diff --git a/t/README b/t/README
index 9028b47d92..eed574dd9b 100644
--- a/t/README
+++ b/t/README
@@ -806,6 +806,28 @@ 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..b94221b951 100755
--- a/t/t0000-basic.sh
+++ b/t/t0000-basic.sh
@@ -821,6 +821,43 @@ 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
 
diff --git a/t/test-lib-functions.sh b/t/test-lib-functions.sh
index 4207af4077..7293041b1e 100644
--- a/t/test-lib-functions.sh
+++ b/t/test-lib-functions.sh
@@ -1155,3 +1155,72 @@ 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}\""
+}