diff --git a/archive-zip.c b/archive-zip.c
index ffb3535e93..ae3d67f9d3 100644
--- a/archive-zip.c
+++ b/archive-zip.c
@@ -448,12 +448,12 @@ static void write_zip_trailer(const unsigned char *sha1)
 	copy_le16(trailer.entries, zip_dir_entries);
 	copy_le32(trailer.size, zip_dir_offset);
 	copy_le32(trailer.offset, zip_offset);
-	copy_le16(trailer.comment_length, sha1 ? 40 : 0);
+	copy_le16(trailer.comment_length, sha1 ? GIT_SHA1_HEXSZ : 0);
 
 	write_or_die(1, zip_dir, zip_dir_offset);
 	write_or_die(1, &trailer, ZIP_DIR_TRAILER_SIZE);
 	if (sha1)
-		write_or_die(1, sha1_to_hex(sha1), 40);
+		write_or_die(1, sha1_to_hex(sha1), GIT_SHA1_HEXSZ);
 }
 
 static void dos_time(time_t *time, int *dos_date, int *dos_time)
diff --git a/archive.c b/archive.c
index 96057ed830..d37c41daf2 100644
--- a/archive.c
+++ b/archive.c
@@ -101,7 +101,7 @@ static void setup_archive_check(struct git_attr_check *check)
 
 struct directory {
 	struct directory *up;
-	unsigned char sha1[20];
+	struct object_id oid;
 	int baselen, len;
 	unsigned mode;
 	int stage;
@@ -177,7 +177,7 @@ static void queue_directory(const unsigned char *sha1,
 	d->stage   = stage;
 	c->bottom  = d;
 	d->len = sprintf(d->path, "%.*s%s/", (int)base->len, base->buf, filename);
-	hashcpy(d->sha1, sha1);
+	hashcpy(d->oid.hash, sha1);
 }
 
 static int write_directory(struct archiver_context *c)
@@ -191,7 +191,7 @@ static int write_directory(struct archiver_context *c)
 	d->path[d->len - 1] = '\0'; /* no trailing slash */
 	ret =
 		write_directory(c) ||
-		write_archive_entry(d->sha1, d->path, d->baselen,
+		write_archive_entry(d->oid.hash, d->path, d->baselen,
 				    d->path + d->baselen, d->mode,
 				    d->stage, c) != READ_TREE_RECURSIVE;
 	free(d);
@@ -354,7 +354,7 @@ static void parse_treeish_arg(const char **argv,
 	time_t archive_time;
 	struct tree *tree;
 	const struct commit *commit;
-	unsigned char sha1[20];
+	struct object_id oid;
 
 	/* Remotes are only allowed to fetch actual refs */
 	if (remote && !remote_allow_unreachable) {
@@ -362,15 +362,15 @@ static void parse_treeish_arg(const char **argv,
 		const char *colon = strchrnul(name, ':');
 		int refnamelen = colon - name;
 
-		if (!dwim_ref(name, refnamelen, sha1, &ref))
+		if (!dwim_ref(name, refnamelen, oid.hash, &ref))
 			die("no such ref: %.*s", refnamelen, name);
 		free(ref);
 	}
 
-	if (get_sha1(name, sha1))
+	if (get_sha1(name, oid.hash))
 		die("Not a valid object name");
 
-	commit = lookup_commit_reference_gently(sha1, 1);
+	commit = lookup_commit_reference_gently(oid.hash, 1);
 	if (commit) {
 		commit_sha1 = commit->object.sha1;
 		archive_time = commit->date;
@@ -379,21 +379,21 @@ static void parse_treeish_arg(const char **argv,
 		archive_time = time(NULL);
 	}
 
-	tree = parse_tree_indirect(sha1);
+	tree = parse_tree_indirect(oid.hash);
 	if (tree == NULL)
 		die("not a tree object");
 
 	if (prefix) {
-		unsigned char tree_sha1[20];
+		struct object_id tree_oid;
 		unsigned int mode;
 		int err;
 
 		err = get_tree_entry(tree->object.sha1, prefix,
-				     tree_sha1, &mode);
+				     tree_oid.hash, &mode);
 		if (err || !S_ISDIR(mode))
 			die("current working directory is untracked");
 
-		tree = parse_tree_indirect(tree_sha1);
+		tree = parse_tree_indirect(tree_oid.hash);
 	}
 	ar_args->tree = tree;
 	ar_args->commit_sha1 = commit_sha1;
diff --git a/bisect.c b/bisect.c
index 8c6d843699..10f5e57ef3 100644
--- a/bisect.c
+++ b/bisect.c
@@ -15,7 +15,7 @@
 static struct sha1_array good_revs;
 static struct sha1_array skipped_revs;
 
-static unsigned char *current_bad_sha1;
+static struct object_id *current_bad_oid;
 
 static const char *argv_checkout[] = {"checkout", "-q", NULL, "--", NULL};
 static const char *argv_show_branch[] = {"show-branch", NULL, NULL};
@@ -404,8 +404,8 @@ static int register_ref(const char *refname, const unsigned char *sha1,
 			int flags, void *cb_data)
 {
 	if (!strcmp(refname, "bad")) {
-		current_bad_sha1 = xmalloc(20);
-		hashcpy(current_bad_sha1, sha1);
+		current_bad_oid = xmalloc(sizeof(*current_bad_oid));
+		hashcpy(current_bad_oid->hash, sha1);
 	} else if (starts_with(refname, "good-")) {
 		sha1_array_append(&good_revs, sha1);
 	} else if (starts_with(refname, "skip-")) {
@@ -564,7 +564,7 @@ static struct commit_list *skip_away(struct commit_list *list, int count)
 
 	for (i = 0; cur; cur = cur->next, i++) {
 		if (i == index) {
-			if (hashcmp(cur->item->object.sha1, current_bad_sha1))
+			if (hashcmp(cur->item->object.sha1, current_bad_oid->hash))
 				return cur;
 			if (previous)
 				return previous;
@@ -607,7 +607,7 @@ static void bisect_rev_setup(struct rev_info *revs, const char *prefix,
 
