &&
 	echo_without_newline "$batch_check_input" >in &&
 	git cat-file --batch-check <in >actual &&
 	test_cmp expect actual
-'
+    '
 
-test_expect_success "--batch-check, -z with multiple sha1s gives correct format" '
+    test_expect_success "--batch-check, -z with multiple oids gives correct format" '
 	tr "\0" "\n" <batch_check_output >expect &&
 	echo_without_newline_nul "$batch_check_input" >in &&
 	git cat-file --batch-check -z <in >actual &&
 	test_cmp expect actual
-'
+    '
 
-test_expect_success "--batch-check, -Z with multiple sha1s gives correct format" '
+    test_expect_success "--batch-check, -Z with multiple oids gives correct format" '
 	echo_without_newline_nul "$batch_check_input" >in &&
 	git cat-file --batch-check -Z <in >actual &&
 	test_cmp batch_check_output actual
-'
-
-test_expect_success FUNNYNAMES 'setup with newline in input' '
-	touch -- "newline${LF}embedded" &&
-	git add -- "newline${LF}embedded" &&
-	git commit -m "file with newline embedded" &&
-	test_tick &&
-
-	printf "HEAD:newline${LF}embedded" >in
-'
-
-test_expect_success FUNNYNAMES '--batch-check, -z with newline in input' '
-	git cat-file --batch-check -z <in >actual &&
-	echo "$(git rev-parse "HEAD:newline${LF}embedded") blob 0" >expect &&
-	test_cmp expect actual
-'
-
-test_expect_success FUNNYNAMES '--batch-check, -Z with newline in input' '
-	git cat-file --batch-check -Z <in >actual &&
-	printf "%s\0" "$(git rev-parse "HEAD:newline${LF}embedded") blob 0" >expect &&
-	test_cmp expect actual
-'
+    '
 
-batch_command_multiple_info="info $hello_sha1
-info $tree_sha1
-info $commit_sha1
-info $tag_sha1
+batch_command_multiple_info="info $boid
+info $loid
+info $coid
+info $toid
 info deadbeef"
 
-test_expect_success '--batch-command with multiple info calls gives correct format' '
+    test_expect_success '--batch-command with multiple info calls gives correct format' '
 	cat >expect <<-EOF &&
-	$hello_sha1 blob $hello_size
-	$tree_sha1 tree $tree_size
-	$commit_sha1 commit $commit_size
-	$tag_sha1 tag $tag_size
+	$boid blob $hello_size
+	$loid tree $lsize
+	$coid commit $csize
+	$toid tag $tsize
 	deadbeef missing
 	EOF
 
@@ -510,22 +540,22 @@ test_expect_success '--batch-command with multiple info calls gives correct form
 	git cat-file --batch-command --buffer -Z <in >actual &&
 
 	test_cmp expect_nul actual
-'
+    '
 
-batch_command_multiple_contents="contents $hello_sha1
-contents $commit_sha1
-contents $tag_sha1
+batch_command_multiple_contents="contents $boid
+contents $coid
+contents $toid
 contents deadbeef
 flush"
 
-test_expect_success '--batch-command with multiple command calls gives correct format' '
+    test_expect_success '--batch-command with multiple command calls gives correct format' '
 	printf "%s\0" \
-		"$hello_sha1 blob $hello_size" \
+		"$boid blob $hello_size" \
 		"$hello_content" \
-		"$commit_sha1 commit $commit_size" \
-		"$commit_content" \
-		"$tag_sha1 tag $tag_size" \
-		"$tag_content" \
+		"$coid commit $csize" \
+		"$ccontent" \
+		"$toid tag $tsize" \
+		"$tcontent" \
 		"deadbeef missing" >expect_nul &&
 	tr "\0" "\n" <expect_nul >expect &&
 
@@ -543,6 +573,33 @@ test_expect_success '--batch-command with multiple command calls gives correct f
 	git cat-file --batch-command --buffer -Z <in >actual &&
 
 	test_cmp expect_nul actual
+    '
+
+}
+
+batch_tests $hello_oid $tree_oid $tree_size $commit_oid $commit_size "$commit_content" $tag_oid $tag_size "$tag_content"
+batch_tests $hello_compat_oid $tree_compat_oid $tree_compat_size $commit_compat_oid $commit_compat_size "$commit_compat_content" $tag_compat_oid $tag_compat_size "$tag_compat_content"
+
+
+test_expect_success FUNNYNAMES 'setup with newline in input' '
+	touch -- "newline${LF}embedded" &&
+	git add -- "newline${LF}embedded" &&
+	git commit -m "file with newline embedded" &&
+	test_tick &&
+
+	printf "HEAD:newline${LF}embedded" >in
+'
+
+test_expect_success FUNNYNAMES '--batch-check, -z with newline in input' '
+	git cat-file --batch-check -z <in >actual &&
+	echo "$(git rev-parse "HEAD:newline${LF}embedded") blob 0" >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success FUNNYNAMES '--batch-check, -Z with newline in input' '
+	git cat-file --batch-check -Z <in >actual &&
+	printf "%s\0" "$(git rev-parse "HEAD:newline${LF}embedded") blob 0" >expect &&
+	test_cmp expect actual
 '
 
