diff --git a/t/README b/t/README
index 3ea6c85460..5e48a043ce 100644
--- a/t/README
+++ b/t/README
@@ -814,6 +814,28 @@ library for your script to use.
    the symbolic link in the file system and a part that does; then only
    the latter part need be protected by a SYMLINKS prerequisite (see below).
 
+ - test_oid_init
+
+   This function loads facts and useful object IDs related to the hash
+   algorithm(s) in use from the files in t/oid-info.
+
+ - test_oid_cache
+
+   This function reads per-hash algorithm information from standard
+   input (usually a heredoc) in the format described in
+   t/oid-info/README.  This is useful for test-specific values, such as
+   object IDs, which must vary based on the hash algorithm.
+
+   Certain fixed values, such as hash sizes and common placeholder
+   object IDs, can be loaded with test_oid_init (described above).
+
+ - test_oid <key>
+
+   This function looks up a value for the hash algorithm in use, based
+   on the key given.  The value must have been loaded using
+   test_oid_init or test_oid_cache.  Providing an unknown key is an
+   error.
+
 Prerequisites
 -------------
 
diff --git a/t/oid-info/README b/t/oid-info/README
new file mode 100644
index 0000000000..27f843fc00
--- /dev/null
+++ b/t/oid-info/README
@@ -0,0 +1,19 @@
+This directory contains various per-hash values that are used in the testsuite.
+
+Each file contains lines containing a key-value pair; blank lines and lines
+starting with `#` are ignored.  The key and value are separated by whitespace
+(specifically, those whitespace in the default `$IFS`).  The key consists only
+of shell identifier characters, and the value consists of a hash algorithm,
+colon, and value.  The hash algorithm also consists only of shell identifier
+characters; it should match the value in sha1-file.c.
+
+For example, the following lines map the key "rawsz" to "20" if SHA-1 is in use
+and to "32" if SHA-256 is in use:
+
+----
+rawsz sha1:20
+rawsz sha256:32
+----
+
+The keys and values used here are loaded by `test_oid_init` (see the README file
+in the "t" directory) and are used by calling `test_oid`.
diff --git a/t/oid-info/hash-info b/t/oid-info/hash-info
new file mode 100644
index 0000000000..ccdbfdf974
--- /dev/null
+++ b/t/oid-info/hash-info
@@ -0,0 +1,8 @@
+rawsz sha1:20
+rawsz sha256:32
+
+hexsz sha1:40
+hexsz sha256:64
+
+zero sha1:0000000000000000000000000000000000000000
+zero sha256:0000000000000000000000000000000000000000000000000000000000000000
diff --git a/t/oid-info/oid b/t/oid-info/oid
new file mode 100644
index 0000000000..a754970523
--- /dev/null
+++ b/t/oid-info/oid
@@ -0,0 +1,29 @@
+# These are some common invalid and partial object IDs used in tests.
+001	sha1:0000000000000000000000000000000000000001
+001	sha256:0000000000000000000000000000000000000000000000000000000000000001
+002	sha1:0000000000000000000000000000000000000002
+002	sha256:0000000000000000000000000000000000000000000000000000000000000002
+003	sha1:0000000000000000000000000000000000000003
+003	sha256:0000000000000000000000000000000000000000000000000000000000000003
+004	sha1:0000000000000000000000000000000000000004
+004	sha256:0000000000000000000000000000000000000000000000000000000000000004
+005	sha1:0000000000000000000000000000000000000005
+005	sha256:0000000000000000000000000000000000000000000000000000000000000005
+006	sha1:0000000000000000000000000000000000000006
+006	sha256:0000000000000000000000000000000000000000000000000000000000000006
+007	sha1:0000000000000000000000000000000000000007
+007	sha256:0000000000000000000000000000000000000000000000000000000000000007
+# All zeros or Fs missing one or two hex segments.
+zero_1		sha1:000000000000000000000000000000000000000
+zero_1		sha256:000000000000000000000000000000000000000000000000000000000000000
+zero_2		sha1:00000000000000000000000000000000000000
+zero_2		sha256:00000000000000000000000000000000000000000000000000000000000000
+ff_1		sha1:fffffffffffffffffffffffffffffffffffffff
+ff_1		sha256:fffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff
+ff_2		sha1:ffffffffffffffffffffffffffffffffffffff
+ff_2		sha256:ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff
+# More various invalid OIDs.
+numeric		sha1:0123456789012345678901234567890123456789
+numeric		sha256:0123456789012345678901234567890123456789012345678901234567890123
+deadbeef	sha1:deadbeefdeadbeefdeadbeefdeadbeefdeadbeef
+deadbeef	sha256:deadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeef
diff --git a/t/t0000-basic.sh b/t/t0000-basic.sh
index 850f651e4e..391f910c6a 100755
--- a/t/t0000-basic.sh
+++ b/t/t0000-basic.sh
@@ -821,9 +821,87 @@ test_expect_success 'tests clean up even on failures' "
 	EOF
 "
 
