diff --git a/builtin/am.c b/builtin/am.c
index 02853b3e05b..3d98e520857 100644
--- a/builtin/am.c
+++ b/builtin/am.c
@@ -1433,7 +1433,7 @@ static void write_index_patch(const struct am_state *state)
 	if (!get_oid_tree("HEAD", &head))
 		tree = lookup_tree(&head);
 	else
-		tree = lookup_tree(&empty_tree_oid);
+		tree = lookup_tree(the_hash_algo->empty_tree);
 
 	fp = xfopen(am_path(state, "patch"), "w");
 	init_revisions(&rev_info, NULL);
diff --git a/builtin/checkout.c b/builtin/checkout.c
index 3faae382de4..e1e157d205a 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -514,7 +514,7 @@ static int merge_working_tree(const struct checkout_opts *opts,
 		}
 		tree = parse_tree_indirect(old->commit ?
 					   &old->commit->object.oid :
-					   &empty_tree_oid);
+					   the_hash_algo->empty_tree);
 		init_tree_desc(&trees[0], tree->buffer, tree->size);
 		tree = parse_tree_indirect(&new->commit->object.oid);
 		init_tree_desc(&trees[1], tree->buffer, tree->size);
diff --git a/builtin/diff.c b/builtin/diff.c
index 9808d062a80..16bfb22f738 100644
--- a/builtin/diff.c
+++ b/builtin/diff.c
@@ -379,7 +379,7 @@ int cmd_diff(int argc, const char **argv, const char *prefix)
 				add_head_to_pending(&rev);
 				if (!rev.pending.nr) {
 					struct tree *tree;
-					tree = lookup_tree(&empty_tree_oid);
+					tree = lookup_tree(the_hash_algo->empty_tree);
 					add_pending_object(&rev, &tree->object, "HEAD");
 				}
 				break;
diff --git a/builtin/pull.c b/builtin/pull.c
index 166b777ed69..511dbbe0f6e 100644
--- a/builtin/pull.c
+++ b/builtin/pull.c
@@ -557,7 +557,7 @@ static int pull_into_void(const struct object_id *merge_head,
 	 * index/worktree changes that the user already made on the unborn
 	 * branch.
 	 */
-	if (checkout_fast_forward(&empty_tree_oid, merge_head, 0))
+	if (checkout_fast_forward(the_hash_algo->empty_tree, merge_head, 0))
 		return 1;
 
 	if (update_ref("initial pull", "HEAD", merge_head, curr_head, 0, UPDATE_REFS_DIE_ON_ERR))
diff --git a/cache.h b/cache.h
index cb5db7bf830..d3e240228c9 100644
--- a/cache.h
+++ b/cache.h
@@ -14,6 +14,7 @@
 #include "hash.h"
 #include "path.h"
 #include "sha1-array.h"
+#include "repository.h"
 
 #ifndef platform_SHA_CTX
 /*
@@ -77,6 +78,8 @@ struct object_id {
 	unsigned char hash[GIT_MAX_RAWSZ];
 };
 
+#define the_hash_algo the_repository->hash_algo
+
 #if defined(DT_UNKNOWN) && !defined(NO_D_TYPE_IN_DIRENT)
 #define DTYPE(de)	((de)->d_type)
 #else
@@ -907,6 +910,7 @@ struct repository_format {
 	int version;
 	int precious_objects;
 	int is_bare;
+	int hash_algo;
 	char *work_tree;
 	struct string_list unknown_extensions;
 };
@@ -1039,22 +1043,22 @@ extern const struct object_id empty_blob_oid;
 
 static inline int is_empty_blob_sha1(const unsigned char *sha1)
 {
-	return !hashcmp(sha1, EMPTY_BLOB_SHA1_BIN);
+	return !hashcmp(sha1, the_hash_algo->empty_blob->hash);
 }
 
 static inline int is_empty_blob_oid(const struct object_id *oid)
 {
-	return !hashcmp(oid->hash, EMPTY_BLOB_SHA1_BIN);
+	return !oidcmp(oid, the_hash_algo->empty_blob);
 }
 