 	/* rev_argv.argv[0] will be ignored by setup_revisions */
 	argv_array_push(&rev_argv, "bisect_rev_setup");
-	argv_array_pushf(&rev_argv, bad_format, sha1_to_hex(current_bad_sha1));
+	argv_array_pushf(&rev_argv, bad_format, oid_to_hex(current_bad_oid));
 	for (i = 0; i < good_revs.nr; i++)
 		argv_array_pushf(&rev_argv, good_format,
 				 sha1_to_hex(good_revs.sha1[i]));
@@ -628,7 +628,7 @@ static void bisect_common(struct rev_info *revs)
 }
 
 static void exit_if_skipped_commits(struct commit_list *tried,
-				    const unsigned char *bad)
+				    const struct object_id *bad)
 {
 	if (!tried)
 		return;
@@ -637,12 +637,12 @@ static void exit_if_skipped_commits(struct commit_list *tried,
 	       "The first bad commit could be any of:\n");
 	print_commit_list(tried, "%s\n", "%s\n");
 	if (bad)
-		printf("%s\n", sha1_to_hex(bad));
+		printf("%s\n", oid_to_hex(bad));
 	printf("We cannot bisect more!\n");
 	exit(2);
 }
 
-static int is_expected_rev(const unsigned char *sha1)
+static int is_expected_rev(const struct object_id *oid)
 {
 	const char *filename = git_path("BISECT_EXPECTED_REV");
 	struct stat st;
@@ -658,7 +658,7 @@ static int is_expected_rev(const unsigned char *sha1)
 		return 0;
 
 	if (strbuf_getline(&str, fp, '\n') != EOF)
-		res = !strcmp(str.buf, sha1_to_hex(sha1));
+		res = !strcmp(str.buf, oid_to_hex(oid));
 
 	strbuf_release(&str);
 	fclose(fp);
@@ -719,7 +719,7 @@ static struct commit **get_bad_and_good_commits(int *rev_nr)
 	struct commit **rev = xmalloc(len * sizeof(*rev));
 	int i, n = 0;
 
-	rev[n++] = get_commit_reference(current_bad_sha1);
+	rev[n++] = get_commit_reference(current_bad_oid->hash);
 	for (i = 0; i < good_revs.nr; i++)
 		rev[n++] = get_commit_reference(good_revs.sha1[i]);
 	*rev_nr = n;
@@ -729,8 +729,8 @@ static struct commit **get_bad_and_good_commits(int *rev_nr)
 
 static void handle_bad_merge_base(void)
 {
-	if (is_expected_rev(current_bad_sha1)) {
-		char *bad_hex = sha1_to_hex(current_bad_sha1);
+	if (is_expected_rev(current_bad_oid)) {
+		char *bad_hex = oid_to_hex(current_bad_oid);
 		char *good_hex = join_sha1_array_hex(&good_revs, ' ');
 
 		fprintf(stderr, "The merge base %s is bad.\n"
@@ -750,7 +750,7 @@ static void handle_bad_merge_base(void)
 static void handle_skipped_merge_base(const unsigned char *mb)
 {
 	char *mb_hex = sha1_to_hex(mb);
-	char *bad_hex = sha1_to_hex(current_bad_sha1);
+	char *bad_hex = sha1_to_hex(current_bad_oid->hash);
 	char *good_hex = join_sha1_array_hex(&good_revs, ' ');
 
 	warning("the merge base between %s and [%s] "
@@ -781,7 +781,7 @@ static void check_merge_bases(int no_checkout)
 
 	for (; result; result = result->next) {
 		const unsigned char *mb = result->item->object.sha1;
-		if (!hashcmp(mb, current_bad_sha1)) {
+		if (!hashcmp(mb, current_bad_oid->hash)) {
 			handle_bad_merge_base();
 		} else if (0 <= sha1_array_lookup(&good_revs, mb)) {
 			continue;
@@ -838,7 +838,7 @@ static void check_good_are_ancestors_of_bad(const char *prefix, int no_checkout)
 	struct stat st;
 	int fd;
 
-	if (!current_bad_sha1)
+	if (!current_bad_oid)
 		die("a bad revision is needed");
 
 	/* Check if file BISECT_ANCESTORS_OK exists. */
@@ -903,7 +903,7 @@ int bisect_next_all(const char *prefix, int no_checkout)
 	struct commit_list *tried;
 	int reaches = 0, all = 0, nr, steps;
 	const unsigned char *bisect_rev;
-	char bisect_rev_hex[41];
+	char bisect_rev_hex[GIT_SHA1_HEXSZ + 1];
 
 	if (read_bisect_refs())
 		die("reading bisect refs failed");
@@ -927,7 +927,7 @@ int bisect_next_all(const char *prefix, int no_checkout)
 		exit_if_skipped_commits(tried, NULL);
 
 		printf("%s was both good and bad\n",
-		       sha1_to_hex(current_bad_sha1));
+		       oid_to_hex(current_bad_oid));
 		exit(1);
 	}
 
@@ -938,10 +938,10 @@ int bisect_next_all(const char *prefix, int no_checkout)
 	}
 
 	bisect_rev = revs.commits->item->object.sha1;
-	memcpy(bisect_rev_hex, sha1_to_hex(bisect_rev), 41);
+	memcpy(bisect_rev_hex, sha1_to_hex(bisect_rev), GIT_SHA1_HEXSZ + 1);
 
-	if (!hashcmp(bisect_rev, current_bad_sha1)) {
-		exit_if_skipped_commits(tried, current_bad_sha1);
+	if (!hashcmp(bisect_rev, current_bad_oid->hash)) {
+		exit_if_skipped_commits(tried, current_bad_oid);
 		printf("%s is the first bad commit\n", bisect_rev_hex);
 		show_diff_tree(prefix, revs.commits->item);
 		/* This means the bisection process succeeded. */
diff --git a/builtin/apply.c b/builtin/apply.c
index 0769b09287..146be97a1a 100644
--- a/builtin/apply.c
+++ b/builtin/apply.c
@@ -208,7 +208,7 @@ struct patch {
 	struct patch *next;
 