+test_expect_success 'test_oid setup' '
+	test_oid_init
+'
+
+test_expect_success 'test_oid provides sane info by default' '
+	test_oid zero >actual &&
+	grep "^00*\$" actual &&
+	rawsz="$(test_oid rawsz)" &&
+	hexsz="$(test_oid hexsz)" &&
+	test "$hexsz" -eq $(wc -c <actual) &&
+	test $(( $rawsz * 2)) -eq "$hexsz"
+'
+
+test_expect_success 'test_oid can look up data for SHA-1' '
+	test_when_finished "test_detect_hash" &&
+	test_set_hash sha1 &&
+	test_oid zero >actual &&
+	grep "^00*\$" actual &&
+	rawsz="$(test_oid rawsz)" &&
+	hexsz="$(test_oid hexsz)" &&
+	test $(wc -c <actual) -eq 40 &&
+	test "$rawsz" -eq 20 &&
+	test "$hexsz" -eq 40
+'
+
+test_expect_success 'test_oid can look up data for SHA-256' '
+	test_when_finished "test_detect_hash" &&
+	test_set_hash sha256 &&
+	test_oid zero >actual &&
+	grep "^00*\$" actual &&
+	rawsz="$(test_oid rawsz)" &&
+	hexsz="$(test_oid hexsz)" &&
+	test $(wc -c <actual) -eq 64 &&
+	test "$rawsz" -eq 32 &&
+	test "$hexsz" -eq 64
+'
+
 ################################################################
 # Basics of the basics
 
+test_oid_cache <<\EOF
+path0f sha1:f87290f8eb2cbbea7857214459a0739927eab154
+path0f sha256:638106af7c38be056f3212cbd7ac65bc1bac74f420ca5a436ff006a9d025d17d
+
+path0s sha1:15a98433ae33114b085f3eb3bb03b832b3180a01
+path0s sha256:3a24cc53cf68edddac490bbf94a418a52932130541361f685df685e41dd6c363
+
+path2f sha1:3feff949ed00a62d9f7af97c15cd8a30595e7ac7
+path2f sha256:2a7f36571c6fdbaf0e3f62751a0b25a3f4c54d2d1137b3f4af9cb794bb498e5f
+
+path2s sha1:d8ce161addc5173867a3c3c730924388daedbc38
+path2s sha256:18fd611b787c2e938ddcc248fabe4d66a150f9364763e9ec133dd01d5bb7c65a
+
+path2d sha1:58a09c23e2ca152193f2786e06986b7b6712bdbe
+path2d sha256:00e4b32b96e7e3d65d79112dcbea53238a22715f896933a62b811377e2650c17
+
+path3f sha1:0aa34cae68d0878578ad119c86ca2b5ed5b28376
+path3f sha256:09f58616b951bd571b8cb9dc76d372fbb09ab99db2393f5ab3189d26c45099ad
+
+path3s sha1:8599103969b43aff7e430efea79ca4636466794f
+path3s sha256:fce1aed087c053306f3f74c32c1a838c662bbc4551a7ac2420f5d6eb061374d0
+
+path3d sha1:21ae8269cacbe57ae09138dcc3a2887f904d02b3
+path3d sha256:9b60497be959cb830bf3f0dc82bcc9ad9e925a24e480837ade46b2295e47efe1
+
+subp3f sha1:00fb5908cb97c2564a9783c0c64087333b3b464f
+subp3f sha256:a1a9e16998c988453f18313d10375ee1d0ddefe757e710dcae0d66aa1e0c58b3
+
+subp3s sha1:6649a1ebe9e9f1c553b66f5a6e74136a07ccc57c
+subp3s sha256:81759d9f5e93c6546ecfcadb560c1ff057314b09f93fe8ec06e2d8610d34ef10
+
+subp3d sha1:3c5e5399f3a333eddecce7a9b9465b63f65f51e2
+subp3d sha256:76b4ef482d4fa1c754390344cf3851c7f883b27cf9bc999c6547928c46aeafb7
+
+root sha1:087704a96baf1c2d1c869a8b084481e121c88b5b
+root sha256:9481b52abab1b2ffeedbf9de63ce422b929f179c1b98ff7bee5f8f1bc0710751
+
+simpletree sha1:7bb943559a305bdd6bdee2cef6e5df2413c3d30a
+simpletree sha256:1710c07a6c86f9a3c7376364df04c47ee39e5a5e221fcdd84b743bc9bb7e2bc5
+EOF
+
 # updating a new file without --add should fail.
 test_expect_success 'git update-index without --add should fail adding' '
 	test_must_fail git update-index should-be-empty
@@ -839,8 +917,8 @@ test_expect_success 'writing tree out with git write-tree' '
 '
 