 static inline int is_empty_tree_sha1(const unsigned char *sha1)
 {
-	return !hashcmp(sha1, EMPTY_TREE_SHA1_BIN);
+	return !hashcmp(sha1, the_hash_algo->empty_tree->hash);
 }
 
 static inline int is_empty_tree_oid(const struct object_id *oid)
 {
-	return !hashcmp(oid->hash, EMPTY_TREE_SHA1_BIN);
+	return !oidcmp(oid, the_hash_algo->empty_tree);
 }
 
 /* set default permissions by passing mode arguments to open(2) */
diff --git a/diff-lib.c b/diff-lib.c
index 5173023cd3c..8104603a3b3 100644
--- a/diff-lib.c
+++ b/diff-lib.c
@@ -218,7 +218,7 @@ int run_diff_files(struct rev_info *revs, unsigned int option)
 			} else if (revs->diffopt.ita_invisible_in_index &&
 				   ce_intent_to_add(ce)) {
 				diff_addremove(&revs->diffopt, '+', ce->ce_mode,
-					       &empty_tree_oid, 0,
+					       the_hash_algo->empty_tree, 0,
 					       ce->name, 0);
 				continue;
 			}
diff --git a/hash.h b/hash.h
index 024d0d3d50b..7d7a864f5dd 100644
--- a/hash.h
+++ b/hash.h
@@ -1,6 +1,8 @@
 #ifndef HASH_H
 #define HASH_H
 
+#include "git-compat-util.h"
+
 #if defined(SHA1_PPC)
 #include "ppc/sha1.h"
 #elif defined(SHA1_APPLE)
@@ -13,4 +15,59 @@
 #include "block-sha1/sha1.h"
 #endif
 
+/*
+ * Note that these constants are suitable for indexing the hash_algos array and
+ * comparing against each other, but are otherwise arbitrary, so they should not
+ * be exposed to the user or serialized to disk.  To know whether a
+ * git_hash_algo struct points to some usable hash function, test the format_id
+ * field for being non-zero.  Use the name field for user-visible situations and
+ * the format_id field for fixed-length fields on disk.
+ */
+/* An unknown hash function. */
+#define GIT_HASH_UNKNOWN 0
+/* SHA-1 */
+#define GIT_HASH_SHA1 1
+/* Number of algorithms supported (including unknown). */
+#define GIT_HASH_NALGOS (GIT_HASH_SHA1 + 1)
+
+typedef void (*git_hash_init_fn)(void *ctx);
+typedef void (*git_hash_update_fn)(void *ctx, const void *in, size_t len);
+typedef void (*git_hash_final_fn)(unsigned char *hash, void *ctx);
+
+struct git_hash_algo {
+	/*
+	 * The name of the algorithm, as appears in the config file and in
+	 * messages.
+	 */
+	const char *name;
+
+	/* A four-byte version identifier, used in pack indices. */
+	uint32_t format_id;
+
+	/* The size of a hash context (e.g. git_SHA_CTX). */
+	size_t ctxsz;
+
+	/* The length of the hash in binary. */
+	size_t rawsz;
+
+	/* The length of the hash in hex characters. */
+	size_t hexsz;
+
+	/* The hash initialization function. */
+	git_hash_init_fn init_fn;
+
+	/* The hash update function. */
+	git_hash_update_fn update_fn;
+
+	/* The hash finalization function. */
+	git_hash_final_fn final_fn;
+
+	/* The OID of the empty tree. */
+	const struct object_id *empty_tree;
+
+	/* The OID of the empty blob. */
+	const struct object_id *empty_blob;
+};
+extern const struct git_hash_algo hash_algos[GIT_HASH_NALGOS];
+
 #endif
diff --git a/merge-recursive.c b/merge-recursive.c
index d00b274381e..a4c280dfc78 100644
--- a/merge-recursive.c
+++ b/merge-recursive.c
@@ -2082,7 +2082,7 @@ int merge_recursive(struct merge_options *o,
 		/* if there is no common ancestor, use an empty tree */
 		struct tree *tree;
 