 	/* three-way fallback result */
-	unsigned char threeway_stage[3][20];
+	struct object_id threeway_stage[3];
 };
 
 static void free_fragment_list(struct fragment *list)
@@ -3426,11 +3426,11 @@ static int try_threeway(struct image *image, struct patch *patch,
 	if (status) {
 		patch->conflicted_threeway = 1;
 		if (patch->is_new)
-			hashclr(patch->threeway_stage[0]);
+			oidclr(&patch->threeway_stage[0]);
 		else
-			hashcpy(patch->threeway_stage[0], pre_sha1);
-		hashcpy(patch->threeway_stage[1], our_sha1);
-		hashcpy(patch->threeway_stage[2], post_sha1);
+			hashcpy(patch->threeway_stage[0].hash, pre_sha1);
+		hashcpy(patch->threeway_stage[1].hash, our_sha1);
+		hashcpy(patch->threeway_stage[2].hash, post_sha1);
 		fprintf(stderr, "Applied patch to '%s' with conflicts.\n", patch->new_name);
 	} else {
 		fprintf(stderr, "Applied patch to '%s' cleanly.\n", patch->new_name);
@@ -4186,14 +4186,14 @@ static void add_conflicted_stages_file(struct patch *patch)
 
 	remove_file_from_cache(patch->new_name);
 	for (stage = 1; stage < 4; stage++) {
-		if (is_null_sha1(patch->threeway_stage[stage - 1]))
+		if (is_null_oid(&patch->threeway_stage[stage - 1]))
 			continue;
 		ce = xcalloc(1, ce_size);
 		memcpy(ce->name, patch->new_name, namelen);
 		ce->ce_mode = create_ce_mode(mode);
 		ce->ce_flags = create_ce_flags(stage);
 		ce->ce_namelen = namelen;
-		hashcpy(ce->sha1, patch->threeway_stage[stage - 1]);
+		hashcpy(ce->sha1, patch->threeway_stage[stage - 1].hash);
 		if (add_cache_entry(ce, ADD_CACHE_OK_TO_ADD) < 0)
 			die(_("unable to add cache entry for %s"), patch->new_name);
 	}
diff --git a/builtin/patch-id.c b/builtin/patch-id.c
index 77db8739b5..ba34dac4d2 100644
--- a/builtin/patch-id.c
+++ b/builtin/patch-id.c
@@ -1,14 +1,14 @@
 #include "builtin.h"
 
-static void flush_current_id(int patchlen, unsigned char *id, unsigned char *result)
+static void flush_current_id(int patchlen, struct object_id *id, struct object_id *result)
 {
 	char name[50];
 
 	if (!patchlen)
 		return;
 
-	memcpy(name, sha1_to_hex(id), 41);
-	printf("%s %s\n", sha1_to_hex(result), name);
+	memcpy(name, oid_to_hex(id), GIT_SHA1_HEXSZ + 1);
+	printf("%s %s\n", oid_to_hex(result), name);
 }
 
 static int remove_space(char *line)
@@ -53,23 +53,23 @@ static int scan_hunk_header(const char *p, int *p_before, int *p_after)
 	return 1;
 }
 
-static void flush_one_hunk(unsigned char *result, git_SHA_CTX *ctx)
+static void flush_one_hunk(struct object_id *result, git_SHA_CTX *ctx)
 {
-	unsigned char hash[20];
+	unsigned char hash[GIT_SHA1_RAWSZ];
 	unsigned short carry = 0;
 	int i;
 
 	git_SHA1_Final(hash, ctx);
 	git_SHA1_Init(ctx);
 	/* 20-byte sum, with carry */
-	for (i = 0; i < 20; ++i) {
-		carry += result[i] + hash[i];
-		result[i] = carry;
+	for (i = 0; i < GIT_SHA1_RAWSZ; ++i) {
+		carry += result->hash[i] + hash[i];
+		result->hash[i] = carry;
 		carry >>= 8;
 	}
 }
 
-static int get_one_patchid(unsigned char *next_sha1, unsigned char *result,
+static int get_one_patchid(struct object_id *next_oid, struct object_id *result,
 			   struct strbuf *line_buf, int stable)
 {
 	int patchlen = 0, found_next = 0;
@@ -77,7 +77,7 @@ static int get_one_patchid(unsigned char *next_sha1, unsigned char *result,
 	git_SHA_CTX ctx;
 
 	git_SHA1_Init(&ctx);
-	hashclr(result);
+	oidclr(result);
 
 	while (strbuf_getwholeline(line_buf, stdin, '\n') != EOF) {
 		char *line = line_buf->buf;
@@ -93,7 +93,7 @@ static int get_one_patchid(unsigned char *next_sha1, unsigned char *result,
 		else if (!memcmp(line, "\\ ", 2) && 12 < strlen(line))
 			continue;
 
-		if (!get_sha1_hex(p, next_sha1)) {
+		if (!get_oid_hex(p, next_oid)) {
 			found_next = 1;
 			break;
 		}
@@ -143,7 +143,7 @@ static int get_one_patchid(unsigned char *next_sha1, unsigned char *result,
 	}
 
 	if (!found_next)
-		hashclr(next_sha1);
+		oidclr(next_oid);
 
 	flush_one_hunk(result, &ctx);
 
@@ -152,15 +152,15 @@ static int get_one_patchid(unsigned char *next_sha1, unsigned char *result,
 
 static void generate_id_list(int stable)
 {
-	unsigned char sha1[20], n[20], result[20];
+	struct object_id oid, n, result;
 	int patchlen;
 	struct strbuf line_buf = STRBUF_INIT;
 
-	hashclr(sha1);
+	oidclr(&oid);
 	while (!feof(stdin)) {
-		patchlen = get_one_patchid(n, result, &line_buf, stable);
-		flush_current_id(patchlen, sha1, result);
-		hashcpy(sha1, n);
+		patchlen = get_one_patchid(&n, &result, &line_buf, stable);
+		flush_current_id(patchlen, &oid, &result);
+		oidcpy(&oid, &n);
 	}
 	strbuf_release(&line_buf);
 }
diff --git a/bulk-checkin.c b/bulk-checkin.c
index 8d157eba45..7cffc3a579 100644
--- a/bulk-checkin.c
+++ b/bulk-checkin.c
@@ -24,7 +24,7 @@ static struct bulk_checkin_state {
 
 static void finish_bulk_checkin(struct bulk_checkin_state *state)
 {
-	unsigned char sha1[20];
+	struct object_id oid;
 	struct strbuf packname = STRBUF_INIT;
 	int i;
 