 # we know the shape and contents of the tree and know the object ID for it.
-test_expect_success SHA1 'validate object ID of a known tree' '
-	test "$tree" = 7bb943559a305bdd6bdee2cef6e5df2413c3d30a
+test_expect_success 'validate object ID of a known tree' '
+	test "$tree" = "$(test_oid simpletree)"
     '
 
 # Removing paths.
@@ -882,16 +960,16 @@ test_expect_success 'showing stage with git ls-files --stage' '
 	git ls-files --stage >current
 '
 
-test_expect_success SHA1 'validate git ls-files output for a known tree' '
-	cat >expected <<-\EOF &&
-	100644 f87290f8eb2cbbea7857214459a0739927eab154 0	path0
-	120000 15a98433ae33114b085f3eb3bb03b832b3180a01 0	path0sym
-	100644 3feff949ed00a62d9f7af97c15cd8a30595e7ac7 0	path2/file2
-	120000 d8ce161addc5173867a3c3c730924388daedbc38 0	path2/file2sym
-	100644 0aa34cae68d0878578ad119c86ca2b5ed5b28376 0	path3/file3
-	120000 8599103969b43aff7e430efea79ca4636466794f 0	path3/file3sym
-	100644 00fb5908cb97c2564a9783c0c64087333b3b464f 0	path3/subp3/file3
-	120000 6649a1ebe9e9f1c553b66f5a6e74136a07ccc57c 0	path3/subp3/file3sym
+test_expect_success 'validate git ls-files output for a known tree' '
+	cat >expected <<-EOF &&
+	100644 $(test_oid path0f) 0	path0
+	120000 $(test_oid path0s) 0	path0sym
+	100644 $(test_oid path2f) 0	path2/file2
+	120000 $(test_oid path2s) 0	path2/file2sym
+	100644 $(test_oid path3f) 0	path3/file3
+	120000 $(test_oid path3s) 0	path3/file3sym
+	100644 $(test_oid subp3f) 0	path3/subp3/file3
+	120000 $(test_oid subp3s) 0	path3/subp3/file3sym
 	EOF
 	test_cmp expected current
 '
@@ -900,20 +978,20 @@ test_expect_success 'writing tree out with git write-tree' '
 	tree=$(git write-tree)
 '
 
-test_expect_success SHA1 'validate object ID for a known tree' '
-	test "$tree" = 087704a96baf1c2d1c869a8b084481e121c88b5b
+test_expect_success 'validate object ID for a known tree' '
+	test "$tree" = "$(test_oid root)"
 '
 
 test_expect_success 'showing tree with git ls-tree' '
     git ls-tree $tree >current
 '
 
-test_expect_success SHA1 'git ls-tree output for a known tree' '
-	cat >expected <<-\EOF &&
-	100644 blob f87290f8eb2cbbea7857214459a0739927eab154	path0
-	120000 blob 15a98433ae33114b085f3eb3bb03b832b3180a01	path0sym
-	040000 tree 58a09c23e2ca152193f2786e06986b7b6712bdbe	path2
-	040000 tree 21ae8269cacbe57ae09138dcc3a2887f904d02b3	path3
+test_expect_success 'git ls-tree output for a known tree' '
+	cat >expected <<-EOF &&
+	100644 blob $(test_oid path0f)	path0
+	120000 blob $(test_oid path0s)	path0sym
+	040000 tree $(test_oid path2d)	path2
+	040000 tree $(test_oid path3d)	path3
 	EOF
 	test_cmp expected current
 '
@@ -924,16 +1002,16 @@ test_expect_success 'showing tree with git ls-tree -r' '
 	git ls-tree -r $tree >current
 '
 
-test_expect_success SHA1 'git ls-tree -r output for a known tree' '
-	cat >expected <<-\EOF &&
-	100644 blob f87290f8eb2cbbea7857214459a0739927eab154	path0
-	120000 blob 15a98433ae33114b085f3eb3bb03b832b3180a01	path0sym
-	100644 blob 3feff949ed00a62d9f7af97c15cd8a30595e7ac7	path2/file2
-	120000 blob d8ce161addc5173867a3c3c730924388daedbc38	path2/file2sym
-	100644 blob 0aa34cae68d0878578ad119c86ca2b5ed5b28376	path3/file3
-	120000 blob 8599103969b43aff7e430efea79ca4636466794f	path3/file3sym
-	100644 blob 00fb5908cb97c2564a9783c0c64087333b3b464f	path3/subp3/file3
-	120000 blob 6649a1ebe9e9f1c553b66f5a6e74136a07ccc57c	path3/subp3/file3sym
+test_expect_success 'git ls-tree -r output for a known tree' '
+	cat >expected <<-EOF &&
+	100644 blob $(test_oid path0f)	path0
+	120000 blob $(test_oid path0s)	path0sym
+	100644 blob $(test_oid path2f)	path2/file2
+	120000 blob $(test_oid path2s)	path2/file2sym
+	100644 blob $(test_oid path3f)	path3/file3
+	120000 blob $(test_oid path3s)	path3/file3sym
+	100644 blob $(test_oid subp3f)	path3/subp3/file3
+	120000 blob $(test_oid subp3s)	path3/subp3/file3sym
 	EOF
 	test_cmp expected current
 '
@@ -943,19 +1021,19 @@ test_expect_success 'showing tree with git ls-tree -r -t' '
 	git ls-tree -r -t $tree >current
 '
 