-		tree = lookup_tree(&empty_tree_oid);
+		tree = lookup_tree(the_hash_algo->empty_tree);
 		merged_common_ancestors = make_virtual_commit(tree, "ancestor");
 	}
 
diff --git a/notes-merge.c b/notes-merge.c
index 4a83b0ebd5c..0f6573cb17c 100644
--- a/notes-merge.c
+++ b/notes-merge.c
@@ -595,7 +595,7 @@ int notes_merge(struct notes_merge_options *o,
 	bases = get_merge_bases(local, remote);
 	if (!bases) {
 		base_oid = &null_oid;
-		base_tree_oid = &empty_tree_oid;
+		base_tree_oid = the_hash_algo->empty_tree;
 		if (o->verbosity >= 4)
 			printf("No merge base found; doing history-less merge\n");
 	} else if (!bases->next) {
diff --git a/repository.c b/repository.c
index bb2fae5446b..998413b8bb8 100644
--- a/repository.c
+++ b/repository.c
@@ -5,7 +5,7 @@
 
 /* The main repository */
 static struct repository the_repo = {
-	NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, &the_index, 0, 0
+	NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, &the_index, NULL, 0, 0
 };
 struct repository *the_repository = &the_repo;
 
@@ -64,6 +64,11 @@ void repo_set_gitdir(struct repository *repo, const char *path)
 	free(old_gitdir);
 }
 
+void repo_set_hash_algo(struct repository *repo, int hash_algo)
+{
+	repo->hash_algo = &hash_algos[hash_algo];
+}
+
 /*
  * Attempt to resolve and set the provided 'gitdir' for repository 'repo'.
  * Return 0 upon success and a non-zero value upon failure.
@@ -136,6 +141,8 @@ int repo_init(struct repository *repo, const char *gitdir, const char *worktree)
 	if (read_and_verify_repository_format(&format, repo->commondir))
 		goto error;
 
+	repo_set_hash_algo(repo, format.hash_algo);
+
 	if (worktree)
 		repo_set_worktree(repo, worktree);
 
diff --git a/repository.h b/repository.h
index 7f5e24a0a24..0329e40c7f5 100644
--- a/repository.h
+++ b/repository.h
@@ -4,6 +4,7 @@
 struct config_set;
 struct index_state;
 struct submodule_cache;
+struct git_hash_algo;
 
 struct repository {
 	/* Environment */
@@ -67,6 +68,9 @@ struct repository {
 	 */
 	struct index_state *index;
 
+	/* Repository's current hash algorithm, as serialized on disk. */
+	const struct git_hash_algo *hash_algo;
+
 	/* Configurations */
 	/*
 	 * Bit used during initialization to indicate if repository state (like
@@ -86,6 +90,7 @@ extern struct repository *the_repository;
 
 extern void repo_set_gitdir(struct repository *repo, const char *path);
 extern void repo_set_worktree(struct repository *repo, const char *path);
+extern void repo_set_hash_algo(struct repository *repo, int algo);
 extern int repo_init(struct repository *repo, const char *gitdir, const char *worktree);
 extern int repo_submodule_init(struct repository *submodule,
 			       struct repository *superproject,
diff --git a/sequencer.c b/sequencer.c
index fa94ed652d2..e90bc316bb7 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -347,7 +347,7 @@ static int read_oneliner(struct strbuf *buf,
 
 static struct tree *empty_tree(void)
 {
-	return lookup_tree(&empty_tree_oid);
+	return lookup_tree(the_hash_algo->empty_tree);
 }
 
 static int error_dirty_index(struct replay_opts *opts)
@@ -706,7 +706,7 @@ static int is_original_commit_empty(struct commit *commit)
 				oid_to_hex(&parent->object.oid));
 		ptree_oid = &parent->tree->object.oid;
 	} else {
-		ptree_oid = &empty_tree_oid; /* commit is root */
+		ptree_oid = the_hash_algo->empty_tree; /* commit is root */
 	}
 