@@ -36,11 +36,11 @@ static void finish_bulk_checkin(struct bulk_checkin_state *state)
 		unlink(state->pack_tmp_name);
 		goto clear_exit;
 	} else if (state->nr_written == 1) {
-		sha1close(state->f, sha1, CSUM_FSYNC);
+		sha1close(state->f, oid.hash, CSUM_FSYNC);
 	} else {
-		int fd = sha1close(state->f, sha1, 0);
-		fixup_pack_header_footer(fd, sha1, state->pack_tmp_name,
-					 state->nr_written, sha1,
+		int fd = sha1close(state->f, oid.hash, 0);
+		fixup_pack_header_footer(fd, oid.hash, state->pack_tmp_name,
+					 state->nr_written, oid.hash,
 					 state->offset);
 		close(fd);
 	}
@@ -48,7 +48,7 @@ static void finish_bulk_checkin(struct bulk_checkin_state *state)
 	strbuf_addf(&packname, "%s/pack/pack-", get_object_directory());
 	finish_tmp_packfile(&packname, state->pack_tmp_name,
 			    state->written, state->nr_written,
-			    &state->pack_idx_opts, sha1);
+			    &state->pack_idx_opts, oid.hash);
 	for (i = 0; i < state->nr_written; i++)
 		free(state->written[i]);
 
diff --git a/cache.h b/cache.h
index 3d3244ba64..b320b1a94e 100644
--- a/cache.h
+++ b/cache.h
@@ -43,6 +43,14 @@ int git_deflate_end_gently(git_zstream *);
 int git_deflate(git_zstream *, int flush);
 unsigned long git_deflate_bound(git_zstream *, unsigned long);
 
+/* The length in bytes and in hex digits of an object name (SHA-1 value). */
+#define GIT_SHA1_RAWSZ 20
+#define GIT_SHA1_HEXSZ (2 * GIT_SHA1_RAWSZ)
+
+struct object_id {
+	unsigned char hash[GIT_SHA1_RAWSZ];
+};
+
 #if defined(DT_UNKNOWN) && !defined(NO_D_TYPE_IN_DIRENT)
 #define DTYPE(de)	((de)->d_type)
 #else
@@ -718,13 +726,13 @@ extern char *sha1_pack_name(const unsigned char *sha1);
 extern char *sha1_pack_index_name(const unsigned char *sha1);
 
 extern const char *find_unique_abbrev(const unsigned char *sha1, int);
-extern const unsigned char null_sha1[20];
+extern const unsigned char null_sha1[GIT_SHA1_RAWSZ];
 
 static inline int hashcmp(const unsigned char *sha1, const unsigned char *sha2)
 {
 	int i;
 
-	for (i = 0; i < 20; i++, sha1++, sha2++) {
+	for (i = 0; i < GIT_SHA1_RAWSZ; i++, sha1++, sha2++) {
 		if (*sha1 != *sha2)
 			return *sha1 - *sha2;
 	}
@@ -732,20 +740,42 @@ static inline int hashcmp(const unsigned char *sha1, const unsigned char *sha2)
 	return 0;
 }
 
+static inline int oidcmp(const struct object_id *oid1, const struct object_id *oid2)
+{
+	return hashcmp(oid1->hash, oid2->hash);
+}
+
 static inline int is_null_sha1(const unsigned char *sha1)
 {
 	return !hashcmp(sha1, null_sha1);
 }
 
+static inline int is_null_oid(const struct object_id *oid)
+{
+	return !hashcmp(oid->hash, null_sha1);
+}
+
 static inline void hashcpy(unsigned char *sha_dst, const unsigned char *sha_src)
 {
-	memcpy(sha_dst, sha_src, 20);
+	memcpy(sha_dst, sha_src, GIT_SHA1_RAWSZ);
 }
+
+static inline void oidcpy(struct object_id *dst, const struct object_id *src)
+{
+	hashcpy(dst->hash, src->hash);
+}
+
 static inline void hashclr(unsigned char *hash)
 {
-	memset(hash, 0, 20);
+	memset(hash, 0, GIT_SHA1_RAWSZ);
 }
 
+static inline void oidclr(struct object_id *oid)
+{
+	hashclr(oid->hash);
+}
+
+
 #define EMPTY_TREE_SHA1_HEX \
 	"4b825dc642cb6eb9a060e54bf8d69288fbee4904"
 #define EMPTY_TREE_SHA1_BIN_LITERAL \
@@ -952,8 +982,10 @@ extern int for_each_abbrev(const char *prefix, each_abbrev_fn, void *);
  * null-terminated string.
  */
 extern int get_sha1_hex(const char *hex, unsigned char *sha1);
+extern int get_oid_hex(const char *hex, struct object_id *sha1);
 
 extern char *sha1_to_hex(const unsigned char *sha1);	/* static buffer result! */
+extern char *oid_to_hex(const struct object_id *oid);	/* same static buffer as sha1_to_hex */
 extern int read_ref_full(const char *refname, int resolve_flags,
 			 unsigned char *sha1, int *flags);
 extern int read_ref(const char *refname, unsigned char *sha1);
diff --git a/combine-diff.c b/combine-diff.c
index 91edce58e1..8eb7278978 100644
--- a/combine-diff.c
+++ b/combine-diff.c
@@ -44,9 +44,9 @@ static struct combine_diff_path *intersect_paths(struct combine_diff_path *curr,
 			memset(p->parent, 0,
 			       sizeof(p->parent[0]) * num_parent);
 
-			hashcpy(p->sha1, q->queue[i]->two->sha1);
+			hashcpy(p->oid.hash, q->queue[i]->two->sha1);
 			p->mode = q->queue[i]->two->mode;
-			hashcpy(p->parent[n].sha1, q->queue[i]->one->sha1);
+			hashcpy(p->parent[n].oid.hash, q->queue[i]->one->sha1);
 			p->parent[n].mode = q->queue[i]->one->mode;
 			p->parent[n].status = q->queue[i]->status;
 			*tail = p;
@@ -77,7 +77,7 @@ static struct combine_diff_path *intersect_paths(struct combine_diff_path *curr,
 			continue;
 		}
 
-		hashcpy(p->parent[n].sha1, q->queue[i]->one->sha1);
+		hashcpy(p->parent[n].oid.hash, q->queue[i]->one->sha1);
 		p->parent[n].mode = q->queue[i]->one->mode;
 		p->parent[n].status = q->queue[i]->status;
 
@@ -284,7 +284,7 @@ static struct lline *coalesce_lines(struct lline *base, int *lenbase,
 	return base;
 }
 