-test_expect_success SHA1 'git ls-tree -r output for a known tree' '
-	cat >expected <<-\EOF &&
-	100644 blob f87290f8eb2cbbea7857214459a0739927eab154	path0
-	120000 blob 15a98433ae33114b085f3eb3bb03b832b3180a01	path0sym
-	040000 tree 58a09c23e2ca152193f2786e06986b7b6712bdbe	path2
-	100644 blob 3feff949ed00a62d9f7af97c15cd8a30595e7ac7	path2/file2
-	120000 blob d8ce161addc5173867a3c3c730924388daedbc38	path2/file2sym
-	040000 tree 21ae8269cacbe57ae09138dcc3a2887f904d02b3	path3
-	100644 blob 0aa34cae68d0878578ad119c86ca2b5ed5b28376	path3/file3
-	120000 blob 8599103969b43aff7e430efea79ca4636466794f	path3/file3sym
-	040000 tree 3c5e5399f3a333eddecce7a9b9465b63f65f51e2	path3/subp3
-	100644 blob 00fb5908cb97c2564a9783c0c64087333b3b464f	path3/subp3/file3
-	120000 blob 6649a1ebe9e9f1c553b66f5a6e74136a07ccc57c	path3/subp3/file3sym
+test_expect_success 'git ls-tree -r output for a known tree' '
+	cat >expected <<-EOF &&
+	100644 blob $(test_oid path0f)	path0
+	120000 blob $(test_oid path0s)	path0sym
+	040000 tree $(test_oid path2d)	path2
+	100644 blob $(test_oid path2f)	path2/file2
+	120000 blob $(test_oid path2s)	path2/file2sym
+	040000 tree $(test_oid path3d)	path3
+	100644 blob $(test_oid path3f)	path3/file3
+	120000 blob $(test_oid path3s)	path3/file3sym
+	040000 tree $(test_oid subp3d)	path3/subp3
+	100644 blob $(test_oid subp3f)	path3/subp3/file3
+	120000 blob $(test_oid subp3s)	path3/subp3/file3sym
 	EOF
 	test_cmp expected current
 '
@@ -964,26 +1042,27 @@ test_expect_success 'writing partial tree out with git write-tree --prefix' '
 	ptree=$(git write-tree --prefix=path3)
 '
 
-test_expect_success SHA1 'validate object ID for a known tree' '
-	test "$ptree" = 21ae8269cacbe57ae09138dcc3a2887f904d02b3
+test_expect_success 'validate object ID for a known tree' '
+	test "$ptree" = $(test_oid path3d)
 '
 
 test_expect_success 'writing partial tree out with git write-tree --prefix' '
 	ptree=$(git write-tree --prefix=path3/subp3)
 '
 
-test_expect_success SHA1 'validate object ID for a known tree' '
-	test "$ptree" = 3c5e5399f3a333eddecce7a9b9465b63f65f51e2
+test_expect_success 'validate object ID for a known tree' '
+	test "$ptree" = $(test_oid subp3d)
 '
 
 test_expect_success 'put invalid objects into the index' '
 	rm -f .git/index &&
-	cat >badobjects <<-\EOF &&
-	100644 blob 1000000000000000000000000000000000000000	dir/file1
-	100644 blob 2000000000000000000000000000000000000000	dir/file2
-	100644 blob 3000000000000000000000000000000000000000	dir/file3
-	100644 blob 4000000000000000000000000000000000000000	dir/file4
-	100644 blob 5000000000000000000000000000000000000000	dir/file5
+	suffix=$(echo $ZERO_OID | sed -e "s/^.//") &&
+	cat >badobjects <<-EOF &&
+	100644 blob $(test_oid 001)	dir/file1
+	100644 blob $(test_oid 002)	dir/file2
+	100644 blob $(test_oid 003)	dir/file3
+	100644 blob $(test_oid 004)	dir/file4
+	100644 blob $(test_oid 005)	dir/file5
 	EOF
 	git update-index --index-info <badobjects
 '
@@ -1006,16 +1085,16 @@ test_expect_success 'git read-tree followed by write-tree should be idempotent'
 	test "$newtree" = "$tree"
 '
 