 test_expect_success 'setup blobs which are likely to delta' '
@@ -569,7 +626,7 @@ test_expect_success 'confirm that neither loose blob is a delta' '
 # we will check only that one of the two objects is a delta
 # against the other, but not the order. We can do so by just
 # asking for the base of both, and checking whether either
-# sha1 appears in the output.
+# oid appears in the output.
 test_expect_success '%(deltabase) reports packed delta bases' '
 	git repack -ad &&
 	git cat-file --batch-check="%(deltabase)" <blobs >actual &&
@@ -583,12 +640,12 @@ test_expect_success 'setup bogus data' '
 	bogus_short_type="bogus" &&
 	bogus_short_content="bogus" &&
 	bogus_short_size=$(strlen "$bogus_short_content") &&
-	bogus_short_sha1=$(echo_without_newline "$bogus_short_content" | git hash-object -t $bogus_short_type --literally -w --stdin) &&
+	bogus_short_oid=$(echo_without_newline "$bogus_short_content" | git hash-object -t $bogus_short_type --literally -w --stdin) &&
 
 	bogus_long_type="abcdefghijklmnopqrstuvwxyz1234679" &&
 	bogus_long_content="bogus" &&
 	bogus_long_size=$(strlen "$bogus_long_content") &&
-	bogus_long_sha1=$(echo_without_newline "$bogus_long_content" | git hash-object -t $bogus_long_type --literally -w --stdin)
+	bogus_long_oid=$(echo_without_newline "$bogus_long_content" | git hash-object -t $bogus_long_type --literally -w --stdin)
 '
 
 for arg1 in '' --allow-unknown-type
@@ -608,9 +665,9 @@ do
 
 			if test "$arg1" = "--allow-unknown-type"
 			then
-				git cat-file $arg1 $arg2 $bogus_short_sha1
+				git cat-file $arg1 $arg2 $bogus_short_oid
 			else
-				test_must_fail git cat-file $arg1 $arg2 $bogus_short_sha1 >out 2>actual &&
+				test_must_fail git cat-file $arg1 $arg2 $bogus_short_oid >out 2>actual &&
 				test_must_be_empty out &&
 				test_cmp expect actual
 			fi
@@ -620,21 +677,21 @@ do
 			if test "$arg2" = "-p"
 			then
 				cat >expect <<-EOF
-				error: header for $bogus_long_sha1 too long, exceeds 32 bytes
-				fatal: Not a valid object name $bogus_long_sha1
+				error: header for $bogus_long_oid too long, exceeds 32 bytes
+				fatal: Not a valid object name $bogus_long_oid
 				EOF
 			else
 				cat >expect <<-EOF
-				error: header for $bogus_long_sha1 too long, exceeds 32 bytes
+				error: header for $bogus_long_oid too long, exceeds 32 bytes
 				fatal: git cat-file: could not get object info
 				EOF
 			fi &&
 
 			if test "$arg1" = "--allow-unknown-type"
 			then
-				git cat-file $arg1 $arg2 $bogus_short_sha1
+				git cat-file $arg1 $arg2 $bogus_short_oid
 			else
-				test_must_fail git cat-file $arg1 $arg2 $bogus_long_sha1 >out 2>actual &&
+				test_must_fail git cat-file $arg1 $arg2 $bogus_long_oid >out 2>actual &&
 				test_must_be_empty out &&
 				test_cmp expect actual
 			fi
@@ -668,28 +725,28 @@ do
 done
 
 test_expect_success '-e is OK with a broken object without --allow-unknown-type' '
-	git cat-file -e $bogus_short_sha1
+	git cat-file -e $bogus_short_oid
 '
 
 test_expect_success '-e can not be combined with --allow-unknown-type' '
-	test_expect_code 128 git cat-file -e --allow-unknown-type $bogus_short_sha1
+	test_expect_code 128 git cat-file -e --allow-unknown-type $bogus_short_oid
 '
 
 test_expect_success '-p cannot print a broken object even with --allow-unknown-type' '
-	test_must_fail git cat-file -p $bogus_short_sha1 &&
-	test_expect_code 128 git cat-file -p --allow-unknown-type $bogus_short_sha1
+	test_must_fail git cat-file -p $bogus_short_oid &&
+	test_expect_code 128 git cat-file -p --allow-unknown-type $bogus_short_oid
 '
 
 test_expect_success '<type> <hash> does not work with objects of broken types' '
 	cat >err.expect <<-\EOF &&
 	fatal: invalid object type "bogus"
 	EOF
-	test_must_fail git cat-file $bogus_short_type $bogus_short_sha1 2>err.actual &&
+	test_must_fail git cat-file $bogus_short_type $bogus_short_oid 2>err.actual &&
 	test_cmp err.expect err.actual
 '
 
