diff --git a/Makefile b/Makefile
index 5776309365..f7e824f25c 100644
--- a/Makefile
+++ b/Makefile
@@ -1073,6 +1073,7 @@ LIB_OBJS += notes-cache.o
 LIB_OBJS += notes-merge.o
 LIB_OBJS += notes-utils.o
 LIB_OBJS += notes.o
+LIB_OBJS += object-file-convert.o
 LIB_OBJS += object-file.o
 LIB_OBJS += object-name.o
 LIB_OBJS += object.o
diff --git a/object-file-convert.c b/object-file-convert.c
new file mode 100644
index 0000000000..4777aba836
--- /dev/null
+++ b/object-file-convert.c
@@ -0,0 +1,57 @@
+#include "git-compat-util.h"
+#include "gettext.h"
+#include "strbuf.h"
+#include "repository.h"
+#include "hash-ll.h"
+#include "object.h"
+#include "object-file-convert.h"
+
+int repo_oid_to_algop(struct repository *repo, const struct object_id *src,
+		      const struct git_hash_algo *to, struct object_id *dest)
+{
+	/*
+	 * If the source algorithm is not set, then we're using the
+	 * default hash algorithm for that object.
+	 */
+	const struct git_hash_algo *from =
+		src->algo ? &hash_algos[src->algo] : repo->hash_algo;
+
+	if (from == to) {
+		if (src != dest)
+			oidcpy(dest, src);
+		return 0;
+	}
+	return -1;
+}
+
+int convert_object_file(struct strbuf *outbuf,
+			const struct git_hash_algo *from,
+			const struct git_hash_algo *to,
+			const void *buf, size_t len,
+			enum object_type type,
+			int gentle)
+{
+	int ret;
+
+	/* Don't call this function when no conversion is necessary */
+	if ((from == to) || (type == OBJ_BLOB))
+		BUG("Refusing noop object file conversion");
+
+	switch (type) {
+	case OBJ_COMMIT:
+	case OBJ_TREE:
+	case OBJ_TAG:
+	default:
+		/* Not implemented yet, so fail. */
+		ret = -1;
+		break;
+	}
+	if (!ret)
+		return 0;
+	if (gentle) {
+		strbuf_release(outbuf);
+		return ret;
+	}
+	die(_("Failed to convert object from %s to %s"),
+		from->name, to->name);
+}
diff --git a/object-file-convert.h b/object-file-convert.h
new file mode 100644
index 0000000000..a4f802aa8e
--- /dev/null
+++ b/object-file-convert.h
@@ -0,0 +1,24 @@
+#ifndef OBJECT_CONVERT_H
+#define OBJECT_CONVERT_H
+
+struct repository;
+struct object_id;
+struct git_hash_algo;
+struct strbuf;
+#include "object.h"
+
+int repo_oid_to_algop(struct repository *repo, const struct object_id *src,
+		      const struct git_hash_algo *to, struct object_id *dest);
+
+/*
+ * Convert an object file from one hash algorithm to another algorithm.
+ * Return -1 on failure, 0 on success.
+ */
+int convert_object_file(struct strbuf *outbuf,
+			const struct git_hash_algo *from,
+			const struct git_hash_algo *to,
+			const void *buf, size_t len,
+			enum object_type type,
+			int gentle);
+
+#endif /* OBJECT_CONVERT_H */