-test_expect_success SHA1 'validate git diff-files output for a know cache/work tree state' '
-	cat >expected <<\EOF &&
-:100644 100644 f87290f8eb2cbbea7857214459a0739927eab154 0000000000000000000000000000000000000000 M	path0
-:120000 120000 15a98433ae33114b085f3eb3bb03b832b3180a01 0000000000000000000000000000000000000000 M	path0sym
-:100644 100644 3feff949ed00a62d9f7af97c15cd8a30595e7ac7 0000000000000000000000000000000000000000 M	path2/file2
-:120000 120000 d8ce161addc5173867a3c3c730924388daedbc38 0000000000000000000000000000000000000000 M	path2/file2sym
-:100644 100644 0aa34cae68d0878578ad119c86ca2b5ed5b28376 0000000000000000000000000000000000000000 M	path3/file3
-:120000 120000 8599103969b43aff7e430efea79ca4636466794f 0000000000000000000000000000000000000000 M	path3/file3sym
-:100644 100644 00fb5908cb97c2564a9783c0c64087333b3b464f 0000000000000000000000000000000000000000 M	path3/subp3/file3
-:120000 120000 6649a1ebe9e9f1c553b66f5a6e74136a07ccc57c 0000000000000000000000000000000000000000 M	path3/subp3/file3sym
+test_expect_success 'validate git diff-files output for a know cache/work tree state' '
+	cat >expected <<EOF &&
+:100644 100644 $(test_oid path0f) $ZERO_OID M	path0
+:120000 120000 $(test_oid path0s) $ZERO_OID M	path0sym
+:100644 100644 $(test_oid path2f) $ZERO_OID M	path2/file2
+:120000 120000 $(test_oid path2s) $ZERO_OID M	path2/file2sym
+:100644 100644 $(test_oid path3f) $ZERO_OID M	path3/file3
+:120000 120000 $(test_oid path3s) $ZERO_OID M	path3/file3sym
+:100644 100644 $(test_oid subp3f) $ZERO_OID M	path3/subp3/file3
+:120000 120000 $(test_oid subp3s) $ZERO_OID M	path3/subp3/file3sym
 EOF
 	git diff-files >current &&
 	test_cmp current expected
@@ -1031,23 +1110,23 @@ test_expect_success 'no diff after checkout and git update-index --refresh' '
 '
 
 ################################################################
-P=087704a96baf1c2d1c869a8b084481e121c88b5b
+P=$(test_oid root)
 
-test_expect_success SHA1 'git commit-tree records the correct tree in a commit' '
+test_expect_success 'git commit-tree records the correct tree in a commit' '
 	commit0=$(echo NO | git commit-tree $P) &&
 	tree=$(git show --pretty=raw $commit0 |
 		 sed -n -e "s/^tree //p" -e "/^author /q") &&
 	test "z$tree" = "z$P"
 '
 
-test_expect_success SHA1 'git commit-tree records the correct parent in a commit' '
+test_expect_success 'git commit-tree records the correct parent in a commit' '
 	commit1=$(echo NO | git commit-tree $P -p $commit0) &&
 	parent=$(git show --pretty=raw $commit1 |
 		sed -n -e "s/^parent //p" -e "/^author /q") &&
 	test "z$commit0" = "z$parent"
 '
 
-test_expect_success SHA1 'git commit-tree omits duplicated parent in a commit' '
+test_expect_success 'git commit-tree omits duplicated parent in a commit' '
 	commit2=$(echo NO | git commit-tree $P -p $commit0 -p $commit0) &&
 	     parent=$(git show --pretty=raw $commit2 |
 		sed -n -e "s/^parent //p" -e "/^author /q" |
diff --git a/t/t0002-gitfile.sh b/t/t0002-gitfile.sh
index 3691023d51..0aa9908ea1 100755
--- a/t/t0002-gitfile.sh
+++ b/t/t0002-gitfile.sh
@@ -92,11 +92,12 @@ test_expect_success 'enter_repo non-strict mode' '
 		mv .git .realgit &&
 		echo "gitdir: .realgit" >.git
 	) &&
+	head=$(git -C enter_repo rev-parse HEAD) &&
 	git ls-remote enter_repo >actual &&
-	cat >expected <<-\EOF &&
-	946e985ab20de757ca5b872b16d64e92ff3803a9	HEAD
-	946e985ab20de757ca5b872b16d64e92ff3803a9	refs/heads/master
-	946e985ab20de757ca5b872b16d64e92ff3803a9	refs/tags/foo
+	cat >expected <<-EOF &&
+	$head	HEAD
+	$head	refs/heads/master
+	$head	refs/tags/foo
 	EOF
 	test_cmp expected actual
 '
@@ -106,21 +107,23 @@ test_expect_success 'enter_repo linked checkout' '
 		cd enter_repo &&
 		git worktree add  ../foo refs/tags/foo
 	) &&
+	head=$(git -C enter_repo rev-parse HEAD) &&
 	git ls-remote foo >actual &&
-	cat >expected <<-\EOF &&
-	946e985ab20de757ca5b872b16d64e92ff3803a9	HEAD
-	946e985ab20de757ca5b872b16d64e92ff3803a9	refs/heads/master
-	946e985ab20de757ca5b872b16d64e92ff3803a9	refs/tags/foo
+	cat >expected <<-EOF &&
+	$head	HEAD
+	$head	refs/heads/master
+	$head	refs/tags/foo
 	EOF
 	test_cmp expected actual
 '
 