-static char *grab_blob(const unsigned char *sha1, unsigned int mode,
+static char *grab_blob(const struct object_id *oid, unsigned int mode,
 		       unsigned long *size, struct userdiff_driver *textconv,
 		       const char *path)
 {
@@ -294,20 +294,20 @@ static char *grab_blob(const unsigned char *sha1, unsigned int mode,
 	if (S_ISGITLINK(mode)) {
 		blob = xmalloc(100);
 		*size = snprintf(blob, 100,
-				 "Subproject commit %s\n", sha1_to_hex(sha1));
-	} else if (is_null_sha1(sha1)) {
+				 "Subproject commit %s\n", oid_to_hex(oid));
+	} else if (is_null_oid(oid)) {
 		/* deleted blob */
 		*size = 0;
 		return xcalloc(1, 1);
 	} else if (textconv) {
 		struct diff_filespec *df = alloc_filespec(path);
-		fill_filespec(df, sha1, 1, mode);
+		fill_filespec(df, oid->hash, 1, mode);
 		*size = fill_textconv(textconv, df, &blob);
 		free_filespec(df);
 	} else {
-		blob = read_sha1_file(sha1, &type, size);
+		blob = read_sha1_file(oid->hash, &type, size);
 		if (type != OBJ_BLOB)
-			die("object '%s' is not a blob!", sha1_to_hex(sha1));
+			die("object '%s' is not a blob!", oid_to_hex(oid));
 	}
 	return blob;
 }
@@ -389,7 +389,7 @@ static void consume_line(void *state_, char *line, unsigned long len)
 	}
 }
 
-static void combine_diff(const unsigned char *parent, unsigned int mode,
+static void combine_diff(const struct object_id *parent, unsigned int mode,
 			 mmfile_t *result_file,
 			 struct sline *sline, unsigned int cnt, int n,
 			 int num_parent, int result_deleted,
@@ -897,7 +897,7 @@ static void show_combined_header(struct combine_diff_path *elem,
 				 int show_file_header)
 {
 	struct diff_options *opt = &rev->diffopt;
-	int abbrev = DIFF_OPT_TST(opt, FULL_INDEX) ? 40 : DEFAULT_ABBREV;
+	int abbrev = DIFF_OPT_TST(opt, FULL_INDEX) ? GIT_SHA1_HEXSZ : DEFAULT_ABBREV;
 	const char *a_prefix = opt->a_prefix ? opt->a_prefix : "a/";
 	const char *b_prefix = opt->b_prefix ? opt->b_prefix : "b/";
 	const char *c_meta = diff_get_color_opt(opt, DIFF_METAINFO);
@@ -914,11 +914,11 @@ static void show_combined_header(struct combine_diff_path *elem,
 			 "", elem->path, line_prefix, c_meta, c_reset);
 	printf("%s%sindex ", line_prefix, c_meta);
 	for (i = 0; i < num_parent; i++) {
-		abb = find_unique_abbrev(elem->parent[i].sha1,
+		abb = find_unique_abbrev(elem->parent[i].oid.hash,
 					 abbrev);
 		printf("%s%s", i ? "," : "", abb);
 	}
-	abb = find_unique_abbrev(elem->sha1, abbrev);
+	abb = find_unique_abbrev(elem->oid.hash, abbrev);
 	printf("..%s%s\n", abb, c_reset);
 
 	if (mode_differs) {
@@ -991,7 +991,7 @@ static void show_patch_diff(struct combine_diff_path *elem, int num_parent,
 
 	/* Read the result of merge first */
 	if (!working_tree_file)
-		result = grab_blob(elem->sha1, elem->mode, &result_size,
+		result = grab_blob(&elem->oid, elem->mode, &result_size,
 				   textconv, elem->path);
 	else {
 		/* Used by diff-tree to read from the working tree */
@@ -1013,12 +1013,12 @@ static void show_patch_diff(struct combine_diff_path *elem, int num_parent,
 			result = strbuf_detach(&buf, NULL);
 			elem->mode = canon_mode(st.st_mode);
 		} else if (S_ISDIR(st.st_mode)) {
-			unsigned char sha1[20];
-			if (resolve_gitlink_ref(elem->path, "HEAD", sha1) < 0)
-				result = grab_blob(elem->sha1, elem->mode,
+			struct object_id oid;
+			if (resolve_gitlink_ref(elem->path, "HEAD", oid.hash) < 0)
+				result = grab_blob(&elem->oid, elem->mode,
 						   &result_size, NULL, NULL);
 			else
-				result = grab_blob(sha1, elem->mode,
+				result = grab_blob(&oid, elem->mode,
 						   &result_size, NULL, NULL);
 		} else if (textconv) {
 			struct diff_filespec *df = alloc_filespec(elem->path);
@@ -1090,7 +1090,7 @@ static void show_patch_diff(struct combine_diff_path *elem, int num_parent,
 		for (i = 0; !is_binary && i < num_parent; i++) {
 			char *buf;
 			unsigned long size;
-			buf = grab_blob(elem->parent[i].sha1,
+			buf = grab_blob(&elem->parent[i].oid,
 					elem->parent[i].mode,
 					&size, NULL, NULL);
 			if (buffer_is_binary(buf, size))
@@ -1139,14 +1139,14 @@ static void show_patch_diff(struct combine_diff_path *elem, int num_parent,
 	for (i = 0; i < num_parent; i++) {
 		int j;
 		for (j = 0; j < i; j++) {
-			if (!hashcmp(elem->parent[i].sha1,
-				     elem->parent[j].sha1)) {
+			if (!oidcmp(&elem->parent[i].oid,
+				     &elem->parent[j].oid)) {
 				reuse_combine_diff(sline, cnt, i, j);
 				break;
 			}
 		}
 		if (i <= j)
-			combine_diff(elem->parent[i].sha1,
+			combine_diff(&elem->parent[i].oid,
 				     elem->parent[i].mode,
 				     &result_file, sline,
 				     cnt, i, num_parent, result_deleted,
@@ -1206,9 +1206,9 @@ static void show_raw_diff(struct combine_diff_path *p, int num_parent, struct re
 