 test_expect_success 'broken types combined with --batch and --batch-check' '
-	echo $bogus_short_sha1 >bogus-oid &&
+	echo $bogus_short_oid >bogus-oid &&
 
 	cat >err.expect <<-\EOF &&
 	fatal: invalid object type
@@ -711,52 +768,52 @@ test_expect_success 'the --allow-unknown-type option does not consider replaceme
 	cat >expect <<-EOF &&
 	$bogus_short_type
 	EOF
-	git cat-file -t --allow-unknown-type $bogus_short_sha1 >actual &&
+	git cat-file -t --allow-unknown-type $bogus_short_oid >actual &&
 	test_cmp expect actual &&
 
 	# Create it manually, as "git replace" will die on bogus
 	# types.
 	head=$(git rev-parse --verify HEAD) &&
-	test_when_finished "test-tool ref-store main delete-refs 0 msg refs/replace/$bogus_short_sha1" &&
-	test-tool ref-store main update-ref msg "refs/replace/$bogus_short_sha1" $head $ZERO_OID REF_SKIP_OID_VERIFICATION &&
+	test_when_finished "test-tool ref-store main delete-refs 0 msg refs/replace/$bogus_short_oid" &&
+	test-tool ref-store main update-ref msg "refs/replace/$bogus_short_oid" $head $ZERO_OID REF_SKIP_OID_VERIFICATION &&
 
 	cat >expect <<-EOF &&
 	commit
 	EOF
-	git cat-file -t --allow-unknown-type $bogus_short_sha1 >actual &&
+	git cat-file -t --allow-unknown-type $bogus_short_oid >actual &&
 	test_cmp expect actual
 '
 
 test_expect_success "Type of broken object is correct" '
 	echo $bogus_short_type >expect &&
-	git cat-file -t --allow-unknown-type $bogus_short_sha1 >actual &&
+	git cat-file -t --allow-unknown-type $bogus_short_oid >actual &&
 	test_cmp expect actual
 '
 
 test_expect_success "Size of broken object is correct" '
 	echo $bogus_short_size >expect &&
-	git cat-file -s --allow-unknown-type $bogus_short_sha1 >actual &&
+	git cat-file -s --allow-unknown-type $bogus_short_oid >actual &&
 	test_cmp expect actual
 '
 
 test_expect_success 'clean up broken object' '
-	rm .git/objects/$(test_oid_to_path $bogus_short_sha1)
+	rm .git/objects/$(test_oid_to_path $bogus_short_oid)
 '
 
 test_expect_success "Type of broken object is correct when type is large" '
 	echo $bogus_long_type >expect &&
-	git cat-file -t --allow-unknown-type $bogus_long_sha1 >actual &&
+	git cat-file -t --allow-unknown-type $bogus_long_oid >actual &&
 	test_cmp expect actual
 '
 
 test_expect_success "Size of large broken object is correct when type is large" '
 	echo $bogus_long_size >expect &&
-	git cat-file -s --allow-unknown-type $bogus_long_sha1 >actual &&
+	git cat-file -s --allow-unknown-type $bogus_long_oid >actual &&
 	test_cmp expect actual
 '
 
 test_expect_success 'clean up broken object' '
-	rm .git/objects/$(test_oid_to_path $bogus_long_sha1)
+	rm .git/objects/$(test_oid_to_path $bogus_long_oid)
 '
 
 test_expect_success 'cat-file -t and -s on corrupt loose object' '
@@ -853,7 +910,7 @@ test_expect_success 'prep for symlink tests' '
 	test_ln_s_add loop2 loop1 &&
 	git add morx dir/subdir/ind2 dir/ind1 &&
 	git commit -am "test" &&
-	echo $hello_sha1 blob $hello_size >found
+	echo $hello_oid blob $hello_size >found
 '
 