 test_expect_success 'enter_repo strict mode' '
+	head=$(git -C enter_repo rev-parse HEAD) &&
 	git ls-remote --upload-pack="git upload-pack --strict" foo/.git >actual &&
-	cat >expected <<-\EOF &&
-	946e985ab20de757ca5b872b16d64e92ff3803a9	HEAD
-	946e985ab20de757ca5b872b16d64e92ff3803a9	refs/heads/master
-	946e985ab20de757ca5b872b16d64e92ff3803a9	refs/tags/foo
+	cat >expected <<-EOF &&
+	$head	HEAD
+	$head	refs/heads/master
+	$head	refs/tags/foo
 	EOF
 	test_cmp expected actual
 '
diff --git a/t/t0064-sha1-array.sh b/t/t0064-sha1-array.sh
index 67484502a0..5dda570b9a 100755
--- a/t/t0064-sha1-array.sh
+++ b/t/t0064-sha1-array.sh
@@ -3,30 +3,30 @@
 test_description='basic tests for the SHA1 array implementation'
 . ./test-lib.sh
 
-echo20 () {
+echoid () {
 	prefix="${1:+$1 }"
 	shift
 	while test $# -gt 0
 	do
-		echo "$prefix$1$1$1$1$1$1$1$1$1$1$1$1$1$1$1$1$1$1$1$1"
+		echo "$prefix$ZERO_OID" | sed -e "s/00/$1/g"
 		shift
 	done
 }
 
 test_expect_success 'ordered enumeration' '
-	echo20 "" 44 55 88 aa >expect &&
+	echoid "" 44 55 88 aa >expect &&
 	{
-		echo20 append 88 44 aa 55 &&
+		echoid append 88 44 aa 55 &&
 		echo for_each_unique
 	} | test-tool sha1-array >actual &&
 	test_cmp expect actual
 '
 
 test_expect_success 'ordered enumeration with duplicate suppression' '
-	echo20 "" 44 55 88 aa >expect &&
+	echoid "" 44 55 88 aa >expect &&
 	{
-		echo20 append 88 44 aa 55 &&
-		echo20 append 88 44 aa 55 &&
+		echoid append 88 44 aa 55 &&
+		echoid append 88 44 aa 55 &&
 		echo for_each_unique
 	} | test-tool sha1-array >actual &&
 	test_cmp expect actual
@@ -34,8 +34,8 @@ test_expect_success 'ordered enumeration with duplicate suppression' '
 
 test_expect_success 'lookup' '
 	{
-		echo20 append 88 44 aa 55 &&
-		echo20 lookup 55
+		echoid append 88 44 aa 55 &&
+		echoid lookup 55
 	} | test-tool sha1-array >actual &&
 	n=$(cat actual) &&
 	test "$n" -eq 1
@@ -43,8 +43,8 @@ test_expect_success 'lookup' '
 
 test_expect_success 'lookup non-existing entry' '
 	{
-		echo20 append 88 44 aa 55 &&
-		echo20 lookup 33
+		echoid append 88 44 aa 55 &&
+		echoid lookup 33
 	} | test-tool sha1-array >actual &&
 	n=$(cat actual) &&
 	test "$n" -lt 0
@@ -52,9 +52,9 @@ test_expect_success 'lookup non-existing entry' '
 
 test_expect_success 'lookup with duplicates' '
 	{
-		echo20 append 88 44 aa 55 &&
-		echo20 append 88 44 aa 55 &&
-		echo20 lookup 55
+		echoid append 88 44 aa 55 &&
+		echoid append 88 44 aa 55 &&
+		echoid lookup 55
 	} | test-tool sha1-array >actual &&
 	n=$(cat actual) &&
 	test "$n" -ge 2 &&
@@ -63,19 +63,24 @@ test_expect_success 'lookup with duplicates' '
 
 test_expect_success 'lookup non-existing entry with duplicates' '
 	{
-		echo20 append 88 44 aa 55 &&
-		echo20 append 88 44 aa 55 &&
-		echo20 lookup 66
+		echoid append 88 44 aa 55 &&
+		echoid append 88 44 aa 55 &&
+		echoid lookup 66
 	} | test-tool sha1-array >actual &&
 	n=$(cat actual) &&
 	test "$n" -lt 0
 '
 
 test_expect_success 'lookup with almost duplicate values' '
+	# n-1 5s
+	root=$(echoid "" 55) &&
+	root=${root%5} &&
 	{
-		echo "append 5555555555555555555555555555555555555555" &&
-		echo "append 555555555555555555555555555555555555555f" &&
-		echo20 lookup 55
+		id1="${root}5" &&
+		id2="${root}f" &&
+		echo "append $id1" &&
+		echo "append $id2" &&
+		echoid lookup 55
 	} | test-tool sha1-array >actual &&
 	n=$(cat actual) &&
 	test "$n" -eq 0
@@ -83,8 +88,8 @@ test_expect_success 'lookup with almost duplicate values' '
 