 	return !oidcmp(ptree_oid, &commit->tree->object.oid);
@@ -959,7 +959,7 @@ static int do_pick_commit(enum todo_command command, struct commit *commit,
 	} else {
 		unborn = get_oid("HEAD", &head);
 		if (unborn)
-			oidcpy(&head, &empty_tree_oid);
+			oidcpy(&head, the_hash_algo->empty_tree);
 		if (index_differs_from(unborn ? EMPTY_TREE_SHA1_HEX : "HEAD",
 				       NULL, 0))
 			return error_dirty_index(opts);
diff --git a/setup.c b/setup.c
index 94768512b79..50c6b2ab11f 100644
--- a/setup.c
+++ b/setup.c
@@ -434,16 +434,15 @@ static int check_repo_format(const char *var, const char *value, void *vdata)
 	return 0;
 }
 
-static int check_repository_format_gently(const char *gitdir, int *nongit_ok)
+static int check_repository_format_gently(const char *gitdir, struct repository_format *candidate, int *nongit_ok)
 {
 	struct strbuf sb = STRBUF_INIT;
 	struct strbuf err = STRBUF_INIT;
-	struct repository_format candidate;
 	int has_common;
 
 	has_common = get_common_dir(&sb, gitdir);
 	strbuf_addstr(&sb, "/config");
-	read_repository_format(&candidate, sb.buf);
+	read_repository_format(candidate, sb.buf);
 	strbuf_release(&sb);
 
 	/*
@@ -451,10 +450,10 @@ static int check_repository_format_gently(const char *gitdir, int *nongit_ok)
 	 * we treat a missing config as a silent "ok", even when nongit_ok
 	 * is unset.
 	 */
-	if (candidate.version < 0)
+	if (candidate->version < 0)
 		return 0;
 
-	if (verify_repository_format(&candidate, &err) < 0) {
+	if (verify_repository_format(candidate, &err) < 0) {
 		if (nongit_ok) {
 			warning("%s", err.buf);
 			strbuf_release(&err);
@@ -464,21 +463,21 @@ static int check_repository_format_gently(const char *gitdir, int *nongit_ok)
 		die("%s", err.buf);
 	}
 
-	repository_format_precious_objects = candidate.precious_objects;
-	string_list_clear(&candidate.unknown_extensions, 0);
+	repository_format_precious_objects = candidate->precious_objects;
+	string_list_clear(&candidate->unknown_extensions, 0);
 	if (!has_common) {
-		if (candidate.is_bare != -1) {
-			is_bare_repository_cfg = candidate.is_bare;
+		if (candidate->is_bare != -1) {
+			is_bare_repository_cfg = candidate->is_bare;
 			if (is_bare_repository_cfg == 1)
 				inside_work_tree = -1;
 		}
-		if (candidate.work_tree) {
+		if (candidate->work_tree) {
 			free(git_work_tree_cfg);
-			git_work_tree_cfg = candidate.work_tree;
+			git_work_tree_cfg = candidate->work_tree;
 			inside_work_tree = -1;
 		}
 	} else {
-		free(candidate.work_tree);
+		free(candidate->work_tree);
 	}
 
 	return 0;
@@ -489,6 +488,7 @@ int read_repository_format(struct repository_format *format, const char *path)
 	memset(format, 0, sizeof(*format));
 	format->version = -1;
 	format->is_bare = -1;
+	format->hash_algo = GIT_HASH_SHA1;
 	string_list_init(&format->unknown_extensions, 1);
 	git_config_from_file(check_repo_format, path, format);
 	return format->version;
@@ -625,6 +625,7 @@ const char *read_gitfile_gently(const char *path, int *return_error_code)
 
 static const char *setup_explicit_git_dir(const char *gitdirenv,
 					  struct strbuf *cwd,
+					  struct repository_format *repo_fmt,
 					  int *nongit_ok)
 {
 	const char *work_tree_env = getenv(GIT_WORK_TREE_ENVIRONMENT);
@@ -650,7 +651,7 @@ static const char *setup_explicit_git_dir(const char *gitdirenv,
 		die("Not a git repository: '%s'", gitdirenv);
 	}
 