 test_expect_success 'git cat-file --batch-check --follow-symlinks works for non-links' '
@@ -941,7 +998,7 @@ test_expect_success 'git cat-file --batch-check --follow-symlinks works for dir/
 	echo HEAD:dirlink/morx >>expect &&
 	echo HEAD:dirlink/morx | git cat-file --batch-check --follow-symlinks >actual &&
 	test_cmp expect actual &&
-	echo $hello_sha1 blob $hello_size >expect &&
+	echo $hello_oid blob $hello_size >expect &&
 	echo HEAD:dirlink/ind1 | git cat-file --batch-check --follow-symlinks >actual &&
 	test_cmp expect actual
 '
diff --git a/t/t1016-compatObjectFormat.sh b/t/t1016-compatObjectFormat.sh
new file mode 100755
index 0000000000..8132cd37b8
--- /dev/null
+++ b/t/t1016-compatObjectFormat.sh
@@ -0,0 +1,281 @@
+#!/bin/sh
+#
+# Copyright (c) 2023 Eric Biederman
+#
+
+test_description='Test how well compatObjectFormat works'
+
+TEST_PASSES_SANITIZE_LEAK=true
+. ./test-lib.sh
+. "$TEST_DIRECTORY"/lib-gpg.sh
+
+# All of the follow variables must be defined in the environment:
+# GIT_AUTHOR_NAME
+# GIT_AUTHOR_EMAIL
+# GIT_AUTHOR_DATE
+# GIT_COMMITTER_NAME
+# GIT_COMMITTER_EMAIL
+# GIT_COMMITTER_DATE
+#
+# The test relies on these variables being set so that the two
+# different commits in two different repositories encoded with two
+# different hash functions result in the same content in the commits.
+# This means that when the commit is translated between hash functions
+# the commit is identical to the commit in the other repository.
+
+compat_hash () {
+    case "$1" in
+    "sha1")
+	echo "sha256"
+	;;
+    "sha256")
+	echo "sha1"
+	;;
+    esac
+}
+
+hello_oid () {
+    case "$1" in
+    "sha1")
+	echo "$hello_sha1_oid"
+	;;
+    "sha256")
+	echo "$hello_sha256_oid"
+	;;
+    esac
+}
+
+tree_oid () {
+    case "$1" in
+    "sha1")
+	echo "$tree_sha1_oid"
+	;;
+    "sha256")
+	echo "$tree_sha256_oid"
+	;;
+    esac
+}
+
+commit_oid () {
+    case "$1" in
+    "sha1")
+	echo "$commit_sha1_oid"
+	;;
+    "sha256")
+	echo "$commit_sha256_oid"
+	;;
+    esac
+}
+
+commit2_oid () {
+    case "$1" in
+    "sha1")
+	echo "$commit2_sha1_oid"
+	;;
+    "sha256")
+	echo "$commit2_sha256_oid"
+	;;
+    esac
+}
+
+del_sigcommit () {
+    local delete=$1
+
+    if test "$delete" = "sha256" ; then
+	local pattern="gpgsig-sha256"
+    else
+	local pattern="gpgsig"
+    fi
+    test-tool delete-gpgsig "$pattern"
+}
+
+
+del_sigtag () {
+    local storage=$1
+    local delete=$2
+
+    if test "$storage" = "$delete" ; then
+	local pattern="trailer"
+    elif test "$storage" = "sha256" ; then
+	local pattern="gpgsig"
+    else
+	local pattern="gpgsig-sha256"
+    fi
+    test-tool delete-gpgsig "$pattern"
+}
+
+base=$(pwd)
+for hash in sha1 sha256
+do
+	cd "$base"
+	mkdir -p repo-$hash
+	cd repo-$hash
+
+	test_expect_success "setup $hash repository" '
+		git init --object-format=$hash &&
+		git config core.repositoryformatversion 1 &&
+		git config extensions.objectformat $hash &&
+		git config extensions.compatobjectformat $(compat_hash $hash) &&
+		git config gpg.program $TEST_DIRECTORY/t1016/gpg &&
+		echo "Hellow World!" > hello &&
+		eval hello_${hash}_oid=$(git hash-object hello) &&
+		git update-index --add hello &&
+		git commit -m "Initial commit" &&
+		eval commit_${hash}_oid=$(git rev-parse HEAD) &&
+		eval tree_${hash}_oid=$(git rev-parse HEAD^{tree})
+	'
+	test_expect_success "create a $hash  tagged blob" '
+		git tag --no-sign -m "This is a tag" hellotag $(hello_oid $hash) &&
+		eval hellotag_${hash}_oid=$(git rev-parse hellotag)
+	'
+	test_expect_success "create a $hash tagged tree" '
+		git tag --no-sign -m "This is a tag" treetag $(tree_oid $hash) &&
+		eval treetag_${hash}_oid=$(git rev-parse treetag)
+	'
+	test_expect_success "create a $hash tagged commit" '
+		git tag --no-sign -m "This is a tag" committag $(commit_oid $hash) &&