 test_expect_success 'lookup with single duplicate value' '
 	{
-		echo20 append 55 55 &&
-		echo20 lookup 55
+		echoid append 55 55 &&
+		echoid lookup 55
 	} | test-tool sha1-array >actual &&
 	n=$(cat actual) &&
 	test "$n" -ge 0 &&
diff --git a/t/t1006-cat-file.sh b/t/t1006-cat-file.sh
index 7f19d591f2..a7c95bb785 100755
--- a/t/t1006-cat-file.sh
+++ b/t/t1006-cat-file.sh
@@ -140,15 +140,17 @@ test_expect_success '--batch-check without %(rest) considers whole line' '
 	test_cmp expect actual
 '
 
+test_oid_init
+
 tree_sha1=$(git write-tree)
-tree_size=33
+tree_size=$(($(test_oid rawsz) + 13))
 tree_pretty_content="100644 blob $hello_sha1	hello"
 
 run_tests 'tree' $tree_sha1 $tree_size "" "$tree_pretty_content"
 
 commit_message="Initial commit"
 commit_sha1=$(echo_without_newline "$commit_message" | git commit-tree $tree_sha1)
-commit_size=177
+commit_size=$(($(test_oid hexsz) + 137))
 commit_content="tree $tree_sha1
 author $GIT_AUTHOR_NAME <$GIT_AUTHOR_EMAIL> 0000000000 +0000
 committer $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> 0000000000 +0000
diff --git a/t/t1400-update-ref.sh b/t/t1400-update-ref.sh
index 02493f14ba..663f17c5fe 100755
--- a/t/t1400-update-ref.sh
+++ b/t/t1400-update-ref.sh
@@ -346,7 +346,7 @@ test_expect_success "verifying $m's log (logged by config)" '
 
 git update-ref $m $D
 cat >.git/logs/$m <<EOF
-0000000000000000000000000000000000000000 $C $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> 1117150320 -0500
+$Z $C $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> 1117150320 -0500
 $C $A $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> 1117150350 -0500
 $A $B $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> 1117150380 -0500
 $F $Z $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> 1117150680 -0500
diff --git a/t/t1405-main-ref-store.sh b/t/t1405-main-ref-store.sh
index a74c38b5fb..331899ddc4 100755
--- a/t/t1405-main-ref-store.sh
+++ b/t/t1405-main-ref-store.sh
@@ -54,7 +54,7 @@ test_expect_success 'for_each_ref(refs/heads/)' '
 '
 
 test_expect_success 'for_each_ref() is sorted' '
-	$RUN for-each-ref refs/heads/ | cut -c 42- >actual &&
+	$RUN for-each-ref refs/heads/ | cut -d" " -f 2- >actual &&
 	sort actual > expected &&
 	test_cmp expected actual
 '
@@ -71,7 +71,7 @@ test_expect_success 'verify_ref(new-master)' '
 '
 
 test_expect_success 'for_each_reflog()' '
-	$RUN for-each-reflog | sort -k2 | cut -c 42- >actual &&
+	$RUN for-each-reflog | sort -k2 | cut -d" " -f 2- >actual &&
 	cat >expected <<-\EOF &&
 	HEAD 0x1
 	refs/heads/master 0x0
diff --git a/t/t1406-submodule-ref-store.sh b/t/t1406-submodule-ref-store.sh
index e093782cc3..d199d872fb 100755
--- a/t/t1406-submodule-ref-store.sh
+++ b/t/t1406-submodule-ref-store.sh
@@ -39,7 +39,7 @@ test_expect_success 'rename_refs() not allowed' '
 '
 
 test_expect_success 'for_each_ref(refs/heads/)' '
-	$RUN for-each-ref refs/heads/ | cut -c 42- >actual &&
+	$RUN for-each-ref refs/heads/ | cut -d" " -f 2- >actual &&
 	cat >expected <<-\EOF &&
 	master 0x0
 	new-master 0x0
@@ -48,7 +48,7 @@ test_expect_success 'for_each_ref(refs/heads/)' '
 '
 
 test_expect_success 'for_each_ref() is sorted' '
-	$RUN for-each-ref refs/heads/ | cut -c 42- >actual &&
+	$RUN for-each-ref refs/heads/ | cut -d" " -f 2- >actual &&
 	sort actual > expected &&
 	test_cmp expected actual
 '
@@ -65,7 +65,7 @@ test_expect_success 'verify_ref(new-master)' '
 '
 
 test_expect_success 'for_each_reflog()' '
-	$RUN for-each-reflog | sort | cut -c 42- >actual &&
+	$RUN for-each-reflog | sort | cut -d" " -f 2- >actual &&
 	cat >expected <<-\EOF &&
 	HEAD 0x1
 	refs/heads/master 0x0
diff --git a/t/t1407-worktree-ref-store.sh b/t/t1407-worktree-ref-store.sh
index 4623ae15c4..9a84858118 100755
--- a/t/t1407-worktree-ref-store.sh
+++ b/t/t1407-worktree-ref-store.sh
@@ -58,7 +58,7 @@ test_expect_success 'for_each_reflog()' '
 	mkdir -p     .git/worktrees/wt/logs/refs/bisect &&
 	echo $ZERO_OID > .git/worktrees/wt/logs/refs/bisect/wt-random &&
 