 		/* Show sha1's */
 		for (i = 0; i < num_parent; i++)
-			printf(" %s", diff_unique_abbrev(p->parent[i].sha1,
+			printf(" %s", diff_unique_abbrev(p->parent[i].oid.hash,
 							 opt->abbrev));
-		printf(" %s ", diff_unique_abbrev(p->sha1, opt->abbrev));
+		printf(" %s ", diff_unique_abbrev(p->oid.hash, opt->abbrev));
 	}
 
 	if (opt->output_format & (DIFF_FORMAT_RAW | DIFF_FORMAT_NAME_STATUS)) {
@@ -1271,16 +1271,16 @@ static struct diff_filepair *combined_pair(struct combine_diff_path *p,
 	for (i = 0; i < num_parent; i++) {
 		pair->one[i].path = p->path;
 		pair->one[i].mode = p->parent[i].mode;
-		hashcpy(pair->one[i].sha1, p->parent[i].sha1);
-		pair->one[i].sha1_valid = !is_null_sha1(p->parent[i].sha1);
+		hashcpy(pair->one[i].sha1, p->parent[i].oid.hash);
+		pair->one[i].sha1_valid = !is_null_oid(&p->parent[i].oid);
 		pair->one[i].has_more_entries = 1;
 	}
 	pair->one[num_parent - 1].has_more_entries = 0;
 
 	pair->two->path = p->path;
 	pair->two->mode = p->mode;
-	hashcpy(pair->two->sha1, p->sha1);
-	pair->two->sha1_valid = !is_null_sha1(p->sha1);
+	hashcpy(pair->two->sha1, p->oid.hash);
+	pair->two->sha1_valid = !is_null_oid(&p->oid);
 	return pair;
 }
 
diff --git a/commit.c b/commit.c
index a8c7577d28..2d9de807ae 100644
--- a/commit.c
+++ b/commit.c
@@ -55,12 +55,12 @@ struct commit *lookup_commit(const unsigned char *sha1)
 
 struct commit *lookup_commit_reference_by_name(const char *name)
 {
-	unsigned char sha1[20];
+	struct object_id oid;
 	struct commit *commit;
 
-	if (get_sha1_committish(name, sha1))
+	if (get_sha1_committish(name, oid.hash))
 		return NULL;
-	commit = lookup_commit_reference(sha1);
+	commit = lookup_commit_reference(oid.hash);
 	if (parse_commit(commit))
 		return NULL;
 	return commit;
@@ -99,7 +99,7 @@ static int commit_graft_alloc, commit_graft_nr;
 static const unsigned char *commit_graft_sha1_access(size_t index, void *table)
 {
 	struct commit_graft **commit_graft_table = table;
-	return commit_graft_table[index]->sha1;
+	return commit_graft_table[index]->oid.hash;
 }
 
 static int commit_graft_pos(const unsigned char *sha1)
@@ -110,7 +110,7 @@ static int commit_graft_pos(const unsigned char *sha1)
 
 int register_commit_graft(struct commit_graft *graft, int ignore_dups)
 {
-	int pos = commit_graft_pos(graft->sha1);
+	int pos = commit_graft_pos(graft->oid.hash);
 
 	if (0 <= pos) {
 		if (ignore_dups)
@@ -138,22 +138,23 @@ struct commit_graft *read_graft_line(char *buf, int len)
 	/* The format is just "Commit Parent1 Parent2 ...\n" */
 	int i;
 	struct commit_graft *graft = NULL;
+	const int entry_size = GIT_SHA1_HEXSZ + 1;
 
 	while (len && isspace(buf[len-1]))
 		buf[--len] = '\0';
 	if (buf[0] == '#' || buf[0] == '\0')
 		return NULL;
-	if ((len + 1) % 41)
+	if ((len + 1) % entry_size)
 		goto bad_graft_data;
-	i = (len + 1) / 41 - 1;
-	graft = xmalloc(sizeof(*graft) + 20 * i);
+	i = (len + 1) / entry_size - 1;
+	graft = xmalloc(sizeof(*graft) + GIT_SHA1_RAWSZ * i);
 	graft->nr_parent = i;
-	if (get_sha1_hex(buf, graft->sha1))
+	if (get_oid_hex(buf, &graft->oid))
 		goto bad_graft_data;
-	for (i = 40; i < len; i += 41) {
+	for (i = GIT_SHA1_HEXSZ; i < len; i += entry_size) {
 		if (buf[i] != ' ')
 			goto bad_graft_data;
-		if (get_sha1_hex(buf + i + 1, graft->parent[i/41]))
+		if (get_sha1_hex(buf + i + 1, graft->parent[i/entry_size].hash))
 			goto bad_graft_data;
 	}
 	return graft;
@@ -302,39 +303,42 @@ int parse_commit_buffer(struct commit *item, const void *buffer, unsigned long s
 {
 	const char *tail = buffer;
 	const char *bufptr = buffer;
-	unsigned char parent[20];
+	struct object_id parent;
 	struct commit_list **pptr;
 	struct commit_graft *graft;
+	const int tree_entry_len = GIT_SHA1_HEXSZ + 5;
+	const int parent_entry_len = GIT_SHA1_HEXSZ + 7;
 
 	if (item->object.parsed)
 		return 0;
 	item->object.parsed = 1;
 	tail += size;
-	if (tail <= bufptr + 46 || memcmp(bufptr, "tree ", 5) || bufptr[45] != '\n')
+	if (tail <= bufptr + tree_entry_len + 1 || memcmp(bufptr, "tree ", 5) ||
+			bufptr[tree_entry_len] != '\n')
 		return error("bogus commit object %s", sha1_to_hex(item->object.sha1));
-	if (get_sha1_hex(bufptr + 5, parent) < 0)
+	if (get_sha1_hex(bufptr + 5, parent.hash) < 0)
 		return error("bad tree pointer in commit %s",
 			     sha1_to_hex(item->object.sha1));
-	item->tree = lookup_tree(parent);
-	bufptr += 46; /* "tree " + "hex sha1" + "\n" */
+	item->tree = lookup_tree(parent.hash);
+	bufptr += tree_entry_len + 1; /* "tree " + "hex sha1" + "\n" */
 	pptr = &item->parents;
 
 	graft = lookup_commit_graft(item->object.sha1);
-	while (bufptr + 48 < tail && !memcmp(bufptr, "parent ", 7)) {
+	while (bufptr + parent_entry_len < tail && !memcmp(bufptr, "parent ", 7)) {
 		struct commit *new_parent;
 