+		eval committag_${hash}_oid=$(git rev-parse committag)
+	'
+	test_expect_success GPG2 "create a $hash signed commit" '
+		git commit --gpg-sign --allow-empty -m "This is a signed commit" &&
+		eval signedcommit_${hash}_oid=$(git rev-parse HEAD)
+	'
+	test_expect_success GPG2 "create a $hash signed tag" '
+		git tag -s -m "This is a signed tag" signedtag HEAD &&
+		eval signedtag_${hash}_oid=$(git rev-parse signedtag)
+	'
+	test_expect_success "create a $hash branch" '
+		git checkout -b branch $(commit_oid $hash) &&
+		echo "More more more give me more!" > more &&
+		eval more_${hash}_oid=$(git hash-object more) &&
+		echo "Another and another and another" > another &&
+		eval another_${hash}_oid=$(git hash-object another) &&
+		git update-index --add more another &&
+		git commit -m "Add more files!" &&
+		eval commit2_${hash}_oid=$(git rev-parse HEAD) &&
+		eval tree2_${hash}_oid=$(git rev-parse HEAD^{tree})
+	'
+	test_expect_success GPG2 "create another $hash signed tag" '
+		git tag -s -m "This is another signed tag" signedtag2 $(commit2_oid $hash) &&
+		eval signedtag2_${hash}_oid=$(git rev-parse signedtag2)
+	'
+	test_expect_success GPG2 "merge the $hash branches together" '
+		git merge -S -m "merge some signed tags together" signedtag signedtag2 &&
+		eval signedcommit2_${hash}_oid=$(git rev-parse HEAD)
+	'
+	test_expect_success GPG2 "create additional $hash signed commits" '
+		git commit --gpg-sign --allow-empty -m "This is an additional signed commit" &&
+		git cat-file commit HEAD | del_sigcommit sha256 > "../${hash}_signedcommit3" &&
+		git cat-file commit HEAD | del_sigcommit sha1 > "../${hash}_signedcommit4" &&
+		eval signedcommit3_${hash}_oid=$(git hash-object -t commit -w ../${hash}_signedcommit3) &&
+		eval signedcommit4_${hash}_oid=$(git hash-object -t commit -w ../${hash}_signedcommit4)
+	'
+	test_expect_success GPG2 "create additional $hash signed tags" '
+		git tag -s -m "This is an additional signed tag" signedtag34 HEAD &&
+		git cat-file tag signedtag34 | del_sigtag "${hash}" sha256 > ../${hash}_signedtag3 &&
+		git cat-file tag signedtag34 | del_sigtag "${hash}" sha1 > ../${hash}_signedtag4 &&
+		eval signedtag3_${hash}_oid=$(git hash-object -t tag -w ../${hash}_signedtag3) &&
+		eval signedtag4_${hash}_oid=$(git hash-object -t tag -w ../${hash}_signedtag4)
+	'
+done
+cd "$base"
+
+compare_oids () {
+    test "$#" = 5 && { local PREREQ=$1; shift; } || PREREQ=
+    local type="$1"
+    local name="$2"
+    local sha1_oid="$3"
+    local sha256_oid="$4"
+
+    echo ${sha1_oid} > ${name}_sha1_expected
+    echo ${sha256_oid} > ${name}_sha256_expected
+    echo ${type} > ${name}_type_expected
+
+    git --git-dir=repo-sha1/.git rev-parse --output-object-format=sha256 ${sha1_oid} > ${name}_sha1_sha256_found
+    git --git-dir=repo-sha256/.git rev-parse --output-object-format=sha1 ${sha256_oid} > ${name}_sha256_sha1_found
+    local sha1_sha256_oid=$(cat ${name}_sha1_sha256_found)
+    local sha256_sha1_oid=$(cat ${name}_sha256_sha1_found)
+
+    test_expect_success $PREREQ "Verify ${type} ${name}'s sha1 oid" '
+	git --git-dir=repo-sha256/.git rev-parse --output-object-format=sha1 ${sha256_oid} > ${name}_sha1 &&
+	test_cmp ${name}_sha1 ${name}_sha1_expected
+'
+
+    test_expect_success $PREREQ "Verify ${type} ${name}'s sha256 oid" '
+	git --git-dir=repo-sha1/.git rev-parse --output-object-format=sha256 ${sha1_oid} > ${name}_sha256 &&
+	test_cmp ${name}_sha256 ${name}_sha256_expected
+'
+
+    test_expect_success $PREREQ "Verify ${name}'s sha1 type" '
+	git --git-dir=repo-sha1/.git cat-file -t ${sha1_oid} > ${name}_type1 &&
+	git --git-dir=repo-sha256/.git cat-file -t ${sha256_sha1_oid} > ${name}_type2 &&
+	test_cmp ${name}_type1 ${name}_type2 &&
+	test_cmp ${name}_type1 ${name}_type_expected
+'
+