-	$RWT for-each-reflog | cut -c 42- | sort >actual &&
+	$RWT for-each-reflog | cut -d" " -f 2- | sort >actual &&
 	cat >expected <<-\EOF &&
 	HEAD 0x1
 	PSEUDO-WT 0x0
@@ -68,7 +68,7 @@ test_expect_success 'for_each_reflog()' '
 	EOF
 	test_cmp expected actual &&
 
-	$RMAIN for-each-reflog | cut -c 42- | sort >actual &&
+	$RMAIN for-each-reflog | cut -d" " -f 2- | sort >actual &&
 	cat >expected <<-\EOF &&
 	HEAD 0x1
 	PSEUDO-MAIN 0x0
diff --git a/t/t5318-commit-graph.sh b/t/t5318-commit-graph.sh
index 0c500f7ca2..75fe09521f 100755
--- a/t/t5318-commit-graph.sh
+++ b/t/t5318-commit-graph.sh
@@ -8,7 +8,8 @@ test_expect_success 'setup full repo' '
 	cd "$TRASH_DIRECTORY/full" &&
 	git init &&
 	git config core.commitGraph true &&
-	objdir=".git/objects"
+	objdir=".git/objects" &&
+	test_oid_init
 '
 
 test_expect_success 'verify graph with no graph file' '
@@ -273,7 +274,7 @@ test_expect_success 'git commit-graph verify' '
 
 NUM_COMMITS=9
 NUM_OCTOPUS_EDGES=2
-HASH_LEN=20
+HASH_LEN="$(test_oid rawsz)"
 GRAPH_BYTE_VERSION=4
 GRAPH_BYTE_HASH=5
 GRAPH_BYTE_CHUNK_COUNT=6
diff --git a/t/test-lib-functions.sh b/t/test-lib-functions.sh
index d82fac9d79..78d8c3783b 100644
--- a/t/test-lib-functions.sh
+++ b/t/test-lib-functions.sh
@@ -1157,3 +1157,72 @@ depacketize () {
 		}
 	'
 }
+
+# Set the hash algorithm in use to $1.  Only useful when testing the testsuite.
+test_set_hash () {
+	test_hash_algo="$1"
+}
+
+# Detect the hash algorithm in use.
+test_detect_hash () {
+	# Currently we only support SHA-1, but in the future this function will
+	# actually detect the algorithm in use.
+	test_hash_algo='sha1'
+}
+
+# Load common hash metadata and common placeholder object IDs for use with
+# test_oid.
+test_oid_init () {
+	test -n "$test_hash_algo" || test_detect_hash &&
+	test_oid_cache <"$TEST_DIRECTORY/oid-info/hash-info" &&
+	test_oid_cache <"$TEST_DIRECTORY/oid-info/oid"
+}
+
+# Load key-value pairs from stdin suitable for use with test_oid.  Blank lines
+# and lines starting with "#" are ignored.  Keys must be shell identifier
+# characters.
+#
+# Examples:
+# rawsz sha1:20
+# rawsz sha256:32
+test_oid_cache () {
+	local tag rest k v &&
+
+	{ test -n "$test_hash_algo" || test_detect_hash; } &&
+	while read tag rest
+	do
+		case $tag in
+		\#*)
+			continue;;
+		?*)
+			# non-empty
+			;;
+		*)
+			# blank line
+			continue;;
+		esac &&
+
+		k="${rest%:*}" &&
+		v="${rest#*:}" &&
+
+		if ! expr "$k" : '[a-z0-9][a-z0-9]*$' >/dev/null
+		then
+			error 'bug in the test script: bad hash algorithm'
+		fi &&
+		eval "test_oid_${k}_$tag=\"\$v\""
+	done
+}
+
+# Look up a per-hash value based on a key ($1).  The value must have been loaded
+# by test_oid_init or test_oid_cache.
+test_oid () {
+	local var="test_oid_${test_hash_algo}_$1" &&
+
+	# If the variable is unset, we must be missing an entry for this
+	# key-hash pair, so exit with an error.
+	if eval "test -z \"\${$var+set}\""
+	then
+		error "bug in the test script: undefined key '$1'" >&2
+	fi &&
+	eval "printf '%s' \"\${$var}\""
+}