-		if (tail <= bufptr + 48 ||
-		    get_sha1_hex(bufptr + 7, parent) ||
-		    bufptr[47] != '\n')
+		if (tail <= bufptr + parent_entry_len + 1 ||
+		    get_sha1_hex(bufptr + 7, parent.hash) ||
+		    bufptr[parent_entry_len] != '\n')
 			return error("bad parents in commit %s", sha1_to_hex(item->object.sha1));
-		bufptr += 48;
+		bufptr += parent_entry_len + 1;
 		/*
 		 * The clone is shallow if nr_parent < 0, and we must
 		 * not traverse its real parents even when we unhide them.
 		 */
 		if (graft && (graft->nr_parent < 0 || grafts_replace_parents))
 			continue;
-		new_parent = lookup_commit(parent);
+		new_parent = lookup_commit(parent.hash);
 		if (new_parent)
 			pptr = &commit_list_insert(new_parent, pptr)->next;
 	}
@@ -342,7 +346,7 @@ int parse_commit_buffer(struct commit *item, const void *buffer, unsigned long s
 		int i;
 		struct commit *new_parent;
 		for (i = 0; i < graft->nr_parent; i++) {
-			new_parent = lookup_commit(graft->parent[i]);
+			new_parent = lookup_commit(graft->parent[i].hash);
 			if (!new_parent)
 				continue;
 			pptr = &commit_list_insert(new_parent, pptr)->next;
@@ -1580,10 +1584,10 @@ struct commit *get_merge_parent(const char *name)
 {
 	struct object *obj;
 	struct commit *commit;
-	unsigned char sha1[20];
-	if (get_sha1(name, sha1))
+	struct object_id oid;
+	if (get_sha1(name, oid.hash))
 		return NULL;
-	obj = parse_object(sha1);
+	obj = parse_object(oid.hash);
 	commit = (struct commit *)peel_to_type(name, 0, obj, OBJ_COMMIT);
 	if (commit && !commit->util) {
 		struct merge_remote_desc *desc;
diff --git a/commit.h b/commit.h
index 9f189cb054..ed3a1d59a5 100644
--- a/commit.h
+++ b/commit.h
@@ -226,9 +226,9 @@ enum rev_sort_order {
 void sort_in_topological_order(struct commit_list **, enum rev_sort_order);
 
 struct commit_graft {
-	unsigned char sha1[20];
+	struct object_id oid;
 	int nr_parent; /* < 0 if shallow commit */
-	unsigned char parent[FLEX_ARRAY][20]; /* more */
+	struct object_id parent[FLEX_ARRAY]; /* more */
 };
 typedef int (*each_commit_graft_fn)(const struct commit_graft *, void *);
 
diff --git a/diff-lib.c b/diff-lib.c
index a85c4971ac..241a8435eb 100644
--- a/diff-lib.c
+++ b/diff-lib.c
@@ -125,7 +125,7 @@ int run_diff_files(struct rev_info *revs, unsigned int option)
 			dpath->next = NULL;
 			memcpy(dpath->path, ce->name, path_len);
 			dpath->path[path_len] = '\0';
-			hashclr(dpath->sha1);
+			oidclr(&dpath->oid);
 			memset(&(dpath->parent[0]), 0,
 			       sizeof(struct combine_diff_parent)*5);
 
@@ -155,7 +155,7 @@ int run_diff_files(struct rev_info *revs, unsigned int option)
 				if (2 <= stage) {
 					int mode = nce->ce_mode;
 					num_compare_stages++;
-					hashcpy(dpath->parent[stage-2].sha1, nce->sha1);
+					hashcpy(dpath->parent[stage-2].oid.hash, nce->sha1);
 					dpath->parent[stage-2].mode = ce_mode_from_stat(nce, mode);
 					dpath->parent[stage-2].status =
 						DIFF_STATUS_MODIFIED;
@@ -339,14 +339,14 @@ static int show_modified(struct rev_info *revs,
 		memcpy(p->path, new->name, pathlen);
 		p->path[pathlen] = 0;
 		p->mode = mode;
-		hashclr(p->sha1);
+		oidclr(&p->oid);
 		memset(p->parent, 0, 2 * sizeof(struct combine_diff_parent));
 		p->parent[0].status = DIFF_STATUS_MODIFIED;
 		p->parent[0].mode = new->ce_mode;
-		hashcpy(p->parent[0].sha1, new->sha1);
+		hashcpy(p->parent[0].oid.hash, new->sha1);
 		p->parent[1].status = DIFF_STATUS_MODIFIED;
 		p->parent[1].mode = old->ce_mode;
-		hashcpy(p->parent[1].sha1, old->sha1);
+		hashcpy(p->parent[1].oid.hash, old->sha1);
 		show_combined_diff(p, 2, revs->dense_combined_merges, revs);
 		free(p);
 		return 0;
diff --git a/diff.h b/diff.h
index b4a624d235..f6fdf49e14 100644
--- a/diff.h
+++ b/diff.h
@@ -6,6 +6,7 @@
 
 #include "tree-walk.h"
 #include "pathspec.h"
+#include "object.h"
 
 struct rev_info;
 struct diff_options;
@@ -207,11 +208,11 @@ struct combine_diff_path {
 	struct combine_diff_path *next;
 	char *path;
 	unsigned int mode;
-	unsigned char sha1[20];
+	struct object_id oid;
 	struct combine_diff_parent {
 		char status;
 		unsigned int mode;
-		unsigned char sha1[20];
+		struct object_id oid;
 	} parent[FLEX_ARRAY];
 };
 #define combine_diff_path_size(n, l) \
diff --git a/hex.c b/hex.c
index cfd9d722fd..899b74a08c 100644
--- a/hex.c
+++ b/hex.c
@@ -38,7 +38,7 @@ const signed char hexval_table[256] = {
 int get_sha1_hex(const char *hex, unsigned char *sha1)
 {
 	int i;
-	for (i = 0; i < 20; i++) {
+	for (i = 0; i < GIT_SHA1_RAWSZ; i++) {
 		unsigned int val;
 		/*
 		 * hex[1]=='\0' is caught when val is checked below,
@@ -56,15 +56,20 @@ int get_sha1_hex(const char *hex, unsigned char *sha1)
 	return 0;
 }
 