+    test_expect_success $PREREQ "Verify ${name}'s sha256 type" '
+	git --git-dir=repo-sha256/.git cat-file -t ${sha256_oid} > ${name}_type3 &&
+	git --git-dir=repo-sha1/.git cat-file -t ${sha1_sha256_oid} > ${name}_type4 &&
+	test_cmp ${name}_type3 ${name}_type4 &&
+	test_cmp ${name}_type3 ${name}_type_expected
+'
+
+    test_expect_success $PREREQ "Verify ${name}'s sha1 size" '
+	git --git-dir=repo-sha1/.git cat-file -s ${sha1_oid} > ${name}_size1 &&
+	git --git-dir=repo-sha256/.git cat-file -s ${sha256_sha1_oid} > ${name}_size2 &&
+	test_cmp ${name}_size1 ${name}_size2
+'
+
+    test_expect_success $PREREQ "Verify ${name}'s sha256 size" '
+	git --git-dir=repo-sha256/.git cat-file -s ${sha256_oid} > ${name}_size3 &&
+	git --git-dir=repo-sha1/.git cat-file -s ${sha1_sha256_oid} > ${name}_size4 &&
+	test_cmp ${name}_size3 ${name}_size4
+'
+
+    test_expect_success $PREREQ "Verify ${name}'s sha1 pretty content" '
+	git --git-dir=repo-sha1/.git cat-file -p ${sha1_oid} > ${name}_content1 &&
+	git --git-dir=repo-sha256/.git cat-file -p ${sha256_sha1_oid} > ${name}_content2 &&
+	test_cmp ${name}_content1 ${name}_content2
+'
+
+    test_expect_success $PREREQ "Verify ${name}'s sha256 pretty content" '
+	git --git-dir=repo-sha256/.git cat-file -p ${sha256_oid} > ${name}_content3 &&
+	git --git-dir=repo-sha1/.git cat-file -p ${sha1_sha256_oid} > ${name}_content4 &&
+	test_cmp ${name}_content3 ${name}_content4
+'
+
+    test_expect_success $PREREQ "Verify ${name}'s sha1 content" '
+	git --git-dir=repo-sha1/.git cat-file ${type} ${sha1_oid} > ${name}_content5 &&
+	git --git-dir=repo-sha256/.git cat-file ${type} ${sha256_sha1_oid} > ${name}_content6 &&
+	test_cmp ${name}_content5 ${name}_content6
+'
+
+    test_expect_success $PREREQ "Verify ${name}'s sha256 content" '
+	git --git-dir=repo-sha256/.git cat-file ${type} ${sha256_oid} > ${name}_content7 &&
+	git --git-dir=repo-sha1/.git cat-file ${type} ${sha1_sha256_oid} > ${name}_content8 &&
+	test_cmp ${name}_content7 ${name}_content8
+'
+
+}
+
+compare_oids 'blob' hello "$hello_sha1_oid" "$hello_sha256_oid"
+compare_oids 'tree' tree "$tree_sha1_oid" "$tree_sha256_oid"
+compare_oids 'commit' commit "$commit_sha1_oid" "$commit_sha256_oid"
+compare_oids GPG2 'commit' signedcommit "$signedcommit_sha1_oid" "$signedcommit_sha256_oid"
+compare_oids 'tag' hellotag "$hellotag_sha1_oid" "$hellotag_sha256_oid"
+compare_oids 'tag' treetag "$treetag_sha1_oid" "$treetag_sha256_oid"
+compare_oids 'tag' committag "$committag_sha1_oid" "$committag_sha256_oid"
+compare_oids GPG2 'tag' signedtag "$signedtag_sha1_oid" "$signedtag_sha256_oid"
+
+compare_oids 'blob' more "$more_sha1_oid" "$more_sha256_oid"
+compare_oids 'blob' another "$another_sha1_oid" "$another_sha256_oid"
+compare_oids 'tree' tree2 "$tree2_sha1_oid" "$tree2_sha256_oid"
+compare_oids 'commit' commit2 "$commit2_sha1_oid" "$commit2_sha256_oid"
+compare_oids GPG2 'tag' signedtag2 "$signedtag2_sha1_oid" "$signedtag2_sha256_oid"
+compare_oids GPG2 'commit' signedcommit2 "$signedcommit2_sha1_oid" "$signedcommit2_sha256_oid"
+compare_oids GPG2 'commit' signedcommit3 "$signedcommit3_sha1_oid" "$signedcommit3_sha256_oid"
+compare_oids GPG2 'commit' signedcommit4 "$signedcommit4_sha1_oid" "$signedcommit4_sha256_oid"
+compare_oids GPG2 'tag' signedtag3 "$signedtag3_sha1_oid" "$signedtag3_sha256_oid"
+compare_oids GPG2 'tag' signedtag4 "$signedtag4_sha1_oid" "$signedtag4_sha256_oid"
+
+test_done
diff --git a/t/t1016/gpg b/t/t1016/gpg
new file mode 100755
index 0000000000..2601cb18a5
--- /dev/null
+++ b/t/t1016/gpg
@@ -0,0 +1,2 @@
+#!/bin/sh
+exec gpg --faked-system-time "20230918T154812" "$@"
diff --git a/t/test-lib-functions.sh b/t/test-lib-functions.sh
index 6eaf116346..2eccf100c0 100644
--- a/t/test-lib-functions.sh
+++ b/t/test-lib-functions.sh
@@ -1655,7 +1655,16 @@ test_set_hash () {
 