-	if (check_repository_format_gently(gitdirenv, nongit_ok)) {
+	if (check_repository_format_gently(gitdirenv, repo_fmt, nongit_ok)) {
 		free(gitfile);
 		return NULL;
 	}
@@ -723,9 +724,10 @@ static const char *setup_explicit_git_dir(const char *gitdirenv,
 
 static const char *setup_discovered_git_dir(const char *gitdir,
 					    struct strbuf *cwd, int offset,
+					    struct repository_format *repo_fmt,
 					    int *nongit_ok)
 {
-	if (check_repository_format_gently(gitdir, nongit_ok))
+	if (check_repository_format_gently(gitdir, repo_fmt, nongit_ok))
 		return NULL;
 
 	/* --work-tree is set without --git-dir; use discovered one */
@@ -737,7 +739,7 @@ static const char *setup_discovered_git_dir(const char *gitdir,
 			gitdir = to_free = real_pathdup(gitdir, 1);
 		if (chdir(cwd->buf))
 			die_errno("Could not come back to cwd");
-		ret = setup_explicit_git_dir(gitdir, cwd, nongit_ok);
+		ret = setup_explicit_git_dir(gitdir, cwd, repo_fmt, nongit_ok);
 		free(to_free);
 		return ret;
 	}
@@ -769,11 +771,12 @@ static const char *setup_discovered_git_dir(const char *gitdir,
 
 /* #16.1, #17.1, #20.1, #21.1, #22.1 (see t1510) */
 static const char *setup_bare_git_dir(struct strbuf *cwd, int offset,
+				      struct repository_format *repo_fmt,
 				      int *nongit_ok)
 {
 	int root_len;
 
-	if (check_repository_format_gently(".", nongit_ok))
+	if (check_repository_format_gently(".", repo_fmt, nongit_ok))
 		return NULL;
 
 	setenv(GIT_IMPLICIT_WORK_TREE_ENVIRONMENT, "0", 1);
@@ -785,7 +788,7 @@ static const char *setup_bare_git_dir(struct strbuf *cwd, int offset,
 		gitdir = offset == cwd->len ? "." : xmemdupz(cwd->buf, offset);
 		if (chdir(cwd->buf))
 			die_errno("Could not come back to cwd");
-		return setup_explicit_git_dir(gitdir, cwd, nongit_ok);
+		return setup_explicit_git_dir(gitdir, cwd, repo_fmt, nongit_ok);
 	}
 
 	inside_git_dir = 1;
@@ -1026,6 +1029,7 @@ const char *setup_git_directory_gently(int *nongit_ok)
 	static struct strbuf cwd = STRBUF_INIT;
 	struct strbuf dir = STRBUF_INIT, gitdir = STRBUF_INIT;
 	const char *prefix;
+	struct repository_format repo_fmt;
 
 	/*
 	 * We may have read an incomplete configuration before
@@ -1053,18 +1057,18 @@ const char *setup_git_directory_gently(int *nongit_ok)
 		prefix = NULL;
 		break;
 	case GIT_DIR_EXPLICIT:
-		prefix = setup_explicit_git_dir(gitdir.buf, &cwd, nongit_ok);
+		prefix = setup_explicit_git_dir(gitdir.buf, &cwd, &repo_fmt, nongit_ok);
 		break;
 	case GIT_DIR_DISCOVERED:
 		if (dir.len < cwd.len && chdir(dir.buf))
 			die(_("Cannot change to '%s'"), dir.buf);
 		prefix = setup_discovered_git_dir(gitdir.buf, &cwd, dir.len,
-						  nongit_ok);
+						  &repo_fmt, nongit_ok);
 		break;
 	case GIT_DIR_BARE:
 		if (dir.len < cwd.len && chdir(dir.buf))
 			die(_("Cannot change to '%s'"), dir.buf);
-		prefix = setup_bare_git_dir(&cwd, dir.len, nongit_ok);
+		prefix = setup_bare_git_dir(&cwd, dir.len, &repo_fmt, nongit_ok);
 		break;
 	case GIT_DIR_HIT_CEILING:
 		prefix = setup_nongit(cwd.buf, nongit_ok);
@@ -1110,6 +1114,8 @@ const char *setup_git_directory_gently(int *nongit_ok)
 			repo_set_gitdir(the_repository, gitdir);
 			setup_git_env();
 		}
+		if (startup_info->have_repository)
+			repo_set_hash_algo(the_repository, repo_fmt.hash_algo);
 	}
 