+int get_oid_hex(const char *hex, struct object_id *oid)
+{
+	return get_sha1_hex(hex, oid->hash);
+}
+
 char *sha1_to_hex(const unsigned char *sha1)
 {
 	static int bufno;
-	static char hexbuffer[4][41];
+	static char hexbuffer[4][GIT_SHA1_HEXSZ + 1];
 	static const char hex[] = "0123456789abcdef";
 	char *buffer = hexbuffer[3 & ++bufno], *buf = buffer;
 	int i;
 
-	for (i = 0; i < 20; i++) {
+	for (i = 0; i < GIT_SHA1_RAWSZ; i++) {
 		unsigned int val = *sha1++;
 		*buf++ = hex[val >> 4];
 		*buf++ = hex[val & 0xf];
@@ -73,3 +78,8 @@ char *sha1_to_hex(const unsigned char *sha1)
 
 	return buffer;
 }
+
+char *oid_to_hex(const struct object_id *oid)
+{
+	return sha1_to_hex(oid->hash);
+}
diff --git a/log-tree.c b/log-tree.c
index 2c1ed0fa90..cf4646b0f4 100644
--- a/log-tree.c
+++ b/log-tree.c
@@ -137,7 +137,7 @@ static int add_ref_decoration(const char *refname, const unsigned char *sha1, in
 
 static int add_graft_decoration(const struct commit_graft *graft, void *cb_data)
 {
-	struct commit *commit = lookup_commit(graft->sha1);
+	struct commit *commit = lookup_commit(graft->oid.hash);
 	if (!commit)
 		return 0;
 	add_name_decoration(DECORATION_GRAFTED, "grafted", &commit->object);
diff --git a/send-pack.c b/send-pack.c
index 2e07ac3339..2a64fec949 100644
--- a/send-pack.c
+++ b/send-pack.c
@@ -182,7 +182,7 @@ static int advertise_shallow_grafts_cb(const struct commit_graft *graft, void *c
 {
 	struct strbuf *sb = cb;
 	if (graft->nr_parent == -1)
-		packet_buf_write(sb, "shallow %s\n", sha1_to_hex(graft->sha1));
+		packet_buf_write(sb, "shallow %s\n", oid_to_hex(&graft->oid));
 	return 0;
 }
 
diff --git a/shallow.c b/shallow.c
index d8bf40ad4b..d08d264dd2 100644
--- a/shallow.c
+++ b/shallow.c
@@ -31,7 +31,7 @@ int register_shallow(const unsigned char *sha1)
 		xmalloc(sizeof(struct commit_graft));
 	struct commit *commit = lookup_commit(sha1);
 
-	hashcpy(graft->sha1, sha1);
+	hashcpy(graft->oid.hash, sha1);
 	graft->nr_parent = -1;
 	if (commit && commit->object.parsed)
 		commit->parents = NULL;
@@ -159,11 +159,11 @@ struct write_shallow_data {
 static int write_one_shallow(const struct commit_graft *graft, void *cb_data)
 {
 	struct write_shallow_data *data = cb_data;
-	const char *hex = sha1_to_hex(graft->sha1);
+	const char *hex = oid_to_hex(&graft->oid);
 	if (graft->nr_parent != -1)
 		return 0;
 	if (data->flags & SEEN_ONLY) {
-		struct commit *c = lookup_commit(graft->sha1);
+		struct commit *c = lookup_commit(graft->oid.hash);
 		if (!c || !(c->object.flags & SEEN)) {
 			if (data->flags & VERBOSE)
 				printf("Removing %s from .git/shallow\n",
@@ -282,7 +282,7 @@ static int advertise_shallow_grafts_cb(const struct commit_graft *graft, void *c
 {
 	int fd = *(int *)cb;
 	if (graft->nr_parent == -1)
-		packet_write(fd, "shallow %s\n", sha1_to_hex(graft->sha1));
+		packet_write(fd, "shallow %s\n", oid_to_hex(&graft->oid));
 	return 0;
 }
 
diff --git a/tree-diff.c b/tree-diff.c
index e7b378c8b2..290a1da4ce 100644
--- a/tree-diff.c
+++ b/tree-diff.c
@@ -64,7 +64,7 @@ static int emit_diff_first_parent_only(struct diff_options *opt, struct combine_
 {
 	struct combine_diff_parent *p0 = &p->parent[0];
 	if (p->mode && p0->mode) {
-		opt->change(opt, p0->mode, p->mode, p0->sha1, p->sha1,
+		opt->change(opt, p0->mode, p->mode, p0->oid.hash, p->oid.hash,
 			1, 1, p->path, 0, 0);
 	}
 	else {
@@ -74,11 +74,11 @@ static int emit_diff_first_parent_only(struct diff_options *opt, struct combine_
 
 		if (p->mode) {
 			addremove = '+';
-			sha1 = p->sha1;
+			sha1 = p->oid.hash;
 			mode = p->mode;
 		} else {
 			addremove = '-';
-			sha1 = p0->sha1;
+			sha1 = p0->oid.hash;
 			mode = p0->mode;
 		}
 
@@ -151,7 +151,7 @@ static struct combine_diff_path *path_appendnew(struct combine_diff_path *last,
 	memcpy(p->path + base->len, path, pathlen);
 	p->path[len] = 0;
 	p->mode = mode;
-	hashcpy(p->sha1, sha1 ? sha1 : null_sha1);
+	hashcpy(p->oid.hash, sha1 ? sha1 : null_sha1);
 
 	return p;
 }
@@ -238,7 +238,7 @@ static struct combine_diff_path *emit_path(struct combine_diff_path *p,
 			}
 
 			p->parent[i].mode = mode_i;
-			hashcpy(p->parent[i].sha1, sha1_i ? sha1_i : null_sha1);
+			hashcpy(p->parent[i].oid.hash, sha1_i ? sha1_i : null_sha1);
 		}
 
 		keep = 1;
diff --git a/upload-pack.c b/upload-pack.c
index aa84576500..745fda8515 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -74,7 +74,7 @@ static int write_one_shallow(const struct commit_graft *graft, void *cb_data)
 {
 	FILE *fp = cb_data;
 	if (graft->nr_parent == -1)
-		fprintf(fp, "--shallow %s\n", sha1_to_hex(graft->sha1));
+		fprintf(fp, "--shallow %s\n", oid_to_hex(&graft->oid));
 	return 0;
 }