 # Detect the hash algorithm in use.
 test_detect_hash () {
-	test_hash_algo="${GIT_TEST_DEFAULT_HASH:-sha1}"
+	case "$GIT_TEST_DEFAULT_HASH" in
+	"sha256")
+	    test_hash_algo=sha256
+	    test_compat_hash_algo=sha1
+	    ;;
+	*)
+	    test_hash_algo=sha1
+	    test_compat_hash_algo=sha256
+	    ;;
+	esac
 }
 
 # Detect the hash algorithm in use.
@@ -1712,6 +1721,12 @@ test_oid () {
 	local algo="${test_hash_algo}" &&
 
 	case "$1" in
+	--hash=storage)
+		algo="$test_hash_algo" &&
+		shift;;
+	--hash=compat)
+		algo="$test_compat_hash_algo" &&
+		shift;;
 	--hash=*)
 		algo="${1#--hash=}" &&
 		shift;;
diff --git a/tree-walk.c b/tree-walk.c
index 690fa6569b..6565d9ad99 100644
--- a/tree-walk.c
+++ b/tree-walk.c
@@ -11,35 +11,19 @@
 #include "json-writer.h"
 #include "environment.h"
 
-static const char *get_mode(const char *str, unsigned int *modep)
-{
-	unsigned char c;
-	unsigned int mode = 0;
-
-	if (*str == ' ')
-		return NULL;
-
-	while ((c = *str++) != ' ') {
-		if (c < '0' || c > '7')
-			return NULL;
-		mode = (mode << 3) + (c - '0');
-	}
-	*modep = mode;
-	return str;
-}
-
 static int decode_tree_entry(struct tree_desc *desc, const char *buf, unsigned long size, struct strbuf *err)
 {
 	const char *path;
-	unsigned int mode, len;
-	const unsigned hashsz = the_hash_algo->rawsz;
+	unsigned int len;
+	uint16_t mode;
+	const unsigned hashsz = desc->algo->rawsz;
 
 	if (size < hashsz + 3 || buf[size - (hashsz + 1)]) {
 		strbuf_addstr(err, _("too-short tree object"));
 		return -1;
 	}
 
-	path = get_mode(buf, &mode);
+	path = parse_mode(buf, &mode);
 	if (!path) {
 		strbuf_addstr(err, _("malformed mode in tree entry"));
 		return -1;
@@ -54,15 +38,19 @@ static int decode_tree_entry(struct tree_desc *desc, const char *buf, unsigned l
 	desc->entry.path = path;
 	desc->entry.mode = (desc->flags & TREE_DESC_RAW_MODES) ? mode : canon_mode(mode);
 	desc->entry.pathlen = len - 1;
-	oidread(&desc->entry.oid, (const unsigned char *)path + len);
+	oidread_algop(&desc->entry.oid, (const unsigned char *)path + len,
+		      desc->algo);
 
 	return 0;
 }
 
-static int init_tree_desc_internal(struct tree_desc *desc, const void *buffer,
-				   unsigned long size, struct strbuf *err,
+static int init_tree_desc_internal(struct tree_desc *desc,
+				   const struct object_id *oid,
+				   const void *buffer, unsigned long size,
+				   struct strbuf *err,
 				   enum tree_desc_flags flags)
 {
+	desc->algo = (oid && oid->algo) ? &hash_algos[oid->algo] : the_hash_algo;
 	desc->buffer = buffer;
 	desc->size = size;
 	desc->flags = flags;
@@ -71,19 +59,21 @@ static int init_tree_desc_internal(struct tree_desc *desc, const void *buffer,
 	return 0;
 }
 
-void init_tree_desc(struct tree_desc *desc, const void *buffer, unsigned long size)
+void init_tree_desc(struct tree_desc *desc, const struct object_id *tree_oid,
+		    const void *buffer, unsigned long size)
 {
 	struct strbuf err = STRBUF_INIT;
-	if (init_tree_desc_internal(desc, buffer, size, &err, 0))
+	if (init_tree_desc_internal(desc, tree_oid, buffer, size, &err, 0))
 		die("%s", err.buf);
 	strbuf_release(&err);
 }
 
-int init_tree_desc_gently(struct tree_desc *desc, const void *buffer, unsigned long size,
+int init_tree_desc_gently(struct tree_desc *desc, const struct object_id *oid,
+			  const void *buffer, unsigned long size,
 			  enum tree_desc_flags flags)
 {
 	struct strbuf err = STRBUF_INIT;
-	int result = init_tree_desc_internal(desc, buffer, size, &err, flags);
+	int result = init_tree_desc_internal(desc, oid, buffer, size, &err, flags);
 	if (result)
 		error("%s", err.buf);
 	strbuf_release(&err);
@@ -102,7 +92,7 @@ void *fill_tree_descriptor(struct repository *r,
 		if (!buf)
 			die(_("unable to read tree (%s)"), oid_to_hex(oid));
 	}
-	init_tree_desc(desc, buf, size);
+	init_tree_desc(desc, oid, buf, size);
 	return buf;
 }
 