 	strbuf_release(&dir);
@@ -1171,7 +1177,8 @@ int git_config_perm(const char *var, const char *value)
 
 void check_repository_format(void)
 {
-	check_repository_format_gently(get_git_dir(), NULL);
+	struct repository_format repo_fmt;
+	check_repository_format_gently(get_git_dir(), &repo_fmt, NULL);
 	startup_info->have_repository = 1;
 }
 
diff --git a/sha1_file.c b/sha1_file.c
index afe4b90f6ee..b44f5247caa 100644
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -39,6 +39,64 @@ const struct object_id empty_blob_oid = {
 	EMPTY_BLOB_SHA1_BIN_LITERAL
 };
 
+static void git_hash_sha1_init(void *ctx)
+{
+	git_SHA1_Init((git_SHA_CTX *)ctx);
+}
+
+static void git_hash_sha1_update(void *ctx, const void *data, size_t len)
+{
+	git_SHA1_Update((git_SHA_CTX *)ctx, data, len);
+}
+
+static void git_hash_sha1_final(unsigned char *hash, void *ctx)
+{
+	git_SHA1_Final(hash, (git_SHA_CTX *)ctx);
+}
+
+static void git_hash_unknown_init(void *ctx)
+{
+	die("trying to init unknown hash");
+}
+
+static void git_hash_unknown_update(void *ctx, const void *data, size_t len)
+{
+	die("trying to update unknown hash");
+}
+
+static void git_hash_unknown_final(unsigned char *hash, void *ctx)
+{
+	die("trying to finalize unknown hash");
+}
+
+const struct git_hash_algo hash_algos[GIT_HASH_NALGOS] = {
+	{
+		NULL,
+		0x00000000,
+		0,
+		0,
+		0,
+		git_hash_unknown_init,
+		git_hash_unknown_update,
+		git_hash_unknown_final,
+		NULL,
+		NULL,
+	},
+	{
+		"sha-1",
+		/* "sha1", big-endian */
+		0x73686131,
+		sizeof(git_SHA_CTX),
+		GIT_SHA1_RAWSZ,
+		GIT_SHA1_HEXSZ,
+		git_hash_sha1_init,
+		git_hash_sha1_update,
+		git_hash_sha1_final,
+		&empty_tree_oid,
+		&empty_blob_oid,
+	},
+};
+
 /*
  * This is meant to hold a *small* number of objects that you would
  * want read_sha1_file() to be able to return, but yet you do not want
diff --git a/submodule.c b/submodule.c
index 95e6aff2bb7..fa25888783a 100644
--- a/submodule.c
+++ b/submodule.c
@@ -587,7 +587,7 @@ void show_submodule_inline_diff(struct diff_options *o, const char *path,
 		struct object_id *one, struct object_id *two,
 		unsigned dirty_submodule)
 {
-	const struct object_id *old = &empty_tree_oid, *new = &empty_tree_oid;
+	const struct object_id *old = the_hash_algo->empty_tree, *new = the_hash_algo->empty_tree;
 	struct commit *left = NULL, *right = NULL;
 	struct commit_list *merge_bases = NULL;
 	struct child_process cp = CHILD_PROCESS_INIT;