@@ -119,7 +109,7 @@ static void entry_extract(struct tree_desc *t, struct name_entry *a)
 static int update_tree_entry_internal(struct tree_desc *desc, struct strbuf *err)
 {
 	const void *buf = desc->buffer;
-	const unsigned char *end = (const unsigned char *)desc->entry.path + desc->entry.pathlen + 1 + the_hash_algo->rawsz;
+	const unsigned char *end = (const unsigned char *)desc->entry.path + desc->entry.pathlen + 1 + desc->algo->rawsz;
 	unsigned long size = desc->size;
 	unsigned long len = end - (const unsigned char *)buf;
 
@@ -633,7 +623,7 @@ int get_tree_entry(struct repository *r,
 		retval = -1;
 	} else {
 		struct tree_desc t;
-		init_tree_desc(&t, tree, size);
+		init_tree_desc(&t, tree_oid, tree, size);
 		retval = find_tree_entry(r, &t, name, oid, mode);
 	}
 	free(tree);
@@ -676,7 +666,7 @@ enum get_oid_result get_tree_entry_follow_symlinks(struct repository *r,
 	struct tree_desc t;
 	int follows_remaining = GET_TREE_ENTRY_FOLLOW_SYMLINKS_MAX_LINKS;
 
-	init_tree_desc(&t, NULL, 0UL);
+	init_tree_desc(&t, NULL, NULL, 0UL);
 	strbuf_addstr(&namebuf, name);
 	oidcpy(&current_tree_oid, tree_oid);
 
@@ -712,7 +702,7 @@ enum get_oid_result get_tree_entry_follow_symlinks(struct repository *r,
 				goto done;
 
 			/* descend */
-			init_tree_desc(&t, tree, size);
+			init_tree_desc(&t, &current_tree_oid, tree, size);
 		}
 
 		/* Handle symlinks to e.g. a//b by removing leading slashes */
@@ -746,7 +736,7 @@ enum get_oid_result get_tree_entry_follow_symlinks(struct repository *r,
 			free(parent->tree);
 			parents_nr--;
 			parent = &parents[parents_nr - 1];
-			init_tree_desc(&t, parent->tree, parent->size);
+			init_tree_desc(&t, &parent->oid, parent->tree, parent->size);
 			strbuf_remove(&namebuf, 0, remainder ? 3 : 2);
 			continue;
 		}
@@ -826,7 +816,7 @@ enum get_oid_result get_tree_entry_follow_symlinks(struct repository *r,
 			contents_start = contents;
 
 			parent = &parents[parents_nr - 1];
-			init_tree_desc(&t, parent->tree, parent->size);
+			init_tree_desc(&t, &parent->oid, parent->tree, parent->size);
 			strbuf_splice(&namebuf, 0, len,
 				      contents_start, link_len);
 			if (remainder)
diff --git a/tree-walk.h b/tree-walk.h
index a6bfa3da3a..0b1067fbc5 100644
--- a/tree-walk.h
+++ b/tree-walk.h
@@ -24,6 +24,7 @@ struct name_entry {
  * A semi-opaque data structure used to maintain the current state of the walk.
  */
 struct tree_desc {
+	const struct git_hash_algo *algo;
 	/*
 	 * pointer into the memory representation of the tree. It always
 	 * points at the current entry being visited.
@@ -83,9 +84,11 @@ int update_tree_entry_gently(struct tree_desc *);
  * size parameters are assumed to be the same as the buffer and size
  * members of `struct tree`.
  */
-void init_tree_desc(struct tree_desc *desc, const void *buf, unsigned long size);
+void init_tree_desc(struct tree_desc *desc, const struct object_id *tree_oid,
+		    const void *buf, unsigned long size);
 
-int init_tree_desc_gently(struct tree_desc *desc, const void *buf, unsigned long size,
+int init_tree_desc_gently(struct tree_desc *desc, const struct object_id *oid,
+			  const void *buf, unsigned long size,
 			  enum tree_desc_flags flags);
 
 /*
diff --git a/tree.c b/tree.c
index 508e5fd76f..7973d3f9a8 100644
--- a/tree.c
+++ b/tree.c
@@ -29,7 +29,7 @@ int read_tree_at(struct repository *r,
 	if (parse_tree(tree))
 		return -1;
 
-	init_tree_desc(&desc, tree->buffer, tree->size);
+	init_tree_desc(&desc, &tree->object.oid, tree->buffer, tree->size);
 
 	while (tree_entry(&desc, &entry)) {
 		if (retval != all_entries_interesting) {
diff --git a/walker.c b/walker.c
index 65002a7220..c0fd632d92 100644
--- a/walker.c
+++ b/walker.c
@@ -45,7 +45,7 @@ static int process_tree(struct walker *walker, struct tree *tree)
 	if (parse_tree(tree))
 		return -1;
 
-	init_tree_desc(&desc, tree->buffer, tree->size);
+	init_tree_desc(&desc, &tree->object.oid, tree->buffer, tree->size);
 	while (tree_entry(&desc, &entry)) {
 		struct object *obj = NULL;